Mortality of non-target flatfishes escaping from demersal trawl codends

Abstract

Mortality of non-target flatfishes that had escaped from various sizes (40, 44 and 50 mm) of codend meshes was determined by capturing and holding the escapees in cages for 7 days. Although three flatfish species were studied, sufficient data for detailed analyses were obtained only for Buglossidium luteum (Risso, 1810) (solenette). Trawl hauls (48 in total) with 15 min towing durations were conducted in the summer and winter seasons of 2011 and 2012. A tailored bottom trawl with 800 meshes around the mouth was used with covers to capture fish. Covers were used as an observation cage, and were fixed to the sea bottom at depths of 3.0–8.2 m. The monitoring period was 7 days. It was estimated that average escape mortality of B. luteum for all test cages (96.5%) in summer was significantly higher (p < .01) than that in winter (56.5%). Mesh size and shape was not a significant factor (p > .01), with the exception of the 2011 winter; however, the mortality rate was inversely related to fish length in most cases (p < .01). High mortalities (~100%) were observed for Arnoglossus laterna (Walbaum, 1792) (Mediterranean scaldfish) and Citharus linguatula (Linnaeus, 1758) (spotted flounder) in all hauls in both summer and winter.