Image caption Sharks are among the species accidentally entangled in purse seing nets

Scientists are embarking on a two-month expedition in the Pacific aimed at finding ways to reduce the damaging accidental toll of tuna fishing.

They want to find techniques that help fishermen find the abundant skipjack tuna without also catching sharks, turtles, or threatened tuna species.

The scientists will sail on board a tuna purse-seine vessel from Ecuador.

Knowledge gained on the trip will be used to develop fishing techniques or new gear that are much more selective.

This could entail fishing at different times of day, at specific depths under the waves, or by more targeted use of fish aggregating devices (FADs).

"The overall objective is to explore some potential options for reducing the mortality of bigeye tunas and other 'undesirable' species while maximising catches of skipjack," said research leader Kurt Schaefer.

"We're looking for ways in which we can learn to harvest the skipjack without impacting other species such as bigeye and yellowfin - we're not yet testing what we consider to be practical solutions," he told BBC News.

Dr Schaefer has been a research scientist with the Inter-American Tropical Tuna Commission (IATTC) - one of the bodies charged with regulating tuna fishing in the open sea - for more than 30 years.

While the small, fecund skipjack (Katsuwonus pelamis) forms the basis of the canned tuna industry, the bigeye (Thunnus obesus) is an endangered species in the Pacific, primarily because of fishing.

The cruise departed from Ecuador on Tuesday, using the chartered commercial fishing vessel Yolanda L.

Modern FADs

Image caption New models of FAD could in future separate different species of tuna, and other fish

For reasons that are not entirely clear, fish and other marine creatures tend to congregate around floating objects such as logs.

Fishermen have learned to take advantage of this, deploying buoys - FADs - equipped with GPS and sonar.

When the sonar senses that fish have gathered, the buoy signals the parent vessel, which steams alongside to collect its haul.

Using a purse seine net, the boat can encircle and capture the entire shoal.

The scientists hope that understanding what makes various species move towards the FAD and then leave it again could open doors to fishing selectively.

This combination of research, training and management is necessary if we want to make these fisheries more sustainable Victor Restrepo, ISSF scientific advisor

"One of the things we're doing is behavioural studies using acoustic tags and telemetry," said Dr Schaefer.

"We'll be tagging these species, and trying to see whether there are times when you see separation eithed horizontally or vertically in the water, and whether you could use this to separate out catches.

"We'll also be looking for times of day at which the species might naturally separate - times when the skipjack, for example, might move away from the FAD."

Smaller species may be trying to shelter from predators, while bigger ones may see it as an easy source of food.

The various species may also be attracted away by different signals, such as water temperatures.

A remotely operated underwater vehicle (ROV) will be deployed to film fish behaviour around the FAD, and after entrapment in the purse seine net.

If different tuna species separate inside the net - some swimming high and others low, for example - that could also form the basis of a separation method.

Local knowledge

Having spent long periods at sea on fishing vessels, Kurt Schaefer believes experienced skippers may already know ways of targeting skipjack.

The scientists will analyse how well the Yolanda L's skipper is able to predict catches.

Image caption An underwater ROV (here being tested) will be deployed to film tuna in the nets

This research cruise is an initiative of the International Seafood Sustainability Foundation (ISSF), which brings scientists together with people from the seafood industry and from environmental groups.

It is the first of a number of cruises planned for different parts of the world's oceans.

Victor Restrepo, chairman of ISSF's scientific advisory committee, said the broader project aims to replicate what has already been achieved in some other fisheries by combining expertise held by fishermen with scientific findings.

"We are sharing what we learn with with skippers through workshops where scientists and fishers exchange ideas on these and other potential techniques," he said.

"And we are working with policymakers in the governments of countries with important purse seine fisheries so that they adopt regulations to implement these techniques.

"I believe that this combination of research, training and management is necessary if we want to make these fisheries more sustainable."

Whereas some environmental groups argue for the abandonment of FADs, the ISSF believes this is neither feasible nor desirable.

"It's the philosophy of ISSF and our partners that abandoning a fishery will not help to improve it," said ISSF president Susan Jackson, previously of food giants Del Monte and Heinz.

"We must help to improve practices that make fishing for tuna more sustainable."

The bluefin - the most talked about tuna species recently, and the most prized for sushi - is not a factor in this cruise.