Diel fish movements in the littoral zone of a temporarily closed South African estuary

The importance of shallow littoral habitat is a long standing paradigm of estuarine ecology, particularly in facilitating the nursery role they play for many fish. Diel cycles have been linked to the redistribution of fish between habitats and may also influence the movement of various sized fish within littoral zone of estuaries. Fine scale temporal (hourly) movements of four size classes of fish (< 100 mm, 100–300 mm, 301–500 mm and > 500 mm total length) onto a shallow (< 0.7 m) littoral zone within a South African estuary was examined using a duel frequency identification sonar (DIDSON) continuously over 24 h periods. Fish within the size classes of 100–300 mm and 301–500 mm were more abundant in the littoral zone during twilight and at night compared to day, whilst the largest size class (> 501 mm) showed no difference in abundance among these three diel periods. The strongest patterns were observed within the small fish (< 100 mm), which were abundant in the littoral zone during the day but never observed in this habitat after dark. Daily commuting onto and off the shallow littoral zone occurred within tight time periods (< 45 min), and were synchronous with sunrise and sunset respectively. It is hypothesized that these movements are linked to the presence/absence of zooplankton (feeding ecology) and predator avoidance. Large predatory fish up to 1010 mm were also observed on the littoral zone, and whilst rare, indicate very large fish will swim into shallow estuarine waters. Our results support the theory that littoral zone habitats harbour high abundances of small fish and may provide refuge from large piscivorous predators in deeper waters. However, observations of the diel movement behaviour of small fishes indicate that the use of littoral habitats is more complicated than often suggested.

► We used a DIDSON to investigate diel patterns of fish abundance in a littoral habitat.
► There was differential abundance of size classes among day, night and twilight.
► Commuting synchronised with twilight was observed by small shoaling fish.
► Our research shows the current littoral paradigm for fish is perhaps too simplistic.