Abstract:
Baited underwater video and underwater census surveys determined that half of all observed fish species in a temperate reef system were active to some extent during both day and night. Species most active at night were considered nocturnal and constituted 35% of the total fish fauna found. The most commonly observed nocturnal predators under both methodologies were the dwarf scorpionfish, Scorpaena papillosa, and the southern bastard cod, Pseudophycis barbata. However, the frequency of particular nocturnal fish species differed between baited underwater video and during underwater census surveys. This was largely attributed to the olfactory related and cryptic behaviour of each species. Nocturnal predators consumed significantly less prey groups than diurnal predators. The nocturnal group consumed mainly fish and mobile invertebrates, especially crustaceans. Predators of spiny lobsters were identified using a novel photographic tethering device. Predation was significantly higher during the day (76%) than during the night (33%) corresponding to the greater number of diurnal predators at the study site. The search strategies of nocturnal predators were studied in the laboratory. The dwarf scorpionfish, Scorpaena papillosa, exhibited saltatory search whereas the southern bastard cod, Pseudophycis barbata, exhibited cruise search. Both fish species were lateral line predators and responded to the respiratory currents of crabs, however, the southern bastard cod also utilised a chemosensory component in prey search. This resulted in the southern bastard cod responding to both live and dead crabs in contrast to the dwarf scorpionfish which responded only to live crabs. The dwarf scorpionfish paused while searching for prey reducing the self-generated noise produced by its own swimming movements and this was reflected in the larger orientation distances observed in this species when compared with the southern bastard cod. However, by pausing a reduction in movement rate was also observed and this reduced encounter rates with prey when compared with the southern bastard cod. The dwarf scorpionfish displayed looping behaviour while searching for prey compared to the more linear movements of the southern bastard cod. The saltatory search strategy seemed more suited to a predator searching for dense, cryptic prey while the cruise search strategy was more beneficial when searching for low density conspicuous prey. The saltatory and cruise strategies adopted by nocturnal predators provide similar advantages and disadvantages as they do for visual predators, but for quite different mechanistic reasons.