Hooking mortality of swordfish in pelagic longlines: comments on the efficiency of minimum retention sizes

Abstract

This paper revises detailed data on hooking (at-haulback) mortality of swordfish captured and discarded by pelagic longlines, using as a case study the shallow night setting Portuguese pelagic longline fishery that targets manly swordfish, in the Atlantic Ocean. The overall at-haulback mortality for swordfish was very high, specifically 85.2% for all sample/sizes combined, and even higher for the smaller sizes classes. Specifically, observed at-haulback mortality was 87.8% for specimens smaller than 125 cm lower jaw fork length (LJFL) and 88.1% for specimens smaller than 119 cm LJFL, corresponding to the minimum landing size options currently in place in the Atlantic Ocean. The mortality was modeled with logistic generalized linear models, showing that specimen size, sea surface temperature (SST) and mode of operation were significant variables, and with the hooking mortality decreasing with specimen size while increasing with SST. Even though this study only focuses one fishery, the data is widespread and covers a wide Atlantic area, being representative of most modern shallow setting longlines targeting swordfish, and including both fresh and freezer operating vessels. Additionally, this study only focuses hooking mortality, with a note that the overall mortality rates are likely higher due to post-release mortality of released specimens. Overall, this work presents new and important information on swordfish mortality and opens the discussion on the efficiency of the minimum landing sizes for swordfish currently in place in the Atlantic that were established mainly to protect juvenile swordfish.