Evaluating vital components of elasmobranch assessment and spatial conservation

Abstract

Elasmobranchs (sharks, skates and rays) have been important predators in marine ecosystems for over 400 million years. However, many species experience an elevated risk of extinction today due to the effects of fishing and bycatch. Yet, scientific information to inform stock assessments and spatial conservation efforts is often scarce and of poor quality. This thesis evaluates (1) how important life history parameters can be estimated more reliably to improve data-poor stock assessments for better fisheries management, and (2) whether existing marine protected area (MPA) networks can act as an additional conservation strategy for threatened elasmobranchs. A new approach is presented to estimate somatic growth parameters from mark-recapture tagging data. This information can be utilized to estimate individual ages indirectly from body length. Another parameter that is crucial to stock assessments is natural mortality. Through comparative analysis, a unified estimator of natural mortality for elasmobranch and teleost (bony fish) is developed that reliably predicts natural mortality from maximum age, as well as a new measure, the length after which natural morality can be assumed constant. On this basis, different approaches to estimate juvenile natural mortality were tested. In cases where independent estimates of natural mortality and growth cannot be obtained this thesis shows that the ratio of natural mortality over growth can be predicted in elasmobranchs, by utilizing more available data on length at maturity and maximum length. The results are critically discussed with regards to life history theory and potential applications in data-poor assessments. Finally, existing MPA networks across northern Europe were evaluated to determine if their increased coverage translated into increased conservation for data-poor and threatened elasmobranch species. It was shown that industrial trawl fishing is 38% higher within MPAs, on average, and that the relative abundance of elasmobranchs within an area decreases significantly with the intensity of trawl fishing. These findings suggest that low protection standards of investigated MPAs undermines conservation outcomes for these and potentially other threatened species. To conclude, this thesis provides approaches to obtain improved ecological insights, stock assessments and spatial protection for elasmobranchs, in order to facilitate more effective fisheries management and conservation.