Shoaling behaviour in the pygmy halfbeak Dermogenys collettei (Beloniformes Zenarchopteridae): Comparing populations from contrasting predation regimes

Exotic species—especially predators—are a potential threat to native species communities and ecosystems worldwide. Introduced exotic species may cause changes in anti-predator behaviour of prey species, thus affecting prey individuals’ time allocations for other crucial behaviours such as feeding and locating mates. To test this hypothesis, we investigated shoaling behaviour of the pygmy halfbeak, Dermogenys collettei, comparing populations with different degrees of exposure to an exotic predator (Cichla orinocensis). Contrary to predictions, halfbeaks exhibited shoaling behaviour in a low predation, forest stream habitat but not in a high predation, more open stream habitat. We argue that behavioural differences are likely driven by competition for resources leading to reduced shoaling, highlighting how costs and benefits of group-living affect population-level shoaling tendencies. Dermogenys collettei also did not increase shoaling behaviour when exposed to C. orinocensis, suggesting that adaptive behavioural responses to immediate predation risk are absent. We discuss the implications of our results for the conservation of small native freshwater fishes in Singapore and Malaysia and identify further areas of research on predator-prey interactions between exotic predators and indigenous aquatic fauna.