Condition cycles in juvenile Pagrus auratus

Nutritional condition was measured in juvenile snapper Pagrus auratus (<200 mm fork length) using three indices: relative condition index (I-C), hepatosomatic index (I-H) and digesto-somatic index (I-D). In a laboratory starvation experiment, all three indices declined substantially over a 24-day period, but I-H was most sensitive. In wild snapper I-C and I-H showed no diel cycles. I-D for 0+ snapper showed a strong diel cycle consistent with continuous feeding during daylight hours and lack of feeding during the night. I-D for 1+ snapper showed no diel cycle. Subsequent analyses were restricted to daytime samples for I-H and morning samples for I-D to minimize the confounding effect of time of day. I-C, I-H and I-D were monitored at one site at approximately bi-monthly intervals over a period of 3.25 years. All three indices varied significantly, but only I-D and I-H displayed seasonal cycles. I-D peaked in late summer-autumn and dropped to a minimum in winter, due to seasonal fluctuations in the feeding rate that probably reflected variations in metabolic and growth rates. I-H peaked in autumn-winter and declined to a minimum in summer, thus lagging 4-6 months behind I-D. I-H varied significantly among four sampling sites for all five combinations of sampling periods and year classes, whereas I-C varied significantly among sites for only one of the five combinations. The Kawau Bay site, which supported the highest density of snapper, had the highest I-H for all except one of the combinations. This suggests that juvenile snapper aggregate selectively at sites that provide optimal feeding conditions. However, no relationship was found between I-H and growth rate, indicating that better nutritional condition may not translate into faster growth. (C) 1997 The Fisheries Society of the British Isles.