Brussels, 08 Oct 2003
The preliminary findings of a recently completed EU project are helping to shed some light on the implications of prawn trawl fishing on stock depletion and ecological conservation.
The Portuguese investigation entitled 'invertebrate discard viability and fitness in prawn trawl fisheries' has received nearly 28,000 euro in EU funding under the energy, environment and sustainable development (EESD) section of the Fifth Framework Programme (FP5).
Prawn trawling is said to be responsible for one third of the world's discarded catch, despite producing less than two per cent of global seafood. Fish and other marine species that are not considered commercially viable either die on deck or are thrown back into the sea. Of those species thrown back, little is known about the trauma suffered and their survival.
As Robert Harris, the project's leading researcher and a Marie Curie fellow, told CORDIS News, the main task of the project was to examine the short term survival rate and long term viability of swimming crabs, young scampi (or nephrops), starfish and other crustacean species, which are caught in prawn nets but are part of the bycatch and hence discarded. 'We were particularly interested in the fate of discards from prawn fishing off the South coast of Portugal where boats trawl in very deep waters and where towing the trawl can last for very long periods of time,' explained Dr Harris.
Over a six-month period, the scientific team went out on commercial trawlers off the Algarve coast in Portugal to examine bycatch originating from waters of depths of up to 700 metres. 'To determine the short term survival rate in our experiments, we treated the crustaceans as they would have been treated under normal trawling circumstances,' explained Dr Harris. 'This meant leaving the trawl once caught on deck for 90 minutes until which time the catch would normally be sorted and the bycatch thrown away.'
The team adopted 90 minutes as a standard period for examining the survival rate. They found that when left on deck for this period, depending on species, 40 to 50 per cent of the bycatch died. 'These deep water species are far more delicate than other species that live in shallow waters, so long periods of towing and exposure to air and heat have fatal implications.'
Those that survive are normally thrown back into sea along with the now dead crustaceans. To determine the survival of discards back in the sea, Dr Harris and his team mimicked sea conditions by placing the crustaceans in darkened plastic aquaria with seawater cooled to a temperature similar to that found at the seabed. 'We went a little further than examining the crustaceans to see if they showed signs of vitality,' said Dr Harris. 'During a ten day period, we checked their response to mechanical stimuli, and their escape and defence capabilities.'
In addition, blood samples were taken from the crustaceans while on deck and in the aquaria to compare the physiological stress levels in and out of the water, and to see whether the crustaceans showed any signs of recovery. Although, some of the crustaceans responded rapidly to stimuli, the team found that there was still a very high level of mortality - 50 to 60 per cent - and, depending on the species, mortality rates rose to as much as 90 per cent after five days. 'Our findings reveal that while the crustaceans are thrown back alive, they are still suffering from trauma and are in fact dying a slow death,' explained Dr Harris. 'They're just food for fish.'
The project's results reveal the negative impact that trawling is having on marine biodiversity. 'Most of the delicate species are gradually disappearing from highly trawled areas, while robust scavenger species are continuing to increase in number,' said Dr Harris, adding that in order to ensure the biodiversity of our oceans, trawling practices have to change. 'Handling the fish needs to be looked at more carefully, ' explained Dr Harris. 'Some suggestions include reducing the amount of time that fish are left on deck; spraying the fish with seawater on deck to keep them moist and cool; or improving the selectivity of trawling nets so that the bycatch can escape.' Dr Harris has concluded from his research that it is worth assessing whether the survival rates would increase if these methods were applied.