An improved method for studying survival of fish that escape through meshes of trawl codends

Abstract

The objective of this study is to develop and test a practical field method for measuring survival of fish that pass through trawl cod-end meshes under commercial fishing conditions. This method permits the collection of escaping fish during any moment of a haul. Hence, survival can be assessed for short and long tows, small and large catch quantities, and for any desired depth using this technique. The method involves the use of a cage (with front and rear gates) that is attached to the terminal end of a cod-end cover. The front and rear gates can be held open during the tow, allowing cod-end escapees to pass freely through the cage and into the open sea. The rear gate is closed using a pre-set timer when a sample of escapees is needed. After a suitable sampling duration, pre-set timers activate the closure of the front gate and trigger the release of the cage from the cover. The cage is then left on the sea bed at the place of release for subsequent monitoring of fish condition (e.g., by divers). Trials were conducted on a demersal trawl vessel in the Baltic Sea in 1996–1997. We found that the method performed well under all circumstances for Baltic cod ( Gadus morhua L.) that escaped through trawl cod-end meshes. The method itself was not detrimental to the escapees.