Disentangling pinniped incidental mortality in a bottom-trawl fishery with seal exclusion devices

Verónica Iriarte^*Andreas Winter

• Fisheries Department, Falkland Islands Government, Stanley, Falkland Islands

Seal exclusion devices (SED) fitted in trawls are recognized as the best technical measure to mitigate pinniped bycatch, but do not completely eliminate incidental mortality. In areas with high pinniped abundance and concentrated fishing effort, identifying cause of death identification of bycaught pinnipeds carcasses in different stages of decomposition is challenging, as the individuals could have died from a variety of causes. In the Falkland Islands, best practice to mitigate and monitor incidental mortality of South American fur seal (Arctocephalus australis) and South American sea lion (Otaria flavescens) in the bottom-trawl Patagonian squid (Doryteuthis gahi) fishery was introduced in 2017. From 2018 through 2023, best practice in the fishery included 100% dedicated observer coverage and the use of SEDs. We analyzed 29,812 trawls (5,748 without a SED and 24,054 with a SED) for pinniped bycatch (total live SED escapes, live deck releases from deck, and mortalities), and corresponding catch and fishing effort, post-mortem gross examinations and multimedia data in the Patagonian squid fishery, including post-mortem gross examinations and bycatch multimedia to confirm cause of mortality. Mortalities in trawls without a SED included 45 drownings (fresh carcasses) and two unknown (carcasses in different stages of decomposition). Mortalities in trawls fitted with a SED included 53 drownings, 28 propeller strikes, and 26 unknown. Our results indicate that SEDs are efficient at reducing incidental mortality, although low levels of drownings prevail continue during trawl hauling. These mortalities comprise mostly males individuals and are related to several operational variables and captain performance, which was found to be heterogenous in facilitating SED escapes. In addition, we link for the first time small marine mammal propeller strike mortality of South American otariids to a particular fishery, evidencing the existence of this type of cryptic mortality in offshore trawl fisheries in the Southwestern Atlantic. Since using SEDs, pinniped mortality levels in the Patagonian squid fishery have been negligible at their population levels. Results of this study can be used to work with captains to enhance best practice and contribute to the sustainable management of the fishery.