Pelagic sharks are taken in many fisheries and reducing the fishery-induced mortality of several shark species is a conservation priority, considering evidences for reduced abundance and vulnerability of these species to overexploitation. While pelagic sharks are not the target of purse seine fisheries, some are sometimes killed in these operations and it is of interest to seek ways to reduce that mortality. Three main issues related to purse seine fisheries have been identified.
Observed catches (taken onboard). The shark bycatch-to-tuna catch ratio in purse seine fisheries is quite small (always less than 0.5% in weight, Fig. 1). However, the global magnitude of the purse seine fishery is quite large so that reducing the mortality caused by these fisheries can contribute towards global conservation efforts. The main species caught by the purse seine fishery is the silky shark (90%), which explains why most conservation efforts are dedicated to this species. A secondary species is the oceanic whitetip shark. Although environmental NGOs often focus most of the attention on FAD fishing, it is noteworthy that in some oceans (e.g. Atlantic, Western Pacific), the bycatch ratios for free-swimming school sets and FAD sets are similar.
Figure 1: Tons of sharks caught by purse seiners per 1000 tons of landed tunas. Data from observers programmes (SPC, IATTC, IRD-IEO-AZTI). FSC : free-swimming school sets. FAD : FAD and log sets
Unobserved mortality due to entanglement. Sharks can get entangled in the underwater FAD netting used by some fleets. This mortality was first considered to be negligible, because observing entangled animals from the deck (by crews or observers) is quite impossible. Such observations are only possible when people dive or lift the entire FAD out of the water. Moreover, it turns out that the sharks do not last a long time entangled before sinking or being eaten. However, a recent assessment made in the Western Indian Ocean showed that between half a million to 1 million small silky sharks could die every year due to entanglement in FAD netting (Filmalter et al., 2013). It is not known what the magnitude of the problem could be in other oceans. Also, not all fleets use the same type of netting; it is likely that netting with smaller mesh sizes will entangle fewer sharks.
Waste through finning. Some fisheries carry out “shark finning”, which is the practice of cutting the fins off and discarding the carcass at sea. This practice is not only wasteful, but it also deteriorates the catch statistics (amounts, species) that scientists need in order to assess all impacts of fishing on these shark populations. Moreover, such practice could sometimes represents an incentive for fishers to increase bycatch of sharks (e.g. not releasing live sharks).
Catches The EU MADE project, the ORTHONGEL-IRD bycatch mitigation project and the ISSF #BycatchProject already found that simple good practices onboard to release live sharks from the deck can reduce the catch induced mortality of silky sharks by 15-30% (Poisson et al. 2011, Filmalter et al. 2012, Hutchinson et al. 2012). Good practices are described in Poisson et al. (2012).
Also, by avoiding setting on small schools of tuna (e.g. < 10 tons), fishers could significantly reduce their catches of silky sharks from 20% to 40%, depending on the oceans (Dagorn et al. 2012).
Fish don’t always stay close to FADs as they sometimes do short excursions (a few hours) away from FADs, likely to forage, and them home back to the FADs. Through the simultaneous electronic tagging of tunas and sharks at FADs, we could determine the periods during the 24-hour cycle when each species is usually present at the FAD, and when it is usually making an excursion. Unfortunately, at least in the Indian Ocean, silky sharks and tunas exhibit very similar temporal patterns: they all make excursions away from FADs at the same period (usually during daytime). Adjusting the fishing time in order to reduce catches of sharks while maintaining good catches of tunas therefore does not appear to be a valid solution in the Indian Ocean.
Entanglement Fishers should use non-entangling FADs. In order to completely eliminate entanglement, this simply consists of avoiding entirely the use of meshed materials when building FADs. Entanglement can also be substantially reduced by other procedures such as tightly wrapping the nets with ropes, resulting in a tight cylinder (or “chorizo”). See ISSF guide for non entangling FADs 2012. Both the IOTC and the IATTC have adopted measures for the fleets to start changing towards FAD designs that minimize entanglement.
Finning Fishers should abandon the practice of finning. If they want to retain the sharks (unless prohibited by national or international legislation), they should retain both the fins and the body. Fishers should ensure that the information (discarded/retained) is recorded in the logbooks. This record-keeping can be greatly improved by the deployment of on-board observers. ISSF has adopted a resolution to prohibit shark finning by purse seine vessels.
The ISSF Bycatch mitigation project is currently investigating other methods to avoid encircling sharks or to release sharks from the net. In particular, scientists are testing the use of escape panels (basically, a large hole in the net that can be opened or closed) to release sharks from the net.
Dagorn L, Filmalter JD, Forget F, Amandè MJ, Hall MA, Williams P, Murua H, Ariz J, Chavance P, Bez N, 2012. Targeting bigger schools can reduce ecosystem impacts of fisheries. Canadian Journal of Fisheries and Aquatic Sciences, 69: 1463-1467.
Filmalter JD, Capello M, Deneubourg JL, Cowley PD, Dagorn L, 2013. Looking behind the curtain: quantifying massive shark mortality in fish aggregating devices. Frontiers in Ecology and the Environment, in press (doi:10.1890/130045).
Filmalter JD, Forget F, Poisson F, Vernet AL, Dagorn L, 2012. An update on the post-release survival of silky sharks incidentally captured by tuna purse seine vessels in the Indian Ocean. IOTC-2012-WPEB08-20.
Hutchinson M, Itano D, Muir J, Leroy B, Holland K, 2012. The Post-release Condition of FAD Associated Silky Sharks (Carcharhinus falciformis) caught in Tuna Purse Seine Gear. WCPFC-SC8-2012/ EB-WP-12 Rev 1.
ISSF Guide on non-entangling FADs, 2012. http://iss-foundation.org/resources/downloads/?did=386
Poisson F, Vernet AL, Filmalter JD, Goujon M, Dagorn L, 2011. Survival rate of silky sharks (Carcharhinus falciformis) caught incidentally onboard French tropical purse seiners. IOTC–2011–WPEB07–28.
Poisson F, Vernet AL, Séret B, Dagorn L, 2012. Good practices to reduce the mortality of sharks and rays caught incidentally by the tropical tuna purse seiners. EU FP7 project #210496 MADE, Deliverable 6.2., 30p.