An examination of some aspects of growth and nutrition of juvenile grey mullet (Mugilidae) in relation to potential mullet farming in Britain
The growth of juvenile (0 - III group) C. labrosus, L. aurata and L. ramada was examined in the Tamar, Lynher and Yealm estuaries in south-west England. Two growth models were used to facilitate comparisons among species, age groups and sites of capture. The growth of C. labrosus and L. ramada was similar and faster than that of L. aurata. The growth and relative condition of 0 group fish tended to be greater in St. John's Lake than at other sites, but amongst older fish the reverse appeared to be true. Annual and daily cycles in feeding intensity were examined. A high daily food intake and fast passage of food through the gut appeared to be characteristic of both C. labrosus and L. aurata. In the spring 100% of fish examined were infected w1th cysts of Myxbolus exiguus but this did not appear to have any significant effect on growth or condition. Three 12-15 week feeding experiments were undertaken to examine aspects of the nutrition of I group C. labrosus using experimental diets of semi-purified rations. In the first and second experiments the effect of dietary protein level and ration size on growth was investigated. There was a significant interaction between these factors with optimum dietary protein level decreasing with increase in ration size, and optimum ration decreasing with increase in dietary protein level. In the third experiment the ability of juvenile mullet to utilise dietary energy supplied as either lipid + carbohydrate or mainly carbohydrate in diets containing 20% and 40% protein was examined. Increase in dietary energy resulted in improved growth, but lipid energy had a greater protein sparing effect than carbohydrate energy. The effects of dietary protein and energy level, energy source and ration size on conversion efficiency, assimilation of the diet, histology and size of certain organs, and body composition were also investigated.