Diet composition and preference of juvenile Murray cod, trout cod and Macquarie perch (Percichthyidae) reared in fertilised earthen ponds

Murray cod, trout cod, and Macquarie perch (family Percichthyidae) have significant commercial, recreational and conservation value in Australia. Juveniles are reared in earthen nursery ponds for stock enhancement purposes to support recreational fisheries and conservation efforts, and for use in aquaculture. Dietary analyses were conducted on fish reared in fertilised earthen ponds to identify and describe diet composition and preference (selectivity). A total of 35 prey taxa were identified. Diet composition between the three species of fish was similar, with 21 prey taxa being recorded from the stomachs of at least two of the fish species. Both the size range and upper size of prey consumed increased with increasing fish size, with larger fish being able to access a wider range of prey. Fish rarely consumed prey smaller than 0.4 mm in length. Numerically, chironomids were the most frequently encountered prey, and were in the highest proportions in the diets of Murray cod and trout cod, while Moina were for Macquarie perch. However, as dry weight biomass, chironomids occurred in higher proportions than any other prey for all three fish species. Diet composition changed as fish grew. In general fish consumed Moina especially in the first 1-2 weeks immediately following stocking into the ponds, but shifted to adult copepods, Daphnia (with the exception of Macquarie perch) and chironomid larvae in latter weeks. Selectivity in all three species of fish changed, often significantly, from 1 week to the next, which in some cases corresponded with changes in the abundance of certain prey taxa in the ponds. Selectivity for rotifers and copepod nauplii was always strongly negative. Identifying the preferred prey of juvenile fish reared in nursery ponds is an important component to improving fish production. This study has considerably broadened the knowledge of diet composition and selection in juvenile, pond-reared, Murray cod, trout cod and Macquarie perch, and notably demonstrated the importance of chironomids as a prey item in these species.