Sardine (Sardina pilchardus) delayed mortality associated with purse seine slipping: contributing stressors and responses

Abstract

This dissertation aims to study the impact of commercial purse seine fishing operations on slipped sardines (Sardina pilchardus), accounting for the survival rates of the escapees and studying the main factors/stressors that may lead to delayed mortality. Field work onboard commercial seiners allowed to study the early dynamics of sardine (Sardina pilchardus) stress reactions and identify likely stressors during fishing operations. Appropriate methodology for capturing, transporting and maintaining sardines in captivity after live capture was optimized in order to use the species for laboratory fishing simulation studies. Laboratory fishing simulations were used to study some operational and environmental factors that lead to variable physical, physiological and behavior responses and result in delayed mortality after slipping. Results showed variable stress responses of sardines during purse seining operations with the significant effect of fishing duration in several physiological blood parameters (cortisol, glucose, haematocrit, haemoglobin, ionic balance, and ATP and its metabolites). Laboratory fishing simulations revealed that survival rates decrease significantly with time inside the net, with temperature having an additional effect on mortality. Fish short term stress responses (physiological) immediately after fishing do not correlate with observed delayed mortality. Scale loss is an important variable of physical injury directly related with the probability of dying. Long term stress responses such as the inhibition of the immune and reproductive systems affects all escapees (whether surviving or not). Sardine suffer behavioral impairments such as lower swimming speed, shorter predator approaches and larger nearest neighbor distances (wider school area) regardless of fishing stressor severity, what indirectly may increase their vulnerability to predation.