An assessment of cod in the Firth of Clyde

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Cook, Robin and Adao, Ana and Heath, Mike (2025) An assessment of cod in the Firth of Clyde. Other. SSRN. (https://doi.org/10.2139/ssrn.5435777)

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Abstract

1.At present Clyde cod are assessed by ICES as part of the Northwestern substock that includes part of the North Sea and all of the West of Scotland. Clyde cod are, however, considered to be a substock separate from the West of Scotland and may have different dynamics. This document reports an assessment of cod in the Clyde as a single stock unit. 2.Data available include a time series of total landings and discards from 1985-2019 based on sampling by the Scottish Government’s Marine Directorate. These data include age compositions for some years. In addition, research vessel surveys provide an index of abundance as well age compositions for the whole period. The level of sampling is generally low and there are gaps in some time series, but data are sufficient to perform a full stock assessment. 3.An age structured stock assessment model is developed that can estimate stock biomass and fishing mortality despite the data gaps. Sensitivity analysis and retrospective analysis suggest the assessment is robust to a variety of assumptions about the data and the fishery. 4.A surplus production model that makes different assumptions about the stock population dynamics is developed and applied to the catch and survey data. The results from this model are very similar to the age structured model which suggests the results are not sensitive to model structure. 5.The assessment estimates very high rates of fishing mortality and a substantial decline in spawning stock biomass. Currently, the high rates of fishing are attributable to bycatch of cod in the Nephrops fishery. 6.Estimates of Maximum Sustainable Yield (MSY) derived from the assessment show that the spawning stock biomass is less than 10% of the biomass expected at MSY. The stock appears to have collapsed around the year 2000. Fishing mortality from 2001-2014 was at or exceeded values that would be expected to cause stock collapse (Fcrash). Current fishing mortality (as of 2019) is close to Fcrash. 7. Investigating the productivity of the stock (the number of recruits produced per tonne of spawning biomass) shows that the current productivity is within the historical range and above rates in the mid-1980s. It does not appear that stock decline is the result of impaired productivity. 8. The most likely cause of the failure of the stock to recover is the high rate of fishing resulting from the bycatch in the Nephrops trawl fishery. Stock recovery needs to focus on measures to reduce bycatch, perhaps by changing gear design. Enhancing productivity through the protection of spawning behaviour is unlikely to be successful at current rates of exploitation.

ORCID iDs

Cook, Robin ORCID logoORCID: https://orcid.org/0000-0002-9604-0204, Adao, Ana and Heath, Mike ORCID logoORCID: https://orcid.org/0000-0001-6602-3107;
CORE (COnnecting REpositories)