Are there really few piscivorous fishes in shallow estuarine habitats?

Abstract

Shallow estuarine habitats throughout the world are recognised as valuable nursery grounds for juvenile fishes. It is often suggested that this is due, at least in part, to the occurrence of low numbers of piscivorous fishes in these habitats. This idea is not confined to particular habitat types, but has been used in reference to a range of habitats, including both vegetated and unvegetated areas. I evaluate the proposition that there are few piscivorous fishes in shallow estuarine habitats, and suggest that the proposition is ill-defined, with little unequivocal supporting or contradictory evidence. To make any sense, the proposition must be evaluated relative to some other habitat, something often not addressed in previous studies. Additionally, including the full range of potentially piscivorous fishes is important if the role of predation is to be understood. Most studies have considered only large piscivorous fishes. This is unfortunate when the proposition logically relates to an advantage for small juvenile fishes. There is considerable evidence that, in addition to large piscivorous fishes, small juvenile fishes are extensively preyed upon both by juveniles of the large piscivorous species and by a range of other small species that include a large component of fish in their diets. In addition, many of the prey species may, themselves, switch opportunistically to piscivorous feeding. Not only are small piscivores numerous, but because of their size they are much more likely to access shallow waters than large piscivores. Even when the full range of piscivorous fishes are considered, it is not clear that the question of the abundance of piscivores in shallow waters should be the real focus of interest. Rather the crucial question is whether or not predator-induced mortality is reduced in shallow estuarine habitats.