Roundfish and <i>Nephrops</i> survival after escape from commercial fishing gear. Final report to the European Commission

ish and Nephrops survival and gear selectivity experiments were carried out in the Gairloch area of north-west Scotland in July and August of the years 1996-8 using a 550HP trawler. Cages were filled with control fish caught on barbless hooks. Only 0.3% of these controls died. An acoustic release system was developed that remotely closed the aft end of the cover collecting fish escaping from the codend and then detached it after a given sampling time. The fish escapees from each survival tow were transferred to a separate seabed cage then fed and monitored. Most tows gave similar high haddock survival rates in the range 86-97%, for 3 tows this reduced to 63-78%. All tows gave high whiting survival rates in the range 90-100%. Comparison of one hour and 3 hour tows revealed that towing time had no significant effect upon survival rate for either species. Analysis of all measurements obtained 1993-1998 showed that sampling time had a significant effect upon the survival of both species. The mean estimated haddock survival rate for a 100mm codend was 86% with a 15 minute sampling time, dropping to 67% with a 45 minute sampling time. Sampling time should therefore be minimised. Mean haddock survival rate in a 70mm codend was estimated to be 74% with a 15 minute sampling time. Survival rate increased significantly with the mean length of the haddock in a cage. The mean length of the haddock mortalities was significantly smaller than that of the survivors in the 1997-8 experiments. Nephrops escapees were transferred from the codend cover to a pen containing isolated artificial burrows for each Nephrops. Creel caught animals were used as controls and subjected to the same transfer processes. Only 0.5% of the controls died. Mean survival rate for codend escapees was 82% (range 73-95%) and for deck discards was 31% (range 19- 39%). These were not significantly dependant upon codend mesh size / type or Nephrops length. The survival rates of discards were significantly lower for females than males. An analysis was made of the relationship between mortality and visual body damage. Mortalities showed a significantly higher amount of visual damage on the abdomen compared to the survivors for discards but not for codend escapees. Total body water content was measured and found to be reduced for codend escapees and further reduced for deck discards with the reduction increasing with time on deck. Deck discards showed significant reductions in blood volume and blood pressure compared to controls. Nephrops escape using a tail-flip mechanism. The number of tail flips that could be elicited reduced by 53% for codend escapees and by 90% for discards emersed for 2 hours. Trawling and exposure to air on deck were found to result in high l-lactate levels in the blood and muscle tissues (anaerobiosis), low muscle glycogen levels, high blood D-glucose levels and high blood ammonia levels. The estimated codend escapee mortality rates for fish and Nephrops were low and have little effect on the results from stock assessments compared to uncertainties in natural mortality. Mean codend selection factors for haddock were in the range 2.7 – 2.9. A comparison was made of tows of 2 hours and 3 hours duration. There was no significant difference in 50% retention lengths but the selection ranges were significantly smaller in the 3-hour tows. Further analysis revealed that the selectivity was solely dependent upon the catch weight in the codend. The selection range decreased with increasing catch weight. The mean Nephrops selection factors for 70mm and 100mm diamond mesh codends were 0.32 and 0.28 based on carapace length. That for a 60mm square mesh codend was significantly higher 0.40.