A study of fish discarded by Scottish demersal fishing vessels
This study focuses on fish discards generated by demersal vessels fishing in Scottish waters (North Sea and West of Scotland). The analysis is based on data collected by scientific observers onboard commercial vessels during the past two decades (1975 - 1993). Chapter 1 introduces the problem of discarding, describes its implications for fisheries and the marine environment, and gives a brief overview of the sampling scheme. Chapter 2 uses generalised additive models (GAMs) and regression trees to investigate the main reasons that lead Scottish fishers to discard commercially important species (haddock - Melanogrammus aeglefinus, whiting - Merlangius merlangus and cod - Gadus morhua). Different discarding patterns are observed in inshore and offshore areas, with vessels fishing offshore discarding larger fish, irrespective of gear type. In inshore areas, discarding decisions are mainly driven by legal landing size, whereas offshore, high grading decisions due to market pressures and, possibly, quota limitations prevail. Chapter 3 explores the performance of several estimators of total discards and their variances in a simulated population of fishing trips. The simulated population is generated from the residuals of temporal models fitted to the catch and discards observations from the trips sampled under the Scottish discards monitoring scheme. The estimator currently applied to obtain annual estimates of total discards in Scotland is shown to be biased and imprecise, whereas several other estimators perform better under limited sampling coverage. The best estimators from chapter 3 are used in chapter 4 to estimate the total discards and their variances for species that were regularly discarded by Scottish vessels in the period 1988 - 1993. Overall, it is estimated that Scottish vessels generate annually around 100,000 tonnes of demersal fish discards in the North Sea and 30,000 tonnes in the West of Scotland. This suggests that close to 40% of the total demersal catch of Scottish vessels is annually wasted through discarding.