Évaluation du potentiel de rétablissement du requin-taupe bleu (Isurus oxyrinchus)

Abstract

The North Atlantic Designatable Unit (DU) of Shortfin Mako Shark is defined as the population that occurs throughout the Northern Hemisphere of the Atlantic Ocean. It was assessed by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) as Endangered in 2019. This Recovery Potential Assessment contains information to support decision-making, develop a recovery strategy, or quantify the impact of a listing decision under the Species at Risk Act. Shortfin Mako have a broad distribution throughout Canadian waters, encompassing areas from the Bay of Fundy, into the Gulf of St. Lawrence, out to the Grand Banks towards the Flemish Cap, and off the eastern coast of Newfoundland. Tagged sharks spent the majority of their time in waters between 10–25°C, remaining primarily in the top 600 m of the water column. Shortfin Mako are vulnerable to fishing pressure given their late age at maturity and relatively slow reproductive rate. The 2019 International Commission for the Conservation of Atlantic Tuna (ICCAT) assessment update predicted a decline of approximately 54% in relative abundance from the 1950s until 2018, with the majority of the decline occurring from the 1980s onwards (approximately 39 years; less than two generations). Mortality from various directed and bycatch fisheries was the only threat to the North Atlantic DU of Shortfin Mako identified by COSEWIC. Since 1994, total international catches of Shortfin Mako have averaged 3,685 mt in the North Atlantic, with an average of 67 mt coming from Canada. In Canada, there has never been a directed fishery for Shortfin Mako. Between 2014–2019, the majority of landings of Shortfin Mako by Canadian fisheries (greater than 99%) come from benthic and pelagic longline fisheries in Maritimes Region. The gear types associated with incidental catches of Shortfin Mako include pelagic or drift longline, bottom longline, and otter trawl; with lesser amounts in purse seine, fixed gillnet, handlines, and troll lines. Relative interception probabilities were highest for the pelagic longline fleet, with an average of 48% of observed sets encountering Shortfin Mako. Although the risk associated with individual fisheries in Canada is low, mortality is cumulative and each fishery contributes to population decline for the North Atlantic DU. An abundance estimate or fisheries assessment specific to Canadian waters is not informative because it would consider a very small component of the entire DU. ICCAT assessments use Biomass at Maximum Sustainable Yield (BMSY) or a proxy for BMSY (e.g., Spawning Stock Fecundity at MSY [SSFMSY]) to assess overfished status, which is proposed as the abundance target for Shortfin Mako. There is no distribution target proposed in this RPA. The available data do not allow for quantitative predictions on distribution or changes in distribution for the North Atlantic DU of Shortfin Mako. Total removals (landings + dead discards + post-release mortality of live releases) of 500 mt or less had a 52% probability of rebuilding the North Atlantic stock to SSFMSY by 2070. Landings prohibitions are expected to be the most effective mitigation measure to reduce fishing mortality, and these were introduced for Canadian fisheries starting in 2020. The efficacy of other types of bycatch mitigation measures (gear or bait modifications, time-area closures, effort restrictions, and shark deterrents) are unclear and would need to be tested following implementation. Current practices of using monofilament leaders, corrodible hooks, and releasing sharks in the water by cutting the line as close as possible to the hook should be maintained. At-Sea-Observer coverage is very low or non-existent in some fisheries, which leads to higher uncertainty in the catch rates, discards, and status of Shortfin Mako, especially when scaling limited information up to entire fisheries. There continues to be unreported bycatch in many fisheries, both in Canadian and international waters. Even if Canadian fisheries removals became zero, total international removals would remain well above 500 mt under current management, representing a Persistent Limitation to recovery. While there is scope for Allowable Harm, efforts should be made to keep future removals from all threats occurring in Canada below approximately 59 mt until a Canadian threshold for Allowable Harm can be developed.