Removing invasive predators from island breeding colonies could save more seabirds for less cost than reductions in fishing, a study of Australia’s Eastern Tuna and Billfish Fishery (ETBF) has found.
According to one of the authors of a paper on the findings in the August edition of Frontiers in Ecology and Environment, CSIRO scientist Dr Chris Wilcox, a major challenge for fisheries worldwide is to reduce their impact on ‘bycatch’ species such as seabirds.
“Australian Commonwealth fisheries have made strong efforts towards reducing bycatch, including modifying fishing gear and restricting areas and periods of fishing, but these measures are not always effective, leading to costly interventions such as fishery closure,” Dr Wilcox says.
“While the priority should always be for fishers to avoid bycatch, they could also ‘offset’ the bycatch that does occur by funding conservation measures that tackle other, often greater, threats to bycatch-affected species.”
Dr Wilcox and C. Josh Donlan of Cornell University explored the offset approach in a study of flesh-footed shearwater bycatch in the ETBF, which targets yellowfin and bigeye tuna, albacore and billfish.
Practices used in the ETBF to reduce the capture of seabirds on longlines are costly and not always effective for all species. A species of concern is the flesh-footed shearwater, which in eastern Australia breeds only on Lord Howe Island where rats are potentially a major predator.
Dr Wilcox and Mr Donlan compared the potential impact of fishing with that of rat predation on Lord Howe Island flesh-footed shearwater populations, and the costs and benefits of rat control and fishery closures.
They found that banning fishing in a 750-kilometre radius of the island would result in a six per cent increase in growth rate of the shearwater population, at a cost of about A$3.5 million. The eradication of rats would result in a 32 per cent increase in the population growth rate, at a cost of about A$580,000.
Rat eradication therefore could yield a conservation return on investment 23 times greater than a fishery closure, and could have broader ecosystem benefits.
“Vessel levies could be set at the cost of offsetting their bycatch,” Dr Wilcox says. “As well as funding actions that effectively offset the bycatch, the levy would encourage fishers to seek innovative ways of avoiding bycatch.”
He says environmental groups have made great strides in drawing attention to the bycatch problem. Fishers, technologists and scientists in turn have reduced bycatch substantially through fishing-method innovation.
“For fisheries to have a zero impact on bycatch, however, they will need to use the full suite of cost-effective tools available, in a responsible and integrated way,” Dr Wilcox says.
Dr Wilcox and Mr Donlan believe that given the number of seabirds and other mammals affected by fisheries and invasive species, the offset approach could prove effective in many scenarios worldwide.
Backgrounder: Island conservation shows promise for bycatch
A major challenge for fisheries worldwide is to reduce their impact on non-target or bycatch species.
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.
A study in compensatory mitigation
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.
Cost effective conservation
The ETBF example shows that bycatch offsets such as rat eradication could achieve conservation benefits exceeding those of bycatch mitigation measures.
Wilcox and Donlan believe that offsets, when used in the proper framework, can constructively address a global conservation concern by providing a mechanism for generating revenue for high-impact conservation actions and forging alliances between conservation and fisheries organisations.
In the fisheries context, setting vessel levies at the cost of offsetting their bycatch:
An important aspect of the approach though is to prioritise the avoidance of impacts, and reduction of those impacts that do occur, leaving offsets as the fallback tool where bycatch cannot be prevented.
An additional benefit is that suites of species, even ecosystems, would benefit from the removal of invasive mammals and other on-island restoration actions.
Given the number of seabirds that are threatened both by fisheries bycatch and invasive species, bycatch offsets are likely to prove applicable and effective in many scenarios worldwide.
Offset approaches could re-establish seabird colonies on islands where they have been extirpated, and be adapted to suit other species affected by fishing.
For example, many endangered populations of sea turtles are affected by factors such as human consumption of adults and eggs, nest predation by invasive mammals, and fisheries bycatch from artisanal, in addition to industrial fishing. Industrial fishing might be able to fund reductions in these other threats, reducing the pressure on the turtles to a sustainable level.
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