Evaluating potential competitive interactions following stocking through trophic niche breadth

Abstract

Responsible fisheries enhancement should include comprehensive monitoring programs to detect ecological effects of fish releases and support adaptive management. Competition for food and shelter is one such impact, and density-dependent effects may occur if release numbers are above levels that the ecosystem can support. Herein we present a novel application of trophic niche breadth estimated from stable isotope data to examine potential competitive interactions arising from eastern king prawn (Melicertus plebejus) stocking. Eastern king prawn and potential competitors (Macrobrachium intermedium, Atherinosoma microstoma, Leptatherina presbyteroides, Ambassis jacksoniensis and Gobiopterus semivestitus) were sampled from two stocked and two reference estuaries, before and after stocking. Stable isotope ratios were measured and used to model trophic overlap and trophic niche breadth. A statistically significant increase in niche breadth was evident for two competing species in one stocked estuary, and one competing species in another. M. intermedium, the species that had the greatest trophic overlap with eastern king prawn, did not show any change in niche breadth. The potential for competition and the role of environmental variability (including rainfall and run-off, mouth opening regime and seasonal changes) in driving the patterns observed are discussed. This approach shows promise for monitoring competitive effects arising from aquaculture-based fisheries enhancements in the future.