AN INDIVIDUAL-BASED MODEL FOR PREDICTING THE EMERGENCE PERIOD OF SEA-TROUT FRY IN A LAKE DISTRICT STREAM

The objective of this study was to predict interannual fluctuations in the emergence period of sea trout fry, using models developed from fi eld data for 70 excavated redds, and laboratory data on egg and alevin development at 30 constant temperatures (range 1.5-10.5 degrees C wit h 100 naturally fertilized eggs at each temperature). Egg weight and n umbers per redd both increased with female length, a power function de scribed the relationship. Early spawners were the largest females layi ng the largest and most numerous eggs, whilst late spawners were the s mallest females laying the smallest and least numerous eggs; middle sp awners being intermediate between these two extremes. Mean values for egg weight and numbers of eggs per redd were obtained for these three groups. Hatching and emergence times in the laboratory decreased with increasing temperature. Of five models tested for hatching time, the b est fit was provided by a three-parameter hyperbolic model which forme d the basis of the individual-based model used to predict egg hatching and fry emergence. Model development was described in detail and the final equations predicted the times taken for 5, 50 and 95% of the fry to emerge, and hence the period over which 90% of the fry emerged. An alogous models were obtained for egg hatching. All models were excelle nt fits to the laboratory data. Hatching times for eggs kept in perfor ated boxes in the stream were almost identical to those kept at simila r mean temperatures in the laboratory. Model predictions of fry emerge nce times were validated by field data for 8 years (1967-1971, 1974, 1 975, 1980). The chief objective was therefore fulfilled, and predictio ns for the 30-year study (1967-1996) revealed a large variation in the timing of emergence (extremes: 11 March-4 April 1989, 15-20 May 1979) . Most of the variation in median emergence date was due to variations in water temperature, with spawning dates as a secondary factor; the latter, however, had a greater effect on the length of the emergence p eriod. (C) 1998 The Fisheries Society of the British Isles.