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Modern management of fish stocks is based on integrating the precautionary approach with the maximum sustainable yield framework. It relies on accurate estimation of precautionary limits, defined as levels of spawning biomass where a stock has reduced reproductive capacity, and harvesting targets aimed to maximise future yields. Therefore, it is heavily depending on productivity assumptions. Most fish stocks are managed assuming that productivity will increase as the stock size decreases (i.e., density dependent compensatory stock and recruitment relationship). However, several biological and ecological processes will result in a decreased productivity below a certain population size, referred to as the Allee effect or depensation. Through a meta‐analysis of 81 Northeast Atlantic fish stocks, we investigated the impact of assuming compensatory recruitment in the presence of depensation in fisheries management. Across life histories, depensation results in a 22% reduction of the fishing mortality rate leading to extinction. On average, the maximum reproductive rate per spawning biomass was found at 35% of BMSY, which was also the biomass where stocks have a 5% risk of extinction without fishing. Finally, the presence of depensation resulted in increased rebuilding times when stock spawning biomass falls below the limit reference point. When depensatory effects are present, assuming increasing productivity at low biomass will generally result in over‐optimistic perceptions of rebuilding and stock status at biomass below 25% and 45% of BMSY in general, and for pelagic stocks respectively. When not accounted for, depensation will potentially lead to unsustainable harvesting practices of marine living resources. [ABSTRACT FROM AUTHOR]

Determining how resilient a stock-recruitment relationship is to environmental variation is crucial for fisheries management. Steepness is a key factor characterizing the resilience of a fish stock and, hence, for establishing management reference points. This study estimates the distribution of steepness for Pacific saury using a simulation approach based on evolutionary ecology and reproductive biology, and how it changes in response to environmental change. The median estimated steepness is 0.82 (80% probable range 0.59, 0.93) based on the best available biological information, which suggests that Pacific saury can produce a relatively high proportion of unfished recruitment when depleted to 20% of unfished spawning biomass. Elasticity analysis indicates that steepness for Pacific saury is most sensitive to the survival rate of early life stages, mean body weight, growth, and length-at-maturity. Environmental change could substantially impact steepness, with unfavorable conditions related to survival rates, length-at-maturity, mean body weight, and growth potentially leading to a reduction in resilience. Understanding these impacts is crucial for the assessment and management of Pacific saury. Our numerical simulation approach provides an analytical tool applicable for calculating the steepness distribution in other small pelagic fish influenced by increases in sea surface temperature due to global warming. [ABSTRACT FROM AUTHOR]

Japanese sardines (Sardinops melanostictus) are environmentally sensitive pelagic fish. We investigated the effects of environmental factors on the stock-recruitment (S-R) relationship of S. melanostictus in the Pacific Ocean from 1984 to 2018. We modeled and analyzed the recruitment of S. melanostictus using the Ricker model, the Ricker environment extension model (Ricker-E), and the generalized additive model (GAM). Different numbers of environmental factors were added to the models for fitting, and the Akaike information criterion (AIC) was used to select the optimal model. The results showed that the nonlinear GAM provided the best fit. The results of the GAM single factor and multifactor analysis showed that environmental factors were significantly correlated with recruitment in S. melanostictus (p < 0.05). The best fitting model was the GAM, with an AIC of 57.9 and a cumulative explanation rate of 95.7%. Sea surface temperature was the most dominant environmental factor and had a negative impact on R/S. Considering both spawning stock biomass (S) and the environmental factors in the S-R relationship is important for studying the mechanisms of complementary changes and population changes in S. melanostictus under the influence of the marine environment. [ABSTRACT FROM AUTHOR]

Japanese sardines (Sardinops melanostictus) are environmentally sensitive pelagic fish. We investigated the effects of environmental factors on the stock-recruitment (S-R) relationship of S. melanostictus in the Pacific Ocean from 1984 to 2018. We modeled and analyzed the recruitment of S. melanostictus using the Ricker model, the Ricker environment extension model (Ricker-E), and the generalized additive model (GAM). Different numbers of environmental factors were added to the models for fitting, and the Akaike information criterion (AIC) was used to select the optimal model. The results showed that the nonlinear GAM provided the best fit. The results of the GAM single factor and multifactor analysis showed that environmental factors were significantly correlated with recruitment in S. melanostictus (p < 0.05). The best fitting model was the GAM, with an AIC of 57.9 and a cumulative explanation rate of 95.7%. Sea surface temperature was the most dominant environmental factor and had a negative impact on R/S. Considering both spawning stock biomass (S) and the environmental factors in the S-R relationship is important for studying the mechanisms of complementary changes and population changes in S. melanostictus under the influence of the marine environment.

The recruitment and biomass of a fish stock are influenced by their environmental conditions and anthropogenic pressures such as fishing. The variability in the environment often translates into fluctuations in recruitment, which then propagate throughout the stock biomass. In order to manage fish stocks sustainably, it is necessary to understand their dynamics. Here, we systematically explore the dynamics and sensitivity of fish stock recruitment and biomass to environmental noise. Using an age-structured and trait-based model, we explore random noise (white noise) and autocorrelated noise (red noise) in combination with low to high levels of harvesting. We determine the vital rates of stocks covering a wide range of possible body mass (size) growth rates and asymptotic size parameter combinations. Our study indicates that the variability of stock recruitment and biomass are probably correlated with the stock's asymptotic size and growth rate. We find that fast-growing and large-sized fish stocks are likely to be less vulnerable to disturbances than slow-growing and small-sized fish stocks. We show how the natural variability in fish stocks is amplified by fishing, not just for one stock but for a broad range of fish life histories.

This study presents age distributions in an exploited population of spawning chokka squid Loligo reynaudii together with their back-calculated spawning times, and considers the results in relation to the exploitation of this species. Samples were collected during two closed fishing seasons, in 2003 and 2004. Age after hatching ranged, in males, from 168 to 484 days, with a mean of 323 days (71–425 mm mantle length [ML]), and in females from 125 to 478 days, with a mean of 316 days (83–263 mm ML). Detailed analysis of catches during nine days of fishing after the end of the closed season indicated strongly that the closed season (October–November) has been beneficial for both the chokka resource and the fishery. The temporal distribution of egg-laying events for parental populations, and a high abundance of squid in the days immediately after the end of the closed season, indicate a link between the parental spawning stock and the resulting spawning stock. This hypothesis was formulated using the distribution of egg-laying events in time, the strength of egg-laying events, and data for the daily catch immediately (i.e. nine days) after the closed season. However, the hypothesis requires rigorous testing using statoliths collected over a longer period. [ABSTRACT FROM AUTHOR]

Impacts of biological uncertainties on estimates of stock–recruitment relationships (SRRs) in elasmobranchs such as lamniform sharks were evaluated using a numerical approach based on an age-structured model considering reproductive ecology in elasmobranchs. The values of steepness were estimated using several combinations of life history parameters for North Pacific shortfin mako to elucidate whether the numerical approach could identify reasonable values for fundamental life history parameters as well as steepness. The results of the numerical approach indicated that the mean values of steepness and their 95% confidence intervals were highly sensitive to combinations of values for growth rate, maturity ogive, longevity, reproductive cycle, and natural mortality rate. Meanwhile, the most plausible combinations of the biological parameters and values of steepness were identified based on the numerical approach with biological knowledge. The mean values and their standard deviation (SD) for steepness with the Beverton-Holt-SRR model were 0.353 (SD =0.057) and 0.273 (SD =0.046) for 2- and 3-year reproductive cycles, respectively. The numerical approach therefore has high potential to become an important tool for estimating SRR in elasmobranchs such as lamniform sharks, and the application of this approach to other elasmobranchs could greatly contribute to improvements in stock assessment and management. [ABSTRACT FROM AUTHOR]

Reference points are used to determine whether overfishing is occurring, a stock is overfished, or the abundance of the stock is approaching an undesirable level. Reference points are often classified as limit reference points, which indicate an undesirable state, or target reference points, which specify a desirable one. Limit reference points frequently indicate a state for which the reproductive potential of the stock is impacted, and are thus based on spawning output. Spawning output is also used in the stock-recruitment curve, which is a key component of stock-assessment models. We investigated the effect on stock assessment results of metrics of spawning output that are used either in the stock-recruitment relationship or to evaluate reference points. We use yellowfin tuna (Thunnus albacares) in the eastern Pacific Ocean as an example. The metrics biomass of mature females and fecundity attribute higher reproductive potential to the older ages, while numbers of mature females and summary biomass attribute more to the younger ages. Both the stock assessment results and the evaluation of reference points are sensitive to the metric used. The most optimistic perceptions of stock status are obtained when using numbers of mature females, female summary biomass or summary biomass to evaluate the reference points. The choice of metric may be as important as the uncertainty about steepness of the stock-recruitment curve. Given the difficulty in determining the appropriate measure of spawning output and the fact that populations fluctuate naturally, it is recommended that fishing mortality-based reference points are given more consideration for management advice aiming to maintain stocks at desirable states (target reference points) and that harvest control rules based on both types of reference points be compared for their relative effectiveness in avoiding recruitment overfishing and achieving other fisheries objectives using Management Strategy Evaluation. [ABSTRACT FROM AUTHOR]

Harvesting can induce rapid evolution in animal populations, yet the role of ecological change in buffering or enhancing that response is poorly understood. Here, we developed an eco‐genetic model to examine how ecological changes brought about by two notorious invasive species, zebra and quagga mussels, influence harvest‐induced evolution and resilience in a freshwater fish. Our study focused on lake whitefish (Coregonus clupeaformis) in the Laurentian Great Lakes, where the species supports valuable commercial and subsistence fisheries, and where the invasion of dreissenid (zebra and quagga) mussels caused drastic shifts in ecosystem productivity. Using our model system, we predicted faster rates of evolution of maturation reaction norms in lake whitefish under pre‐invasion ecosystem conditions when growth and recruitment of young to the population were high. Slower growth rates that occurred under post‐invasion conditions delayed when fish became vulnerable to the fishery, thus decreasing selection pressure and lessening the evolutionary response to harvest. Fishing with gill nets and traps nets generally selected for early maturation at small sizes, except when fishing at low levels with small mesh gill nets under pre‐invasion conditions; in this latter case, evolution of delayed maturation was predicted. Overall, the invasion of dreissenid mussels lessened the evolutionary response to harvest, while also reducing the productivity and commercial yield potential of the stock. These results demonstrate how ecological conditions shape evolutionary outcomes and how invasive species can have a direct effect on evolutionary responses to harvest and sustainability. [ABSTRACT FROM AUTHOR]

Stock-recruitment relationships in elasmobranchs are uncertain due to a lack of data, especially regarding pre-recruit survivorship. A pre-recruit survival model for the early life history of elasmobranchs was developed and parameterized using biological data. Then, an existing age-structured model for the reproductive ecology of teleost fishes was modified for elasmobranchs and combined with the pre-recruit model. The combined model was then applied to the North Pacific blue shark ( Prionace glauca ). The stock-recruitment relationships of this species were clarified using the biological data collected from wide areas of the western North Pacific. The model provides a point estimate for steepness, which represents a fraction of the unfished recruitment when spawning stock biomass is 20% of the unfished spawning stock biomass. Numerical simulations were conducted to incorporate uncertainties in the biological parameters and produce the variance of steepness. The mean value and its standard deviation for steepness with the Beverton-Holt model were 0.584 (standard deviation = 0.099). The results suggest that the stock-recruitment relationship in North Pacific blue shark remains highly density-dependent and that its productivity is higher than that of other viviparous elasmobranchs. [ABSTRACT FROM AUTHOR]

The relationship between spawning fish abundance and the number of offspring, the so-called stock-recruitment relationship, is crucial for fisheries management and conservation measures. Using the most comprehensive data set ever assembled, we quantify this relationship for 211 fish stocks worldwide, revealing a global pattern with a pervasive asymptotic shape that shows increasing recruitment reaching an upper limit for values around half to two-thirds of parental biomass. This corroborates previous theoretical and modeling results. However, parental biomass is a predictor for only 5% to 15% of the variance in recruitment, demonstrating the weak predictive power of the stock-recruitment relationship in marine fish populations. Thus, there is a need to move rapidly toward models that integrate environmental conditions and species interactions in fisheries stock assessment and management, as suggested by Johan Hjort 100 years ago

The demographic Allee effect, or depensation, implies positive association between per capita population growth rate and population size at low abundances, thereby lowering growth ability of sparse populations. This can have far-reaching consequences on population recovery ability and colonization success. In the context of marine fishes, there is a widespread perception that Allee effects are rare or non-existent. However, studies that have failed to detect Allee effects in marine fishes have suffered from several fundamental methodological and data limitations. In the present study, we challenge the prevailing perception about the rarity of Allee effects by analysing nine populations of Atlantic herring (Clupea harengus), using Bayesian statistical methods. We find that populations of the same species can show either strong evidence for Allee effects or compensation. We explicitly demonstrate how the evidence for Allee effects is strongly provisional on observations made at low population abundances. We contrast our statistical approach with previous attempts to detect Allee effects and illustrate methodological issues that can lead to erroneous conclusions about the nature of population dynamics at low abundance. The present study demonstrates that there is no substantive scientific basis to support the perception that Allee effects are rare or non-existent in marine fishes. [ABSTRACT FROM AUTHOR]

This study presents a Monte Carlo method ( CMSY) for estimating fisheries reference points from catch, resilience and qualitative stock status information on data-limited stocks. It also presents a Bayesian state-space implementation of the Schaefer production model ( BSM), fitted to catch and biomass or catch-per-unit-of-effort ( CPUE) data. Special emphasis was given to derive informative priors for productivity, unexploited stock size, catchability and biomass from population dynamics theory. Both models gave good predictions of the maximum intrinsic rate of population increase r, unexploited stock size k and maximum sustainable yield MSY when validated against simulated data with known parameter values. CMSY provided, in addition, reasonable predictions of relative biomass and exploitation rate. Both models were evaluated against 128 real stocks, where estimates of biomass were available from full stock assessments. BSM estimates of r, k and MSY were used as benchmarks for the respective CMSY estimates and were not significantly different in 76% of the stocks. A similar test against 28 data-limited stocks, where CPUE instead of biomass was available, showed that BSM and CMSY estimates of r, k and MSY were not significantly different in 89% of the stocks. Both CMSY and BSM combine the production model with a simple stock-recruitment model, accounting for reduced recruitment at severely depleted stock sizes. [ABSTRACT FROM AUTHOR]

Pacific bluefin tuna (PBF) ( Thunnus orientalis ) is commercially important in the North Pacific Ocean. Although its stock has been relatively low for decades, international discussions on a long-term management framework, including the definition of a limit reference point (LRP), have only recently started. This paper argues that an LRP for PBF could be developed by determining a biomass level that would prevent recruitment overfishing. First, it reviews the development of LRPs for various tuna species and demonstrates that most of these limits are not necessarily based on biological information on the respective species. Then, the current management of PBF is also reviewed as background information for considering an LRP for PBF. Finally, a variety of simple analyses of the stock–recruitment relationship of PBF are conducted to find a biomass level that would prevent recruitment overfishing—i.e. an LRP below which stocks should not fall. It is concluded that, for the first time to our knowledge, defining such an LRP for a tuna species is possible (about 30 thousand tonnes or 5% of estimated unfished spawning stock biomass in our calculation). Not only is the LRP based on actual experience, but also the logic behind it would be easier for stakeholders to understand than the theoretical LRPs used elsewhere. This LRP should be useful in future in more comprehensive management framework, such as one through management strategy evaluations, in which stakeholder involvement in decision-making is crucial. [ABSTRACT FROM AUTHOR]

In this work I introduce a probabilistic stock-recruitment function, of the Cushing family, that stands as an alternative to the canonical formulas provided by Beverton-Holt and Ricker, among others. I embed this function in a mathematically tractable (dynamic linear) population model, which renders inference of abundance-at-age and unknown biological parameters (natural mortality, virgin stock's egg production rate, and steepness), as well as fisheries parameters (catchability and selectivity), easier than with canonical representations. To assist management, I provide formulas for exact and approximate reference points, associated with Maximum Sustainable Yield (MSY) and Maximum Excess Recruitment (MER). I also introduce a new summary statistic, called bottleneck abundance ratio, which requires no knowledge of steepness under the proposed stock-recruitment function. With simulated data and the concept of Pretty Good Yield, I generate bounds for MSY- and MER-based reference points and show that those based on the new function have greater resilience to uncertainty about steepness. As a case study, I apply a state-space model to the US Gulf menhaden fishery, 1964–2004. Results suggest higher than previously considered natural mortality and a discernible connection between parental stock abundance and recruitment, undetected with Beverton-Holt and Ricker models. [ABSTRACT FROM AUTHOR]

We review the precautionary approach to fisheries management, propose a framework that will allow a systematic assessment of insufficient precaution and provide an illustration using an Antarctic fishery. For a single-species fishery, our framework includes five attributes: (1) limit reference points that recognize gaps in our understanding of the dynamics of the species; (2) accurate measures of population size; (3) ability to detect population changes quickly enough to arrest unwanted declines; (4) adequate understanding of ecosystem dynamics to avoid adverse indirect effects; and (5) assessment of the first four elements by a sufficiently impartial group of scientists. We argue that one or more of these elements frequently fail to be present in the management of many fisheries. Structural uncertainties, which characterize almost all fisheries models, call for higher limit points than those commonly used. A detailed look into the five elements and associated uncertainties is presented for the fishery on the Antarctic toothfish in the Ross Sea ( FAO/ CCAMLR Area 88.1, 88.2), for which management was recently described as 'highly precautionary'. In spite of having features that make the Ross Sea fishery ideal for the application of the precautionary approach, gaps in our knowledge and failure to acknowledge these gaps mean that current regulation falls short of being sufficiently precautionary. We propose some possible remedies. [ABSTRACT FROM AUTHOR]

The recruitment processes of Baltic cod and sprat were analysed and critical periods were identified by addressing the major impact factors on individual early life history stages separately and relating observed abundance data between successive stages. For cod, recruitment appeared to be dependent on egg survival, with low oxygen concentration in dwelling depths and predation by clupeids as the major causes for egg mortality. Surviving egg production and larval abundance were weakly correlated, whereas larval abundance was significantly related to year class strength. This indicated that the period between the late egg and the early larval stage is critical for cod recruitment. A potential variable identified to affect this life stage was prey availability for larvae. For sprat, early and late egg stage production as well as late egg stage production and larval abundance were significantly related. However, year class strength was largely independent of larval abundance. Thus, the period between the late larval and early juvenile stage appeared to be critical for sprat recruitment. Potential variables identified to affect this life stage were ambient temperature and wind stress. Environmental factors showing statistically significant covariance with the survival of one of these critical life stages were incorporated into stock-recruitment models for individual spawning areas separately and for the Central Baltic combined.

The resilience of a stock-recruitment relationship is a key characteristic for modeling the population dynamics of living marine resources. Steepness determines the expected resiliency of a fish stock to harvest and is fundamentally important for the estimation of biological reference points such as maximum sustainable yield. Stock-recruitment steepness was the primary uncertainty for the determination of stock status and biological reference points in recent stock assessments of Western and Central North Pacific striped marlin ( Kajikia audax ). We therefore applied the method of Mangel et al. to estimate probable values of steepness for striped marlin using new information on the mean batch fecundity, spawning frequency, and spawning season duration under an assumption of Beverton–Holt stock-recruitment dynamics. Results indicated that the median steepness was 0.87 with an 80% probable range of (0.38, 0.98). It is very likely that North Pacific striped marlin is highly resilient to reductions in spawning potential. Variation in reproductive and life history parameters had an important influence on the distribution of steepness. Sensitivity analyses showed that steepness was most sensitive to body girth, mean egg weight, and most importantly, early life history stage survival. Sensitivity analyses also confirmed that the effects of changes in life history parameters on steepness were consistent with expected increases or decreases in reproductive output due to changes in body weight or fecundity. Our approach can be applied to pelagic fish species to directly assess the probable distribution of stock-recruitment resiliency when sufficient information on reproductive ecology and life history parameters is available. [ABSTRACT FROM AUTHOR]

Delay-difference models are intermediate between simple surplus-production models and complicated age-structured models. Such intermediate models are more efficient and require less data than age-structured models. In this study, a delay-difference model was applied to fit catch and catch per unit effort (CPUE) data (1975-2011) of the southern Atlantic albacore ( Thunnus alalunga) stock. The proposed delay-difference model captures annual fluctuations in predicted CPUE data better than Fox model. In a Monte Carlo simulation, white noises (CVs) were superimposed on the observed CPUE data at four levels. Relative estimate error was then calculated to compare the estimated results with the true values of parameters α and β in Ricker stock-recruitment model and the catchability coefficient q. a is more sensitive to CV than β and q. We also calculated an 80% percentile confidence interval of the maximum sustainable yield (MSY, 21756 t to 23408 t; median 22490 t) with the delay-difference model. The yield of the southern Atlantic albacore stock in 2011 was 24122 t, and the estimated ratios of catch against MSY for the past seven years were approximately 1.0. We suggest that care should be taken to protect the albacore fishery in the southern Atlantic Ocean. The proposed delay-difference model provides a good fit to the data of southern Atlantic albacore stock and may be a useful choice for the assessment of regional albacore stock. [ABSTRACT FROM AUTHOR]

The English Channel cuttlefish (Sepia officinalis) is the most abundant cephalopod resource in the Northeast Atlantic and one of the three most valuable resources for English Channel fishers. Depletion methods and age-structured models have been used to assess the stock, though they have shown limitations related to the model assumptions and data demand. A two-stage biomass model is, therefore, proposed here using, as input data, four abundance indices derived from survey and commercial trawl data collected by Ifremer and Cefas. The model suggests great interannual variability in abundance during the 17 years of the period considered and a decreasing trend in recent years. Model results suggest that recruitment strength is independent of spawning–stock biomass, but appears to be influenced by environmental conditions such as sea surface temperature at the start of the life cycle. Trends in exploitation rate do not reveal evidence of overexploitation. Reference points are proposed and suggestions for management of the sustainable utilization of cuttlefish in the English Channel are advanced. [ABSTRACT FROM AUTHOR]

Highlights: [•] B MSY estimates are needed to manage Alaska crab fisheries. [•] Proxy methods for estimating B MSY are currently used for management. [•] Methods which use stock and recruitment data and allow for changes in productivity due to the environment are outlined. [Copyright &y& Elsevier]

Highlights: [•] A stock–recruitment model was compiled for pikeperch, Sander lucioperca. [•] Water temperature was incorporated in the model as environmental factor. [•] In the stock assessment, commercial and recreational catches were taken into account. [•] The model was able to closely predict the year-class strengths of pikeperch (R 2 =0.8). [Copyright &y& Elsevier]

Highlights: [•] Meta-analysis can be used to develop prior distributions for the steepness parameter of the stock–recruitment relationship. [•] Three alternative methods for conducting steepness meta-analysis are outlined. [•] A method based on profile likelihood performed best. [ABSTRACT FROM AUTHOR]

When viewed on a discrete time step (e.g., one year) rather than instantaneously, the Ricker and Beverton-Holt stock recruitment relationships describe a strong influence of changes in adult abundance on juvenile survival even at low biomass levels. In fact, they describe a stronger influence at lower adult abundance than at high adult abundance. This is not consistent with a simplistic interpretation of carrying capacity, where survival reduces more rapidly as the juvenile abundance approaches its carrying capacity due to resource limitation. It implies that other processes (e.g., changes in predation risk with increased abundance as described by arena theory) control density dependence. A simple habitat limitation model suggests that a similar relationship can occur if the number of juveniles produced by the adults is always more than the juvenile carrying capacity. However, the resulting stock-recruitment relationship is constant recruitment, while that of the Beverton-Holt is increasing asymptotically and the Ricker is dome shaped. Fluctuations around an average recruitment level for a wide range of adult abundance levels is common for highly fecund pelagic spawners (e.g., tuna), suggesting that the survival and recruitment relationships implied by the simple habitat limitation model might be appropriate for these species. Underpinning this argument is the concept that the adult population will saturate the juvenile carrying capacity unless the adult population is fished to extremely low levels. However, it is reasonable to expect that survival should be density independent, or possibly even decompensatory, at some low level of adult abundance. Ignoring depensation, the survival relationship would therefore be well described by a general logistic equation where the decline in survival initially accelerates as abundance increases and then deaccelerates. The resulting stock-recruitment relationship would be a hockey stick shape with the possibility of reduced recruitment at large stock sizes. The model could also be used for low fecund species (e.g., sharks) for which the juvenile carrying capacity is not saturated until the adult abundance is large (i.e., survival is density-independent for a wide range of adult population sizes). In our model, particularly for high fecund species, temporal variation in recruitment would be controlled by changes in the juvenile carrying capacity. We provide equations for the generalized logistic survival stock-recruitment model and investigate the yield consequences. We suggest that an important management consideration is the level of adult abundance at which the juvenile carrying capacity becomes saturated (i.e., when does density dependence become influential on juvenile survival). [ABSTRACT FROM AUTHOR]

The maximum annual reproductive rate (i.e., the slope at the origin in a stockrecruitment relationship) is one of the most important biological reference points in fisheries; it sets the upper limit to sustainable fishing mortality. Estimating the maximum reproductive rate by fitting parametric models to stock-recruitment data may not be a robust approach because two statistically indistinguishable models can generate radically different estimates. To mitigate this issue, we developed a flexible, semiparametric Bayesian approach based on a conditional Gaussian process prior specifically designed to estimate the maximum annual reproductive rate, and applied it to analyze simulated stock-recruitment data sets. Compared with results based on other Gaussian process priors, we found that the conditional Gaussian process prior provided superior results: the accuracy and precision of estimates were enhanced without increasing model complexity. Moreover, compared with parametric alternatives, performance of the conditional Gaussian process prior was comparable to that of the datagenerating model and always better than the wrong model. [ABSTRACT FROM AUTHOR]

Abstract: Understanding the relationship between abundance of spawners and subsequent recruitment is one of the central issues in fisheries stock assessment. We developed a new, pre-recruit survival based stock–recruitment model that enables explicit modeling of survival between embryos and age 0 recruits, and allows the description of a wide range of pre-recruit survival curves. The model is especially useful for low fecundity species that produce relatively few offspring per litter and exhibit a more direct connection between spawning output and recruitment than species generating millions of eggs. The proposed model provides additional flexibility in the stock–recruitment options that may be explored in any fishery stock assessment, and it is now available within the Stock Synthesis assessment platform. In this paper, we describe the mathematical formulation of the new stock–recruitment model, explain how this model can be specified within Stock Synthesis, and use it to model the stock–recruitment relationship of the spiny dogfish shark in the Northeast Pacific Ocean. We compare the results of the application of our new stock–recruitment model, with those from traditional Beverton–Holt relationship, and illustrate why the new approach is more appropriate for this species. [Copyright &y& Elsevier]

We examined the effects of process and observation errors on the selection of the stock-recruitment relationship (SRR) curve using simulations. When the process and observation errors were added to both spawning stock biomass and recruitment, the results were as follows: (1) When the proportional model was set as the true SRR model, there was a high probability that the Ricker or Beverton and Holt model was selected in response to the errors; (2) When the Ricker or Beverton and Holt model was set as the true SRR model, the proportional model was seldom selected in response to the errors; (3) The proportional SRR model should be accepted as the optimum SRR model for the Pacific stock of Japanese sardine; (4) We should use an SRR model that is constructed from more than two independent variables (spawning stock biomass, environmental factors, etc.) when we discuss management of fisheries resources. [ABSTRACT FROM AUTHOR]

The relationship between the biomass of reproductively mature individuals (spawning stock) and the resulting offspring added to the population (recruitment), the stock–recruitment relationship, is a fundamental and challenging problem in all of population biology. The steepness of this relationship is commonly defined as the fraction of recruitment from an unfished population obtained when the spawning stock biomass is 20% of its unfished level. Since its introduction about 20 years ago, steepness has become widely used in fishery management, where it is usually treated as a statistical quantity. Here, we investigate the reproductive ecology of steepness, using both unstructured and age-structured models. We show that if one has sufficient information to construct a density-independent population model (maximum per capita productivity and natural mortality for the unstructured case or maximum per capita productivity, natural mortality and schedules of size and maturity at age for the structured model) then one can construct a point estimate for steepness. Thus, steepness cannot be chosen arbitrarily. If one assumes that the survival of recruited individuals fluctuates within populations, it is possible, by considering the early life history, to construct a prior distribution for steepness from this same demographic information. We develop the ideas for both compensatory (Beverton–Holt) and over-compensatory (Ricker) stock–recruitment relationships. We illustrate our ideas with an example concerning bluefin tuna ( Thunnus thynnus/orientalis, Scombridae). We show that assuming that steepness is unity when recruitment is considered to be environmentally driven is not biologically consistent, is inconsistent with a precautionary approach, and leads to the wrong scientific inference (which also applies for assigning steepness any other single value). [ABSTRACT FROM AUTHOR]

Tjelmeland, S., and Røttingen, I. 2009. Objectives and harvest control rules in the management of the fishery of Norwegian spring-spawning herring. – ICES Journal of Marine Science, 66: 1793–1799.The main element in the management of the Norwegian spring-spawning herring, as implemented by the coastal states, is to conduct the fishery based on a maximum fishing mortality (F) of 0.125. As the appropriateness of this rule (given the stated objectives) has not yet been tested thoroughly, we set out to do this by long-term simulations, in which we applied a range of alternative stock–recruitment relationships. These different relationships are estimated from historical replicates of the stock, as calculated by the herring-stock assessment model SeaStar. During prognostic simulations, a recruitment model is selected probabilistically for each historical replicate based on Akaike weights. We evaluate whether the management objectives are met by applying the present harvest control rule. Results are given for the current assessment option of natural mortality (M = 0.5) in the oldest aggregated age group and for the assessment option used in 2005 and earlier (M = 0.15). These show that perceptions of the long-term yield differ considerably and that the current management is somewhat on the conservative side from the perspective of maximum sustainable yield. [ABSTRACT FROM PUBLISHER]

Abstract: We used a spatially explicit stochastic simulation model to evaluate whether source–sink population dynamics would affect performance of alternative harvest policies for yellow perch, Perca flavescens, in southern Lake Michigan. The model contained four management areas in southern Lake Michigan representing each U.S. state''s waters. We parameterized the model such that all recruitment was produced by only one management area, considering each of the four areas to be the sole source in turn, and contrasted results with a base scenario where all areas produced recruits. We evaluated three types of harvest policies: constant-F, where fishing mortality was constant, and two state-dependent policies, where fishing mortality was constant above either 40% or 70% of unfished spawning stock biomass (B 0) and decreased to 0 at 0% B 0. We used four performance statistics to evaluate polices: (1) average percentage of B 0 remaining, (2) percentage of years with low spawning stock biomass, (3) average recreational harvest, and (4) percentage of years with low recreational harvest. Performance of harvest policies differed predictably depending on which management area was the source because relative productivity of stock-recruitment relationships and growth patterns differed among source areas. Thus, if management areas on the western side of Lake Michigan were the source of most of the recruits, the fishery could support higher fishing mortality rates than if areas on the eastern side of the lake were sources. State-dependent harvest policies were less sensitive to assumptions about the source of recruits than were constant fishing mortality rate policies. The 0–70 policy was most robust to source–sink dynamics across most of the performance statistics and levels of fishing mortality. [Copyright &y& Elsevier]

Åström M., and Dekker W. 2007. When will the eel recover? A full life-cycle model. – ICES Journal of Marine Science, 64: 000–000: –.The European eel population has declined over the past decades in most of its distribution area, and the stock is outside safe biological limits. The EU has taken up the challenge to design a management system that ensures the escapement of 40% of spawning-stock biomass, relative to unexploited, unpolluted circumstances in unobstructed rivers. This ultimately aims to restore the spawning stock to a level at which glass eel production is not impaired, i.e. to restore to full historical glass eel recruitment. To explore the trajectory from the current depleted state to full recruitment recovery, we developed a simple model of stock dynamics, based on a simplified stock–recruitment relationship and the conventional dynamic pool assumptions. Recruitment trajectories under different future fishery regimes are explored, for the medium (one generation time) and long time-span (until full recruitment recovery). Reducing fisheries to zero, recovery is expected within ∼80 years, whereas under an ultimately sustainable fishing regime of just 10% of the current rate of fishing mortality, recovery may take more than 200 years. Moreover, management regimes, apparently leading to slight recovery of the stock in the coming 5–15 years, might still be unsustainable in the long run. [ABSTRACT FROM PUBLISHER]

This study assessed the stock-recruitment relationship (SRR) for the Japanese sardine Sardinops melanostictus in the North-western Pacific. Of the 20 SRR models investigated, the Akaike information criterion (AIC) was the minimum (AIC = 551.2) when the data were separated into two groups (A and B) and the log-normal distribution was applied as the error term. Group A was constructed with SRR data from 1976–1987 and 1992–2004. Group B consisted of data from 1988–1991. The AIC minimum model was R = 22.8 S × e ε for Group A, where R, S, and ε denote the recruitment of sardine (individual number of 0-year old fish), spawning stock biomass (SSB), and error term, respectively. This model indicated that recruitment was proportional to the SSB and that no density-dependent effect operated over the range of SSB investigated (51 000–11.3 million t). Recruitment was markedly higher (lower) when the sea surface temperature (SST) of the Kuroshio Extension area in February was low (high). The essential SRR can simply be expressed as R = 22.8 S × e ε with the level of recruitment deviating from the model to a greater or lesser degree depending on the environmental conditions. [ABSTRACT FROM AUTHOR]

Kimoto, A., Mouri, T., and Matsuishi, T. 2007. Modelling stock–recruitment relationships to examine stock management policies. – ICES Journal of Marine Science, 64: 870–877. Simulation studies are used widely for fish stock management. In such studies, forecasting future recruitment, which can vary greatly between years, has become an essential part of evaluating management strategies. We propose a new forecasting algorithm to predict recruitment for short- or medium-term stochastic projections, using a stock–recruitment relationship. We address cases in which the spawning stock has dropped below previously observed levels, or in which predicted recruitment is situated close to the maximum observed level. The relative prediction error of seven existing algorithms was compared with that of the new model using leave-one-out cross-validation for 61 data sets from ICES, the Japanese Fisheries Agency, and PICES. The new algorithm had the smallest prediction error for 49 of the data sets, but was slightly biased by the precautionary treatment of predictions of high recruitment. [ABSTRACT FROM AUTHOR]

To measure and predict the response of fish communities to exploitation, it is necessary to understand how the direct and indirect effects of fishing interact. Because fishing and predation are size-selective processes, the potential response can be explored with size-based models. We use a simulation approach to describe the relationship between size spectrum slope and overall fishing mortality and to try to understand how a linear spectrum might be maintained. The model uses 15 parameters to describe a 13-“species” fish community, where species are defined by their maximum body size and the general relationship between size and life-history characteristics. The simulations allow us to assess the role of changes in the strength and type of density dependence in controlling the response to fishing, and to investigate the trade-off between catches and stock status of the different species. The outputs showed that the linear slope of the size spectrum was a function of community exploitation rate. Density-dependent controls, specifically predation mortality and the extent of compensation in the stock-recruit relationship, were key mechanisms in maintaining a linear spectrum. A linear spectrum emerged independent of the rate of compensation in the stock-recruit relationship. When this rate was low, the effects of changes in fishing mortality on predator abundance dominated those on spawning-stock biomass, whereas the dominance was reversed when the compensation rate in the stock-recruit relationship was high. The approach allows us to explore the effects of different fishing mortality schedules on properties of the fish community, to assess how fishing affects species with different life histories in mixed fisheries, and to assess the effects of selectively fishing different size classes. The simulations indicate that the size classes to be included when developing and interpreting size-based metrics must be carefully considered in relation to the trophic structure and likely strength of predatory interactions in the community. Runs with different fishing mortality by size suggest that the dynamics of predation cannot compensate fully for changing rates and patterns of exploitation, implying that the effects of selectively fishing different size classes should be assessed on a case-by-case basis. [Copyright &y& Elsevier]

This study investigated the main causes of population abundance fluctuations. Particularly, attention was paid to whether a density-dependant factor, such as a stock-recruitment relationship (S-R relationship) or a density-independent factor such as an environmental factor, is more important. Using data pertaining to the number of eggs of the Pacific stock mackerel and information about regime shifts and sea surface temperature, the shape of the S-R relationship was discussed and these shapes with the results of simulation trails were compared. Further other historical S-R relationship data were analyzed. The results are as follows: (i) a new mechanism that causes population fluctuations could be proposed, that is, (a) the recruitment is proportional to the spawning stock biomass (SSB) and the relation is expressed by several lines with the same slope, and (b) the shift between the lines occurs due to environmental conditions; and (ii) the density-dependent S-R relationship, which suggests that recruitment decreased due to high density in SSB, proposed by Ricker or Beverton and Holt, would not exist. [ABSTRACT FROM AUTHOR]

The article presents a study which we explore the dynamic response of fertilization efficiency and zygote production to different combinations of adult density, adult spatial distributions, and gamete distributions on broadcast spawning populations in fishery. The results of the study indicate that potential Allee effects at low density in broadcast spawning populations rarely exhibit threshold behavior.

A comparative analysis of the fish condition (Fulton’s K) of 10 cod stocks in the north Atlantic in relation to the temperature of their habitat, growth rates and their reproductive potential is presented. It is shown that the cod stocks in the north Atlantic display different levels of mean condition, which is partly due to the different temperature regimes of their habitats. Cod living in colder waters, e.g. Greenland, Labrador and Grand Bank stocks, were found to be in poorer condition than cod living in warmer waters, e.g. North Sea and Irish Sea stocks.Poor condition causes reduced productivity in terms of slow growth and low recruitment potential. Stocks in better condition display significantly higher weights at age 4 than stocks in poor condition. The a coefficients (function’s slopes) obtained from standardised Ricker’s recruitment–spawning stock biomass (SSB) relationships were defined as indicators for the recruitment potential of stocks. These a coefficients were found to be positively correlated with the mean condition factor of the 10 stocks analysed. This indicates that stocks consisting of individuals in poor condition appear to be very susceptible to reduced recruitment at low SSB, while the stocks that consist of fish in good condition seem to behave more robustly with a higher probability of good recruitment at low SSB. The positive effect of the cod condition on their reproductive potential generally implies that the stocks in good condition in the temperate regions of the northeast and west Atlantic can sustain higher exploitation rates than stocks in poor condition in the colder regimes of the northwest Atlantic (Greenland, Labrador and Grand Bank). This is confirmed by the positive relationship established between the estimated biological management reference points Fmed and the mean cod condition factors, as well as by the recent status of these stocks. [Copyright &y& Elsevier]

The relationship between spawning stock and recruitment in penaeid shrimp fisheries has been questioned because they exhibit unique characteristics with respect to population dynamics and fisheries. This study used meta-analyses to test the null hypothesis that recruitment is a series of random, independent events based on the penaeid shrimp stocks found in published sources. Both the derivative hypothesis test and Granger causality test rejected the null hypothesis. Thus, recruitment is related to spawner abundance. Shrimp populations should be managed so as to maintain sufficient spawning stock abundance to yield high recruitment. [ABSTRACT FROM PUBLISHER]

According to the theory of compensatory dynamics, depleted populations should recover when the threat responsible for their decline is removed because per capita population growth is assumed to be highest when populations are at their smallest viable sizes. Yet, many seriously depleted fish populations have failed to recover despite threat mitigation. Atlantic cod ( Gadus morhua ) stocks off Newfoundland, despite 30 years of dramatically reduced fishing mortality and numerous fishery closures, have not recovered, suggesting that drivers other than fishing can regulate the growth of collapsed fish populations, inhibiting or preventing their recovery. Here, using Bayesian inference, we show strong evidence of Allee effects in a south Newfoundland cod population, based on data on recruitment and spawning stock biomass. We infer the Allee-effect threshold, below which recovery is impaired. We demonstrate the necessity of data at low population sizes to make inferences about the nature of low-abundance dynamics. Our work indicates that Allee effects are not negligible in commercially exploited fish populations, as commonly projected, and that they represent an inhibitory force that can effectively prevent recovery from overfishing. Our findings contrast with prevailing fisheries management practices that assume compensatory dynamics at low abundances with potential to seriously overestimate the recovery potential of collapsed populations.

The framework of this article is modeling applied to fisheries. It aims at relating some classical stock–recruitment curves to a dynamic structured model, consisting of the adults stages of the stock and a pre-recruits stage (i.e., eggs, larvae, juveniles). The main result is that no functional stock–recruitment relationship can be found, unless further restrictive assumptions are added to the model (slow–fast dynamics and uniformity). Among the curves thus obtained is the Beverton–Holt curve, but not Ricker's. [ABSTRACT FROM AUTHOR]