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

Protecting juvenile fish habitat is of particular importance for their survival. However, in many cases fish habitat requirements are poorly understood. Stereo-video methods can provide non-destructive quantitative information on fish abundance and size in relation to their surrounding environment. Stereo-video SCUBA transects were conducted during daylight hours from June to September 2013 within a proposed Marine Protected Area (MPA) in the Firth of Clyde, west of Scotland. More juvenile Atlantic cod Gadus morhua of size 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 fish size 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.