BACKGROUND
The demand for fishery resources to feed the worlds burgeoning human population continues unabated, placing increasing pressure on the oceans finite fish resources. Since the late 1980s, decreased catch rates due to overexploitation, plus increasing management restrictions (e.g. reduced quota or time-area closures) and operational costs (e.g. fuel prices and crew wages), have all impacted negatively on the economic viability of affected commercial fisheries. Over the same period, many marine mammal populations have recovered numerically from the industrial harvesting period of the 18th to early 20th century. Additionally, post WW2 advances in materials, mechanical and electronic technology have enabled fishing vessels to become larger, stay at sea longer and fish in deeper waters. As a result, competition between commercial fisheries and other marine predators, such as marine mammals and seabirds, is on the increase. With less food to share among them, the chance of encounters, or operational interactions, also inevitably increases. Operational interactions can be divided into two groups…
DEPREDATION
Depredation of the catch typically involves marine mammals, specifically odontocetes (toothed whales), pinnipeds (mostly the otariids or fur seals and sea lions, although sometimes phocid species). Depredation of bait is confined to longline and trap fisheries, with the latter mostly involving the smaller odontocetes (dolphins and porpoises) and seabirds (mostly albatross). This occurs in most fisheries, including trawl, gill-net, purse-seine and longline, trap and cage.
Marine mammals and seabirds are intelligent, quickly learning to forage opportunistically on bait or caught fish in or on fishing gear. Depredation is the outcome of searching for food around fishing gear, which can result in the (i) removal of bait, (ii) deterrence of target fish from approaching fishing gear (although strictly not depredation perse, it does have the same result), (iii) complete removal of caught fish, or (iv) damaging of caught fish that are landed on the vessel. Some fish may feature in the natural diet of odontocetes, while others may be too large, too quick, or reside in waters that are too deep for them to catch when foraging naturally.
Today, few fisheries remain unaffected by depredation. Active fishing methods, such as trawling and purse-seining may be minimally impacted in many cases, but gill-net, longline, trap and cage fisheries may suffer considerable losses in some areas. Some sources report losses of up to US$300M in Pacific Ocean pelagic tuna longline fisheries alone to toothed whales. The rock lobster trap fishery in south-eastern Australia reported losses in the millions attributable to pinnipeds.
BY-CATCH
By-catch of marine mammals and seabirds is almost inevitable wherever the two overlap geographically. Marine mammals are most at risk in purse-seine, trawl, trap and gill-net fisheries, while seabirds are mostly at risk in longline fisheries and trawl fisheries, with occasional reports in gill-net fisheries.
When marine mammals and seabirds depredate the bait or catch, they are at considerable risk of becoming by-caught on or in the fishing gear. Individuals may misjudge their movements around the fishing gear when depredating, or may be naïve to the associated dangers, resulting in the ingestion of a hook, entrapment in a pot, or entanglement in a net fold. These scenarios may lead to immediate drowning death if the animal is unable to reach the surface to breath. Many individuals may break free with external entanglements or ingested fishing gear, which may impede foraging efficiency and cause further injury and infection, leading to prolonged and painful death.
To date, reports suggest the conservation of many seabird species, particularly albatrosses, have been most at risk. Until very recently, large numbers were by-caught in longline fisheries and to a lesser although still significant extend in trawl fisheries. Several dolphin populations have also been negatively impacted by high levels of by-catch in purse-seine fisheries, as have several populations of larger odontocete species in longline fisheries. Some sea lion populations may be under threat of decline or even extinction due to by-catch in gill-net and trawl fisheries. Aside from the obvious threats to the conservation of many of the species involved, the welfare of the individuals that are drowned, or that sustain life threatening injuries also needs to be considered. Today, many countries have legislation that mandates the need to improve fishing practices to mitigate the negative conservation and welfare impact on affected marine mammal and seabird populations and individuals.
