Properly designed effort management for highly fluctuating small pelagic fish populations: a case study in a purse seine fishery targeting chub mackerel

摘要

Proper fisheries management is difficult in fisheries targeting small pelagic fish that are subject to climate-induced oceanic regime shifts. A favourable regime tends to induce capacity investments, but the results of these investments turn out to be irreducible overcapacity under the next unfavourable regime. The Japanese purse seine fishery studied here, which targets small pelagic fish, is a typical example of such overcapacity. However, we demonstrate that this fishery represents a best practice of properly designed effort management in recent years through its quantitative evaluation. Generalized linear mixed models (GLMMs) applied to daily logbook data revealed that effort restrictions on daily purse seine operations (in terms of the duration and the total number of operations per ship and day) significantly reduced total fishing efforts on chub mackerel. Stochastic simulations based on the parameters estimated in GLMMs quantified the extent of the potential catch reduction by the effort reductions at approximately 20%. Combined with the effect of another concurrently implemented management measure, i.e. day closures, the net effects of the effort management on the total catch reduction were estimated at 30%. Because the quota uptake percentages in this fishery were 60-80% during the study period, we conclude that the effort management helped avoid overshooting of the quota and consequent seasonal closures.