Impacts of Commercial Fishing on Marine Wildlife
...ity (ie birth rates) and long survival and therefore are especially vulnerable to the effects of an increase in adult mortality (Barnes, Ryan & Reid 1997). Many procellariiform species take one mate and hence the loss of one adult bird results in loss of its mate to the breeding population for some time. In addition two adults may be necessary to fledge chicks so in the absence of both parent birds new hatchlings will perish (Bergin 1997). Clearly any reduction of the adult population has the potential to reduce recruitment through the loss of breeding pairs. Studies have shown that populations of several albatross species are declining (Tomkins 1985, Weimerskirck & Jouventin 1987, Croxall et al 1990 in Klaer & Polacheck 2000). Bergin (1997) reports that some populations of albatross have declined by nearly 90% and some species are facing extinction if present bycatch rates are not reduced. In the Mediterranean the most commonly caught species in longline bycatch include Coryâs shearwate (Calonectris diomedea), Balearic shearwater (P. mauretanicus), Audouinâs gull (Larus audouinni) and yellow-legged gull (L. cachinnans) (Belda & Sanchez 2001). Observers on longline vessels in the Western Mediterranean in 1998-1999 estimated between 656-2829 birds were caught annually, of which 66% were Coryâs shearwaters. Mortality was also highest in adult birds, which affected reproductive success. Therefore longline fisheries in the Mediterranean may cause declines in some seabird populations especially if longline fishing efforts are increased (Belda & Sanchez 2001). Gillnet entanglement is another threat to marine bird species (Darby & Dawson 2000). Gillnets set near the surface can entangle and drown large numbers of seabirds (Piatt et al 1984, Salzman 1989, Carter et al 1995 in Darby & Dawson 2000). Vulnerable species such as the yellow-eyed penguin (Megadyptes antipodes) have been found to be impacted by this technique; an issue of conservation importance given that this is one of the least abundant extant penguins. For example Darby & Dawson (2000) have reported that gillnet entanglement is a significant threat to populations of yellow-eyed penguins on the South Island of New Zealand. Impacts on marine mammals The commercial fishing industry impacts indirectly on marine mammals by reducing prey species availability by altering marine ecological processes, and also by direct capture and entanglement in fishing equipment (bycatch) (Northridge & Hofman 1999, Whitehead, Reeves, & Tyack 2000). The mortality associated with bycatch is a cause for concern for many cetaceans (dolphins, porpoises, whales), sea turtles and pinnipeds (seals, sea lions, walrus) (Northridge & Hofman 1999, Whitehead, Reeves, & Tyack 2000). Offshore driftnet fishing has been reported to take large numbers of marine mammals in bycatch (Perrin et al 1994 in Silvani, Gazo & Aguilar 1999). For example one study reported that an illegal Spanish driftnet fishery operating on the Mediterranean side of the Gibraltar Straits took sea turtles, especially loggerhead turtles (Caretta caretta), common dolphins (Delphinus delphis), and striped dolphins (Stenella coeruleoalba). The levels of bycatch were estimated to be potentially unsustainable when the impacts of other Mediterranean fisheries were also considered (Silvani, Gazo & Aguilar 1999). Small coastal cetaceans such as Hectorâs dolphin (Cephalorhychus hectori) that is endemic to New Zealand are also known to be victims of gillnet and trawl fishery bycatch (Pichler, Slooten & Dawson 2001). The Hectorâs dolphin has a low reproductive rate and population viability analyses have shown that the current level of bycatch is unsustainable for most Hectorâs dolphin populations (Pichler, Slooten & Dawson 2001). Another small cetacean under critical threat from gillnet fishing activities is the Vaquita dolphin (Phocoena sinus) which is the most endangered marine cetacean in the world and is caught in gillnet fisheries throughout much of its range (Dâagrosa, Lennert-Cody & Vidal 2000). A study of the incidental mortality of vaquita dolphins in the upper Gulf of California, Mexico from 1993-1995 found an estimated catch of 39 vaquitas per year which is over 17% of the estimated population size. This rate is totally unsustainable when the impacts of all gillnet fisheries in the vaquitaâs range are considered and a recommended ban on gillnets in the area is indicated to prevent this species being driven to extinction (Dâagrosa, Lennert-Cody & Vidal 2000). Pinnipeds are commonly victims of fishing bycatch (Read & Wade 2000, Wickens 1995, Wilkinson, Burgess & Cawthorn 2001) although little quantitative data exists for most fisheries (Shaughnessy et al 2001). The most complete data for current fisheries in Australian waters is for the South East Trawl Fishery (Shaughnessy et al 2001). Documented bycatch includes the death of 89 fur seals in trawls targeting blue grenadier in 1999 of North West Tasmania. In addition observers in Tasmania have noted bycatch involving killer whales and fur seals in the dropline fishery and fur seals in the longline and gillnet fisheries (Shaughnessy et al 2001). The 1995 world Review of Operational Interactions Between Pinnipeds and Fisheries indicates that all four of the pinnipeds found in Australian and New Zealand waters (Australian sea lions, Hookerâs sea lions, Australian fur seals, and New Zealand fur seals) are killed accidentally or deliberately during net and trap fishing operations, line fishing and fish farming (Wickens 1995). For instance Australian fur seals drown when caught in lobster pots and fish traps and trawling operations (Warneke 1982, Goldsworthy & Shaughnessy et al 1994 in Wickens 1995) and New Zealand fur seals are caught in the hoki trawl fishery in New Zealand (Mattlin & Cawthorn 1991, Mattlin 1994 in Wickens 1995). Some pinnniped species impacted by fishery operations are of particular conservation concern due to low population levels. The Hookerâs sea lion (Phocarctos hookeri) is classified as a threatened species and is one of the rarest species of sea lion in the world (Wilkinson, Burgess & Cawthorn 2001). It has a highly localised breeding range in the New Zealand subantarctic on the Aukland Islands. These sea lions are caught and drowned by trawlers fishing for arrow squid around the islands (Wilkinson, Burgess & Cawthorn 2001). Since the mid 1980âs the annual mortality of Hookerâs sea lions caused by incidental capture is ~110 animals (74-163) of which approximately 72% are females and this consequently reduces the survival of pups (Cawthorn 1985, Mattlin & Cawthorn 1991 in Wickens 1995). Other pinnipeds under threat from deliberate or accidental killing during fishing operations for whole or part of their populations are the Stellerâs sea lion (vulnerable), South American sea lion, northern fur seal, Australian fur seal, New Zealand fur seal, Eastern Atlantic harbour seal, Western Pacific harbour seal, harp seal, Baltic ringed seal (vulnerable), Saimaa seal (endangered), Baikal seal, Mediterranean monk seal (endangered), and the Hawaiian monk seal (endangered) (Wickens 1995). Other impacts of commercial fishing on marine mammals are less observable than the direct evidence of bycatch. For instance fishing activities can reduce the availability of key food resources for cetaceans and pinnipeds. This is an issue of which marine scientists are aware but which is extremely difficult to demonstrate quantitatively. Consequently it can be fundamentally problematic predicting the impacts of fishery exploitation (Christensen 1996). A study of the diet of New Zealand fur seals (Arctocephalus forsteri) at Otago Peninsula, New Zealand showed that the prey species of fur seals were being targeted by commercial fisheries in the area (Fea et al 1999). . Although the impacts of this are unclear these findings highlight the potential that competition from fisheries may have negative outcomes for New Zealand fur seals or other similarly affected marine mammals. Another impact of commercial fishing only recently discovered is behavioural modification of populations of bottlenose dolphins (Tursiops aduncus) subjected to the influences of trawl fishing in Moreton Bay, Queensland (Chilvers 2001). The study indicated that two populations of bottlenose dolphins existed in the bay and these were socially separated. The separation was due to the apparent dependency of one group and non-dependency of the other on the use of trawler discard as a food source. Significant differences were found in the behaviour, social structure, group sizes, distribution, and habitat use and range of the two populations. This study is the first to report social separation of two sympatric groups of small cetaceans that seems to be the result of the anthropogenic effects of fishing (Chilvers 2001). Mitigation measures The most serious threat to small cetaceans (Delphinidae and Phocoenidae) is incidental mortality in gillnets with many tens of thousands of animals killed annually (IWC 1994 in Dawson et al 1998). In 1994 a study in the New Hampshire area demonstrated that a 92% reduction in bycatch of harbour p...