Sharks and Rays

Incidental shark and ray capture occurs in longline, purse seine and gillnet tuna fisheries. The most effective mitigation measures for longline fisheries include deep-setting, reduced soak time, avoiding wire leaders and hook and bait changes [2], though in applying these, cross-taxa effects need careful consideration. Silky and oceanic whitetip sharks are the most common shark bycatch species in purse seine fisheries, captured mostly when vessels fish on FADs but also through entanglement in FAD netting. Limiting FAD use and modifying FAD designs and practices are the most promising mitigation methods [2]. As with cetaceans, entanglement is a significant problem in medium-scale, semi-industrial and artisanal tuna drift gillnet fisheries. There has been some success with sub-surface gillnets [3] but further validation of this technique is required. For all gear types, safe handling and release procedures have been developed which significantly reduce post-release mortality of sharks, e.g., try to avoid landing sharks onboard; instead bring them close to the vessel and cut line as close to the hook as possible to reduce the length of trailing line [1].

Recent studies have looked at physiological factors affecting post-release mortality of sharks, e.g., stationary-respiring shark species have been shown to experience lower post-release mortality rates than obligate ram-ventilators on longlines [2]. Such information helps to understand the vulnerability of different shark species and thus feeds into improved management strategies for shark bycatch.

For detailed biological and distributional data about shark species, refer to FishBase, Sharkipedia (includes abundance time-series) or Shark-References. FAO has published a series of regional catalogues describing shark/ batoid/chimeras, such as this one for the North Atlantic. Look for others in the series here. NOAA's 'Find a Species - Highly Migratory Species' has information on protection status, biology and population status, distribution, threats, conservation measures and research. See also FAO's database of management measures. Handy species ID guides can be found here.

STAGIS, a database of shark tagging FIELD STUDIES conducted in the Pacific Ocean, can be accessed here.

References

1. Francis MP, Lyon WS, Clarke SC, et al (2023) Post-release survival of shortfin mako (Isurus oxyrinchus) and silky (Carcharhinus falciformis) sharks released from pelagic tuna longlines in the Pacific Ocean. Aquatic Conservation: Marine and Freshwater Ecosystems 33:366–378. https://doi.org/10.1002/aqc.3920
2. Dapp DR, Huveneers C, Walker TI, et al (2016) Moving from Measuring to Predicting Bycatch Mortality: Predicting the Capture Condition of a Longline-Caught Pelagic Shark. Front Mar Sci 2:126. https://doi.org/10.3389/fmars.2015.00126
3. Swimmer Y, Zollett E, Gutierrez A (2020) Bycatch mitigation of protected and threatened species in tuna purse seine and longline fisheries. Endang Species Res 43:517–542. https://doi.org/10.3354/esr01069
4. Moazzam M (2019) Species composition of elasmobranchs in the surface and subsurface gillnet operation in the Northern Arabian Sea. In: IOTC - 15th Working Party on Ecosystems and Bycatch. IOTC-2019-WPEB15-13, La Saline Les Bains, Reunion Island