FAO FISHERIES TECHNICAL PAPER 478

FAO FISHERIES TECHNICAL PAPER 478

Mortality of fish escaping trawl gears

by
Petri Suuronen
Finnish Game and Fisheries Research Institute
Helsinki, Finland
   
FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS
Rome, 2005
 

Table of Contents


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ISSN 0429-9345
ISBN 92-5-105323-5

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Contents

Preparation of this document

Abstract

Abbreviations, definitions and terms

Introduction

Major factors causing stress, injury and mortality of fish escaping from trawl codends

Species-specific variation in escape mortalities
Effects of mesh size and shape
Effects of fish size
Causes and effects of skin injury: potential healing
Effects of swimming exhaustion
Effects of codend catch
Effects of sea state
Effects of escape during haul back
Effects of water temperature
Effects of time and ambient light
Effects of tow duration
Effects of sex
Predation mortality
Effects of repeated escape and the potential role of learning
Effects of collisions with the ground gear
Cumulative effects
Conclusions

Major factors causing mortality of trawl-caught and discarded fish

Main factors affecting the fate of discarded fish
Presence of gas-filled organs and overall fragility
On-deck exposure time and air temperature
Water temperature
Tow duration and catch quantity
Mortality of trawl-caught and discarded invertebrates
Conclusions

Mortality associated with fisheries other than trawling

Mortality associated with being hooked and released
Mortality associated with gillnetting
Mortality associated with lost pots
Mortality associated with purse seine operations
Mortality associated with (scallop) dredging
Conclusions

Methodologies and techniques used in escape survival experiments: general guidelines for field experiments

Potential flaws in methodology
Cover effect and sampling duration
Methods for collecting escapees
Collection strategies
Duration of sample collection and size of sample
Construction of the codend cover
Transfer of escapees to cage site
Location and design of monitoring cages
Duration of monitoring
Control samples - what they should represent
Catching adequate controls
Tag and recapture methodology
Laboratory studies
Investigating fish capture behaviour in the field
Conclusions

Improvement of escape survival by technical and operational means

Reduce the time that fish swim with the gear
Allow fish to escape before they reach the codend
Facilitate rapid, voluntary escape
Avoid excessive catch sizes and prevent debris from entering the codend
Use escape panels, grids and bycatch reduction devices
Use selective ground gear and cutback headrope
Avoid grounds where young fish or non-target species are abundant
Conclusions

Estimating the magnitude of escape mortality and its impact on fisheries management

Incorporating mortality in assessment and management processes: case studies

Baltic cod demersal trawl fishery
Northeast Arctic cod trawl fishery
Northern Baltic pelagic herring fishery
Hauraki Gulf snapper fishery in New Zealand
North Sea haddock
North Sea mixed-species trawl fishery
Short-term effects: an example from the Baltic cod fishery

Other components of unaccounted fish mortality
Conclusions

Conclusion

References

Annex 1 - Species and their Latin names mentioned in this study

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