Predator driven diel variation in abundance and behaviour of fish in deep and shallow habitats of an estuary

Our traditional understanding of the behaviour of large predatory fish and their smaller prey in estuarine ecosystems is often restricted by different gear types and visibility. In this study we determined the diel distribution and inferred movements of fish in an estuary in shallow and deep habitats (<1 m and 4 m deep respectively), using an unbaited acoustic camera (DIDSON). Baitfish (<100 mm TL) formed small and large shoals during the day in both shallow and deep habitats, compared to loose aggregations during the night or when they were inactive and not observed. Three larger size classes of fish (small, 100–300 mm Total Length (TL); medium, 301–500 mm TL and large >500 mm TL) were also more abundant during the day, likely due to general higher activity. This coincided with predatory activity with attacks by larger fish (301–500 mm and >500 mm) witnessed during the day but not at night. This heightened activity is the likely cause for changes in the schooling behaviour of the baitfish. The proportion of medium and large fish in the shallow habitat at night increased by over 50% as they moved from deeper areas of the estuary, showing the abundance of large predators in shallow water can be related to diel period. This highlights the pervasive top down influence even small numbers of predators can exert on the behaviour and distribution of estuarine fish assemblages.