Equilibrium concepts and the expectation of compensatory density dependence remain fundamental to fisheries science, but stock collapses and an increasing appreciation of environmental factors have raised questions about their real-world applicability. To explore the demographic variability of harvested marine fishes, we have calculated metrics commonly used in conservation biology to describe the demographics for 77 assessed stocks from the North Atlantic and Northeast Pacific Oceans using life-tables. We found that median annual population growth rates ([Formula: see text]) were centered around 1, and surprisingly, they were only slightly higher when the effect of fishing was excluded. For most stocks, as abundance declined, [Formula: see text] tended to increase and become more variable as would be expected from compensatory dynamics. The population growth of several stocks was sustained by a limited number of years with exceptionally high rates. However, the ability of a stock to increase from low abundance appeared largely independent of life history characteristics and exhibited stronger geographical differences among stocks of the same species (notably Atlantic cod). Life history characteristics alone were poor predictors of annual population growth or future recovery potential, whereas regional factors appeared to be more influential. Overall, recovery potential remained relatively high, with simulations indicating that 62 of the stocks would be highly likely to double in size within 20 years in the absence of fishing. Low recovery potential was exclusively observed in stocks with a low median [Formula: see text] and low variability in [Formula: see text]. These results suggest that understanding stock-specific (rather than species-specific) demographic parameters is necessary to promote sustainable management or develop rebuilding plans for collapsed stocks.

Quantifying a species' distribution and identifying its ecological drivers are essential for effective ecosystem monitoring, assessment, and management. In marine ecosystems, conventional fishery-based methods are often inadequate for producing indicators of the abundance and distribution of large, fast-moving pelagic predators such as Atlantic Bluefin Tuna (ABFT, Thunnus thynnus). In the Gulf of St. Lawrence (GSL), Canada, fisheries provide limited presence-only data due to technical and economic limitations, while ABFT predation on key preys has likely intensified in recent years. To enhance our understanding of ABFT distribution and its ecological context in the GSL, we developed a spatial presence-absence model using multispecies environmental DNA (eDNA) detections for ABFT and its primary preys, alongside environmental covariates. Water samples were collected at over 300 locations across four years. ABFT occurrence probability was best predicted by maximum depth and Atlantic mackerel (Scomber scombrus) presence, but not sea surface temperature. Model-predicted ABFT presence diverged from distribution inferred from fishery catch data and that predicted by basin-scale habitat suitability models, suggesting that conventional methods may underestimate ABFT presence in certain areas. These discrepancies highlight the value of eDNA as a complementary ecological indicator for elusive or under-sampled marine species. Despite inherent uncertainties, as any method, eDNA-based inference can fill critical gaps in biodiversity monitoring. Simultaneous detection of predators and prey offers a novel avenue for tracking trophic interactions and ecosystem dynamics. We propose integrating eDNA-based indicators into science-based management frameworks, particularly for mobile marine predators and complex oceanic environments.

There is widespread recognition that existing regulatory and advisory systems for fisheries management in Canada influence our ability to address ecosystem-level objectives, particularly relative to bycatch mitigation. However, the monitoring programs, information storage systems (i.e., databases), and data extraction workflows used for fisheries analyses have rarely been considered in this context. Management goals for bycatch mitigation represent a substantial departure from the original objectives used to structure monitoring programs and their associated databases. Using a detailed example from multispecies groundfish fleets in Atlantic Canada, we demonstrate that the way information from fisheries is stored, as well as the analytical workflows used to identify and summarize information from commercial fishing trips, complicate research on bycatch. Employing the same workflow and data extraction process to answer all types of fisheries management questions is fraught with pitfalls. We hope this perspective will inform the way in which bycatch analyses are incorporated into Canadian fisheries management and provide the impetus to redesign current data management systems to meet emerging fisheries challenges in Canada.

This study presents the first systematic assessment of trends in white shark Carcharodon carcharias presence in Atlantic Canadian waters (ACWs) using 2 standardized acoustic monitoring arrays-the Halifax and Cabot Strait lines-deployed by the Ocean Tracking Network between 2014 and 2023. We evaluated annual changes in the probability that acoustically tagged white sharks (n = 260) migrate from US waters to these arrays and the extent of their seasonal detection period in ACWs. Our analysis revealed an increase in the probability of white sharks migrating into ACWs, starting around 2019-2021. The probability of migrating to the Halifax line increased 2.4-fold [1.8, 3.3] (median [95% credible interval]), from 0.18 [0.13, 0.24] to 0.44 [0.38, 0.49], and to the Cabot Strait line increased 3.7-fold [2.4, 6.2], from 0.08 [0.05, 0.11] to 0.28 [0.24, 0.33], between 2018 (n = 63) and 2022 (n = 159). We also observed a 1.4-fold [1.2, 1.8] increase in the seasonal detection period in ACWs, extending from 48 d [40, 58] during 2014-2018 (n = 13) to 70 d [63, 78] in 2023 (n = 76). While our analysis robustly detected an increase in white shark presence in ACWs, limited data in the early years introduced uncertainty in quantifying the amplitude of the migration probability trend. The reported numbers should be interpreted carefully. The rapid timespan of this increase highlights the need to investigate causal mechanisms structuring white shark presence in ACWs, which is essential for the management of the Northwest Atlantic population.

For highly mobile marine species such as pelagic elasmobranchs, the development of effective spatial management requires a comprehensive understanding of movement ecology. Research incorporating movement data across seasons and life stages, including reproductive states, is valuable for informing spatial management, yet is absent for most species. In the Northwest (NW) Atlantic Ocean (hereafter referred to as NW Atlantic), the porbeagle shark Lamna nasus is a pelagic species that is overfished, has a live retention ban (Canada) or landings regulations (United States), and is also commonly captured incidentally as bycatch. Research on the spatiotemporal dynamics of NW Atlantic porbeagle habitat use is limited, with all previous research utilizing pop-off satellite archival tags that are prone to large uncertainty in location estimates. This study used higher-accuracy fin-mount satellite tags to identify patterns in habitat use across life stages and seasons for porbeagle sharks tagged off the northeastern coast of the United States. During the summer and fall, the 95% kernel density estimate (referred to as “activity space”) of tagged porbeagles occurred almost exclusively on the continental shelf in the Gulf of Maine and Georges Bank. Activity space expanded and shifted southwards to include offshore environments during the winter and spring for juveniles, mature non-gravid females, and mature females of unknown reproductive states, while the activity space of mature males and gravid females remained in shelf waters year-round. This finding differs from the previous assumption that southward migrations are linked to reproduction for NW Atlantic porbeagles. Tagged porbeagles were also found to have a relatively small 50% kernel density estimate (referred to as “high occupancy area”) located around Cape Cod, Massachusetts that was well-conserved across life stages and seasons. This relatively static, small high occupancy area has implications for the population’s conservation given the high amount of fishing activity (rod-and-reel, trawl, gillnet) occurring within this region. Given the overlap between porbeagle high occupancy area and fishing activity, as well as the relatively high recapture rate of tagged sharks (10.5%), the coastal waters around Cape Cod, Massachusetts should be considered for spatial management of the NW Atlantic porbeagle.

To support ecosystem-based fisheries management, monitoring data from at-sea observer (ASO) programs should be leveraged to understand the impact of fisheries on discarded species (bycatch). Available techniques to estimate fishery-scale quantities from observations range from simple mean estimators to more complex spatiotemporal models, each making assumptions with differing degrees of support. However, the resulting implementation and analytical trade-offs are rarely discussed when applying these techniques in practice. Using blue shark (Prionace glauca) bycatch in the Canadian pelagic longline fishery as a case study, we evaluated the performance of seven contrasting approaches to estimating total annual discard amounts and assessed their trade-offs in application. Results demonstrated that simple approaches such as mean estimator and nearest neighbors are feasible to implement and can be as efficient for prediction as complex models such as random forest and mixed-effects models. The traditionally used catch-ratio estimator consistently underperformed among all tested models, likely due to misspecified correlative relationships between target and bycatch species. Overall, efforts in model-based approaches were rewarded with very small gains in predictive ability, suggesting that such models relying on environmental, biological, spatial, and/or temporal patterns to improve prediction of bycatch may lack sufficient foundation in data-limited contexts.

A recent Letter to the Editor by Gennari et al. (2024) contends that methodological issues and data uncertainties may be obscuring declines in abundance of the white shark population in South Africa in the analyses of Bowlby et al. (2023). We have addressed their critiques using scientifically accepted analytical understanding to demonstrate why our ecological conclusions remain unchanged: (1) the relative abundance of white sharks has not exhibited systematic increases or declines at a regional level since protection in 1991, and (2) observed data on human-shark incidents are consistent with the hypothesis that white sharks have partially redistributed along the South African coastline. Future long-term, standardized monitoring throughout South Africa would be expected to substantially reduce uncertainty about the population trend and status of white sharks. Ultimately, the lack of abundance increase following protection remains concerning and necessitates continued conservation efforts.

Tuna Regional Fishery Management Organizations (tRFMOs) are increasingly interested in spatiotemporal management as a tool to reduce interaction rates with vulnerable species. We use blue shark ( Prionace glauca ) as a case study to demonstrate the critical first steps in the implementation process, highlighting how predictions of global habitat for vulnerable life stages can be transformed into a publicly -accessible spatial bycatch mitigation tool. By providing examples of possible management goals and an associated threshold to identify essential habitats, we show how these key areas can represent a relatively low percentage of oceanic area on a monthly basis (16-24% between 50 degrees S and 60 degrees N), yet can have relatively high potential protection efficiency (similar to 42%) for vulnerable stages if fishing effort is redistributed elsewhere. While spatiotemporal management has demonstrable potential for blue sharks to effectively mitigate fishing mortality on sensitive life stages, we identify inherent challenges and sequential steps that require careful consideration by tRFMOs as work proceeds. We also discuss how our single-species framework could be easily extended to a multispecies approach by assigning relative conservation risk before layering habitat model predictions in an integrated analysis. Such broader application of our approach could address the goals of tRFMOs related to reducing the ecosystem effects of fishing and pave the way for efficient fisheries co-management using an ecosystem-based approach.

Context For threatened marine species, data on their vertical habitat use patterns can reveal risk of interactions with fishing gear and can inform bycatch avoidance strategies. Such data are lacking for young porbeagles (Lamna nasus), which are captured as bycatch in north-western Atlantic fisheries. Aims We aimed to examine temporal patterns in diving and characterise vertical habitat use of young porbeagles during summer and autumn. Methods We used data from short-term (28-day), high-resolution (5-min interval) pop-off satellite tags attached to 14 young (young-of-the-year and 1-year-old) porbeagles to model depth use. Key results Occupied depths ranged from the sea surface to 679 m, with ambient water temperatures of −0.2 to 26°C. Diel period and season were factors related to depth use. Conclusions Sharks exhibited a diel activity pattern characterised by more extensive use of the water column during the day while remaining primarily at the surface at night. Depth use differed between seasons, with summer characterised by greater affinity for surface waters (0–10 m) compared to autumn. Implications Young porbeagles are at risk of interaction with active fisheries on the continental shelf, but interactions may be reduced by setting gear deeper at night or during summer.

While lethal sampling can be the most effective technique to collect critical reproductive data for elasmobranchs, non-lethal techniques need to be validated for future use. Concentrations of reproductive hormones in plasma and muscle have been found to correlate to sexual maturity and/or reproductive cycles in oviparous as well as yolk-sac, placental, and histotrophic viviparous elasmobranchs, offering a potentially non-lethal technique to study reproduction. However, reproductive hormone analysis is scant for oophagous sharks. This study utilized muscle tissues from porbeagles Lamna nasus that were dissected for other life history studies and were stored frozen for up to 37 years to quantify testosterone (T) and estradiol (E2) concentrations in relation to previously-known maturity and reproductive stage. A total of 207 samples (92 males, 115 females) from porbeagles ranging in size from 80 to 256.5 cm fork length were analyzed. Muscle T and E2 concentrations were related to maturity and reproductive stage in porbeagles, with the highest T concentrations found in mature males during the spermatogenic season (summer) and the highest E2 concentrations found in gravid females. These results suggest muscle hormone concentrations have the potential to serve as a non-lethal proxy of reproductive stage in oophagous sharks. This study also demonstrates the value of specimen sharing and the potential for continued use of stored vertebral muscle tissue for reproductive hormone analysis in order to optimize the amount of data gained from biological samples. Future use of these methods would be particularly valuable for threatened species for which lethal sampling is restricted.

Unprecedented levels of change in ocean ecosystems bring an ever-increasing need for re-analyses of existing data to explore pressing conservation questions. Substantial declines in white shark (Carcharodon carcharias) presence at two primary aggregation sites have raised concerns about the species’ status throughout South Africa. Using the most comprehensive suite of abundance indices compiled to date, we evaluated temporal trends and the strength of evidence for regional redistribution. Individual indices from all primary aggregation sites in South Africa were highly variable. The overall temporal trend from a log-linear Generalized Additive Model was relatively flat, indicating largely unchanged abundance throughout South Africa since protection in 1991. However, reports of human-shark incidents showed a general shift from the Western to the Eastern Cape. Correlations among individual abundance indices demonstrated that movements were not as simple as animals leaving one site to inhabit another. Further research is needed to explore the effect of movement on monitoring data. Our results reaffirm the need for better standardization in data collection methods to generate abundance indices and to develop long-term monitoring programs on the Eastern Cape. Ideally, environmental or operational factors affecting abundance indices should also be explored in future status assessments at a regional level. Our results provide a baseline for future work, directing research to understand drivers of localized and regional changes and focusing management on reducing anthropogenic sources of mortality within their Southwest Indian Ocean range.

Fisheries can adversely affect threatened bycatch species and vulnerable marine ecosystems (VMEs). Thresholds are unique amongst bycatch management methods in providing flexibility in individual participants’ approaches to avoid exceeding limits, and particularly for individual vessel quotas, in incentivizing the innovation of effective and commercially viable solutions. This study assessed bycatch thresholds for sharks and relatives, air-breathing marine species and macroinvertebrate indicators for identifying benthic VMEs of 21 intergovernmental organizations and arrangements (IGOs). Seven IGOs lacking bycatch thresholds, who tended to have fewer members, might rely on bycatch management by national authorities. Sharks were the predominant focus. IGOs did not know if thresholds were reached for almost half of measures, likely due to compliance monitoring deficits. Individual vessel limits may be more equitable and prevent a race for fish. However, risk pools and fleetwide thresholds may be more effective when mitigation approaches for individual vessels are limited. No IGO uses individual transferable bycatch quotas or risk pools, which would be challenging to implement regionally. No thresholds were reference points of a harvest strategy. There were limited incidences of thresholds being reached. Thresholds might be set too high to meet objectives. When reached, there was high variability in management responses being systematically implemented. Addressing deficits of thresholds being set too low, inadequate compliance monitoring and inconsistent management response implementation could improve performance. Thresholds have the potential to be an effective component of regional bycatch management strategies, incentivizing fishers to minimize their individual and collective bycatch fishing mortality and adverse effects on VMEs.

Apex and mesopredators such as elasmobranchs are important for maintaining ocean health and are the focus of conservation efforts to mitigate exposure to fishing and other anthropogenic hazards. Quantifying fishing mortality components such as at-vessel mortality (AVM) is necessary for effective bycatch management. We assembled a database for 61 elasmobranch species and conducted a global meta-synthesis to estimate pelagic longline AVM rates. Evolutionary history was a significant predictor of AVM, accounting for up to 13% of variance in Bayesian phylogenetic meta-regression models for Lamniformes and Carcharhiniformes clades. Phylogenetically related species may have a high degree of shared traits that explain AVM. Model-estimated posterior mean AVM rates ranged from 5% (95% HDI 0.1%-16%) for pelagic stingrays and 76% (95% HDI 49%-90%) for salmon sharks. Measures that reduce catch, and hence AVM levels, such as input controls, bycatch quotas and gear technology to increase selectivity are appropriate for species with higher AVM rates. In addition to reducing catchability, handling-and-release practices and interventions such as retention bans in shark sanctuaries and bans on shark finning and trade hold promise for species with lower AVM rates. Robust, and where applicable, phylogenetically-adjusted elasmobranch AVM rates are essential for evidence-informed bycatch policy.

Assessing progress for the endangered white shark (Carcharodon carcharias) relative to Canadian conservation objectives requires understanding distribution patterns. From the largest tagging data set in the Northwest Atlantic (2010-2020; 272 deployments), we determined the proportion of the population detected in Canadian waters, characterized patterns in occupancy, and explored the behavioural characteristics of animals while in Canadian waters versus elsewhere in their range. The component of the population detected in Canadian waters annually was highly variable, yet proportionately small. Juveniles and subadults were 4.7 and 3.4 times more likely, respectively, to move northward than adults. From June to November, all pop-up satellite archival tagged white sharks remained primarily in coastal locations within the 200 m bathymetric contour and exhibited shallow diving behaviour within the top 100 m of the water column. However, individuals in Canadian waters experienced a more restricted temperature range and used proportionately less of the water column. Accounting for behavioural effects on distribution when predicting habitat use from environmental associations will become critical to evaluate the population-level impact of recovery actions implemented under Canadian legislation.

Knowledge of the three-dimensional movement patterns of elasmobranchs is vital to understand their ecological roles and exposure to anthropogenic pressures. To date, comparative studies among species at global scales have mostly focused on horizontal movements. Our study addresses the knowledge gap of vertical movements by compiling the first global synthesis of vertical habitat use by elasmobranchs from data obtained by deployment of 989 biotelemetry tags on 38 elasmobranch species. Elasmobranchs displayed high intra- and interspecific variability in vertical movement patterns. Substantial vertical overlap was observed for many epipelagic elasmobranchs, indicating an increased likelihood to display spatial overlap, biologically interact, and share similar risk to anthropogenic threats that vary on a vertical gradient. We highlight the critical next steps toward incorporating vertical movement into global management and monitoring strategies for elasmobranchs, emphasizing the need to address geographic and taxonomic biases in deployments and to concurrently consider both horizontal and vertical movements.

Blue shark (Prionace glauca) is amongst the most abundant shark species in international trade, however this highly migratory species has little effective management and the need for spatio-temporal strategies increases, possibly involving the most vulnerable stage or sex classes. We combined 265,595 blue shark observations (capture or satellite tag) with environmental data to present the first global-scale analysis of species’ habitat preferences for five size and sex classes (small juveniles, large juvenile males and females, adult males and females). We leveraged the understanding of blue shark biotic environmental associations to develop two indicators of foraging location: productivity fronts in mesotrophic areas and mesopelagic micronekton in oligotrophic environments. Temperature (at surface and mixed layer depth plus 100 m) and sea surface height anomaly were used to exclude unsuitable abiotic environments. To capture the horizontal and vertical extent of thermal habitat for the blue shark, we defined the temperature niche relative to both sea surface temperature (SST) and the temperature 100 m below the mixed layer depth (Tmld+100). We show that the lifetime foraging niche incorporates highly diverse biotic and abiotic conditions: the blue shark tends to shift from mesotrophic and temperate surface waters during juvenile stages to more oligotrophic and warm surface waters for adults. However, low productivity limits all classes of blue shark habitat in the tropical western North Atlantic, and both low productivity and warm temperatures limit habitat in most of the equatorial Indian Ocean (except for the adult males) and tropical eastern Pacific. Large females tend to have greater habitat overlap with small juveniles than large males, more defined by temperature than productivity preferences. In particular, large juvenile females tend to extend their range into higher latitudes than large males, likely due to greater tolerance to relatively cold waters. Large juvenile and adult females also seem to avoid areas with intermediate SST (~21.7-24.0°C), resulting in separation from large males mostly in the tropical and temperate latitudes in the cold and warm seasons, respectively. The habitat requirements of sensitive size- and sex-specific stages to blue shark population dynamics are essential in management to improve conservation of this near-threatened species.

It can be difficult to determine whether a prohibition to exploitation ensures effective conservation or recovery for species that remain exposed to fishing effort and other sources of mortality throughout their range. Here we used simulation modeling of four life history scenarios (different productivity and population size) to contextualize potential population response to multiple levels of mortality, using white sharks (Carcharodon carcharias) in South Africa as a case study. The species has been protected since 1991, yet substantial uncertainty about population dynamics persists and recent declines at two aggregation sites have renewed conservation concern. All scenarios indicated that annual removals in the 10s of individuals would substantially limit the potential for and magnitude of any abundance increase following prohibition. Because average known removals from the KwaZulu-Natal Sharks Board’s Bather Protection Program have typically remained higher than these thresholds, they likely eliminated much of the conservation benefit derived from prohibition. The only life history scenario to achieve appreciable increase when simulated removals were similar to published averages assumed maturation occurred at a much younger age than currently understood. Our results demonstrate why general application of life history-based simulations can provide a useful mechanism to evaluate the biological plausibility of life history information and abundance trends, and to explore the scope for population response to recovery actions. For South Africa, our results suggest that even known levels of white shark removals, which likely underestimate total removals within their range, may be sufficient to drive abundance decline and new mitigation measures may be required to ensure population recovery.

The Northwest Atlantic (NWA) population of porbeagles Lamna nasus is susceptible to capture in rod-and-reel fisheries and most individuals are discarded alive due to catch and size limits. To estimate post-release survival, pop-off satellite archival tags were attached to porbeagles captured with rod-and-reel. Fourteen tags were deployed, of which 13 transmitted. All sharks for which we had data survived, giving a post-release survival rate of 100%. Following release, 6 individuals remained in surface waters for several hours to days, while 2 individuals immediately resumed normal diving behaviors. For the remaining sharks (n = 5), low tag transmission resolution precluded the detection of fine scale post-release behavior. The duration of initial depth-holding behavior was characterized using a break-point analysis of dive track variance, which suggests porbeagles exhibited a median post-release recovery period of 116 h (10th and 90th percentiles = 68.8 and 280.1 h) following capture and handling. Our preliminary study suggests immature porbeagles are resilient to capture and handling, although more data would provide stronger support for management recommendations.

Accurately characterizing the biology of a pelagic shark species is critical when assessing its status and resilience to fishing pressure. Natural mortality (M) is well known to be a key parameter determining productivity and resilience, but also one for which estimates are most uncertain. While M can be inferred from life history, validated direct estimates are extremely rare for sharks. Porbeagle (Lamna nasus) and shortfin mako (Isurus oxyrinchus) are presently overfished in the North Atlantic, but there are no directed fisheries and successful live release of bycatch is believed to have increased. Understanding M, post-release mortality (PRM), and variables that affect mortality are necessary for management and effective bycatch mitigation. From 177 deployments of archival satellite tags, we inferred mortality events, characterized physiological recovery periods following release, and applied survival mixture models to assess M and PRM. We also evaluated covariate effects on the duration of any recovery period and PRM to inform mitigation. Although large sample sizes involving extended monitoring periods (>90 days) would be optimal to directly estimate M from survival data, it was possible to constrain estimates and infer probable values for both species. Furthermore, the consistency of M estimates with values derived from longevity information suggests that age determination is relatively accurate for these species. Regarding bycatch mitigation, our analyses suggest that juvenile porbeagle are more susceptible to harm during capture and handling, that keeping lamnid sharks in the water during release is optimal, and that circle hooks are associated with longer recovery periods for shortfin mako.

Recovered archival satellite tags afford a rare opportunity to evaluate diving behavior relative to ocean conditions at fine spatiotemporal scales. Ocean temperature and mixed layer depth (MLD) were strongly related to daily and seasonal patterns in diving behavior of an adult female porbeagle shark Lamna nasus. The shark remained below MLD when waters were strongly stratified in summer or in the Gulf Stream; the daily timing of dives was associated with local sunrise and sunset. In the presence of a hurricane, diving activity abruptly increased, coincident with upwelling causing an abrupt decrease in surface temperature. Our first report of the behavioral response to a hurricane as well as the seasonal patterns in depths, occupied temperatures, and vertical speeds suggest environmental preferences of porbeagle are not static and individuals may use horizontal, as well as vertical, movement to take advantage of areas with specific characteristics.