Survivorship of discarded sole (Solea solea) characterised via telemetry, vitality, and physiology

Abstract

While the discarding practices of commercial fisheries might greatly contribute to fish mortality, a better understanding of discard survival patterns and their drivers is crucial to assess fishing mortality and develop effective mitigation measures. Bottom trawling in particular is likely to induce multiple forms of stress in captured fish. In this study, we used three experimental methods to explore discard survivorship of the common sole (Solea solea). We focused on fish captured via a coastal bottom trawler of the Bay of Biscay and conducted sampling during commercial fishing operations. First, we used acoustic telemetry to determine the survival rate of 160 discarded soles. While this technique makes it possible to study discarded fish in their natural habitat, inferring fish fate from these data can be challenging, especially when acoustic receiver network coverage is fragmented. Second, we scored the vitality of the sole before their release. Third, we examined three plasma markers of fish physiological status over a 90-min period following capture. The method based on acoustic telemetry estimated a minimum discard survival rate of 41%. This metric was positively correlated to the vitality. Furthermore, both survival and vitality were associated to tow duration and the presence of algae in the catch consistently. The three physiological markers reflected the presence of various levels of stress and fatigue among discarded individuals, but also a fish’s ability to recover after being discarded. Individuals with excellent vitality (33% of discards in this study) suffered less stress, were less fatigued and, therefore, were greater equipped to react to other sources of stress than individuals in lower vitality. The consistency of the results demonstrates the reliability of our approach to estimate post-release survival from acoustic tagging even from fragmented network coverage, and that vitality can serve as a proxy for discard survivorship.