A comparison of the stress response and mortality of sea bream Pagrus major captured by hook and line and trammel net

Abstract

Fisheries management policy to increase escapement of non-target species and sizes of fish from fishing gears is generally based on the assumption that injury and stress received during the capture-escape process do not result in a significant level of mortality after escape. However, there has been little research to validate this assumption. To investigate the stress response and mortality associated with capture-escape trauma, a series of comparative experiments were carried out using red sea bream Pagrus major captured by hook and line and trammel net. Cortisol levels in fish captured by hook and line were measured as an index of the stress associated with capture duration using commercially available plasma Cortisol RIA kits. Plasma cortisol levels of resting fish (12.21 ng ml −1, n = 12) were significantly lower than those for all fish captured. In hook and line caught fish, stress increased as capture duration was increased from 10 min to 3 h. Stress levels for capture periods of 6, 9, 12 and 18 h were lower than those for 1 and 3 h but above 10 min capture time and resting values. Fish behaviour during capture included initial flight response, successive struggles of decreasing magnitude, reverse swimming and finning to maintain position. Struggle activity reduced as time of capture increased. No mortalities occurred during capture or 38 days of post release monitoring. On the other hand, stress levels of fish after 10 min, 1.5, 3, 6, 9, 12 and 18 h captured by trammel net were significantly higher than resting fish and continued to rise with capture duration. Forty four percent of all fish captured by trammel net died, of which 28% occurred in the net in fish whose gill cover was held closed by the small mesh netting. Five percent of the fish died within 48 h post release and 11% were delayed mortalities between 8–18 days post release in fish with severe open wounds. No mortalities occurred between 19–38 days post release. We suggest that hook and line caught fish are able to exhibit an adaptive response to capture, in which a cessation of struggling resulted in the captured fish regaining their normal swimming position and allowed the fish to recover from struggling which reduces the probability of mortality. Adaptive behaviour was not possible in fish caught by trammel net in which the degree of entanglement remained constant or increased with capture duration. Ceasing or continuing to struggle did not reduce the degree of entanglement or reduce the constriction of netting around the fish body and gills. These results show that fish stress during capture may vary by gear type and that mortalities may be immediate or delayed after release from the fishing gear. Stress and injury incurred during encounter and subsequent escape may result in an unaccounted level of fishing mortality.