Physiological condition, short-term survival, and predator avoidance behavior of discarded Norway lobsters (Nephrops norvegicus)

Abstract

The landing obligation of the reformed European Union Common Fisheries Policy is designed to encourage more selective fishing strategies and improve recording of catches. There are allowable exemptions to this landing obligation including for species with high post-release survivability. Discarding patterns of prawns (Nephrops norvegicus) were evaluated in a trawl fishery in the Firth of Clyde, West Scotland which supplies the live-catch market. Around 30% of the Nephrops caught were discarded but the reasons for discarding changed seasonally. Using visual indices, physiological biomarkers and video recordings this study evaluated the physiological condition linked to short-term survival and predator avoidance behaviour of the discarded animals. Although short-term survival after 48 h recovery was high (around 90%) and physiological measures indicated that discarded Nephrops can recover from trawling, survival was negatively affected by levels of physical damage and Hematodinium infection. Taking into consideration these factors a conservative estimate for discard survival was 63-88%. Underwater video showed that Nephrops discarded in good condition rapidly recovered normal behaviour when placed on the seabed. Moribund animals, however, took up to 10 minutes to return to an upright posture and this time was sufficient for predators to be attracted. Since around 20% of Nephrops were in a moribund condition immediately after trawling, the survival estimates based upon enclosed recovery experiments may need correcting by up to this amount to account for potential interactions with predators on the seabed. The post-release survival rates in discarded Nephrops suggested for this fishery are relatively high compared with other Nephrops trawl-fisheries which have been studied. This could be explained because this fishery targets the live-market, prioritises product quality over volume and uses short-duration tows leading to relatively low levels of physical damage to the Nephrops.