Modified hooks reduce incidental mortality of snapper (Pagrus auratus: Sparidae) in the New Zealand commercial longline fishery

摘要

In longline fisheries, decreasing the catch of undersized fish and minimizing the rate of gut-hooking over all sizes will reduce incidental mortality, and improve the landed value of the commercial catch. In this study, standard Tainawa 16R longline hooks were simultaneously fished for snapper (Pagrus auratus) with the same hook pattern modified by the addition of 20-mm and 40-mm wire appendages. The experimental design also included three bait types. Gut-hooking rates were markedly lower on modified hooks relative to normal hooks. Normal hooks gut-hooked 17% and 30% (pooled across baits) of snapper caught in January and June, respectively, whereas 20-mm modified hooks gut-hooked 7% and 12%, and 40-mm hooks gut-hooked only 2% in both seasons. Overall catch rates were significantly lower on modified hooks, however most of the loss of catch comprised undersized fish and “deads” (unsuitable for export). There was no significant reduction in the weight of export-quality snapper landed using modified hooks. Modified hooks reduced both the catch rate and gut-hooking rate of undersized snapper. If it is assumed that all gut-hooked discards are likely to die, the estimated annual reduction in discard mortality at the stock level would be 78% if 20-mm hook modifications are used, and 96% if 40-mm modifications are used. These estimates are consistent for scenarios where minimum legal size is set at both 25 cm and 27 cm, however they are based on the assumption that observed catch and mortality rates are representative of the commercial fishery.