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Newsletter February 2015

How many flatfish survive after being discarded?

A staggering amount of commercially-caught fish is being thrown overboard. Some say that all of those discarded fish are either dead before they hit the water or they die soon after, victims of predation or injury. But others argue that some of those species are strong enough to survive after being discarded and live long enough to reproduce. The European Common Fisheries Policy was recently reformed and will now phase in a ban on discards, meaning that fishers will have to land everything they catch. The idea behind the ban is to stimulate more selective fishing techniques, because it will be in the interest of the fisher to only catch the most valuable fish. However, by landing everything, this ban risks killing more fish than before. If a juvenile fish lives long enough after being discarded to spawn new fish, it should be given that opportunity. For this reason, the discard policy provides an important exception: if a certain species can be scientifically proven to have a high chance of survival, fish of that species should be thrown back after catch. Researchers at the Institute for Agricultural and Fisheries Research (or ILVO) in Ostend, Belgium are testing the most commercially important species of flatfish - plaice, sole and dab – for their likelihood of survival.

To study the fate of discarded fish, ILVO scientists have opted for two approaches. The first one monitors the fate of flatfish by observing whether they are dead or alive, after onboard sorting (immediate mortality assessment) and also after a number of days (short-term mortality assessment). For these, a sample of flatfish retained during the sorting process aboard of commercial Belgian beam trawlers will be used. From this fraction, the ratio of dead and live fish will be determined shortly after leaving the sorting conveyor. Subsequently, some of the live fish will be placed for at least three days inside a holding shelf with plastic containers onboard the vessel. This shelf is made out of steel and holds two stacks of 8 containers which can be pulled out like drawers. Each container is continuously supplied with fresh seawater to maintain favourable conditions for the fish while aboard the vessel. At regular intervals the fish will be checked for their survival. During the catch and monitoring process, measurements will be made of important technical, biological and environmental variables that could potentially influence the vitality of the fish.

The second approach seeks to develop a reliable proxy for survival based on the Reflex Action Mortality Predictor method (RAMP). This method scores the presence or absence of reflex responses and injuries in flatfish as an indicator of their health. Possibly, those fish that are stressed and damaged and do not respond to these reflex tests, will be also the ones to be more likely to die. If a strong correlation exists between survival and the RAMP score, it will be possible to predict flatfish survival simply by scoring RAMP without actually investing time and money in monitoring their fate in captivity. Both methods will be applied during 15 commercial trips.

This project will generate estimates of discard survival for flatfish, mainly of plaice, but also sole and dab. The results will be included in regional discard plans which may support application to exclude certain species from the discard ban.

Project: Overleving
Funding: De Europese Commissie (EVF – Europees Visserijfonds), De Vlaamse overheid (FIVA – Financieringsinstrument voor de Vlaamse Visserij- en Aquacultuursector) & ILVO – Instituut voor Landbouw- en Visserijonderzoek
Term: 01/03/2014 – 31/10/2015
Partners: The Rederscentrale (boat owners’ association) (promotor) + five Belgian fishing vessels

Contact: Ruben Theunynck, Sebastian Uhlmann

container system installed container system

Photo left: Container system with transparent lids without the front sliding doors and insulation wall - Photo left: Installed containeer system aboard a fishing boat (with sliding doors and insulation layer).