How has gadoid biomass in the Clyde Sea been sustained

Start date: October 2013
Stipend: £13,590

What is sustaining gadoid biomass in the Clyde Sea?

Worldwide many fish stocks are below levels that are considered safe and this reduced abundance is often linked to a decline in the average size of individuals. Understanding how to return fish stocks and size compositions to healthy levels is of great scientific interest. Unfortunately simply removing the original fishing pressure does not always achieve this aim and the reasons for this failure are not clear. A key problem is that reducing fishing in one region may not reduce the overall mortality fish are exposed to because they can move over large distances during their life cycle and offspring from a carefully managed area might subsequently swim off into areas where they are caught.

This PhD will study demeral fish in the Firth of Clyde on the west coast of Scotland. This is a large but semi-enclosed sea area with a long and well-documented history of fishing. Following intense fishing pressure from the 1960s to 1980s, the species diversity and size composition of demersal fish in the Clyde changed substantially (Heath and Speirs 2011). In particular, stocks of the main commercial demersal species; cod, haddock and whiting collapsed. Fishing pressure was then severely reduced. Fish biomass is now greatly increased, but the majority of these are very young whiting (1 year or less), with few haddock or cod of any size and very few large fish. The Clyde’s ecosystem has fundamentally changed. It is vital to understand why this has happened so that appropriate management can be put in place.

We suspect that an important factor is the spawning and dispersal of the demersal fish and it is vital to know whether the lack of large fish in the Clyde is due to them leaving for other areas of the west coast or because they are being killed before reaching maturity.

The main approach to be used will be the analysis of gadoid otolith chemistry. Otoliths are calcareous structures in the ear of fish that accrete daily and annual increments. The chemistry of these increments reflects the chemistry of the surrounding water. Distinct differences in the otolith chemistry of young whiting (Tobin et al., 2010) and cod (Wright et al., 2006) have been found off the west coast of Scotland making it possible to trace the origin of these species in the Clyde and the contribution of this region to spawning further afield. Together with survey data this method will help build a full picture of the life history of commercial demersal fish populations in the Clyde and should provide an important demonstration of the way in which fish dispersal affects fisheries management.

The project will be jointly supervised between the University of Glasgow and Marine Scotland Science.

Heath, M.R. and D.C. Speirs (2011). Proceedings of the Royal Society, B, doi, 10.1098/rspb.2011.1015.
Tobin, D. et al. (2010). Marine Biology 157(5): 1063-1073.
Wright, P.J. et al. (2006). Journal of Fish Biology 69: 181-199.