Island conservation shows promise for bycatch

Traditional methods of bycatch reduction include modifying fishing gear and restricting the areas fished and periods of fishing.

Gear modifications such as the use of turtle exclusion devices have effectively reduced the capture of some bycatch species. In other cases, avoiding unacceptable mortality levels has been difficult, leading to costly interventions such as fishery closure. High-value fisheries that have been closed due to their impact on endangered marine vertebrate species include New Zealand’s squid fishery and Hawaii’s pelagic longline fishery.

For many bycatch species, however, fishing is not the only cause of mortalities, and a redirection of resources to alleviate a greater threat could prove a more effective means of conservation. This ‘offset’ approach, known as ‘compensatory mitigation’, will be outlined in a paper by Chris Wilcox of CSIRO and C. Josh Donlan of Cornell University in the journal Frontiers in Ecology and Environment in August 2007.

Wilcox and Donlan evaluate the potential costs and benefits of compensatory mitigation using the case study of seabird bycatch in Australia’s Eastern Tuna and Billfish Fishery (ETBF). They conclude that bycatch offsets, in conjunction with direct bycatch mitigation efforts, are an effective, enforceable, and cost-effective approach to seabird conservation.

Invasive mammals a greater threat
Many bycatch species spend part of their life on land where they are subject to predation by invasive mammals. Predation by invasive mammals on islands is the cause of the majority of vertebrate extinctions in the past six centuries.

Predators such as feral cats and rats have decimated seabird breeding colonies worldwide, preying on eggs, chicks, and adults of many species. Three-quarters of seabirds listed by the World Conservation Union, or IUCN, are threatened by invasive species, compared with less than half threatened directly or indirectly by fisheries.

Given this situation, the removal of invasive predators from island breeding colonies may save more seabirds for less cost than a reduction in fishing pressure. The relatively low cost and high impact of predator control at key seabird breeding colonies suggests that it might be a feasible offset for fisheries bycatch, in cases where it is difficult to reduce bycatch by the more direct means of avoidance or gear modifications.

Wilcox and Donlan tested the concept of bycatch offsets in the ETBF, a longline fishery that extends from Cape York, Queensland, to the South Australian/Victorian border. The fishery targets billfish along with yellowfin, bigeye, and albacore tunas in the Australian fishing zone and adjacent high seas.

Seabird capture is a major issue for the ETBF and mitigation measures include prohibiting the setting of longlines during daylight, mandatory use of heavily-weighted lines, and fisheries closures. But some of these measures are thought to be costly, difficult to enforce, and in the end may not provide adequate protection for some species.

While effective in reducing albatross bycatch, Australian operated vessels have been reported to kill 1800–4500 flesh-footed shearwaters (Puffinus carneipes) annually. With the entire eastern Australian population breeding on Lord Howe Island and evidence of a decline, fishery closures are possible.

However, fishing is not the only threat to flesh-footed shearwaters. Other threats include habitat loss, predation by invasive predators including rats and ingestion of plastic.

Wilcox and Donlan used annual population growth rates of flesh-footed shearwaters and estimates of rat consumption of shearwaters on Lord Howe Island to compare the impact of predation by rats with that of fishing. They also compared the expected cost and conservation benefit of rat control and fishery closures.

Their analysis showed that the closure to fishing of a 750-kilometre radius around Lord Howe Island resulted in a 6% increase in population growth rate of the shearwater population, at a cost of about A$3.5 million.

Eradication of rats resulted in a 32% increase of population growth, at a cost of about A$580,000.

Rat eradication from Lord Howe Island therefore could yield a conservation return on investment 23 times greater than a fishery closure.-CSIRO Australia