Life history, ecology and management of blue sharks (Prionace glauca) in the North Atlantic

Abstract

Overfishing has caused a decline of oceanic sharks and rays by over two thirds since 1970. Resilience to fishing pressure is informed by species-specific life history strategies, which are shaped by phylogeny, environment and individual energy budgets. Blue sharks (Prionace glauca) are potentially more resilient to fishing pressure, with their relatively high reproductive output compared to most other sharks. However, blue sharks are listed as Near Threatened on the IUCN red list, with declines of more than 50% across three generations in the North Atlantic. To support life history assessments and halt population declines, low-cost methods to assess blue shark growth and to mitigate fisheries bycatch are needed. In addition, the genetic population structure of blue sharks is unresolved, with contrasting evidence for panmixia and regional substructure, preventing effective conservation and management. This thesis aimed to investigate the relationship between shark life history strategies and conservation biology, assess blue shark genetic population structure, and test low-cost methods to assess body size and to reduce bycatch. The thesis starts by exploring the potential for sensory deterrents to reduce marine megafauna mortality in fisheries. The efficacy of sensory deterrents is context specific, depending on species biology, fishery and environmental characteristics. Next, life history strategies of 151 elasmobranch species are assessed using energy budget models. Elasmobranch life history strategies are structured along two axes: pace of life (the fast-slow continuum) and a reproductive strategy axis. Species’ life history strategies, population growth rates and demographic resilience changed between a low and high feeding level. The thesis then develops and tests designs for low-cost stereo-video and laser photogrammetry systems. Stereo video performed significantly better than laser photogrammetry in both pool and field trials, offering a cheap and accurate method to attain non-invasive length measurements of pelagic sharks. The thesis further assesses genetic structure of blue sharks for 307 individuals across eight global sampling locations. Subtle population substructure was found between one cluster in the eastern and southeastern Pacific and a second cluster for all other regions, which suggests they should be treated as independent management units. Finally, the thesis investigates behavioural response of blue sharks to ferrite magnet deterrents deployed on fishing lines. There was no significant difference in bait choices between control and magnet lines. Further development in sensory deterrents is required before implementation in fisheries. Overall, this thesis investigates conservation challenges and multidisciplinary assessment of blue sharks, with potential applications for fisheries management