Observations of discards in the Scottish mixed demersal trawl fishery

The Marine Laboratory in Aberdeen (now part of Marine Scotland Science) has operated a scientific observer programme in the Scottish mixed demersal trawl fishery for more than 20 years. Discards of the main demersal species (cod, haddock, whiting, and saithe) have been sampled according to a stratified design based on gear, area, and month, with quota sampling used to select vessels. The aim of the programme is to estimate the quantity (at age) of the discarded component of the catch, to contribute to estimates of total catch for these species so that assessments of each stock can be made. Trends of discards from this observer programme from 1987 to 2009 are reported. Notable features include the sudden practice of discarding marketable cod in 2007. This was due to a number of factors including, paradoxically, improvements in compliance measures which meant that fish could no longer be landed illegally. Methods for raising the sampled discards to the level of the fleet are also explored and discussed in relation to changes in discarding patterns. These are particularly relevant at a time when, in northern Europe at least, there is a move towards standardizing methods for estimating total quantities of discards.