MODELS OF TEMPERATURE-DEPENDENT RATE OF DEVELOPMENT OF PILCHARD SARDINOPS-SAGAX EGGS, TO BE USED IN ROUTINE PROCEDURES FOR ESTIMATING DAILYEGG-PRODUCTION

Routine ageing of planktonic eggs requires mathematical models to desc ribe the relationship between the time elapsed after spawning to reach a developmental stage and the ambient temperature of the water in whi ch the eggs are developing. In this study, the temperature-dependent d evelopment time of 11 stages of pilchard Sardinops sagax eggs was dete rmined for 10 temperatures between 12,55 and 22,05 degrees C. The pilc hard eggs were collected in late summer in the northern Benguela and i ncubated on board ship. Stages were not equal in duration, the earlies t and latest stages being shorter than the intermediate stages at all temperatures. The duration of Stages VII and VIII deviated most with t emperature, varying from 2 to 13 hours over the temperature range of t he experiment. Two models, an exponential and a power function, were a ssessed to describe the development of the stages in relation to tempe rature. At lower temperatures, the power function fitted the observed data better than did the exponential function, and it was therefore ch osen as the appropriate model for ageing pilchard eggs. These function s can now be used to construct automated procedures for ageing eggs, a s a step in the estimation of daily egg production of pilchard.