Abstract
High-grading is the decision by fishers to discard fish of low value that allows them to land more valuable fish. A literature review showed high-grading is reported in commercial and non-commercial fisheries around the world, although the number of observations is small. High-grading occurs in fisheries that are restricted to land their total catch due to management, market or physical constraints. Using the mixed flatfish fishery as a model system, a dynamic state variable model simulation showed that high-grading of certain grades occurs throughout the year when their ex-vessel price is low. High-grading increases with the degree of quota restriction, while the level of over-quota discarding is unrelated to the quota level. The size composition of the high-graded catch differs from the landed catch. Due to the differences in the seasonal variation in size specific ex-vessel price, the effect of quota restrictions on the size composition of the discarded catch is non-linear. High-grading is difficult to detect for the fishery inspection as it occurs on-board during the short period when the catch is processed. We conclude that high-grading is under-reported in fish stocks managed by restrictive quota, undermining the quality of stock assessments and sustainable management of exploited fish stocks.
Highlights
Many fisheries around the world capture fish that are subsequently discarded back into the sea (Kelleher 2005)
Whiting were discarded at markedly longer lengths than the established minimum landing size (MLS) (23 cm) before 2000 and discard ogives obtained for haddock and megrim are consistent with high-grading for some areas and fleets
L50 is the estimated length at which 50 % of the caught fish is discarded * These papers explicitly mention the existence of high-grading in the fishery in the title or abstract ** These are references in papers resulting from the Scopus query
Summary
Many fisheries around the world capture fish that are subsequently discarded back into the sea (Kelleher 2005). Since high-grading affects the age and length structure of the landed fish, the resulting stock assessment may lose accuracy in estimating the mortality and stock size, undermining the credibility of fisheries management (McCay 1995; Daan 1997; Rijnsdorp et al 2007). We present a case study that applies a behavioural model to study high-grading decisions of fishers in a Dutch beam trawl mixed fishery under individual quota management (Gillis et al 2008). This fishery is known for discarding marketable fish (Quirijns et al 2008; Poos et al 2010). The results allow us to forecast over-quota discarding and high-grading and explore the effect on age composition and the implications for stock assessment
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