Threshold Management Policies for Exploited Populations

Abstract

Under a threshold management policy, harvesting occurs at a constant rate but ceases when a population drops below a threshold. A simulation model of an age-structured population with stochastic recruitment was constructed with such a harvest policy with several threshold levels. Other factors were fishing mortality, recruitment, and initial biomass. The objective function was a weighted function of average yield and standard deviation over a planning horizon. First, we determined the optimal threshold given fishing mortality. Secondly, we determined optimal threshold and fishing mortality, simultaneously. In application to eastern Bering Sea pollock, a threshold management policy always increased average yield over a non-threshold policy. For the first problem, optimal threshold levels ranged from 20 to 30% of pristine biomass. For the second problem, each scenario had a unique threshold and fishing mortality, with fishing mortality slightly above the maximum sustainable yield (MSY) level and a threshold range of 25–50%. These results were robust in regard to other factors. Benefits of the threshold policy were greater with a Ricker spawner-recruit model and with higher fishing mortality. The success of the threshold management policy is due to the relatively rapid rebuilding of a population to levels producing MSY.

Harvesting Strategies in a Fish Stock Dominated by Low-frequency Variability: The Norwegian Spring-spawning Herring (Clupea harengus)

“…However, similar results have been reached in simulation studies (e.g. , Ludwig 1980Ludwig , 1998Quinn, Fagen, and Zheng 1990;Mace 1994), and it has been proposed that threshold strategies also perform well in the presence of critical depensation (a stock tends to collapse when below a critical size), catastrophes (sudden unexpected collapses in stock size), and with a heavy-tailed distribution of fluctuations in net reproduction (Ludwig 1998).…”

Sustainable yield indicators from biomass: are there appropriate reference points for use in tropical fisheries?

“…The FAO Code of Conduct for Responsible Fisheries (FAO, 1995a) and the UN Agreement on Straddling Fish Stocks and Highly Migratory Fish Stocks (UN, 1995) distinguish between benchmarks for exploitation which should be used as targets for safe exploitation (socalled target reference points) and those reference points which should not be approached and certainly not exceeded (Quinn et al, 1990;Mace, 1994, and see Caddy and Mahon, 1995, for a recent review). Experience has shown that MSY is not a safe target reference point for exploitation, but, under some circumstances, may be used as a limit reference point (Caddy and McGarvey, in press).…”

Avaliação dos estoques pesqueiros de quatro espécies de grandes bagres (Siluriformes, Pimelodidae) na bacia do rio Cuiabá, Pantanal norte, Brasil, utilizando alguns Pontos de Referência Biológicos

“…Além disto, é preciso levar em consideração, por exemplo, qual a porcentagem de redução que este nível de pesca traria para a biomassa por recruta inicial (quando F = 0). Dentro desta perspectiva o limite ótimo estaria entre 20 e 30% da biomassa do estoque virgem (QUINN II et al 1990) e em estoques pouco conhecidos, principalmente em relação ao estoque-recrutamento como neste caso, este limite deveria ser 30% (CADDY & MAHON 1995). Esta recomendação é sustentada pela análise teó-rica de MACE (1994).…”