Evaluation of different harvest strategies for a vendace population with highly variable recruitment: a simulation approach

In the auto-regulative commercial vendace fishery, the fishing effort is increased if stock decreases in order to reduce interannual variation in supply and revenue. When the revenue per unit effort reaches a profitability threshold, fishing ceases. The performance of this strategy was compared with constant effort and constant revenue strategies combined with different threshold levels of fishing shut-down. A simulation model of an age-structured vendace population was applied. Qualitatively the results were very similar for the Ricker and Cushing stock-recruitment curves with or without negative dependence between consecutive recruitments. The interannual variability of revenue increased with an increase in mean revenue when approaching maximum mean annual revenue. Given a low threshold, the constant effort strategy performed best and constant yield worst with respect to the objectives of maximum mean revenue, low mean effort, and low risk for spawning biomass. For the constant effort strategy, the revenue variation was already high at low revenue levels. For the strategy aiming at constant revenue, the variation was low at a low revenue level but increased rapidly with increasing revenue. At a moderate revenue level, the intermediate strategy produced good trade-offs of rather high mean revenue with reasonable variation and risk to reproduction. A tendency for two-year-cyclicity appeared at a low mean spawning stock level. When the threshold of fishery shut-down was increased, the variability in revenue increased for every strategy and differences in their performance decreased. Simultaneously, the risk of low spawning stock decreased. The threshold regulation increased the instability of stock in cases of high fishing effort. Means for regulating the access to fishing should be established in order to ensure the possibility of flexibility of fishing effort and tolerable revenue variation. Some threshold regulation to protect the spawning stock is also necessary.