Links between seabed type and quality are closely related to the abundance and size of young commercially fished species such as cod, haddock and whiting. A new study, led by the University of Glasgow and published today in Marine Ecology Progress Series, examines the abundance and size of these three types of commercial fish over the course of two years in the South Arran Nature Conservation Marine Protected Area in the Firth of Clyde. The Firth of Clyde was once an important area for a variety of fish such as haddock and cod and whiting. However, the populations of these fish have changed to the extent that they no longer support the fisheries they once did.

Previous studies have shown that there is a now a lower diversity of species, and the current fish population in the Clyde (while remaining high in biomass) is dominated by small whiting below the minimum landing size. In this study researchers analysed factors which could be affecting the species from recovering. They found that the biodiversity of the seabed affects the abundance and growth of juvenile demersal fish. The early stages of a fish’s life may depend on “nursery” areas where they can feed and shelter from predators. But until now we knew little about what makes a good nursery area for the most important commercial fish. Cod were found to be most abundant in shallow, sheltered areas where the seabed was composed of gravels and pebbles that contained maerl – a protected red algae which supports a particularly high diversity of species.

The study also reports that in 2014 while fewer cod were observed than in 2013, cod numbers remained fairly constant within gravel/pebble seabed types, whereas they reduced in other seabed types (such as sediment types with boulders and sand). Researchers believe that this hints at gravel/pebble seabed being important for the young of this species. However this type of seabed could be more vulnerable to disturbance, such as by fishing by dredges which can take place in these shallower areas. In contrast more haddock and whiting were observed over sheltered seabed made up of sand or mud. Interestingly, despite previous research which suggested that whiting did not have a preference for any particular nursery habitat, this study did find that sandy seabeds were favoured by them. Click here to read more