Suspected Key Predators of Long-Spined Urchins Fail to Show Signs of Significant Predation in Gut Contents Analyses

Abstract Worldwide, fish predation by labrid and sparid species is thought to regulate urchin populations. In South East (SE) Australia, Centrostephanus rodgersii (long-spined urchin) has become a focal species due to its recent incursion south, where it has created new urchin barrens. It has since been suggested that reduced large fish predators across the native C. rodgersii range of New South Wales (NSW) may have caused extensive barrens there. However, if large fish eat urchins frequently, then urchins should occur more frequently in fuller guts. Predation patterns may also vary with location and sex due to prey availability and fish behaviour. To test these hypotheses, we analysed gut contents of 39 Achoerodus viridis (Family Labridae; eastern blue groper) and 111 Chrysophrys auratus (Family Sparidae; pink snapper) across temperate NSW. For both A. viridis and C. auratus , gastropods (87.2%, 46.8%), crustaceans (58.9%, 46.0%) and macroalgae (66%, 69.4%) occurred more frequently than urchins (30.7%, 13.5%) which did not include C. rodgersii . We found low occurrences of C. rodgersii in A. viridis (5.1%) and C. auratus (2.7%) guts, with C. rodgersii occurring in only one medium (405 mm standard length, SL) and one large (725 mm SL) A. viridis and three small C. auratus (330 mm to 390 mm SL) despite sampling fishes up to 782 mm SL. Primarily, Heliocidaris spp. urchins were encountered. There were no consistent trends between gut occurrence of urchins and fish size, sex or location. Fuller guts were positively correlated with urchin consumption for C. auratus but not A. viridis . Our results suggest predation by these species on C. rodgersii in SE Australia is limited, but there are frequent predation events on H. erythrogramma .