Shark longline fishery of Papua New Guinea: size and species composition and spatial variation of the catches

Abstract

This study provides the first detailed investigation of the catches of the shark longline fishery in Papua New Guinea. Fisheries observers collected data on shark catches from a total of 318 longline sets between May and June 2014, before its closure in July 2014. In all, 14694 sharks were recorded with a total estimated biomass of 439 tonnes (Mg). Eighteen species of sharks were recorded in the observer data, with the most dominant species being Carcharhinus falciformis, which constituted more than 90% of the total catches by both weight and number of individuals. The level of observer misidentification was low (<10%), which reflected the use of region-specific identification guides by well-trained fisheries observers. The most diverse catches were in the Solomon Sea area, whereas catches in most other areas, particularly the Bismarck Sea areas, were less diverse and more strongly dominated by C. falciformis. Size and sex ratios varied by species, highlighting the importance of obtaining species-level information from the fishery being investigated. Any consideration by fisheries managers to reopen this fishery needs to consider the effect this will have on the species targeted and the livelihoods of coastal fishers who also rely on the same resources.

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

“…The principle requirement of gauntlet fisheries is the ability to successful target juveniles. Therefore, as longlines typically select against the youngest age classes (either through gear, spatial or behavioural selectivity; Clarke et al., 2018; White et al, 2019), a maximum size approach is impractical as it becomes analogous with a harvest slot approach. Therefore, minimum legal lengths that correspond to the appropriate age classes are the most effective size‐limit approach for sharks caught in longline fisheries.…”

“…For minimum length limits to be effective, it is imperative that mortality is restricted to the correct age classes. Unfortunately, this is unlikely for the PNG shark longline fishery as current gear selectivity predominantly catches individuals under the length‐at‐maturity for most species (White et al, 2019). If post‐release mortality of these species was low then the release of undersized animals could still allow sustainable harvest.…”

“…These were constructed from life‐history parameters for 12 shark species from PNG. Necessary life‐history parameters either were estimated from recent sampling conducted in PNG (White et al, 2019) or were sourced from the literature when more robust estimates were available (Table A1). Species included the silky shark C. falciformis , oceanic whitetip shark Carcharhinus longimanus , grey reef shark Carcharhinus amblyrhynchos , silvertip shark Carcharhinus albimarginatus , common blacktip shark Carcharhinus limbatus , spinner shark Carcharhinus brevipinna , blue shark P. glauca , shortfin mako Isurus oxyrinchus , scalloped hammerhead Sphyrna lewini , tiger shark Galeocerdo cuvier , bigeye thresher Alopias superciliosus and the pelagic thresher Alopias pelagicus , C. amblyrhynchos , C. albimarginatus , C. limbatus and C. falciformis from PNG have recently been the subject of demographic analyses in PNG (Grant et al., 2019; Smart et al., 2017; Smart, Punt, Espinoza, White, & Simpfendorfer, 2018).…”

“…Lack of such reporting has typically been market driven because ‘shark’ (or other various market names) has been used as an umbrella term that includes numerous species (Walker, 1998). These data limitations are exacerbated when many species are caught simultaneously and have diverse life histories (White et al, 2019). The simultaneous harvest of species with divergent productivities is problematic as it can produce a ‘two‐speed fishery’, where some species can be underexploited while others are overfished.…”

“…Prior to 2002, the reporting rate was <30% and while having improved since then, information on species composition remained poor (Kumoru, 2003). Recent studies have demonstrated that this fishery caught mostly pelagic species (White et al, 2019), which are often vulnerable to overexploitation (Dulvy et al., 2008).…”