Age validation, growth and mortality of introduced Tilapia zillii in Crater Lake Nkuruba, Uganda.
Periodicity and timing of opaque zone formation in otoliths of introduced redbelly tilapia, Tilapia zillii (Gervais), in Crater Lake Nkuruba, Uganda, were validated using marginal increment. Age and growth were assessed through readings of biannuli in thin-sectioned sagittal otoliths. Deposition of opaque zone formation in T. zillii otoliths was bimodal (March-May and September-November), corresponding to two seasonal peaks of precipitation characteristic of this equatorial region. Ages of T. zillii ranged from 2 to 8 years, with fish gillnetted offshore having a faster growth and attaining larger size-at-age than fish captured inshore in minnow traps, suggesting that use of multiple gears is needed when estimating the growth of T. zillii. Total instantaneous mortality (Z), estimated using catch curve analysis, was 0.74 for gillnetted fish and 0.71 for trapped fish. These estimates were at the low end of the total mortality reported for other tilapia species. Natural mortality (M) was estimated as 0.52-0.54 by applying Rikhter and Efanov's maximum age at maturity and Hoenig's maximum age methods, respectively. Fishing mortality (F) in Lake Nkuruba was 0.17-0.22, indicating a low exploitation level in the lake.