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Climate change is causing fish stocks to shift, upending the social-ecological systems that rely on the historic distributions of these stocks and creating or exacerbating fisheries conflicts. The movements of internationally shared stocks between Exclusive Economic Zones (EEZs) or between EEZs and the high seas are especially concerning because they bring into play a variety of geopolitical factors and equity issues surrounding missing or conflicting regulations of jurisdictional boundary zones. Though many studies have explored the responses to and repercussions of shifting stocks on fisheries management, there is a dearth of interdisciplinary case studies that provide insight into the complexity of conflict formation in shifting transboundary fisheries, and that highlight the initial response stages where inclusion of proactive and cooperative measures can greatly improve a system's resilience to conflict. Our study helps to fill this gap by drawing on the knowledge of a diverse group of experts to analyze four case studies where transboundary stock shifts, geopolitical or governance tensions, and uncertainty regarding the future of the marine environment collide. Through synthesis of case study findings, we create a causal model of fishery conflict, within which we highlight factors that may heighten or mitigate the risk of conflict over shifting resources such as complex histories of power imbalance, unequal access to resources, or a lack of consistent and transparent data collection. Cooperation and equitable decision-making processes are recognized as vital components of internationally shared stock management which can promote lasting, effective, and conflict-resilient fisheries.

Marine heatwaves have been linked to negative ecological effects in recent decades1,2. If marine heatwaves regularly induce community reorganization and biomass collapses in fishes, the consequences could be catastrophic for ecosystems, fisheries and human communities3,4. However, the extent to which marine heatwaves have negative impacts on fish biomass or community composition, or even whether their effects can be distinguished from natural and sampling variability, remains unclear. We investigated the effects of 248 sea-bottom heatwaves from 1993 to 2019 on marine fishes by analysing 82,322 hauls (samples) from long-term scientific surveys of continental shelf ecosystems in North America and Europe spanning the subtropics to the Arctic. Here we show that the effects of marine heatwaves on fish biomass were often minimal and could not be distinguished from natural and sampling variability. Furthermore, marine heatwaves were not consistently associated with tropicalization (gain of warm-affiliated species) or deborealization (loss of cold-affiliated species) in these ecosystems. Although steep declines in biomass occasionally occurred after marine heatwaves, these were the exception, not the rule. Against the highly variable backdrop of ocean ecosystems, marine heatwaves have not driven biomass change or community turnover in fish communities that support many of the world's largest and most productive fisheries.

In the Northwest Atlantic Ocean, Atlantic halibut (Hippoglossus hippoglossus) range from Greenland to Virginia, United States (Bigelow and Schroeder, 1953; Collette and Klein-MacPhee, 2002). In Canada, it is currently managed [...]

Interactions between spatial dynamics and stock structure in marine fishes have largely focused on stocks in decline; stock structure is rarely re-visited for expanding species. Here, the spatial ecology of Atlantic halibut (Hippoglossus hippoglossus L.), managed as four stocks in the Northwest Atlantic, is reviewed. Halibut collapsed under high exploitation in the mid-19th century, but the Canadian fisheries value has increased seven-fold since the early 2000s. Atlantic halibut's thermal habitat has increased due to warming, possibly contributing to its expansion. Genomic evidence differentiates two populations in the four management units, whereas there is non-genetic spatial structure within each of the stock boundaries. There are different core juvenile areas and a diversity of spawning migration patterns influenced by timing, fish size, maturity state, and distance between summer-feeding and over-wintering habitats. From tagging studies, multiple estimates of median distance at recapture (⁓3-90 km) are much less than the spatial domain of each stock. Growth rates are faster in the warmer south, as predicted by growing degree day. The current perspective of Atlantic halibut spatial structure is that there are two distinct populations, and within each, there are subpopulations composed of multiple migratory contingents. The level of mixing on common spawning grounds both among and within subpopulations is only partly understood. [ABSTRACT FROM AUTHOR]

Over the last two decades, low-trophic-level fisheries have rapidly expanded in Atlantic Canada, largely compensating for collapsed groundfisheries; however, concerns have been raised regarding the limited background knowledge for many newly targeted species and their overexploitation in other regions. Using government stock assessments, we evaluated the amount of information available to assess population, fisheries, and ecosystem status in emerging (new since 1988), developing (expanding since 1988), and established fisheries on the Scotian Shelf. Emerging fisheries had significantly lower levels of population knowledge than developing and established fisheries. Importantly, knowledge was often lacking in basic population parameters such as growth rates, current biomass, and geographic range. In contrast, ecosystem knowledge, such as habitat disruption and recovery, was higher in emerging than established fisheries. Overall, quantitative knowledge was positively related to fishery value and greatest for 30- to 100-year-old fisheries. Although the number of government and general scientific publications greatly increased since 1990 for developing and established fisheries, publications for emerging fisheries remained at low levels. Emerging fisheries represent important socio-economic value in Atlantic Canada but may be progressing too rapidly for adequate knowledge to be gained, presenting a risk for their sustainable development. Au cours des deux dernieres decennies, les peches de niveau trophique inferieur ont connu une expansion rapide dans l'Atlantique canadien, compensant l'effondrement des peches de fond; des inquietudes ont, cependant, ete formulees concernant le peu de connaissances de base disponibles sur ces nouvelles especes ciblees et leur surexploitation dans d'autres regions. A l'aide d'evaluations de stocks gouvernementales, nous determinons la quantite de donnees disponibles pour estimer le statut des populations, des peches et des ecosystemes dans les peches commerciales en emergence (nouvelles depuis 1988), en croissance (en expansion depuis 1988) et les peches etablies sur la plate-forme neo-ecossaise. Il existe significativement moins de connaissances demographiques sur les peches en emergence que sur les peches croissantes ou etablies. En particulier, il manque souvent de donnees sur les variables demographiques de base, telles que les taux de croissance, la biomasse courante et la repartition geographique. En revanche, les connaissances de l'ecosysteme, telles que les perturbations des habitats et leur recuperation, sont plus grandes dans le cas des peches en emergence que dans celui des peches etablies. Globalement, il existe une relation positive entre les informations quantitatives disponibles et la valeur de la peche commerciale; ces informations sont le plus abondantes dans le cas de peches en existence depuis 30 A 100 ans. Bien que le nombre de publications scientifiques gouvernementales et generales sur les peches en croissance et les peches etablies ait augmente depuis 1990, il reste bas dans le cas des peches emergentes. Les peches commerciales en emergence representent une valeur socio-economique considerable dans l'Atlantique canadien, mais il se peut qu'elles progressent trop rapidement pour permettre l'acquisition de connaissances adequates, ce qui represente un risque pour leur developpement durable. [Traduit par la Redaction], Introduction The collapse of the groundfishery on the Canadian Scotian Shelf resulted in major ecosystem changes and economic challenges (Roy 1996; Worm and Myers 2003; Frank et al. 2005). This [...]

The NAFO divisions 3NOPs4VWX5Zc Atlantic halibut (Hippoglossus hippoglussus L.) stock is managed over a large spatial scale, spanning over 20° of longitude and 6° of latitude. Previous studies have shown that female halibut in the warmer southern area mature at sizes 10-20 cm smaller than female halibut in the colder north. Our goal was to examine the role of growing degree-day (GDD, the number of days X temperature °C above a minimum threshold) on growth rate and length at maturity (LM50%). Two separate datasets, associated with the stock-wide DFO-Industry Halibut Longline Survey, were used to estimate these life history traits. GDD had a significant effect on both growth rates and probability of maturity. Females grew faster and matured at a larger size than males. Female growth rate at a reference length of 90 cm corresponding to the southerly NAFO 4X was ~11.5 cm/year, significantly faster than in the northerly NAFO division 3N (8 cm/year). Male growth rate at 90 cm corresponding to NAFO divisions 4X and 4W were 7.2 and 7.5 cm/year, significantly faster than in NAFO divisions 3N and 3Ps (5.6 and 5.8 cm/year). Both sexes matured at smaller sizes in the southern areas with higher GDD. Females in the warmer southern divisions were more likely to mature ~10-28cm less than the more northerly divisions. However, the corresponding prediction intervals for NAFO divisions were very wide. On average, fish mature at a smaller size in 4WX, but we cannot predict length at maturity for any new observations in any division. We have identified large variation in life history traits across the stock domain associated with the thermal regime. Such variation in life history traits could be used to improve the assessment models and may also be indicative of stock structure that could be eroded if not addressed in the management of the fishery. [ABSTRACT FROM AUTHOR]

We have learned much about the impacts of warming on the productivity and distribution of marine organisms, but less about the impact of warming combined with other environmental stressors, including oxygen depletion. Also, the combined impact of multiple environmental stressors requires evaluation at the scales most relevant to resource managers. We use the Gulf of St. Lawrence, Canada, characterized by a large permanently hypoxic zone, as a case study. Species distribution models were used to predict the impact of multiple scenarios of warming and oxygen depletion on the local density of three commercially and ecologically important species. Substantial changes are projected within 20-40 years. A eurythermal depleted species already limited to shallow, oxygen-rich refuge habitat (Atlantic cod) may be relatively uninfluenced by oxygen depletion but increase in density within refuge areas with warming. A more stenothermal, deep-dwelling species (Greenland halibut) is projected to lose ~55% of its high-density areas under the combined impacts of warming and oxygen depletion. Another deep-dwelling, more eurythermal species (Northern shrimp) would lose ~4% of its high-density areas due to oxygen depletion alone, but these impacts may be buffered by warming, which may increase density by 8% in less hypoxic areas, but decrease density by ~20% in the warmest parts of the region. Due to local climate variability and extreme events, and that our models cannot project changes in species sensitivity to hypoxia with warming, our results should be considered conservative. We present an approach to effectively evaluate the individual and cumulative impacts of multiple environmental stressors on a species-by-species basis at the scales most relevant to managers. Our study may provide a basis for work in other low-oxygen regions and should contribute to a growing literature base in climate science, which will continue to be of support for resource managers as climate change accelerates. [ABSTRACT FROM AUTHOR]

According to density-dependent habitat selection theory, areas of high density can be indicative of high population productivity and have positive individual fitness consequences. Here, we explore six groundfish populations on the Scotian Shelf, Canada, where a decline in areas of high density beyond a certain threshold is associated with disproportionately large declines in Spawning Stock Biomass (SSB). This is evidenced by empirical, concave, positive relationships between high-density areas (HDAs) and SSB. We introduce a methodology to estimate the threshold below which SSB declines increasingly faster per unit of HDA decline. The spatial threshold among these six stocks was remarkably consistent; when stocks lose 70-80% of HDAs, disproportionately large SSB declines are likely to occur. We propose that spatial thresholds could serve as spatial reference points to complement existing SSB limit reference points (LRPs). For some stocks we identify spatial thresholds which correspond to SSB levels that exceed those associated with the designated SSB LRP, suggesting that a review of these SSB LRPs warrants merit. For other stocks, spatial reference points can be used in concert with SSB reference points, strengthening efforts to incorporate a precautionary approach to fisheries management. Our results warrant further research into the general application of HDA as spatial limit and target reference points for fisheries management in addition to other population status indicators within a broad recovery framework. [ABSTRACT FROM AUTHOR]

A juvenile haddock (Melanogrammus aeglefinus) closed area was established on the offshore banks (Emerald and Western) of the central Scotian Shelf (NAFO Div. 4W) in 1987. The management objective associated with this meausre was to protect incoming recruits and thereby allow the stock to rebuild. Our evaluation of the effectiveness of the closed area revealed that the management objective was not fully met. The expected trend of declining juvenile mortality after, and high mortality preceding its imposition, was not readily apparent. The lack of response may have been due to several factors: (i) the proportion of juveniles within the closed area steadily declined and a majority of year classes during the post-closure period remained unprotected; (ii) the closed area remained open to fishing by fixed gear whose catches inside the closed area and surrounding areas steadily increased; and (iii) the resident haddock stock deteriorated in terms of growth and condition due to a combination of historical over-exploitation and large-scale environmental changes. The closed area does appear to have had some benefit to other groundfish species in terms of increased abundance, notably American plaice (Hippoglossoides americanus) and winter flounder (Pseudopleuronectes americanus). [ABSTRACT FROM PUBLISHER]