Can multi‐species shark longline fisheries be managed sustainably using size limits? Theoretically, yes. Realistically, no

Abstract

Size limits are a common fisheries management strategy that are applied to many fisheries and species. Most size limits use a minimum legal size to protect adult fish as per the ‘reproduce at least once’ paradigm, where stock collapse becomes impossible if every adult can produce one spawner prior to harvest. These approaches can be useful in fisheries where determining catch is difficult and therefore catch limits can be ineffective and potentially cause stock decline. Shark longline fisheries can be complicated to manage due to a deficiency of species‐level reporting. Catch limits are therefore difficult to determine, creating a need for other management measures. Here, three different size‐based management approaches were tested using the shark longline fishery from Papua New Guinea as a case study. These approaches included minimum size, maximum size and harvest slot approaches. Age‐structured Leslie matrix models revealed a broad range of productivities for 12 commonly caught species, ranging from low population growth rates of 1% per year for pelagic thresher sharks Alopias pelagicus to >33% per year for blue sharks Prionace glauca. Therefore, different harvest strategies were required for each species to be fished sustainably. However, management could be applied by grouping similar and easily distinguishable groups of species together. Synthesis and applications. The most pragmatic harvest strategy for all shark species included in this study was to limit harvest to mature individuals using minimum legal sizes. This provided sufficient resilience for all species to tolerate higher levels of fishing mortality than the Papua New Guinea (PNG) longline fishery could impose. Legal minimum lengths were derived from the largest length‐at‐maturity within each species group. However, while this produced a sustainable harvest strategy, issues with selectivity, post‐release mortality, interactions with other fisheries, economic viability and illegal, underreported and unregulated fishing were challenges to restricting fishing to the correct size classes and overall management efficacy. Therefore, while theoretically viable harvest strategies were outlined, it is apparent that size limits would not ensure that these shark species are fished sustainably. Therefore, the PNG longline fishery cannot operate sustainably if it were to reopen using size limits as its sole management strategy.

Evaluating artisanal fishing of globally threatened sharks and rays in the Bay of Bengal, Bangladesh

“…Strategies include size or catch limits or live release of bycatch. Size or catch limit may be ineffective if there is a lack of understanding about stocks [ 128 ]; the success of live release is also dependent on post-release mortality and its effects on the species [ 129 ], indicating more research is needed. While doing so, Bangladesh’s main challenge will be to ensure pre-cautionary and proactive approaches for policies, implementation, and enforcement of laws.…”

Estimating economic losses to small‐scale fishers from shark conservation: A hedonic price analysis

“…Despite these economic findings, we acknowledge that it may be impractical to reduce catches of conservation‐priority species in the absence of an entire fishery closure or large reduced catches of associated species [e.g., due to issues with selectivity, post‐release mortality, interactions with other fisheries and economic viability (Smart et al, 2020)]. The temptation for high‐grading and discarding might be high, particularly if there is no operationally‐feasible adaptation available to avoid catching restricted species.…”

Is restricting catch to young sharks only more sustainable? Exploring a controversial management strategy for bull, tiger, blue and bonnethead sharks

“…For example, for an MHA of age 3, proportions of the stable age distribution from age 0 to age 2 sharks were summed to quantify the exploited segment of the population. We assumed that the most suitable harvest strategy maintained the highest harvest without causing the population to decline (Prince, 2005; Smart et al, 2020, 2017).…”