The effect of hook type and trailing gear on hook shedding and fate of pelagic stingray (Pteroplatytrygon violacea): New insights to develop effective mitigation approaches
Introduction
Pelagic stingray (Pteroplatytrygon violacea) occurs in tropical and subtropical waters of all the Oceans, including the Mediterranean. It is the only species from the family Dasyatidae to be encountered in pelagic ecosystems [1,2] and one of the most productive oceanic elasmobranchs that, in captivity, can produce two litters of 1–13 pups per year, giving a potential annual rate of population increase of 31% [3]. Pelagic stingray can be caught in shelf seas and open oceans, mainly by pelagic longlines and, to a lesser extent trawls and nets [[4], [5], [6]].
Of limited commercial value, pelagic stingrays are not usually retained and catch data from commercial fisheries are incomplete. Their at-vessel mortality (AVM) in pelagic longline fisheries is generally low, in the range of 1–18.5% [[7], [8], [9]] possibly because they are not obligate ram ventilators and so can survive longer when hooked. Furthermore, regardless of hook shape, pelagic stingrays are almost always hooked in the mouth or body, and not deep-hooked in the esophagus or stomach [10,11]. Common practices to remove the hook consist of swinging the animal against the rail, cutting the jaws with a knife, or pulling strongly on the trace until either the jaw breaks or the line parts. As pelagic stingray can inflict serious injuries to the crew [12], the tail may sometimes be cut off before being discarded. Consequently the post-release mortality (PRM) rate could be high [13], and highly dependent on fisher behavior and discarding practices [14,15]. Moreover, in response to the increased fish welfare concerns [16,17], higher standards of care of captured fish should be considered [18,19] and implemented onboard fishing vessels.
In the Mediterranean, different longline types are traditionally used to target swordfish, albacore or bluefin tuna. Each type is characterized by differences in the gear's components (e.g. mainline material, hook shape and size, bait type and size, etc.) which affect the selectivity and the impact on potential bycatch species [20,21]. After the ban of Atlantic bluefin tuna (ABFT) driftnet fishery, French fishers switch steadily to longline fishing. The number of permits has doubled in one decade. Around 100 hundred small-scale vessels were operating in 2018. This surface longline fishery operates mainly in the Gulf of Lions (France) and around Corsica Island between April and December. The number of hook deployed range from 400 to 900 hooks per set and the soaking time very short (less than 5 h). The quota for this fleet has been increasing in the last years from 225 mt in 2014 to 389 mt in 2018. A recent study showed that pelagic stingray accounted around 50% of the catch in numbers, ranking first of the five major species caught [22].
Studies conducted mainly on recreationally-caught freshwater fish showed that hooks lodged in fish jaws, or even deeply internally, can be evacuated naturally over time [[23], [24], [25], [26], [27]]. The influence of hook type, size and shape on hook retention, injuries and mortality, and the ability to ingest food has been also investigated on bonefish [24,26] but never to our knowledge on pelagic fish.
There are at least three types of hooks commonly used in the domestic ABFT longline fisheries: circle hook, J-hook and tuna hook. The point of the circle hook directed inwards and perpendicular to the shank prevents the deep engagement in the esophagus and the stomach [28] while the sharp point of J-hook (or jabbing) oriented parallel to the shank [29,30] can penetrate the flesh and stay embedded thanks of the reversed barb. However, the anatomical location of hooking is directly correlated with the potential for lethal injuries and mortality. Retained deep hooks in blue shark (Prionace glauca) can have long-term pathological consequences [31,32].
The main objectives of this study were to (1) examine the effects of hook shape (circle versus J-type hooks) and trailing gear on hook retention, feeding behavior, fate of pelagic stingray and recovery from injuries, (2) monitor any delayed mortality in captive-held specimens (3) propose potential and effective mitigation approaches for the fishery.
Section snippets
Field collection
Fieldwork was conducted by researchers aboard longliners operating in the ABFT fishery in the Gulf of Lions. Longlines were rigged with two hook types commonly used by the fleet (Circle hook: VMC ref. 9788PS, size n°7 and J-type hook, size 5/0 ARG. Ref 1.20*10 MTRS). Hooks were baited with sardine (Sardina pilchardus). Ten pelagic stingrays were caught under normal commercial operations, of which six were caught with J-type hooks and four with circle hooks. All ten specimens retained had hooks
Hook shedding and healing
During the 6 day quarantine, the ten stingrays were left unattended to reduce stress, some food was provided but no inspection of the fish was implemented. Therefore, it was not possible to identify the specific day when any hooks were shed. At the completion of the quarantine period, seven hooks (one circle and six J-type hooks) were found on the bottom of the tank. The number of hooks shed was conservatively assigned to the sixth day after the capture event. For the remainder of the
Effects of the hook type and the hook size
Circle hooks have been considered as one of the more promising mitigation options for reducing deep hooking of hard-shelled turtles and lethal injuries associated [33]. They increase jaw-hooking, facilitating life release of unwanted or protected species but usually do not reduce catch rate. Indeed, the use of circle hooks is already mandatory in certain areas in the world [34,35]. In the case of sharks species, they can increase catch rate on monofilament gears reducing bite-offs due to
Conclusions
The use of circle hooks is widely promoted to reduce deep hooking and lethal injuries associated regardless the species. The current study shows that for the stingray J-type hook had a faster self-shedding rate than circle hook (for a similar size), highlighting the fact that it is crucial when implementing mitigation methods to consider all possible conflicting effects on other vulnerable taxa. The adoption of good practices to handle and release the stingrays identified could reduce
Acknowledgements
The present study is the outcome of a project “RéPAST”, funded by France Filière Pêche (FFP), IFREMER and the Regional councils of Hérault, Pyrénées-Oriental, Languedoc Roussillon (France), carried out in collaboration with the fishing sector: AMOP (Sathoan and OP du Sud) and CEPRALMAR. The authors are grateful to all the skippers of the fleet who took part of the programme and especially Christophe, Kevin and Dorian from the “Dochris” and Frédéric Aversa from the “Narval” for facilitating the
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