The Protein Metabolism of Sunfishes of Different Ages

Abstract

The utilization of protein for growth by sunfish (Lepomis m. megolotis) and green sunf ish (L. cyanellus) of different ages was investigated. The fish were kept in individual aquaria and fed a weighed amt. of meal worms (Tenebrio molitor larvae) each day for 50 days. N was measured by the Kjeldahl technic. There was no difference in the ability of fish of different ages to absorb protein. The efficiency of protein absorption averaged 96.4% for all fish. Fecal N was measured by filtering the aquarium water and analyzing its residue. The chitin content of the meal worms was taken into consideration. N retention was measured by the difference in N content between the exptl. fish at the end of the expt. and a sample fish of the same size at the beginning of the expt. The utilization of protein for growth was measured by relating the protein N retained to the protein N absorbed. The utilization of protein declined in a linear fashion from 33% in young fish (10 g.) to 5% in old fish (105 g.). Both species showed this characteristic. Young fish increased about 12 mm. in length for each g. of protein retained while old fish increased about 3.5 mm. for each g. of protein stored. The amt. of protein required for 1 mm. linear growth for fish of different sizes was detd., and it was possible to estimate the amt. of protein required to grow any specified increment in length from these data. It was suggested that fish growth is much more determinate than it is usually considered to be, since fish of max. age are able to utilize very little of available protein for growth. The concept of physiologic age seems to be applicable to fish as interpreted from the data presented.