Hind-casting the quantity and composition of discards by mixed demersal fisheries in the North Sea.

Michael R Heath,Robin M Cook,Shing Yip Lee

doi:10.1371/journal.pone.0117078

Abstract

Many commercial fisheries seek to maximise the economic value of the catch that they bring ashore and market for human consumption by discarding undersize or low value fish. Information on the quantity, size and species composition of discarded fish is vital for stock assessments and for devising legislation to minimise the practice. However, except for a few major species, data are usually extremely sparse and reliant on observers aboard a small sample of fishing vessels. Expanding these data to estimate total regional discards is highly problematic. Here, we develop a method for utilising additional information from scientific trawl surveys to model the quantities of fish discarded by the commercial fisheries. As a case-study, we apply the model to the North Sea over the period 1978-2011, and show a long-term decline in the overall quantity of fish discarded, but an increase in the proportion of catch which is thrown away. The composition of discarded catch has shifted from predominantly (∼80%) roundfish, to >50% flatfish. Undersized plaice constitute the largest single fraction of discards, unchanged from the beginning of the 20th century. Overall, around 60% of discarded fish are rejected on the basis of size rather than for reasons of species value or quota restrictions. The analysis shows that much more information can be gained on discarding by utilising additional sources of data rather than relying solely on information gathered by observers. In addition, it is clear that reducing fishing intensity and rebuilding stocks is likely to be more effective at reducing discards in the long term, than any technical legislation to outlaw the practice in the short term.

Highlights

Most commercial fisheries supplying fish for human consumption will seek to capture species which are economically valuable and in demand by consumers
We first identified the subset of demersal species categories in the International Bottom Trawl Survey (IBTS) survey data which corresponded with species categories in the ICES/FAO landings data
All hauls located in the ICES North Sea regions IVa, IVb and IVc combined were extracted from the data set, and the annual averages of number density per 1cm length class calculated for each taxonomic class and year

Summary

Introduction

Most commercial fisheries supplying fish for human consumption will seek to capture species which are economically valuable and in demand by consumers. The catch often includes fish, benthos, macrophytes, and occasionally marine mammals and seabirds which are unsuitable for consumption and/or have little or no market value. This material is sorted from the catch and discarded at sea [1,2,3,4,5,6]. Regarding the fish components of the catch, modern-day discarding decisions made by fishers depend on a complex interaction between. Hind-Casting Fishery Discards in the North Sea

Objectives

Methods

Results

Conclusion