Pelagic longline fishing trials to shape a mitigation device of the depredation by toothed whales
Highlights
► We design an innovative toothed whale depredation mitigation device. ► Our device aims at physically protecting longline-caught fish. ► We report an important toothed whale depredation rate. ► We face some operational constraints when trying to routinely deploy our devices. ► We need to improve our devices but our results are nevertheless promising.
Introduction
Negative interactions between marine mammals and fisheries are known to occur worldwide, affecting many species and many fisheries (Northridge, 1991). Depredation is part of those interactions and is defined as the removal or damage of fish from fishing gear by large marine predators, mostly toothed whales and sharks (Donoghue et al., 2002, Gilman et al., 2008). Over the past decades, the scale of interactions between toothed whales and pelagic longline fisheries in particular has expanded, altogether with an increase of the fishing effort. Depredation damage evolved in time from a few capture among the whole catch in the first years up to the entire catch of longliners currently (Nishida and Tanio, 2001). In pelagic longline fisheries targeting tuna (Thunnus spp.) and swordfish (Xiphias gladius), depredation on catch involves false-killer whales (Pseudorca crassidens), short-finned pilot whales (Globicephala macrorhynchus), killer whales (Orcinus orca) and pelagic sharks (Dalla Rosa and Secchi, 2007, IOTC, 2007, Mandelman et al., 2008, Nishida and Tanio, 2001, Secchi and Vaske, 1998, Sivasubramanian, 1964).
The monitoring of the extent and magnitude of toothed whale depredation is of a great importance since it leads to many negative consequences affecting assessment, biological, ecological and commercial aspects (Gilman et al., 2006, Perrin, 1991). As an impact on assessment, fish loss due to depredation is not taken into account in stock assessment analysis (Donoghue et al., 2002). As an impact on biology and ecology, toothed whales hunting behaviour is changing as they will get used to search after boats to get easy-to-catch preys instead of hunting their common feeding preys (McPherson et al., 2003, Ramos-Cartelle and Mejuto, 2007, Secchi and Vaske, 1998). Moreover increased risks of injury or mortality of cetaceans occur, firstly in a deliberate way from fishermen (retaliation) and second in an accidental way due to negative interactions with the fishing gear (bycatch). For instance, previous photo-identification studies of false-killer whales evolving around Hawaii and short-finned pilot whales around Mayotte Island exhibited obvious signs of their interactions with longline fisheries (Baird and Gorgone, 2005, Kiszka et al., 2009). As an impact on commercial aspects, depredation represents an economic loss as fishermen spend extra money when fixing fishing gears damaged by predators, altogether with an increased fuel expenditure when they move away to avoid areas of high depredation rate (Secchi and Vaske, 1998). However the main loss of profits is related to fish damage.
In the Indian Ocean, toothed whale depredation on pelagic longliners is characterised by a data-limited situation. Few scientific papers or grey literature deal with this issue (Nishida and Shiba, 2007, Nishida and Tanio, 2001, Rabearisoa et al., 2007, Romanov et al., 2007, Sivasubramanian, 1964). In Seychelles, the toothed whale depredation rate for swordfish was estimated at 10.3% (Rabearisoa et al., 2007). Therefore, professional longliners are in an urgent need of mitigation devices which could help them to significantly reduce the financial impact of depredation. Moreover they might produce beneficial effects for the conservation of toothed whales and the management of pelagic target species.
In the US, first depredation mitigation measures consisted in the promulgation of permits by the NMFS (National Marine Fisheries Service) allowing fishermen to do whatever may be necessary to protect their gear and their catch from damage by toothed whales (including killing and harassing), which meant that a significant number of them may have been killed (Mate, 1980). Over the last decades, non-lethal control actions (conservation strategy) progressively replaced those lethal ones (eradication strategy) (Breitenmoser et al., 2005). Most research, less radical, are currently focusing on the use of active acoustic means to deter depredation from cetaceans. ADD (Acoustic Deterrent Device), or pingers, and AHD (Acoustic Harassment Device) acoustically bother toothed whales and aim at preventing them from approaching the fishing gear to steal the fish and/or the bait and from being incidentally captured (McPherson et al., 2008). If active acoustic can be efficient at short term it generates opposite effect at medium term as pingers end up at being used as an acoustic attractor by cetaceans (Brotons et al., 2008, Jefferson and Curry, 1995).
There is good evidence that cetaceans use their hearing to locate the gear and/or the boat (Thode et al., 2007). In response, some mitigation measures regarding noise reduction onboard fishing vessels were also proposed (Purves et al., 2004), but despite those advices, depredation still leads to important fish loss. Other depredation mitigation methods have been tested so far, such as using explosives, chemical deterrents, flare guns or predators sounds, but none of them were proved to be successful (Gilman et al., 2006, Jefferson and Curry, 1995, Werner et al., 2006). This lack of success may be due to the behavioural adaptability of toothed whales to new stimuli, and this adaptability is the main difficulty met by researchers dealing with depredation mitigation (Nitta and Henderson, 1993). Changing fishing strategies, such as setting shorter lines or travelling long distances to avoid predators, was somewhat efficient in reducing depredation by killer whales (O. orca) (Tixier et al., 2010).
As depredation deterrence by using acoustic devices or other preventive methods did not prove to be efficient so far, we suggest acting on the end of the depredation process, i.e. once toothed whales have located the fishing gear, get close to it and prepare for attacking the captured fish. Therefore, we propose to develop devices shaped to produce a physical protection of capture to mitigate depredation events and to test them on field. It must be noted that fishing trials to assess the impact of a device developed to mitigate the depredation in pelagic longlining were very rare so far. For the first time, our study presents results dedicated to the physical protection of capture to deter predators in pelagic longlining. This depredation mitigation principle is also currently in the development phase in both Tropical South Pacific and Indian oceans, but has not been already trialled (Hamer et al., 2012).
In this context, a first trip was conducted off the Seychelles archipelago onboard a commercial longliner in November 2006. It allowed us to study the fishing operations in order to design appropriate depredation mitigation devices (DMD) to be deployed on the fishing gear. Two types of DMD, named “spider” and “sock”, were designed and tested at sea during commercial fishing operations respectively in November 2007 and November 2008. Both surveys aimed at checking the efficiency of each DMD regarding toothed whale depredation and assessing whether they fit the fishing gear and fishing technique parameters and constraints.
Section snippets
The “spider”
The first depredation mitigation device (DMD) was named “spider” after its eight strands (Fig 1). We opted for a dissuasive device made up of a 100 mm diameter plastic disc with sixteen holes in its outer range and a 37 mm diameter central hole. Four polyester strands were inserted in those outer holes, forming eight 1200 mm long hanging legs. Theoretically, the whole system can only be triggered by a biting fish. The triggering system was made up of a beta pin and an elastic ring. The branchline
Catch and depredation events
Results related to catch and depredation events were summarised in Table 3. For both surveys, more fishing sets were affected by shark depredation but the average depredation rate was higher when toothed whale depredation occurred. A Kruskal–Wallis test showed that the presence of socks did not affect the catch per unit effort (CPUE) during the second survey (H = 1.87, p = 0.17). There was no significant difference between shark or toothed whale depredation rates observed for sets equipped or not
Discussion
Acoustic device (ADD = Acoustic Deterrent Device named also pingers and AHD = Acoustic Harassment Device) is the mostly used measure to mitigate toothed whale depredation occurring in various fisheries, from longlines to gillnets (Bordino et al., 2002, Buscaino et al., 2009, López and Mariño, 2011, Mooney et al., 2009, Reeves et al., 1996). Longlines due to their length are almost impossible to protect efficiently with AHD. Furthermore the long-term effect and cetacean habituation to such
Acknowledgements
This study was achieved in the frame of an action plan produced in Seychelles in 2006 by the Seychelles Fishing Authority and the French Embassy and aiming at reducing toothed whale depredation on longline-caught swordfish and tunas in the southwestern of Indian Ocean.
Many thanks are due to the crew of Albacore which invested a lot in helping us carrying out experiments at sea during both surveys, and especially to Elvis and Beatty Hoarau who warmly welcomed us on board their boat and gave us
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