To effectively protect at‐risk sharks, resource managers and conservation practitioners must have a good understanding of how fisheries removals contribute to changes in abundance and how regulatory ...restrictions may impact a population trajectory. This means they need to know the number of animals being removed from a population and whether a given number of removals will lead to population increases or declines. For white shark (Carcharodon carcharias), theoretical quantities like the intrinsic rate of population increase or rebound potential (ability to increase in size following decline) are difficult to conceptualize in terms of real‐world abundance changes, which limits our ability to answer practical management questions. To address this shortfall, we designed a simulation model to evaluate how our understanding of longevity and life history variability of white shark affects our understanding of population trends in the Northwest Atlantic. Then, we quantified the magnitude of removals that could have caused historical population declines, compared these to biologically based reference points, and explored the removal scenarios which would result in population increase. Our results suggest that removals on the order of 100s of juveniles per year could have resulted in population‐level declines in excess of 60% during the 1970s and 1980s. Conservation actions implemented since the 1990s would have needed to be nearly 100% effective at preventing fishing mortality in order for the population to double in abundance over the last 30 years. Total removals from all fleets needed to be exceptionally small to keep them below biological reference points for white shark in the Northwest Atlantic. The population's inherent vulnerability to fishing pressure reaffirms the need for restrictive national and international conservation measures, even under a situation of abundance increase.
We designed a simulation model to evaluate how our understanding of longevity and life history variability of white shark affects our understanding of population trends in the North Atlantic. Then, we quantified the magnitude of removals that could have caused historical population declines, compared these to biologically based reference points, and explored the removal scenarios which would result in population increase. Our results demonstrate the inherent vulnerability of the species to fishing pressure and reaffirm the need for restrictive international conservation measures to protect the population, even under a situation of abundance increase.
Metapopulation structure is typically thought to increase regional species abundance, promote population persistence, and aid in the re-establishment of extirpated populations. However, the ...underlying theoretical models tended to assume high productivity, making the conservation benefit of metapopulation structure uncertain for endangered species with low productivity. We simulated population assemblages (N = 50) of diadromous fishes under high to low productivity scenarios to explicitly assess how straying (movement from natal to non-natal rivers) contributes to changes in species abundance and extinction risk. The population aggregation exhibited greater total abundance from source–sink dynamics and also exhibited the rescue effect when productivity remained moderately high. However, straying did not ensure persistence of nonviable populations or enable population re-establishment when productivity was low. These results were robust to a wide range of alternate spatial and life-history parameterizations of the simulation model. Relative to a real-world population aggregation of endangered Atlantic salmon (Salmo salar), our results would argue for a shift in remediation priorities to prevent extinction. Although there is strong evolutionary justification for maintaining widespread distributions of endangered diadromous species, the immediate numerical consequences of this approach may hinder recovery.
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.
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.
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.
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•Non-traditional monitoring data allow evaluation of abundance and distribution trends.•White sharks’ regional status remains stable despite localized decline.•Human-shark incidents ...demonstrate a shift eastwards from traditional aggregation sites.•Future monitoring would benefit from improved standardization to track population status.•Understanding ecological drivers and mortality sources is crucial.
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.
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.
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.
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.
Supplementation programs based on captive breeding and rearing are increasingly being used in recovery planning for endangered or threatened salmonid populations. However, it is largely unknown if ...increased abundance from these programs can offset deleterious genetic changes from the captive environment and lead to viable populations in the wild. In this paper, we developed a life-history-based population viability analysis that explicitly incorporates declines in fitness attributable to captive breeding and rearing using the breeder's equation as part of the projection model. Using endangered inner Bay of Fundy Atlantic salmon as a case study (a population assemblage for which supplementation is a major component for the recovery plan), we evaluated how genetic changes influence abundance trajectories and extinction risk. Based on the population projections, continual supplementation enables the population to build from critically low abundance levels, even under high rates of fitness loss. However, beyond 4-6 generations, loss of fitness (>15%%) outweighs any increase in abundance and causes the population projection to start to decline. For the majority of the scenarios, abundances were predicted to increase and remain in excess of the current population size for 10 to upward of 30 years, albeit at progressively lower population-level fitness as compared to current values. Although the captive breeding and rearing program does prevent extinction in the short term in this case study, associated fitness costs limit the population's overall probability of recovery, as well as increase the length of time to recovery. Under the assumption of interbreeding between wild and captive-reared individuals, declines greater than 10%% in relative fitness at a population level are sufficient to counter abundance increases resulting from supplementation. Although extinction risk over the short term can be reduced by increasing the proportion of the population that is reared in captivity, this comes at a cost to the probability of recovery for the population over the longer term, particularly as environmental conditions change. Generalizing from this case study, the useful duration of supplementation programs may be limited to short-term population increase (i.e., to prevent extinction) and may not be a workable strategy for longer term recovery planning.
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