Morphometrical, behavioural and chemical changes during growth and starvation of herring and plaice larvae.

Abstract

Growth rates of herring (Clupea harengus) and plaice (Pleuronectes platessa) larvae were 0.22 and 0.16 mm/day respectively. The slope of the length-dry weight line for plaice larvae was 3.92. That for herring larvae was 4.57, which remained constant throughout starvation, although the intercepts decreased. If wet weights were used the slopes decreased. Relative condition factors were used to estimate nutritive condition over a wide size range. Condition factors based on length and weight were not very good for estimating nutritive condition, due to concurrent losses of length and weight. The ratio of eye to head height rapidly increased during starvation due to head shrinkage. The sinking rate of herring larvae in sea water decreased from hatching to the end of the yolk sac stage but increased with further growth. Newly hatched plaice larvae were positively buoyant, but their sinking rate increased with development. In both species the rate decreased during starvation. This was suggested to be a mechanism of energy conservation. Water content was inversely related to the sinking rate, but other body components also influenced it. Water provided the major upward vector, followed by fat; protein was responsible for the downward force. The decrease in sinking rate during starvation was partially due to the increasing percentage of water, but the largest proportion was from nitrogen catabolism. The days of starvation to reach irreversible starvation increased during development; the rate of increase was greater in plaice. Over 50j6 of the life span of herring and plaice starved from the end of the yolk sac stage was beyond irreversible starvation. Ontogenetic changes in chemical composition were dependent upon larval size rather than age. Percent water decreased throughout development from the end of the yolk sac stage. In the period of initial post-hatching growth (up to 20 mm in herring and stage 2 in plaice) nitrogen and carbohydrate were laid down faster than triglyceride, suggesting that it was advantageous to the larvae to convert food largely into growth rather than simultaneously accumulating energy stores. During starvation percent water increased about 4$ above the unstarved level; percent ash also increased. The percentage of triglyceride, carbohydrate, and carbon decreased in both species, as did nitrogen in plaice. In herring the percentage of nitrogen did not change throughout starvation, although the actual amount decreased. The C/N ratio decreased in starved herring, but it did not show a consistent pattern in plaice. It was suggested that the use of nitrogen throughout starvation could be an adaptation by pelagic marine larvae to the planktonic environment, since nitrogen catabolism was responsible for their decreasing sinking rate. Herring egg composition from different females was related to hatching success. The size of the larvae at the end of the yolk sac stage was compared to their chemical composition. Survival and chemical composition of 100 day-old herring and 50 day-old plaice larvae were altered after 20 days of feeding on diets of Artemi a . rotifers, or plankton, but larval length was not 'affected. The size of the larval fat store was influenced by the amount of dietary fat. Herring larvae started feeding on rotifers 5 days post-hatching, 2 days earlier than on a mixture of barnacle and shrimp brine/nauplii and plankton. By 28 days the rotifer-feeders were significantly larger. 20 mm herring larvae ate 190 rotifers/ larvae/day.