Diurnal and Tidal Variation in the Abundance of the Fish Fauna of a Temperate Tidal Mudflat

Mud and sand intertidal flats are a dominant feature of many estuaries and harbours, and may comprize a significant component of the total estuarine habitat available to fish. Information on what fish utilize such habitats, and when, is sparse. Small fish usage of a medium sized tidal flat ( 1 by 1 km) in Manukau Harbour, New Zealand has been quantified. Fish assemblages were sampled with respect to tide (high, low) and diurnal (night, day) phases, using beach seine and outrigger trawl (a form of push net). Catches were dominated by fish smaller than 100 mm, and included juveniles of larger species, and adults of smaller species. For beach seine, low tide samples had substantially higher species diversity and abundance than high tide samples. In addition, low tide night samples had greater species diversity and abundance than low tide day samples. High tide samples varied little between day and night. Data from the outrigger trawl, a novel method able to sample across the full tidal flats, showed that most fish species occurred across the full extent of the tidal flat at high tide. This indicates that fish move 1000 m or more per tidal cycle between the deep, low tide channels and the shoreward margin of the tidal flat. The fish assemblage was not concentrated along the tideline. Species that were expected to be less mobile (a goby and a triplefin), declined in abundance with increasing distance from the low tide channel. Overall, no particular tidal zone was more important than any other. Most species were present over both day and night periods, although catch rates were higher at night for many species. The degree to which gear avoidance may have contributed to this is not known.