For many species, reproductive failure may occur if abundance drops below critical Allee thresholds for successful breeding, in some cases impeding recovery. At the same time, extreme environmental ...events can cause catastrophic collapse in otherwise healthy populations. Understanding what natural processes and management strategies may allow for persistence and recovery of natural populations is critical in the face of expected climate change scenarios of increased environmental variability. Using a spatially explicit continuous-size fishery model with stochastic dispersal parameterized for abalone—a harvested species with sedentary adults and a dispersing larval phase—we investigated whether the establishment of a system of marine protected areas (MPAs) can prevent population collapse, compared with nonspatial management when populations are affected by mass mortality from environmental shocks and subject to Allee effects. We found that MPA networks dramatically reduced the risk of collapse following catastrophic events (75%–90% mortality), while populations often continued to decline in the absence of spatial protection. Similar resilience could be achieved by closing the fishery immediately following mass mortalities but would necessitate long periods without catch and therefore economic income. For species with Allee effects, the use of protected areas can ensure persistence following mass mortality events while maintaining ecosystem services during the recovery period.
Abstract
Atlantic bluefin tuna (ABT) are large, wide-ranging pelagic predators, which typically migrate between foraging regions in the North Atlantic and two principal spawning regions, the Gulf of ...Mexico and the Mediterranean Sea. A new spawning area has been described in the Slope Sea (SS) region off New England; however, the relationship between ABT that spawn in the SS and ABT using the principal spawning regions remains poorly understood. We used electronic tags to examine the location, temperature, and diving behaviour of ABT in the SS, and identified 24 individuals that were present during the spawning season (June–August) with tag data showing temperatures and behaviour consistent with spawning ABT. In general, the SS spawners had similar spatial ranges to Mediterranean-spawning ABT; however, some individuals displayed distinct behaviours that were identified first in the Gulf of Mexico spawners. Using monthly spatial distributions, we estimated that the SS spawners have high exposure to fishing pressure relative to other ABT and may represent a disproportionate share of the West Atlantic catch. This analysis provides the first description of the behaviour of ABT frequenting this spawning ground, creating a foundation for integrating this region into multi-stock management and, potentially, conserving an important source of genetic diversity.
Atlantic bluefin tuna (Thunnus thynnus) are a large, highly migratory fish distributed throughout the North Atlantic Ocean and adjacent seas currently managed as two discrete stocks: western and ...eastern. Both stocks forage in the North Atlantic, and a high degree of intermixing occurs, which combined with limited single-stock survey data makes it difficult to assess the abundance and status of individual populations. In this study, we used movement patterns from a multidecadal tagging dataset to create monthly distribution maps for these two major stocks. We then used these maps to separate the overall catch records into stock-specific catch (catch per unit effort, CPUE) time series. We identified an increase in the past two decades in the proportion of catch estimated to come from the eastern stock, attributable to a decrease in CPUE in regions dominated by the western stock, relative to other regions. The stock-specific catch series can be used to improve the accuracy of stock assessments and inform spatial management.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Fisheries typically truncate target species’ size distributions through an increase in mortality, especially if harvest is size-selective. Such truncation can push a harvested species’ size ...distribution into classes most vulnerable to gape-limited predation, such that predator–prey dynamics might affect the rate of recovery from fishing. Understanding this rate of recovery is crucial to adaptive management of no-take reserves and fisheries closures. We used a 2-species size-structured model to examine how gape-limited predation alters post-harvest recovery for 3 example prey species, viz. cod, haddock, and whiting. We found little difference in recovery patterns of prey abundance and size structure between systems with gape-limited generalist predation and those with no size-dependent predation. However, gape-limited obligate predation can lead to fluctuations, with transient declines, that delay recovery to the pre-harvest equilibrium for both biomass and mean size due to a time lag in predator recovery. Fluctuating recovery dynamics under obligate predation were most likely for predators that experienced greater declines due to prey loss during harvest and for slow-growing prey with an adult size refuge. We conclude that through these delays, the presence of a strong dynamically linked predator can alter the expectations of the time scale for different recovery metrics used in adaptive management.
Introduction
Atlantic bluefin tuna (ABT) are large, migratory pelagic predators of high economic importance. ABT are currently managed as two independent stocks assigned to discrete spawning areas ...(Gulf of Mexico, and Mediterranean); however, stock overlap outside spawning areas makes accurate assignment of catch to stock-of-origin difficult.
Methods
Within this two-stock paradigm, we characterised stock-specific spatial distributions and behaviours by comparing habitat usage and vertical movement behaviours of 118 electronically tagged adult ABT spatially assigned to the GOM and Med spawning grounds. These spatial and behavioural differences were used in tests to probabilistically assign unknown individuals (which did not visit the GOM/Med spawning areas) to a stock.
Results
This new methodological approach using existing tag data, enables increased assignment of a track to a potential stock, to be achieved before genetic assignments. We identified certain markedly different movement patterns, range extents, depth use preferences (and associated area usage), migration directness and speeds, corresponding distance from shore, and mesopelagic-layer visitation. The probabilistic assignment approach had 97% in-bag testing accuracy, then assigned 190 individuals to a stock, doubling the number of assigned stock-of-origin tracks in our dataset, and potentially revealing novel movement and behaviour patterns among pre-spawning-age ABT.
Discussion
This approach can be easily adapted to other study species, more stocks, and different testing variables, hopefully serving as a useful addition to the fisheries management toolkit.
If a species is bycatch in a fishery targeted at its competitor or predator, it experiences both direct anthropogenic mortality and indirect positive effects through species interactions. If the ...species involved interact strongly, the release from competition or predation can counteract or exceed the negative effects of bycatch. We used a set of two- and three-species community modules to analyze the relative importance of species interactions when modeling the overall effect of harvest with bycatch on a nontarget species. To measure the trade-off between direct mortality and indirect positive effects, we developed a "bycatch transition point" metric to determine, for different scenarios, what levels of bycatch shift overall harvest impact from positive to negative. Under strong direct competition with a targeted competitor, release from competition due to harvest leads to a net increase in abundance even under moderate levels of bycatch. For a three-species model with a shared obligate predator, the release from apparent competition exceeds direct competitive release and outweighs the decrease from bycatch mortality under a wide range of parameters. Therefore, in communities where a shared predator forms a strong link between the target and nontarget species, the effects of indirect interactions on populations can be larger than those of direct interactions. The bycatch transition point metric can be used for tightly linked species to evaluate the relative strengths of positive indirect effects and negative anthropogenic impacts such as bycatch, habitat degradation, and introduction of invasive species.
Many fishery production models implicitly incorporate a single time lag for both recruitment and mortality despite the fact that in populations of breeding adults, deaths occur yearly while the entry ...of new adults comes from juveniles born potentially many years prior to adulthood. Models that do not account for this difference in timing will overestimate abundance for a decreasing stock and underestimate increases during a recovery period. We investigated the effect of incorporating unequal recruitment and mortality time lags into depletion-based stock reduction analysis (DB-SRA), a stock assessment method for data-poor species. Using both simulated data and catch series of Pacific rockfish (Sebastes spp.), we found that for declining stocks with no mortality delay and a recruitment time lag equal to age-at-maturity, estimated overfishing limits were up to 40% lower than those from the model with both time lags equal to age-at-maturity. Deviation between the two models’ predictions increases with age-at-maturity and natural mortality rate, suggesting that time lag separation is most important for long-lived species. We propose a correction factor for net production models that eliminates stock overestimation due to implicitly equal time lags.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
•Large Atlantic bluefin tuna tagged off the coast of Ireland utilize the warm North Atlantic Current to access foraging areas in the North Atlantic Ocean.•Five hotspots of Atlantic bluefin tuna occur ...in the central and eastern North Atlantic Ocean. Each are in regions with long-lived, quasi-stationary anticyclonic features (i.e., eddies or recirculation).•Daily maximum depth and time at mesopelagic depths (i.e., greater than 200 m) are positively correlated with absolute dynamic topography in the open ocean.•In the Winter, a majority of Atlantic bluefin tuna tagged in Irish waters travel to the Newfoundland Basin, a region with intense mesoscale eddy activity and high mesopelagic fish biomass.•Some Atlantic bluefin tuna migrated in the Spring to the Mediterranean Sea, an important spawning ground for this species.
Electronic tagging of Atlantic bluefin tuna (ABT; Thunnus thynnus) has shaped our understanding of their movements and migrations throughout the Atlantic basin. In this study, we used pop-up satellite archival tagging data to examine the movements of 51 large (CFL µ ± σ: 215 ± 15 cm) ABT tagged off the coast of Ireland. When combined with satellite oceanographic data, we found that ABT take advantage of the warm North Atlantic Current to access foraging areas in the North Atlantic Ocean. We identified four potential foraging regions: (1) off the coast of Ireland, (2) the Bay of Biscay, (3) the Newfoundland Basin, and (4) the West European Basin. In addition, 14 ABT migrated to their spawning grounds in the Mediterranean Sea, entering by May 16 and exiting by July 7, on average. In all five regions, anticyclonic ocean features (i.e., eddies or recirculation) were present. In the open ocean, these features often co-occurred with areas where the daily maximum depth of tuna exceeded 400 m and tuna spent extended time at mesopelagic depths (i.e., greater than 200 m). We hypothesize that ABT exploit anticyclonic structures to forage on the abundant mesopelagic fish communities. Additionally, our results suggest that ABT are travelling across the North Atlantic Ocean in a directed migration to the Newfoundland Basin to reach what may be one of the best mesopelagic feeding grounds in the world.
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