An assessment of juvenile Atlantic cod Gadus morhua distribution and growth using diver operated stereo-video surveys

Abstract

While the relatively infamous Atlantic cod (Gadus morhua) fisheries have recovered in some areas, the cod fishery in the Firth of Clyde on the west coast of Scotland remains depleted. The role of juvenile fish survival in determining the future cohort sizes is important, yet the key habitats for juvenile marine fishes in the U.K. have received little attention. Many juvenile fish inhabit shallow coastal areas, where the monitoring of fish is not possible using fisheries dependent methods. Here, we conducted 31 stereo-video scuba transects during daylight hours from June to September 2013 within a proposed marine protected area (MPA) in the Firth of Clyde. More juvenile Atlantic cod Gadus morhua of fork length (LF) range 6–11 cm were observed in substrata containing mixed gravel, including maerl, than in boulder-cobble substrata with high algal cover, or sand with low density seagrass. Community composition was significantly different between substratum types. A decrease in G. morhua abundance was observed over the period of data collection. Over time, mean and variance in G. morhua LF increased, indicating multiple recruitment events. Protecting mixed gravel substrata could be a beneficial management measure to support the survival and recruitment of juvenile G. morhua; other substrata might be important at night given their diel migratory behaviour. Stereo-video cameras provide a useful non-destructive fisheries-independent method to monitor species abundance and length measurements.