Thesis

The role of discarding in the dynamics of the demersal fish community of the Firth of Clyde

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Awarding institution
  • University of Strathclyde
Date of award
  • 2025
Thesis identifier
  • T17187
Person Identifier (Local)
  • 201957769
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Department, School or Faculty
Abstract
  • The Firth of Clyde is one of the main grounds of the Scottish Nephrops (Nephrops norvegicus, or Norway lobster) trawl fishery. The fishery takes a bycatch of demersal fish that includes cod, haddock and whiting. Almost 100% of these fish are discarded due to trawlers not possessing licenses to land whitefish or because the fish are below the minimum conservation reference size. Even though targeted fishing for whitefish had effectively ceased by the early 2000s, there are still no signs of stock recovery in the Clyde. One hypothesis for the lack of recovery is that fish discards in the Nephrops trawl fishery are sufficient to maintain a high mortality rate on the stocks, despite the best efforts from the industry to minimize bycatch. To explore this hypothesis, quantities of cod, haddock and whiting discarded by the Nephrops fishery in the Clyde were estimated. Second, an age-structured stock assessment model was developed that uses scientific survey data and commercial fisheries data to estimate fishing mortality rates and stock biomass, and applied to the three main species of whitefish in the Clyde. Lastly, the populations were projected forward 30 years, under different mortality and recruitment scenarios to identify potential stock recovery. Results from the assessment model show historically high levels of mean fishing mortality for all three stocks and low levels of spawning biomass. Projections suggest that only haddock has some chance of recovery under current fishing conditions. For whiting and cod stocks, the projections show recovery only when substantial reductions in fishing mortality are made where the strength of recovery is dependent on the level of recruitment. Auto-correlated discard estimates were used as model inputs which may lead to stable but unrealistically precise model fits in the more recent years (after 2002). Further work on this more recent period that explores alternative discard estimates would be useful to characterise uncertainties in stock recovery. Prior to 2002, when the age-based assessment uses only survey data, fishing mortality rates are also estimated to be high and stock biomass declines. This is consistent with fishing mortality from bycatch in the Nephrops fishery being a likely significant factor in the lack of recovery of the cod and whiting stocks in the Clyde.
Advisor / supervisor
  • Cook, Robin M.
  • Heath, M. (Mike)
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DOI

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