Refugees or ravenous predators: detecting predation on new recruits to tropical estuarine nurseries

Abstract

Many of the most abundant small and juvenile fishes within shallow water estuarine nursery habitats consume other fish to some degree but have rarely been considered as potentially important predators in the functioning of these systems because of the low (\50%) average occurrence of fish in their diets. Predation by abundant minor piscivores on new recruits when they first enter the nursery may make a significant contribution to the predation mortality of this critical life-history stage. To determine the potential importance of minor piscivores as predators on new recruits, temporal patterns in the diets of 15 common species of minor piscivores were examined and related to the abundance of new recruits (B20 mm FL) in biweekly seine samples over 13 months in shallow (\1.5 m) sandy habitats in the Ross River estuary in north-eastern Queensland, Australia. The high spatial patchiness of new recruits made it difficult to correlate their abundance with their consumption by minor piscivores, and there was no relationship detected between the abundance of new recruits and the occurrence of fish in the diets of minor piscivores.To gain broader insight into spatio-temporal patterns in the consumption of fish prey by minor piscivores, we utilised a collection of fishes sampled during various studies over 6 years from 17 estuaries in the region to examine the diets of[3500 individuals from 20 spp. of minor piscivores. Patterns in the consumption of fish prey by these minor piscivores, especially the highly abundant sparids, sillaginids and ambassids, revealed that the low average occurrence of fish in their diet greatly underestimated the predation pressure imposed by these on fish prey at particular locations and times. For most sampling occasions and locations few minor piscivores consumed fish prey (consumed by 0% of individuals examined), while occasionally a large proportion of individuals within a taxon did so (50-100% of individuals consumed fish prey). Often at such times/locations multiple species of minor piscivores simultaneously preyed heavily on fish. When minor piscivores consumed fish, they preyed mainly on small new recruits. Because many of these minor piscivores are relatively recent recruits, many of the small and juvenile fishes believed to gain refuge in shallow estuarine nurseries may themselves be important predators on fish subsequently recruiting to these habitats, and so potentially play a significant role in structuring estuarine fish faunas and the functioning of shallow water nurseries.

True Value of Estuarine and Coastal Nurseries for Fish: Incorporating Complexity and Dynamics

“…Predation is usually the largest source of mortality for juvenile fish (Harter and Heck 2006), so high risk areas, such as transition zones between refuge and feeding areas (Hammerschlag et al 2010) may represent ecological bottlenecks. For example, predatory activity at these locations can control the supply of recruits to nursery grounds (MacGregor and Houde 1994, Brown et al 2004) and the supply of juveniles from nursery grounds to adult populations (Yurk andTrites 2000, Friedland et al 2012), and so provide the opportunity for predatory control of nursery populations (Baker and Sheaves 2009b). In addition, these refuge-food acquisition trade-offs vary between species (Camp et al 2011) meaning that nursery ground values may differ markedly depending on the species involved.…”

Nursery Function Drives Temporal Patterns in Fish Assemblage Structure in Four Tropical Estuaries

“…At the same time, these periods of high abundance would provide large numbers of prey fish that could support an abundance of predators (e.g. Juanes and Conover 1995;Baker and Sheaves, 2009a), but predator abundances would be limited by low numbers of prey fish when few offshore spawners were present. This could be offset by predators switching to feed on estuary residents, alternate prey, or moving to other habitats, but any of these responses again adds considerable complexity to overall community dynamics (Baker and Sheaves 2009b).…”

Predators and habitat association of post‐settlement snapper (Chrysophrys auratus)

“…Despite the lack of post‐settlement snapper, set netting did confirm the presence of some piscivorous fish species, specifically kahawai and hammerhead shark. Further investigation of the diet of these two species may be warranted, although obtaining appropriate sample sizes may prove challenging considering that catch rates are likely to be low and that the consumption of small fishes by these predators can be extremely sporadic (Baker & Sheaves, 2009b ). As such, a much more extensive and temporally replicated set netting effort would likely be required to identify important predators.…”