Mortality of non-target flatfishes escaping from demersal trawl codends

Abstract

Summary 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.

Fish welfare in capture fisheries: A review of injuries and mortality

“…In trawls, mortality is higher after escape via codend meshes than after escape via sorting grid (Suuronen et al, 1996c), in line with the sorting grid's purpose of reducing escapee mortality. Within codend meshes, mortality was higher after escape from smaller rather than larger meshes (Düzbastilar et al, 2016) and after escape via diamond meshes rather than square meshes (Düzbastilar et al, 2010a(Düzbastilar et al, , 2015(Düzbastilar et al, , 2016, in line with square meshes' purpose of reducing escapee mortality. In purse seining, the influence of selectivity device on mortality was variable, with higher mortality among mackerel that escaped via a sorting grid than for controls, but with no such difference in mortality for saithe (Misund and Beltestad, 2000).…”

“…In purse seining, the influence of selectivity device on mortality was variable, with higher mortality among mackerel that escaped via a sorting grid than for controls, but with no such difference in mortality for saithe (Misund and Beltestad, 2000). In several studies on trawls and purse seines, however, there was no relation between selectivity devices and mortality (Düzbastilar et al, 2010b(Düzbastilar et al, , 2016Ingólfsson et al, 2007;Suuronen et al, 1996aSuuronen et al, , 2005). Table 3 shows that fish length and species influence mortality.…”

“…Table 3 shows that fish length and species influence mortality. In general, mortality is higher among smaller than among larger fish in all gear types, i.e., trawls (Depestele et al, 2014;Düzbastilar et al, 2010aDüzbastilar et al, , 2010bDüzbastilar et al, , 2015Düzbastilar et al, , 2016Hyvärinen et al, 2008;Ingólfsson et al, 2007;Richards et al, 1995;Suuronen et al, 1996aSuuronen et al, , 1996cUhlmann et al, 2016), hooks (Milliken et al, 1999;Pálsson et al, 2003;Stephen and Harris, 2010), purse seines (Marçalo et al, 2010;Tenningen et al, 2012), gillnets (Broadhurst et al, 2009;Purbayanto et al, 2001;Smith and Scharf, 2011), traps (Stewart, 2008) and seines (Broadhurst et al, 2008). The higher mortality among smaller fish could be explained by their poorer swimming ability and endurance as compared to larger fish (Suuronen and Erickson, 2010), which disables them to avoid contact with the trawl, purse seine or seine net (Olla et al, 1997).…”

“…Table 3 shows that the context variables change in water temperature, capture depth, fishing duration, duration of air exposure and density in the net affect mortality across several gear types. A larger change in water temperature due to higher temperatures of surface water resulted in higher mortality of fish caught by trawl Hyvärinen et al, 2008;Olla et al, 1998;Suuronen et al, 2005;Uhlmann et al, 2016;Van Beek et al, 1990), hook Düzbastilar et al, 2016;Milliken et al, 2009), purse seine (Marçalo et al, 2008(Marçalo et al, , 2010, gillnet (Bettoli and Scholten, 2006;Broadhurst et al, 2009) and trap (MacMillan and Roth, 2012). A larger change in water temperature can expose fish to thermoclines, i.e., changes in water temperature that exceed fish' tolerance limits (Olla et al, 1998).…”

An overview of bottom trawl selectivity in the Mediterranean Sea

“…Little information is available on the survival probability of the individuals escaped from a trawl net. Metin et al (2004) and Düzbastilar et al (2010Düzbastilar et al ( , 2016) collected escapees using covers, which were detached after a short tow, fixed to the sea bottom and monitored by divers for a few days. Survival probability depended on species, fish size and water temperature, and was higher in red mullet (Metin et al, 2004) than in flat fish (Düzbastilar et al, 2016).…”