Weed preference and growth of young grass carp in New Zealand.
See also WA 22, 2080. In each of eight 5-day pond experiments carried out at intervals over a year, grass carp (Ctenopharyngodon idella), ranging from fingerlings of 17.6-44.5 g average wt. in June 1972 to fish >1 year old weighing about 500 g in February 1973, were offered a choice of at least 10 main species of water weeds from the Rotorua area. Bunches of weeds were suspended from a rope tied across the pond at the water surface and floating weeds (Azolla rubra and Lemna minor) were floated inside frames; the drained wt. of bunches was compared at intervals with that of controls which increased their wt. by about 7.5% in 5 days. Water temp. was measured daily. Food selectivity decreased with increasing temp. and size of fish but the order of preference did not change. The preference was for Nitella hookeri > Lemna minor > Elodea canadensis > Callitriche stagnalis > Paspalum sp. > Nasturtium officinale > Potamogeton crispus > Azolla rubra > Myriophyllum propinquum > Lagarosiphon major. crispus > Azolla rubra The fish preferred dry food (oatmeal and commercial trout pellet) to weeds. Fish weighing 14 gr would not eat the last 3 species whereas fish weighing 500 g consumed them in considerable amounts. Fish averaging 500 g could be expected to consume substantial amounts of the major problem weeds M. propinquum, L. major and Ceratophyllum demersum and could be expected to grow from 20 g fingerlings to 500 g fish in one summer. At summer temps. (about 21 deg C), the fish grew at 4 g/day but in winter (9-10 deg C) little food was taken and growth was slow.