Fish missed by bottom trawlers go hungry, researchers find

By Lewis Smith
March 09 2011 Fish left behind after fishing boats have dragged their trawl nets over the sea bottom are left hungry, skinnier and less virile, scientists have found. Cod, lemon sole and megrim suffer after surviving the bottom-trawling nets because they cannot find as much to eat. Researchers behind the study warned that the damage to the health of the surviving fish could impair “the recovery of threatened stocks”. Bottom-trawling equipment “lifts and grinds” so many of the prey species that scientists believe there is a food shortage once the boats have gone. Shellfish, sea urchins, prawns, crabs and even polychaete worms are among the prey that are killed are driven away by the disturbance. The shortage of food leaves the fish hungry which in the longer term damages their health, life expectancy and ability to reproduce, while the sea bed itself is dramatically changed from a rich and varied eco-system to a much more limited and empty habitat. Fishermen facing declining stocks are also likely to find that fillets from the fish they catch in trawled zones of the seas are thinner than on those that are the same length but come from other regions. Researchers who analysed UK and Irish data on fish caught in bottom-trawling nets in the Celtic Sea concluded the overall health of surviving fish is reduced by up to 20 per cent. Dr Samuel Shephard, of the Galway-Mayo Institute of Technology in Ireland, led the study and said: “What is left is fewer in number and the individuals are generally smaller. The target species are skinnier and being skinnier are going to be less reproductively successful.” He added: “We found that trawling in the Celtic Sea was related to poorer condition of lemon sole, megrim and cod – important commercial species. “This indirect effect of trawling could mean that fish are less able to resist disease and reproduce successfully, and they may have lower market value. Heavily trawled areas with impoverished seabeds may support fewer fish, possibly impairing recovery of threatened stocks.” … Scientists involved in the study, published in the journal Proceedings of the Royal Society B, said in their report: “Declines in fish condition have been implicated in reduced fecundity, increased natural mortality and reduced market value.” …