Abstract
Globally, the bycatch of marine mammals in fisheries represents the greatest source of human-caused mortality that threatens the sustainability of many populations and species. The Australian sea lion (Neophoca cinerea) is an endangered species, whose populations off South Australia (SA) have been subject to bycatch in a demersal gillnet fishery targeting sharks since the 1960s. A comprehensive assessment was undertaken of sea lion bycatch mortality that combined independent fishery observer data with species distribution models (underpinned by satellite tracking, abundance data and population modeling) to model the relationship between at-sea foraging effort and bycatch rate. Combined with the distribution of fishing effort, these models enabled the overall level of bycatch mortality to be estimated by age, sex and subpopulation, facilitating population viability analyses that indicated most subpopulations were declining, and subject to unsustainable levels of bycatch mortality. To reduce this mortality, the Australian Fisheries Management Authority implemented an Australian Sea Lion Management Strategy that included an independent observer program (ultimately 100% electronic monitoring of gillnet fishing off SA), permanent spatial gillnet closures around all sea lion breeding sites, bycatch mortality limits that triggered temporal (18 months) spatial closures when zone-specific bycatch trigger limits were reached, and incentives for gillnet fishers to switch to an alternate fishing method (longlines). The Strategy had immediate impacts on the fishery: it resulted in significant reductions in gillnet fishing effort and on the reported bycatch of sea lions. In the next decade, there was an estimated 98% reduction in sea lion bycatch mortality from gillnet interactions and an apparent stabilization of the decline in sea lion pup abundances at some impacted breeding sites. There was an almost complete transition in the fishery from gillnets to longlines, and fishing catches returned to pre-management levels. The successful implementation of management measures to mitigate sea lion bycatch mortality in the gillnet fishery off SA was rapid, science informed, adaptive, comprehensive, and backed by strong compliance and monitoring of the fishery. It provides an important case study which demonstrates how management measures can be effectively applied to mitigate bycatch mortality of marine mammals and other marine protected species.
Highlights
This study confirms previous reports indicating that high levels of bycatch mortality of ASL occurred in the demersal gillnet sector of the Gillnet Hook and Trap (GHAT) fishery off South Australia (SA), prior to the introduction of management measures to mitigate bycatch in 2010 (Goldsworthy et al, 2010; Hamer et al, 2013)
This study provides a unique and comprehensive assessment of bycatch impact that combined independent observer data with species distribution models, enabling the relationship between sea lion foraging effort and observed bycatch rate to be modeled
These models allowed sea lion bycatch to be estimated across the fishery, the impacts on subpopulations to be assessed, and the potential benefit of alternate management options to be evaluated (Goldsworthy et al, 2010)
Summary
Incidental catch or bycatch in fisheries presents a major threat to many marine species and is one of the most significant sources of anthropogenic mortality of marine mammals (Read et al, 2006; Sims et al, 2008; Lewison et al, 2009; Kovacs et al, 2012; Komoroske and Lewison, 2015; Peltier et al, 2016, 2021; Gray and Kennelly, 2018; Luck et al, 2020). Australian sea lions (Neophoca cinerea) (ASLs) are an endangered pinniped, endemic to southern Australia (Goldsworthy, 2015; Goldsworthy et al, 2021). They are unique among pinnipeds in having a non-annual breeding cycle (∼18 months between successive breeding seasons) in which breeding can occur at any time of the year, breeding seasons last 4 to 9+ months and occur asynchronously across the species range (i.e., neighboring colonies can breed at different times). The non-annual and asynchronous breeding habit of ASL makes systematic monitoring of their status and trends in abundance extremely challenging (Goldsworthy et al, 2021). Breeding season pup production is a common measure of abundance in seals, because pups form the only age-class that is identifiable (natal pelage), and most pups are ashore at the end of a breeding season
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