Distribution, abundance, growth and food of freshwater eels (Anguilla spp.) in the Douglas River, Tasmania

Abstract

Population structure, growth and diet of the freshwater eels A. reinhardtii Steindachner and A. australis australis Richardson were studied in the Douglas River, eastern Tasmania. Fish population structure was assessed by electrofishing, and efficiency of capture was determined for 'eels' and 'other fish'. The other fishes encountered during quantitative sampling were the native Galaxias maculatus (Jenyns), G. truttaceus Valenciennes, Pseudaphritis urvillii (Cuvier & Valenciennes), Favonigobius tamarensis Johnston, and the introduced brown trout, Salmo trutta L. A. reinhardtii dominated fish biomass in the lower freshwater reaches; an estimate of 115 gm-2, recorded at the lowest sampling site, is the highest eel biomass recorded in a Tasmanian stream. A decline in the number and weight of A. reinhardtii with increased distance from the sea was related to avoidance of low water temperatures in winter and/or a parallel decline in abundance of suitable forage fish. Measurements of marginal growth increments in burnt eel otoliths indicated the formation of annual growth rings in both species. Age groups 0: 1 and 2 of A. reinhardtii were poorly represented; this was attributed to a residence in the estuary or deep pools of the river, rather than to failure in recruitment. Growth rate of A. a. australis was slower than that of A. reinhardtii. The diet of both eel species changed significantly with size. The diet of larger individuals of A. reinhardtii had an increased dependence on fish; fish were not recorded in the diet of A. a. australis. The diet of small (less than 20 cm) eels was similar for both species, a feature that may lead to direct competition during periods of low summer flows.