Get them off the deck: Straightforward interventions increase post-release survival rates of manta and devil rays in tuna purse seine fisheries

Abstract

Bycatch remains a major challenge in commercial fisheries, with large numbers of threatened species impacted by incidental capture. One of the most vulnerable bycatch groups in global tuna fisheries are the manta and devil rays (mobulids), which have experienced significant population declines in response to both targeted and incidental capture. The retention of mobulids has been banned by many countries and Regional Fisheries Management Organizations (RFMOs), but major knowledge gaps exist in the group's survival rates following release after incidental capture. Mobulids are accidentally captured in purse seine fisheries, and in recent years, many RFMOs have mandated handling and release procedures for mobulids in an effort to maximize survivorship and reduce impacts of fishery bycatch. We synthesize data from four empirical studies using satellite tags (n = 89) to estimate survival rates of four species of mobulids (Mobula birostris, M. mobular, M. tarapacana and M. thurstoni) released from tuna purse seine vessels in three global regions. We directly estimate the effects of intrinsic, environmental, and operational factors, and handling and release methods on mobulid survival rates. We found a significant negative effect of time spent on deck; likely negative effects of the brailer number in which a ray was brought out of the sacked net, and remaining in the sacked purse seine net until after brailing was complete; and likely positive effects of being captured in a floating object set and body size on survival probability. The observed survival rates of mobulids with known fates were 50 \% for M. birostris, 74.2 \% for M. mobular, 33.3 \% for M. tarapacana, and 20 \% for. M thurstoni. The median predicted survival probability under optimal handling conditions was 83.7 \% for M. birostris, 95.3 \% for M. mobular, 82.2 \% for M. tarapacana, and 53.7 \% for M. thurstoni. These empirical estimates can improve handling methods and vulnerability assessments of these endangered species in global fisheries.