Behavioural observations in and around various fishing gears have been used to develop more species and size selective fishing gears. Especially in mixed fisheries, like the North Sea, species selectivity is of major importance to avoid discard.
The Nordmøre Grid for separating shrimp and fish was developed in the late eighties, and has been adopted in all North Atlantic shrimp fisheries. Following the success of this separating grid, further grid developments have been tried in a number of demersal and pelagic fisheries to reduce by-catch. In 2005 a grid for sorting out bycatch of cod and saithe in a herring trawl was developed and is now used in the commercial pelagic trawlfisheries for herring.
Survival of fish escaping from fish gear
In order for technical measures to work, it is important to ensure that fish that escape from the gear actually survive. Work has been carried out to develop methodology to study survival both during demersal trawling and purse seining.
In 2003 the SURVIVAL-project – a three year project, partly funded by the EU Commission – was started to assess the survival of fish (haddock, whiting, saithe and cod) escaping from towed fishing gear. Scientists from Institute of Marine Research, Norway, Fisheries Research Services, Scotland and Danish Institute for Fisheries Research participated in the project. A fourth project partner, The North Sea Museum, Denmark produced a pamphlet and a short video to describe the work and the results. During the project, a range of experiments were conducted to provide information and data on escape-survival under different conditions. The work was done on the west coast of Scotland and in the Barents Sea. Both these locations have known fishing grounds.
The experiments showed that survival of fish that had passed through a trawl codend was generally good. On average the survival of both whiting and haddock was around 95%.There was, however, a marked difference in survival between different length-groups of whiting and haddock. For both species the smallest fish had the lowest chance of survival. To capture fish the trawl first herds the fish and forces them to swim at large speed for a relatively long time. These fish become fatigued and fall back into the mouth of the trawl. The smallest fish have the poorest swimming ability and will tire quickly, which means they are more likely to be stressed and injured. This may explain why it is the smallest fish that are most likely to die.
The experiments investigating whether fish escaping at the surface survive showed that surface-escapees had a substantially lower chance of surviving than fish escaping during the tow.
Survival experiments conducted in the Barents Sea compared the survival of fish escaping from a trawl codend in an area which was first closed to commercial trawlers and then reopened to intensive trawling activity. Cages which were an integrated part of the codend cover were left anchored to the seabed after being released from the trawl. The fish were monitored every day using cameras installed in the cages. When the cages were retrieved after 6 days, the living and dead fish were counted and examined for injuries.
Survival rates were very high for saithe and cod, close to 99% during both the high and low fishing intensity periods. Although some fish can encounter and escape from trawls more than once, high fishing intensity does not appear to lower the survival of escaping saithe and cod.
Haddock survival was substantially lower and varied considerably from haul to haul. Strong tides in this area may have moved the cages and this is probably what caused the high mortality – consequently no conclusions could be made about the effect of fishing intensity for haddock.
The SURVIVAL-project showed that the survival of haddock, whiting, cod and saithe escaping from trawl codends were generally better than previous investigations have suggested. It has also helped scientists to better understand what kills the fish that do die. Finally these results can now be used to reduce uncertainty in stock assessments and improve the management of these fisheries, helping conserving these stocks for the future.
Unaccounted mortality in the purse seine fisheries
Catch regulation by slipping whole or parts of the catch has traditionally been used in purse seine fisheries for pelagic species like mackerel (Scomber scombrus L.) and herring (Clupea harengus) if the catches are considered too big or size and/or quality of the fish are considered unsatisfactory (high grading). This is particularly the case when the prize differs between sizes or quality groups of fish. Until today little has been known about the fate of pelagic fish that has been caught, crowded and thereafter released from a purse seine. Recently a new offshore method has been used to study the effect of crowding with subsequent slipping from a purse seine on the mortality of Atlantic mackerel (Scomber scombrus L).
In these experiments mackerel were carefully transferred from a purse seine into two identical large floating net-pens through a transfer channel. One pen was used as control and left floating in the sea without further treatment. The other was used for simulating crowding and slipping from purse seines. The water volume in the pen was gradually decreased by hoisting the bottom of the pen by a crane until the fish started to show flash expansion behaviour (or started to ‘boil’ as denoted by fishermen), at which point the density was kept for 15 (2006) or 10 min (2007). The volume was then returned to normal and the net-pens left drifting freely in the open sea for 3 to 6 days.
A total of five experiments were conducted, and it was evident that crowding had a major effect on survival of mackerel. In all five experiments, the mortality was significantly higher among the crowded fish (80 – 100 % mortality) than among the controls (0.1 – 46 % mortality). The experiments showed that slipping of mackerel from purse seines has the potential of killing fish and should ideally be avoided. However, if inevitable, slipping must be done in an early stage of pursing before the fish exerts any sign of panic behaviour.
Last update 2 October