Fine-scale spatial and diel dynamics of zooplanktivorous fish on temperate rocky and artificial reefs

Abstract

Plankton is an important component of the food web in coastal reef ecosystems. Ocean currents subsidise local production by supplying plankton to resident and reef-associated planktivorous fishes. Measuring the fine-scale distribution of these schooling fishes provides insight into their habitat use and how they balance risk and reward while foraging for plankton. Maintaining their proximity to benthic structure can provide refuge from predation but may also limit foraging opportunities. We used a portable multibeam echosounder to survey schooling fish at 5 natural and 3 artificial reefs, during day and night and under different current conditions. We isolated midwater acoustic targets and used generalised linear models to explain the distribution of schools as a function of current exposure, distance from structure and seafloor complexity. We also isolated individual schools and used generalised least squares to model how school characteristics differed between night and day, using spatial metrics of school area, perimeter length and height above the seafloor. Modelling revealed that the occurrence of schools was almost twice as likely upstream versus downstream of artificial reefs, although distance to reef structure was the main influence. School occurrence was also more likely on artificial versus natural reefs. Schools at artificial reefs exhibited greater volume and areal coverage at night, and during the day they rose higher in the water column while aggregating more closely around the reef. These findings suggest that artificial and natural reefs featuring enhanced vertical relief and direct exposure to the prevailing current are preferred habitats for planktivorous fish.