For most reef fish species, small juveniles swim in the mid-water (rather than on the sea surface or floor) for several days to weeks before settling on the reef where they will live as adults. The behaviour of juvenile fish during this presettlement time is very important as it can influence their dispersal and even the distribution of their adult populations.

Understanding what happens to fish during this phase of their life is important for determining how to manage fisheries and marine parks.

Studying small fish in such a vast and dynamic environment can be very difficult so our knowledge on this early life stage of fishes is limited and only available for a small number of species. In order to obtain a window to the pelagic (mid-water) environment and see fish behaviour in their natural habitat, I used pelagic stereo-BRUVs Baited Remote Underwater stereo-Video Systems). This method uses bait to attract fish to the field of view of two stereo-video cameras so that species can be identified and individuals counted and accurately measured.

In 2012 we deployed 48 of these camera systems off the coast in Ningaloo Reef (Western Australia) in order to study pelagic fish, those that like tunas and mackerel swim in mid-water. Among almost 7,000 fish individuals recorded, we were surprised to observe many juvenile reef fish during their pelagic phase, prior to settlement on the reef.

Because it's so difficult to identify juvenile fish, I contacted Dr. Jeff Leis from the Australian Museum Research Institute for assistance with the identification. Among the captions and clips received, he was especially intrigued by the juvenile Purplespotted Bigeyes (Priacanthus tayenus) that were recorded schooling.

Bigeyes (or priacanthids) are a family of tropical marine fishes characterised by extremely large and bright eyes, a deep body, rough scales and red coloration in adults. Presettlement schooling behaviour had not yet been reported for Bigeyes and most aspects of their biology and behaviour in this early life-history stage were largely unknown.

Using stereo-video we were able to measure the fish (on average just 24 mm in length!) and estimate their swimming speed and the distance between individuals in the school. These results provide the first detailed information on the presettlement schooling behaviour of any bigeye fish and, they add to the limited but growing body of literature on the schooling behaviour of larval and juvenile reef fishes.Our study also shows the potential of stereo-video techniques to study the behaviour of juvenile fishes in a non-intrusive way.

Julia Santana Garcon
PhD Candidate at The University of Western Australia

More information:

Santana-Garcon, J., Leis, J.M., Newman, S.J., Harvey, E.S., 2014. Presettlement schooling behaviour of a priacanthid, the Purplespotted Bigeye Priacanthus tayenus (Priacanthidae: Teleostei). Environmental Biology of Fishes 97(3), 277-283.