The blue whiting fishery that takes place west of Ireland and Scotland in the spring is know for tight concentrations of fish, often resulting in big bags after short tows.
Researchers at the Norwegian Institute of Marine Research have been examining the performance of trawl gear, and by placing 10-15 sensors on the trawls of five fishing vessels have been tracking how gear handles during a tow, as they look for ways of preventing codends from bursting under the pressure of the fish inside.
There is heavy fishing at this time of year, and with hundreds of tonnes of blue whiting in the codend, there’s always a risk of bursting the netting and losing the catch.
‘In addition, the swim bladders of blue whiting expand as the fish are brought to the towards the surface from depths of several hundred metres. This causes the codends to lift to the surface like a torpedo,’ said marine scientist Ólafur Arnar Ingólfsson.
By collecting data and simulating the codend behaviour, it is possible to see the whole process.
‘We wanted to see the shape of the trawl below the surface. We were most interested in the last phase – the ascent,’ he said, adding that as the codend approaches the surface, it gathers pace and lifts almost vertically.
‘We had to find a way to be more precise about the catch, so that we avoid bursting,’ he said.
During a research survey in 2021, they tried different solutions to release excess fish. a number of different options with large openings ahead of the codend were tested, and the challenge is to find something that ensures there is no loss of catch until the codend is full.
‘I think we have now found a decent solution,’ Ólafur Arnar Ingólfsson said. ‘Blue whiting largely seek upwards, so openings in the lower panel in front of the codend mean that there is little loss of fish until it’s full,’ he said.
The research team will continue testing various gear arrangements in March.