Bycatch in the Maldivian pole-and-line tuna fishery

Abstract

Tropical tuna fisheries are among the largest worldwide, with some having significant bycatch issues. However, pole-and-line tuna fisheries are widely believed to have low bycatch rates, although these have rarely been quantified. The Maldives has an important pole-and-line fishery, targeting skipjack tuna (Katsuwonus pelamis). In the Maldives, 106 pole-and-line tuna fishing days were observed between August 2014 and November 2015. During 161 fishing events, tuna catches amounted to 147 t: 72% by weight was skipjack, 25% yellowfin tuna (Thunnus albacares) and 3% other tunas. Bycatch (all non-tuna species caught plus all tuna discards) amounted to 951 kg (0.65% of total tuna catch). Most of the bycatch (95%) was utilized, and some bycatch was released alive, so dead discards were particularly low (0.02% of total tuna catch, or 22 kg per 100 t). Rainbow runner (Elagatis bipinnulata) and dolphinfish (Coryphaena hippurus) together constituted 93% of the bycatch. Live releases included small numbers of silky sharks (Carcharhinus falciformis) and seabirds (noddies, Anous tenuirostris and A. stolidus). Pole-and-line tuna fishing was conducted on free schools and schools associated with various objects (Maldivian anchored fish aggregating devices [aFADs], drifting FADs from western Indian Ocean purse seine fisheries, other drifting objects and seamounts). Free school catches typically included a high proportion of large skipjack and significantly less bycatch. Associated schools produced more variable tuna catches and higher bycatch rates. Fishing trips in the south had significantly lower bycatch rates than those in the north. This study is the first to quantify bycatch rates in the Maldives pole-and-line tuna fishery and the influence of school association on catch composition. Ratio estimator methods suggest roughly 552.6 t of bycatch and 27.9 t of discards are caught annually in the fishery (based on 2015 national catch), much less than other Indian Ocean tuna fisheries, e.g. gillnet, purse-seine, and longline.

Highest risk abandoned, lost and discarded fishing gear

“…A majority of the catch from pole-and-line fisheries comes from fisheries where some of the fishing effort occurs on anchored FADs 35 , 44 – 48 , 66 ,89 . Some tuna pole-and-line fisheries also fish on drifting FADs, likely deployed by tuna purse seine fisheries 44 , 47 , 66 , 67 . While FADs are also used by some handline, jig, troll and driftnet fisheries 44 – 46 , 64 , 68 , this study did not include FADs as part of the assessment of risks from ALDFG for these gear types as it is unclear what proportion of global effort by these gears occurs on FADs.…”

Rethinking sustainability in seafood

“…With respect to bycatch impacts, there are some important considerations. First, fleets that employ selective gears (e.g., troll and surface methods) often lack observer coverage, and thus, bycatch impacts are relatively unknown compared to longline and purse seine fleets (Miller et al, 2017). Second, some highly selective fishing methods require a substantial quantity of baitfish whose sustainability and capture emissions were not assessed here (Gillett, 2011).…”

“…The trade-offs inherent in these catches are illustrated across different gear types. More selective fishing gears such as pole-and-line and trolling gears typically have limited bycatch but lower catch rates (Miller et al, 2017). Meanwhile, less selective gears such as longlines and purse seines are capable of catching more fish at a time, which is often also associated with a greater bycatch risk (Watson et al, 2009;Gilman, 2011;Watson and Bigelow, 2014).…”

Tuna behaviour at anchored FADs inferred from Local Ecological Knowledge (LEK) of pole-and-line tuna fishers in the Maldives

“…In the Maldives skipjack and small yellowfin tuna are caught by targeting (i) free swimming schools (45.4%), (ii) logs or other drifting objects (11.3%), (which includes drifting fish aggregating devices (DFADs) deployed by purse seiners that pass through the Maldives EEZ), (iii) seamount associated schools (10.4%), and (iv) anchored fish aggregating devices (AFADs) (32.8%) [33]. Studies showed that AFADs related catches contribute to nearly one third of all the tuna caught by pole-and-line in the Maldives [33]. Traditionally, Maldivian tuna fishers have fished at logs or drifting objects associated schools for centuries and they refer to these objects that attract tuna as 'oivaali' (a local name given to drifting objects).…”