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The Acceptable Level of Impact (ALI) methodology defines acceptable limits for the predicted population effect of mortality imposed by offshore wind farms (OWFs) on marine bird populations and was first developed in 2021 as a replacement for the Potential Biological Removal (PBR) method. The ALI is a probabilistic framework and defined as: ‘The probability of a population decline of X% or more, 30 years after the impact, cannot exceed Y’. The methodology is based on a comparison of future population abundance between two scenarios: one unimpacted scenario without impact from OWFs, and one impacted scenario which includes additional mortality resulting from bird collisions with offshore wind turbines and/or habitat loss from avoiding OWFs. In the ALI definition, X represents the threshold value above which the population effects of OWFs are considered undesirable and Y is the threshold value for the probability that a population decline larger than X would still occur. Following its development and first use, several reviews and an in-depth analysis revealed a number of methodological issues. These were related to the use of a causality measure. This quantified the probability that an unacceptable decline in population abundance was caused by OWFs, instead of by uncertainty or biological variability (e.g. environmental stochasticity). Discounting the effect of uncertainty and variation was considered contrary to the precautionary principle. In addition, this may lead to the counterintuitive result that a more strict threshold for the acceptable level of population decline (X) results in a lower probability that such a decline is judged as unacceptable. Thus, only choosing a more strict X threshold would permit larger impacts of OWFs. Although these dependencies can be accounted for through the choice of threshold values, this requires expert judgement and complicates the methodology. It was therefore recently recommended to revise the ALI methodology and abandon the

Density dependence is likely to act as a regulatory mechanism in fish stocks that are recovering from overfishing. In general, density dependence in fish stocks is assumed to only occur in reproduction and early life stages and is therefore usually modelled as a stock-recruitment relationship. Recent research shows that density dependence can also reduce individual growth in body size later in life. In this study, we show how optimal fishing effort changes with the strength of density dependence in individual growth for four stocks of North Sea flatfish species. Using size-structured population models we show that density dependence arises due to a mechanistic link between the resource availability and life history processes at the individual level. We furthermore show that the stock response to harvesting is either driven by changes in individual reproduction when density dependence in individual growth is weak or by changes in individual growth rate when individual growth is strongly affected by density dependence. These two types or regimes are separated by a sudden shift in dynamics. It is therefore of great importance to account for density dependence in growth when managing fish stocks.

The Acceptable Level of Impact (ALI) methodology defines acceptable limits for the predicted population effects of mortality imposed by offshore wind farms (OWFs) for marine birds. These population effects are quantified using stochastic population models tailored for specific seabird species. The ALI is defined as: ‘The probability of a population decline of X% or more, 30 years after the impact, cannot exceed Y’. In this definition, the X threshold determines the reduction in population abundance due to OWFs that is considered acceptable, evaluated 30 years after the onset of the impact. The Y threshold of the ALI is derived from a threshold value for the ‘causality probability’ (PC). PC is the probability that a violation of X is caused by the impact from OWFs, instead of being caused by variability or uncertainty inherent to predictions of future population abundance. The ALI methodology was reviewed in November 2021 – January 2022. Reviewers concluded that the approach and assumptions of the proposed ALI methodology were a considerable improvement in relation to existing impact-evaluation frameworks (ORNIS, PBR). However, reviewers also raised concerns about the legal tenability and definition of the causality level PC. Several points put forward in the reviews were addressed within the current report, namely a derivation of the causality threshold PC and results of a sensitivity analysis of the ALI methodology, which contained several components. First, an analysis of the effect of uncertainty on the outcome of the ALI was performed. This analysis was done by: 1) changing the way in which parameters of the population model were sampled to simulate year-to-year variation in demographic rates and, 2) changing the standard deviation of the parameters of the population model. Second, the effect of changing the X threshold value on the outcome of the ALI was evaluated. Lastly, the effect of time-varying, as opposed to constant, mortality levels on the outcome of th

The Working Group on Fisheries Benthic Impact and Trade-offs (WGFBIT) develops methods and performs assessments to evaluate benthic impact from fisheries at regional scale, while considering fisheries and seabed impact trade-offs. In this report, new fishery benthic impact assessments are carried out for several sub-regions in the Mediterranean (Greek waters, South Adriatic Sea, Sicily waters). For other regions, updates of the whole assessment or specific steps only were presented. A standard advice sheet for the regional benthic assessments, intended as input to the next generation of the ICES Ecosystem and Fisheries Overviews, was finalised and compiled for some regions as example (Greek waters, Baltic Sea). A validation of the longevity relationships using new data was executed for the Kattegat area and the Southern North Sea. In relation to the methodology, some recommendations were formulated concerning the update on depletion rates, the use of epifauna- or infauna-based data, guidance on which set of epibenthic species to include and the time scale for setting the average swept-area-ratio (SAR) used in model fitting and assessment. A benchmarking process comparing available benthic impact assessment approaches for MSFD descriptor 6 “Seafloor integrity” is needed, as the WGFBIT approach (relative benthic state) is not the only way to assess benthic impacts from physical disturbances. A start was made to explore how to incorporate more explicitly ecosystem functioning in to the WGFBIT seafloor assessment methodology. An improved understanding of the relationships between total community biomass and ecosystem functioning may assist in setting acceptable thresholds for ecosystem impacts from trawling. Furthermore, an improved understanding of the link between species functional effect traits and proxies and processes for specific ecosystem functions could help increase our ability to predict the impact of fishing disturbance on benthic ecosystem function

The correct prediction of the shape and strength of density dependence in productivity is key to predicting future stock development and providing the best possible long-term fisheries management advice. Here, we identify unbiased estimators of the relationship between somatic growth, recruitment and density, and apply these to 80 stocks in the Northeast Atlantic. The analyses revealed density-dependent recruitment in 68% of the stocks. Excluding pelagic stocks exhibiting significant trends in spawning stock biomass, the probability of significant density dependence was even higher at 78%. The relationships demonstrated that at the commonly used biomass limit of 0.2 times maximum spawning stock size, only 32% of the stocks attained three quarters of their maximum recruitment. This leaves 68% of the stocks with less than three quarters of their maximum recruitment at this biomass limit. Significantly lower recruitment at high stock size than at intermediate stock size was seen in 38% of the stocks. Density dependence in late growth occurred in 54% of the stocks, whereas early growth was generally density-independent. Pelagic stocks were less likely to exhibit density dependence in recruitment than demersal and benthic stocks. We recommend that both the degree to which productivity is related to density and the degree to which the relationship changes over time should be investigated. Both of these aspects should be considered in evaluations of whether sustainability and yield can be improved by including density dependence in forecasts of the effects of different management actions.

Growth and growth limitation are important indicators of density dependence and environmental limitation of populations. Estimating individual growth trajectories is therefore an important aspect of understanding and predicting the life history and dynamics of a population. Variation in individual growth trajectories arises due to variation in the environmental factors limiting individual growth. This environmental limitation can vary over time, between cohorts and between individuals within a cohort. For a complete and accurate understanding of individual growth in a population, it is important to include all these sources of variation. So far, statistical models only accounted for a subset of these factors or required an extensive growth history of individuals. Here, we present a novel model describing the growth curves of cohorts in a population. This model is derived from a stochastic form of the Von Bertalanffy growth equation describing individual growth. The model is specifically tailored for use on length-at-age data in which the growth trajectory of an individual is unknown and every individual is only measured once. The presented method can also be used if growth limitation differs strongly between age or length classes. We demonstrate the use of the model for length-at-age data of North Sea plaice (Pleuronectes platessa) from the last 30 years. Fitting this model to length-at-age data can provide new insights in the dynamics of the environmental factors limiting individual growth and provides a useful tool for ecological research and management.

The csv data file “SAR_TBB.csv” contains data on habitat characteristics and fishing effort of the Dutch beam trawl fleet by grid cells of 1 minute longitude * 1 minute latitude in the North Sea used to study the changes in trawling impact on the benthic ecosystem due to the transition from conventional beam trawling to pulse trawling. Habitat variables include %sand, %gravel, %mud, bed shear stress (N.m-2) and level 3 EUNIS habitat type. Fishing effort, expressed as the annual swept area ratio (area swept by the gear in km2 / surface area of the grid cell (km2)), is given for the total Dutch beam trawl fleet and for a subset of vessels holding a pulse license (PLH) when fishing with the conventional beam trawl gear (PLH.T.year) or with the innovative pulse trawl (PLH.P.year).

Bottom trawl fisheries have significant effects on benthic habitats and communities, and these effects have been studied intensively in the last decades. Most of these studies have related the changes in benthic community composition to direct effect of trawl gears on benthos, through imposed mortality. This line of argumentation ignores the fact that benthic organisms themselves form a complex food web and that bottom trawling may trigger secondary effects through this food web. We studied the potential consequences of such food web effects using a model of benthic predators, filter feeders, deposit feeders and fish. Our analysis shows how inclusion of ecological interactions complicates the relationship between bottom trawling intensity and the state of the benthic community and causes a non-linear and non-monotonic response of the benthic community to trawling. This shows that indirect food web effects can fundamentally alter the response of a benthic ecosystem to bottom trawling, compared to the direct effects of mortality. In light of our results, we argue that indicators of fishing impact on benthos need to account for positive as well as negative effects of bottom trawling, in order to accurately quantify the impact. Our findings highlight that understanding the food web ecology of the benthic ecosystem is crucial for understanding and predicting the effects of trawling on the seafloor. Work that promotes such understanding of the food web ecology seems a more productive research strategy than conducting ever more empirical trawling effect measurements.

Historic hunting has led to severe reductions of many marine mammal species across the globe. After hunting ceased, some populations have recovered to pre-exploitation levels and may have regained their prominent position as top predator in marine ecosystems. Also, the harbor seal population in the international Wadden Sea grew at an exponential rate following a ban on seal hunting in 1960s, and the current number ~38,000 is close to the historic population size. Here we estimate the impact of the harbor seal predation on the fish community in the Wadden Sea and nearby coastal waters. Fish remains in fecal samples and published estimates on the seal’s daily energy requirement were used to estimate prey selection and the magnitude of seal consumption. Estimates on prey abundance were derived from demersal fish surveys, and fish growth was estimated using a Dynamic Energy Budget model. GPS tracking provided information on where seals most likely caught their prey. Harbor seals hauling-out in the Dutch Wadden Sea fed predominantly on demersal fish, for example, flatfish species (flounder, sole, plaice, dab), but also on sandeel, cod, and whiting. Although harbor seals acquire the majority of prey further offshore in the adjacent North Sea, and only spend 14% of their diving time in the Wadden Sea, seal predation was still estimated to cause an average annual mortality of 43% of the remaining fish in the Wadden Sea and 60% in the nearby shallow coastal waters (<20 m). There were however large sources of uncertainty in the estimated impact of seals on fish, including the migration of fish between the North Sea and Wadden Sea, and catchability estimates of the fish survey sampling gear, particularly for sandeel and other pelagic fish species. Our estimate suggested a considerable top-down pressure by harbor seals on demersal fish. However, predation by seals may also alleviate density-dependent competition between the remaining fish, allowing for increased fish growth, an

Offshore activities elevate ambient sound levels at sea, which may affect marinefauna. We reviewed the literature about impact of airgun acoustic exposure on fish in terms of damage, disturbance and detection and explored the nature of impact assessment at population level. We provided a conceptual framework for how to address this interdisciplinary challenge, and we listed potential tools for investigation. We focused on limitations in data currently available, and we stressed the potential benefits from cross-species comparisons. Well-replicated and controlled studies do not exist for hearing thresholds and dose–response curves for airgun acoustic exposure. We especially lack insight into behavioural changes for free-ranging fish to actual seismic surveys and on lasting effects of behavioural changes in terms of time and energy budgets, missed feeding or mating opportunities, decreased performance in predator-prey interactions, and chronic stress effects on growth, development and reproduction. We also lack insight into whether any of these effects could have population-level consequences. General “population consequences of acoustic disturbance” (PCAD) models have been developed for marine mammals, but there has been little progress so far in other taxa. The acoustic world of fishes is quite differentfrom human perception and imagination as fish perceive particle motion and sound pressure. Progress is therefore also required in understanding the nature and extent to which fishes extract acoustic information from their environment. We addressed the challenges and opportunities for upscaling individual impact to the population, community and ecosystem level and provided a guide to critical gaps in our knowledge

The bioconcentration of waterborne geosmin in rainbow trout, Oncorhynchus mykiss (Waldbaum) was assessed. Fifty rainbow trout with a mean (SD) weight of 226.6 (29.0) g and lipid content of 6.2 (0.6) % (w/w) were exposed to geosmin in static water for 0, 2, 4, 6, 8, 12, 24, 36, 48 and 120 hr, with one tank containing five fish for eac h exposure period. Geosmin concentrations were measured in fish tissue and water samples collected over time. With time the geosmin concentration in the fish increased and decreased in the water. However, the total absolute amount of geosmin in the system declined over time which could be explained by induction of biotransformation. This is in accordance with the decreasing lipid normalized geosmin levels in the liver compared with the liver-free carcass. Geosmin distribution within rainbow trout clearly is not exclusively governed by the lipid content of tissues. In vivo geosmin bioconcentration in rainbow trout is slower and the body burden reached is lower than the generally accepted theoretical model predicts.

Bottom fishing such as trawling and dredging may pose serious risks to the seabed and benthic habitats, calling for a quantitative assessment method to evaluate the impact and guide management to develop mitigation measures. We provide a method to estimate the sensitivity of benthic habitats based on the longevity composition of the invertebrate community. We hypothesize that long‐lived species are more sensitive to trawling mortality due to their lower pace of life (i.e., slower growth, late maturation). We analyze data from box‐core and grab samples taken from 401 stations in the English Channel and southern North Sea to estimate the habitat‐specific longevity composition of the benthic invertebrate community and of specific functional groups (i.e., suspension feeders and bioturbators), and examine how bottom trawling affects the longevity biomass composition. The longevity biomass composition differed between habitats governed by differences in sediment composition (gravel and mud content) and tidal bed‐shear stress. The biomass proportion of long‐lived species increased with gravel content and decreased with mud content and shear stress. Bioturbators had a higher median longevity than suspension feeders. Trawling, in particular by gears that penetrate the seabed >2 cm, shifted the community toward shorter‐lived species. Changes from bottom trawling were highest in habitats with many long‐lived species (hence increasing with gravel content, decreasing with mud content). Benthic communities in high shear stress habitats were less affected by bottom trawling. Using these relationships, we predicted the sensitivity of the benthic community from bottom trawling impact at large spatial scale (the North Sea). We derived different benthic sensitivity metrics that provide a basis to estimate indicators of trawling impact on a continuous scale for the total community and specific functional groups. In combination with high resolution data of trawling pressure, our appr

One quarter of marine fish production is caught with bottom trawls and dredges on continental shelves around the world. Towed bottom-fishing gears typically kill 20–50 per cent of the benthic invertebrates in their path, depending on gear type, substrate and vulnerability of particular taxa. Particularly vulnerable are epifaunal species, which stabilize the sediment and provide habitat for benthic invertebrates. To identify the habitats, fisheries or target species most likely to be affected, we review evidence of the indirect effects of bottom fishing on fish production. Recent studies have found differences in the diets of certain species in relation to bottom fishing intensity, thereby linking demersal fish to their benthic habitats at spatial scales of ~10 km. Bottom fishing affects diet composition and prey quality rather than the amount of prey consumed; scavenging of discarded by-catch makes only a small contribution to yearly food intake. Flatfish may benefit from light trawling levels on sandy seabeds, while higher-intensity trawling on more vulnerable habitats has a negative effect. Models suggest that reduction in the carrying capacity of habitats by bottom fishing could lead to lower equilibrium yield and a lower level of fishing mortality to obtain maximum yield. Trawling effort is patchily distributed – small fractions of fishing grounds are heavily fished, while large fractions are lightly fished or unfished. This patchiness, coupled with the foraging behaviour of demersal fish, may mitigate the indirect effects of bottom fishing on fish productivity. Current research attempts to scale up these localized effects to the population level.

An important consideration in assessing the impacts of fishing on seabed habitats is to understand the functional links (as trophic interactions) between populations of demersal fish species and potential benthic invertebrate prey (food) which live on or in the seabed. The type of sea-bed fauna has been shown to respond to both natural variation in habitat conditions and in response to different levels of fishing pressure. The extent to which different commercial fish species will depend on specific combinations of habitat type and fishing disturbance to feed will likely be species specific. It has been suggested that positive changes in growth rates of different demersal fish species are not only related to density-dependent processes, but may also be dependent on increased bottom-trawl disturbance and eutrophication (Millner and Whiting 1996, Rijnsdorp and van Leeuwen 1996, Shephard et al., 2010). However, different trawling and habitat specific responses in relation to fish feeding in different size classes of fish has not been investigated before. The aim1 of this study is therefore to examine such relationships using biological traits analysis (BTA) through a selected quantification of demersal fish stomach contents and habitat fauna using grab and epi-benthic trawl data previously analysed as part of BENTHIS deliverable D3.4. The present study addresses two important questions, namely; i. what type of sea-bed habitats serve as important feeding areas for different species of demersal fish, and ii. what, if any, differences do we observe in habitat preference and diet associated with different size classes of demersal fish, both within and between different fish species. Central to answering these two questions is the need to ensure that the sea-bed habitat characteristics are assessed and described at a scale appropriate to the operation of the fisheries and the assessment of demersal fish stocks. The study found strong associations between community trait com

This project looked at the effect of closing an area for fisheries on the development of the benthic community. An existing data set of benthic species densities sampled at different distances and angles from a platform was used for this purpose. The particular area was closed due to the installation of a gas production platform, but is likely to function as a marine protected area (MPA), with the expectation that the benthic community develops differently from the benthic community in the surrounding areas as it no longer has to cope with the impact of fisheries. Differences in development might be linked to the impact of fisheries and the effects of closure might provide an expectation for the effect of planned MPAs in similar areas.

A recent publication about balanced harvesting (Froese et al., ICES Journal of Marine Science; 73: 1640-1650) contains several erroneous statements about size-spectrum models. We refute the statements by showing that the assumptions pertaining to size-spectrum models discussed by Froese et al. are realistic and consistent. We further show that the assumption about density-dependence being described by a stock recruitment relationship is responsible for determining whether a peak in the cohort biomass of a population occurs late or early in life. Finally, we argue that there is indeed a constructive role for a wide suite of ecosystem models to evaluate fishing strategies in an ecosystem context.