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Individuals must reproduce to survive and thrive from generation to generation. In fish, the fecundity of individuals and estimates of total reproductive output are critical for evaluating reproductive success and understanding population dynamics. Estimating fecundity is an onerous task; therefore, many populations lack contemporary estimates of fecundity and size‐fecundity relationships. However, reproductive dynamics are not static in time; therefore, it is important to develop contemporary fecundity estimates to better inform conservation and management action. To highlight the importance of contemporary fecundity estimates, we examined the fecundity of southern Gulf of St. Lawrence (sGSL) spring and fall spawning Atlantic herring in 2022, developed size‐fecundity models, and compared these to historical fecundity estimates and models. Our results suggest that the average fecundity of sGSL spring and fall herring has undergone a substantial temporal decline of approximately 47% and 58%, respectively, since the 1970s and 1980s. The size‐fecundity relationships for fall spawning herring have shifted, with fish of a given size exhibiting lower fecundity in 2022 compared to the 1970s. Alternatively, the size‐fecundity relationships for spring spawning herring have remained relatively static. Furthermore, simulations highlighted a substantial reduction in potential reproductive output in 2022 compared to 1970 of approximately 32% and 68% for spring and fall spawners, respectively, based on a fixed number of mature females, which may have negative implications for stock rebuilding. Overall, our study provides support for periodic estimates of fecundity in fish populations to better understand temporal changes in reproductive and population dynamics. [ABSTRACT FROM AUTHOR]

Examining temporal changes in the growth and generation time of fish species can be valuable for understanding population responses to different management measures and environmental conditions. Atlantic herring (Clupea harengus-Linnaeus, 1758) is an ecologically and commercially important pelagic forage fish species that occupies the southern Gulf of St. Lawrence. Here, we developed von Bertalanffy growth curves and estimated yearly values of generation time for NAFO Division 4TVn Atlantic herring for the years from 1988 to 2021. The results indicate a temporal reduction in the growth and generation time of both spring and fall spawning herring. Over the time series, the generation time of spring and fall spawners reduced by approximately 1 and 2 years, respectively. Furthermore, the average generation time of spring spawners across the time series (6.23 years (95% CI: 5.78–6.85 years)) was approximately 1 year lower than that of fall spawners (7.52 years (6.82–8.34 years)). Overall, the findings of this study can be used to better inform stock assessments and rebuilding plans for 4TVn spring and fall spawning Atlantic herring and highlight the importance of examining temporal trends in growth and generation time for more effective management of fish stocks. [ABSTRACT FROM AUTHOR]

Atlantic herring (Clupea harengus) are economically and ecologically significant but have been in decline in Atlantic Canada due to an uncertain combination of environmental recruitment controls, predation, and fishing (commercial fishery and poorly documented bait removals). Fisheries and Oceans Canada partially closed Atlantic mackerel (Scomber scombrus) and herring fisheries in March 2022 amid controversy and disagreement. Here, we develop a conceptual model for natural and anthropogenic controls on herring abundance centered on the southern Gulf of St. Lawrence (sGSL) and evaluate the likely importance of each. We provide the first estimates of the magnitude of bait fishery withdrawals in the sGSL. The decline in sGSL herring is likely driven by natural predation exacerbated by fishing. Bait fishery removals were 182 tonnes in 2021, suggesting that previous estimates for removals of spring-spawning herring were ∼30% too low. Fisheries closure is consistent with a precautionary approach given uncertainties and irreversibility of stock collapse. Ecosystemic models are needed but will be difficult to develop given the incomplete understanding of prey substitutability and net effect of interacting environmental processes. [ABSTRACT FROM AUTHOR]

Atlantic Herring in the southern Gulf of St. Lawrence (sGSL) are found in the area extending from the north shore of the Gaspé Peninsula to the northern tip of Cape Breton Island, including the Magdalen Islands. Fall spawning Herring in the sGSL are assessed using regionallydisaggregated assessment models (North, Middle, South regions). Since 2015, hydroacoustic surveys were conducted annually on six major Herring fall spawning grounds in the sGSL. To account for missing samples, a predictive model of nightly Herring biomass was used to obtain a complete data grid and derive unbiased biomass indices. The covariates included in the negative-binomial model allowed to predict data for missing values in the observed data set. The trends in Herring biomass generated from this hydroacoustic survey closely match the trends observed in other population indices and stock status estimates from the population model. The North region biomass showed a decline to very low values in 2021. However, the Middle region decline in biomass seemed to be slower and with more interannual variation. Finally, a declining trend in the South region seemed to have reversed and biomass levels in 2021 were as high as in the beginning of the time series. The quality of the sampling and potential biases are discussed. [ABSTRACT FROM AUTHOR]

Lake trout (Salvelinus namaycush) exhibit substantial life history variation range-wide and at a local scale. This study addresses two hypotheses that have been proposed to account for this: (i) over the zoogeographic range, climatic conditions are associated with life history differences; and (ii) within smaller geographic regions, physical lake attributes are associated with life history differences. Multivariate statistics (Procrustean analysis and canonical correlation analysis) identified a strong, range-wide association between climate and life history variables. Colder climates were associated with slower prematuration growth, older age at maturity, and increased longevity. Winter conditions were also important; longer, warmer winters were associated with slower prematuration growth, smaller maximum sizes, and increased weight at a standard length of 425 mm. In southern populations, these general trends were further modified by physical lake attributes. High productivity lakes had lake trout with faster prematuration growth and larger maximum sizes; deeper lakes were associated with larger maximum sizes and later ages at maturity; and larger lakes were associated with slower prematuration growth, greater longevity, and larger maximum sizes. This study identifies abiotic variables that should be incorporated into existing lake trout management models, thus extending their applicability range-wide and permitting them to deal with possible impacts of climate change. [ABSTRACT FROM AUTHOR]

Understanding how marine organisms adapt to local environments is crucial for predicting how populations will respond to global climate change. The genomic basis, environmental factors and evolutionary processes involved in local adaptation are however not well understood. Here we use Atlantic herring, an abundant, migratory and widely distributed marine fish with substantial genomic resources, as a model organism to evaluate local adaptation. We examined genomic variation and its correlation with environmental variables across a broad environmental gradient, for 15 spawning aggregations in Atlantic Canada and the United States. We then compared our results with available genomic data of northeast Atlantic populations. We confirmed that population structure lies in a fraction of the genome including likely adaptive genetic variants of functional importance. We discovered 10 highly differentiated genomic regions distributed across four chromosomes. Nine regions show strong association with seasonal reproduction. One region, corresponding to a known inversion on chromosome 12, underlies a latitudinal pattern discriminating populations north and south of a biogeographic transition zone on the Scotian Shelf. Genome–environment associations indicate that winter seawater temperature best correlates with the latitudinal pattern of this inversion. The variation at two so‐called 'islands of divergence' related to seasonal reproduction appear to be private to the northwest Atlantic. Populations in the northwest and northeast Atlantic share variation at four of these divergent regions, simultaneously displaying significant diversity in haplotype composition at another four regions, which includes an undescribed structural variant approximately 7.7 Mb long on chromosome 8. Our results suggest that the timing and geographic location of spawning and early development may be under diverse selective pressures related to allelic fitness across environments. Our study highlights the role of genomic architecture, ancestral haplotypes and selection in maintaining adaptive divergence in species with large population sizes and presumably high gene flow. [ABSTRACT FROM AUTHOR]

Senescence is the age-specific decline in fitness of adult organisms principally associated with a decline in survival rate (actuarial senescence) and fecundity (reproductive senescence). Although common in natural populations of many taxa, there are few examples in fishes. A recent study found age-specific increases in the relative frequency of macroscopically non-reproductive Atlantic herring (Clupea harengus Linnaeus, 1758), consistent with reproductive senescence in a number of Canadian populations. However, a non-adaptive explanation for these patterns, unrelated to senescence, could not be definitively ruled out. Here, we present the results of histological examination of herring gonads undertaken to examine this and another hypothesis. The sample size of macroscopically senescent gonads was small (n = 4 females), a function of the low abundance of large, older, purportedly senescent, herring in these populations subjected to high mortality rates, and constraints on obtaining fresh samples. The results indicate that these fish were senescent and not merely skip spawning, providing further evidence of senescence and the occurrence of a post-reproductive period in herring in Atlantic Canada. Based on existing theory for the evolution of senescence, observations of actuarial senescence in Norwegian spring-spawning herring suggest that this population may also experience reproductive senescence and that the phenomenon may occur broadly in the species. [ABSTRACT FROM AUTHOR]

The abundance of top predators in the southern Gulf of St. Lawrence, Canada, has fluctuated dramatically in recent decades. The associated increase in predation and its effect on the lack of recovery of many fish stocks in the system generates the need for a better understanding of predator-prey relationships and the implementation of an ecosystem approach to fisheries management. This study used stomach content analysis to further describe the diet of Atlantic bluefin tuna in the southern Gulf of St. Lawrence. Teleost fish largely dominated the stomach contents in all years. Previous studies established that Atlantic herring was the main component of the diet by weight, whereas herring was almost absent from the diet in this study. A shift in the diet of Atlantic bluefin tuna has been observed, as it now feeds almost exclusively on Atlantic mackerel. The estimated daily meal varied between years, ranging from 1026 g per day in 2019 to 2360 g per day in 2018. Daily meals and daily rations were calculated and showed substantial year-to-year variation. • The diet of Atlantic bluefin tuna was studied in the southern Gulf of St Lawrence between 2010 and 2021. • Stomach contents were identified visually and by DNA barcoding. • Teleosts (Atlantic herring and Atlantic mackerel) largely dominated the diet in all years. • A shift in Atlantic bluefin tuna diet was observed in recent years. • The diet was almost exclusively composed of Atlantic mackerel in recent years. [ABSTRACT FROM AUTHOR]

Forage fish form a critical trophic link in marine ecosystems, and yet, for many species, there is limited information available. As nations move from single species stock assessments to ecosystem approaches to fisheries management (EAFMs), more information on forage fish will be required. In this study, 50 years of scientific literature were systematically mapped for 11 forage fish species in Canada's Arctic, Atlantic, and Pacific oceans. The objectives were to identify (1) knowledge clusters and gaps and (2) the pressures studied in relation to forage fish outcomes. Of the 2897 articles mapped, the majority studied adults and the distribution, productivity, growth, and life history of commercially fished species. Knowledge gaps were identified for forage fish: (1) that were noncommercially exploited; (2) egg and larval life history stages of most species and juveniles of noncommercial species; (3) diets of most species; (4) migration and performance for all species and survival of noncommercial species; and (5) the effects of some pressures (e.g., large-scale climate pressures). Addressing these knowledge gaps would improve the application of EAFMs. [ABSTRACT FROM AUTHOR]

Atlantic herring (Clupea harengus), a vital ecosystem component and target of the largest Northwest Atlantic pelagic fishery, undergo seasonal spawning migrations that result in elusive sympatric population structure. Herring spawn mostly in fall or spring, and genomic differentiation was recently detected between these groups. Here we used a subset of this differentiation, 66 single nucleotide polymorphisms (SNPs) to analyze the temporal dynamics of this local adaptation and the applicability of SNP subsets in stock assessment. We showed remarkable temporal stability of genomic differentiation corresponding to spawning season, between samples taken a decade apart (2005 N = 90 vs. 2014 N = 71) in the Gulf of St. Lawrence, and new evidence of limited interbreeding between spawning components. We also examined an understudied and overexploited herring population in Bras d'Or lake (N = 97); using highly reduced SNP panels (NSNPs > 6), we verified little‐known sympatric spawning populations within this unique inland sea. These results describe consistent local adaptation, arising from asynchronous reproduction in a migratory and dynamic marine species. Our research demonstrates the efficiency and precision of SNP‐based assessments of sympatric subpopulations; and indeed, this temporally stable local adaptation underlines the importance of such fine‐scale management practices. Herring spawn mostly in fall or spring, and genomic differentiation was recently detected between these groups. Here, we used a subset of this differentiation, 66 single nucleotide polymorphisms (SNPs) to analyze the temporal dynamics of this local adaptation and the applicability of SNP subsets in stock assessment. We showed remarkable temporal stability of this genomic differentiation between samples taken a decade apart (2005 N = 90 vs. 2014 N = 71) in the Gulf of St. Lawrence, alongside new evidence of limited interbreeding. [ABSTRACT FROM AUTHOR]

Cold-water fishes are becoming increasingly vulnerable as changing thermal conditions threaten their future sustainability. Thermal stress and habitat loss from increasing water temperatures are expected to impact population viability, particularly for inland populations with limited adaptive resources. Although the long-term persistence of cold-adapted species will depend on their ability to cope with and adapt to changing thermal conditions, very little is known about the scope and variation of thermal tolerance within and among conspecific populations and evolutionary lineages. We studied the upper thermal tolerance and capacity for acclimation in three captive populations of brook trout (Salvelinus fontinalis) from different ancestral thermal environments. Populations differed in their upper thermal tolerance and capacity for acclimation, consistent with their ancestry: the northernmost strain (Lake Nipigon) had the lowest thermal tolerance, while the strain with the most southern ancestry (Hill's Lake) had the highest thermal tolerance. Standard metabolic rate increased following acclimation to warm temperatures, but the response to acclimation varied among strains, suggesting that climatic warming may have differential effects across populations. Swimming performance varied among strains and among acclimation temperatures, but strains responded in a similar way to temperature acclimation. To explore potential physiological mechanisms underlying intraspecific differences in thermal tolerance, we quantified inducible and constitutive heat shock proteins (HSP70 and HSC70, respectively). HSPs were associated with variation in thermal tolerance among strains and acclimation temperatures; HSP70 in cardiac and white muscle tissues exhibited similar patterns, whereas expression in hepatic tissue varied o among acclimation temperatures but not strains. Taken together, these results suggest that populations of brook trout will vary in their ability to cope with a changing climate. [ABSTRACT FROM AUTHOR]