SEASONAL VAlUATIONS DETERMlNE DIET QUALITY FOR OREOCHROMlS NlLOTICUS L. (PISCES: CICHLIDAE) IN LAKE TANA, ETHIOPIA

Abstract

The diet composition and its use by Oreochromis nilolicus in
Lake Tana, Ethiopia, were studied from monthly samples taken over 13 months. The composition of the diet varied seasonally, with dramatic increases in quality following seasonal climatic events (rainfall, vertical mixing) that bring plant nutrients to the photic zone. Thus, organic matter in the diet increased four-fold, from 150 to 600 mg g"1 of diet. Protein content increased from 71· to 256 mg g-\ ash free dry weight (AFDw). Assimilation efficiency increased from 8.3 % to 43.2% for AFDW, and from near 0 to 45.8% for protein; The fraction of organie matter that is refractory declined from 800 to 21 mg ,-1 AFDW. Condition factor
followed the same pattern, but lagged 1 to 2 months behind..changes in diet quality. It was concluded that seasonal pattern of rai.nfall and temperatllre affect pelagic food resources, hence heterogeneity in diet quality is temporal to a great extent for O. niloticus in Lake Tw.