Top-down pressure on small pelagic fish by eastern Australian salmon Arripis trutta; estimation of daily ration and annual prey consumption using multiple techniques

- Three independent techniques quantified Arripis trutta consumption in SE Australia.
- Annual food consumed/biomass (Q/B) ranged from 3.20 to 5.25.
- Annual prey consumption was estimated to be 42,200 t of mainly small pelagic fish.
- A. trutta may play an important top-down role in this pelagic ecosystem.
- This approach produced estimates with improved reliability and repeatability.

Ecosystem-level models that trace flows of mass and energy to quantify species interactions are important tools in whole ecosystem-based approaches to managing marine resources. A critical input parameter for such models is an estimate of the consumption rates and biomasses of prey consumed by fish populations. Here we estimate the annual prey consumption of an abundant coastal piscivore, Arripis trutta, which may exert substantial top-down control on its prey over its broad latitudinal range (28-43°S) in the coastal waters off south-eastern (SE) Australia. Three independent techniques were used to estimate annual prey consumption at two environmentally-relevant temperatures, 15 and 20 °C: experimental gastric evacuation experiments (coupled with stomach content analyses); bioenergetics modelling, and; empirical regression modelling. Each technique yielded a similar range of estimates with annual food consumed/biomass (Q/B) ranging from 3.20 to 4.02 at 15 °C and 4.23 to 5.25 at 20 °C. Using an estimated A. trutta stock biomass in SE Australia of 10,000 t, total mean annual prey consumption was estimated to be 42,200 t, consisting primarily (93%) of small zooplanktivorous teleosts, particularly Australian sardines Sardinops sagax (35%) and scads Trachurus spp. (30%). These results indicate that A. trutta do possess a top-down influence on the pelagic ecosystem of coastal SE Australia via consumption of considerable biomasses of small pelagic fish. However, despite the use of a maximum regional estimate of A. trutta abundance and the presence of at least two other ecologically similar predatory fish species with higher annual prey consumption rates in SE Australian coastal waters, A. trutta may remove only a small proportion (~ 15%) of the estimated spawning biomasses of its major prey species in this region annually. The multiple-technique approach used in this study has for the first time quantified the diet of A. trutta in SE Australia and provided broad and repeatable estimates of its annual prey consumption for use in ecosystem models designed to manage coastal fisheries resources in this region.