The Experts below are selected from a list of 285 Experts worldwide ranked by ideXlab platform
Sturla Furunes Kvamsdal - One of the best experts on this subject based on the ideXlab platform.
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stochastically induced critical Depensation and risk of stock collapse
Marine Resource Economics, 2015Co-Authors: Diwakar Poudel, Leif Kristoffer Sandal, Sturla Furunes KvamsdalAbstract:This article investigates the risk of stock collapse due to stochastically induced critical Depensation in managed fisheries. We use a continuous-time surplus production model and an economic model with downward-sloping demand and stock-dependent costs. First, we derive an optimal exploitation policy as a feedback control rule by applying the Hamilton-Jacobi-Bellman approach and analyze the effects of stochasticity on the optimal policy. Then, we characterize the long-term sustainable optimal state and estimate the risk of stock collapse due to stochastically induced critical Depensation. We find that the optimal harvest policy in the stochastic setting is conservative at low stochasticity and approaches the myopic solution at high stochasticity. The risk of stock collapse is increasing with the stochasticity and decreasing with stock sizes.
Ana Rakitin - One of the best experts on this subject based on the ideXlab platform.
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Depensation, probability of fertilization, and the mating system of Atlantic cod (Gadus morhua L.)
ICES Journal of Marine Science, 2004Co-Authors: Sherrylynn Rowe, Jeffrey A. Hutchings, Dorte Bekkevold, Ana RakitinAbstract:Rowe, S., Hutchings, J. A., Bekkevold, D., and Rakitin, A. 2004. Depensation, probability of fertilization, and the mating system of Atlantic cod (Gadus morhua L.). e ICES Journal of Marine Science, 61: 1144e1150. Atlantic cod (Gadus morhua L.) have been severely overexploited and are currently at historic population lows, having declined 90% in the North Sea and 99% off northeast Newfoundland in recent decades. Slow rates of recovery and continuing declines may be attributable to Depensation, defined as a reduction in per capita growth rate concomitant with reduced population size. Several potential causes of Depensation relate to low mating success and consequent reduced production of offspring. We explore the empirical basis of one of these in Atlantic cod using egg fertilization and male abundance data obtained from 21 experimental populations generated by three independent research programmes. We find support for the hypotheses that (a) fertilization rate declines with abundance and (b) variance in fertilization rate increases as population size declines. The former identifies one potential mechanism underlying Depensation in Atlantic cod. The latter has negative genetic consequences for effective population size (Ne), resulting in a decline in the ratio of Ne to census population size (Ne/Nc) with declining abundance. Our results may have general implications for the conservation biology of broadcast-spawning marine fish, particularly those with mating systems similar to that of Atlantic cod.
James F Kitchell - One of the best experts on this subject based on the ideXlab platform.
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cultivation Depensation effects on juvenile survival and recruitment implications for the theory of fishing
Canadian Journal of Fisheries and Aquatic Sciences, 2001Co-Authors: Carl J Walters, James F KitchellAbstract:Large, dominant fish species that are the basis of many fisheries may be naturally so successful due partly to "cultivation effects," where adults crop down forage species that are potential competitors/predators of their own juveniles. Such effects imply a converse impact when adult abundance is severely reduced by fishing: increases in forage species may then cause lagged, apparently depensatory decreases in juvenile survival. Depensatory effects can then delay or prevent stock rebuilding. Cultivation effects are apparently common in freshwater communities and may also explain low recruitment success following severe declines of some major marine stocks such as Newfoundland Atlantic cod (Gadus morhua). Risk of depensatory effects should be a major target of recruitment research, and management policies should aim for considerably higher spawning abundances than has previously been assumed necessary based on recruitment data collected during adult stock declines associated with fishery development.
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cultivation Depensation effects on juvenile survival and recruitment implications for the theory of fishing
Canadian Journal of Fisheries and Aquatic Sciences, 2001Co-Authors: Carl J Walters, James F KitchellAbstract:Large, dominant fish species that are the basis of many fisheries may be naturally so successful due partly to "cultivation effects," where adults crop down forage species that are potential compet...
Diwakar Poudel - One of the best experts on this subject based on the ideXlab platform.
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stochastically induced critical Depensation and risk of stock collapse
Marine Resource Economics, 2015Co-Authors: Diwakar Poudel, Leif Kristoffer Sandal, Sturla Furunes KvamsdalAbstract:This article investigates the risk of stock collapse due to stochastically induced critical Depensation in managed fisheries. We use a continuous-time surplus production model and an economic model with downward-sloping demand and stock-dependent costs. First, we derive an optimal exploitation policy as a feedback control rule by applying the Hamilton-Jacobi-Bellman approach and analyze the effects of stochasticity on the optimal policy. Then, we characterize the long-term sustainable optimal state and estimate the risk of stock collapse due to stochastically induced critical Depensation. We find that the optimal harvest policy in the stochastic setting is conservative at low stochasticity and approaches the myopic solution at high stochasticity. The risk of stock collapse is increasing with the stochasticity and decreasing with stock sizes.
Jeffrey A. Hutchings - One of the best experts on this subject based on the ideXlab platform.
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Renaissance of a caveat: Allee effects in marine fish
2013Co-Authors: Jeffrey A. HutchingsAbstract:The population dynamics of marine fish at low abundance has long been of interest. One century ago, Johan Hjort drew attention to the importance of understanding “the laws which govern the renewal of the animal population”. Integral to the current work on the recovery of collapsed fish stocks is the association between abundance and per capita population growth rate (r), a negative correlation being rep-resentative of compensation and a positive correlation indicative of an Allee effect, also termed Depensation. Allee effects are predicted to slow the rate, and increase the uncertainty, of recovery. Based on studies having sufficient data at low abundance, the magnitude of depletion experienced by some fish populations appears to have been sufficient to have generated either an Allee effect or a transition from strong to weak (or absent) compensatory dynamics. To a first approximation, empirically based Allee-effect reference points are consistent with suggested thresholds for overfishing and stock collapse. When evaluating Allee effects in marine fish, it is important not to conflate causal mechanism(s) with the pattern between r and abundance; the latter is of greater practical import. An additional caveat is that the longer a population remains at low abundance, the more likely it is that the environment around it will change in ways that are unfavourable to recovery. It might be this “temporal tyranny ” of small population size that is most likely to produce an emergent Allee effect and depensatory dynamics in some collapsed marine fish populations
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marine fish population collapses consequences for recovery and extinction risk
BioScience, 2004Co-Authors: Jeffrey A. Hutchings, John D ReynoldsAbstract:Abstract Rapid declines threaten the persistence of many marine fish. Data from more than 230 populations reveal a median reduction of 83% in breeding population size from known historic levels. Few populations recover rapidly; most exhibit little or no change in abundance up to 15 years after a collapse. Reductions in fishing pressure, although clearly necessary for population recovery, are often insufficient. Persistence and recovery are also influenced by life history, habitat alteration, changes to species assemblages, genetic responses to exploitation, and reductions in population growth attributable to the Allee effect, also known as Depensation. Heightened extinction risks were highlighted recently when a Canadian population of Atlantic cod (Gadus morhua) was listed as endangered, on the basis of declines as high as 99.9% over 30 years. Unprecedented reductions in abundance and surprisingly low rates of recovery draw attention to scientists' limited understanding of how fish behavior, habitat, ecol...
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Depensation, probability of fertilization, and the mating system of Atlantic cod (Gadus morhua L.)
ICES Journal of Marine Science, 2004Co-Authors: Sherrylynn Rowe, Jeffrey A. Hutchings, Dorte Bekkevold, Ana RakitinAbstract:Rowe, S., Hutchings, J. A., Bekkevold, D., and Rakitin, A. 2004. Depensation, probability of fertilization, and the mating system of Atlantic cod (Gadus morhua L.). e ICES Journal of Marine Science, 61: 1144e1150. Atlantic cod (Gadus morhua L.) have been severely overexploited and are currently at historic population lows, having declined 90% in the North Sea and 99% off northeast Newfoundland in recent decades. Slow rates of recovery and continuing declines may be attributable to Depensation, defined as a reduction in per capita growth rate concomitant with reduced population size. Several potential causes of Depensation relate to low mating success and consequent reduced production of offspring. We explore the empirical basis of one of these in Atlantic cod using egg fertilization and male abundance data obtained from 21 experimental populations generated by three independent research programmes. We find support for the hypotheses that (a) fertilization rate declines with abundance and (b) variance in fertilization rate increases as population size declines. The former identifies one potential mechanism underlying Depensation in Atlantic cod. The latter has negative genetic consequences for effective population size (Ne), resulting in a decline in the ratio of Ne to census population size (Ne/Nc) with declining abundance. Our results may have general implications for the conservation biology of broadcast-spawning marine fish, particularly those with mating systems similar to that of Atlantic cod.
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population dynamics of exploited fish stocks at low population levels
Science, 1995Co-Authors: Ransom A Myers, Jeffrey A. Hutchings, Nicholas J Barrowman, A A RosenbergAbstract:Models of population dynamics in which per capita reproductive success declines at low population levels (variously known as Depensation, the Allee effect, and inverse density-dependence) predict that populations can have multiple equilibria and may suddenly shift from one equilibrium to another. If such depensatory dynamics exist, reduced mortality may be insufficient to allow recovery of a population after abundance has been severely reduced by harvesting. Estimates of spawner abundance and number of surviving progeny for 128 fish stocks indicated only 3 stocks with significant Depensation. Estimates of the statistical power of the tests strengthen the conclusion that depensatory dynamics are not apparent for fish populations at the levels studied.