Fish behaviour in response to tidal variability and internal waves over a shelf sea bank

The availability of fish to top mobile predators such as seabirds and fishermen and to fisheries surveys depends on their schooling behaviour. In temperate coastal ecosystems, tide is the main driver of ecosystem dynamics. To investigate the mechanisms linking fish schooling behaviour to bio-physical oceanography in a tidal ecosystem, we combined novel fine scale survey data of fish behaviour using fisheries acoustics with co-located oceanographic data from contrasting topographical locations. The schooling behaviour of pelagic fish was examined in relation to bio-physical oceanography by carrying out repeated surveys of the same locations over the daily tidal cycle at spring and neap tides over a small bank (Jones Bank) and a nearby flat region. Different fish species were found over the bank and in the flat region 14 km to the south-east, and fish schooling behaviour varied similarly. Results showed that the bank played an important role in influencing fish distribution and behaviour, with shallow pelagic schools concentrated over the bank and closer to the surface at times of high internal wave activity. Behaviour of schools identified close to the bottom was partially influenced by oceanographic variables that drove internal waves over the bank slopes, but were most strongly influenced by the spring-neap cycle. These deep schools were larger, closer to the bottom and less dense at neap tides. Identifying some the of the bio-physical variables which drive changes in fish distribution and behaviour will help quantify fish catchability and assist in the design and interpretation of better targeted surveys for top predators and prey fish species.