Odontocete bycatch and depredation in longline fisheries: A review of available literature and of potential solutions

Abstract

Operational interactions between odontocetes (i.e., toothed whales) and longline gear are a global phenomenon that may threaten the conservation of odontocete populations and the economic viability of longline fisheries. This review attempts to define the issue, summarize the trends and geographical extent of its occurrence over the last half century, explore the potential impact on odontocetes and on fisheries, and describe potential acoustic and physical mitigation solutions. Reports of odontocete bycatch rates are highly variable (between 0.002 and 0.231 individuals killed per set) and at least 20 species may be involved. Information about marine mammal population size, migration patterns and life history characteristics are scarce, although at least one population may be in decline due to losses attributable to longline bycatch. Information about the financial impact of depredation on pelagic longline fisheries is also scarce, although estimates of daily fleet‐wide losses range between US$1,034 and US$8,495 (overall fleet income was not reported). Such biological and financial losses may be unsustainable. Recent developments in acoustic and physical mitigation technologies have yielded mixed results. Acoustic mitigation technologies have no moving parts, although require complex electronics. To date, they are insufficiently developed and their efficacy has been difficult to assess. Physical mitigation technologies generally require complex moving parts, although they are relatively simple to develop and assess. Further development and testing remains necessary before widespread implementation would be possible. Development of these approaches should be prioritized and a “toolbox” of various strategies and solutions should be compiled, because a single panacea to the problem is unlikely to emerge.

Pelagic longline fishing trials to shape a mitigation device of the depredation by toothed whales

“…This differs from pelagic longline fishery by the fact that the line is exposed to depredation during all the fishing time (Moreno et al, 2008) and why in ideal conditions the deployment of the DMD occurs when the fish is biting the baited hook. As far as we know our fishing trials were the first carried out in 2007 and in 2008 to test DMD on field for pelagic longline fisheries, while some are currently ongoing (Hamer et al, 2012). Obviously our results show that socks and spiders are not yet ready to be deployed during commercial longline operations to mitigate toothed whale depredation.…”

“…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).…”

Depredation of Patagonian toothfish (Dissostichus eleginoides) by two sympatrically occurring killer whale (Orcinus orca) ecotypes: Insights on the behavior of the rarely observed type D killer whales

“…Other studies have shown that most odontocete species that have been observed depredating are naturally fish‐eaters (Read , Hamer et al . ). For instance, sperm whales depredating in the Southern Ocean are known to feed naturally on toothfish (Yukhov ).…”

“…), as well as for other odontocete species that have been documented to depredate worldwide (Clark and Agnew , Hamer et al . , Peterson et al . ).…”

Combined gut-content and stable isotope trophic analysis of the pelagic stingray Pteroplaytrygon violacea (Bonaparte, 1832) diet from the western North Atlantic Ocean

“…The dietary shift could thus be due to the availability of prey items at different times of the year; for example, Neer (2008) reported that pelagic stingrays seasonally target schools of mating squids in the eastern Pacific Ocean. Alternatively, pelagic stingrays could be opportunistically depredating squid bait from pelagic longline sets, which may explain the wide range of estimated contributions by squid in the isotope modeling efforts; such depredation behaviors by odontocete whales have been described previously in longline fisheries (e.g., Hamer, Childerhouse, & Gales, 2012). Pelagic stingrays show a high rate of survival at gear retrieval in the pelagic longline fishery, especially with the circle hooks now required for the US commercial fleet (Kerstetter & Graves, 2006).…”