A decline in the quality of the freshwater ®sh seed available to small-scale farmers in Asia has been identi®ed as an important constraint on the development of aquaculture in the region. There is a need for simple and reliable methods of determining the quality of seed at the farm gate or hatchery. The present study was carried out, in the ®eld, to develop and evaluate a stress challenge test for Nile tilapia (Oreochromis niloticus, Linn.) fry as the effects of stress on ®sh performance have been well documented and similar tests are used elsewhere in the sector. Formalin and iodized salt were used as reference toxicants. LC 50 values were estimated using mortality ®gures resulting from exposure of marketable-sized fry to saline and formalin solutions. The two test concentrations, 24 ppt for saline and 500 p.p.m. for formalin, were used as reference toxicants in acute, static bioassays to challenge fry reared under different conditions during the post sex-reversal, nursery period. Using existing farm practice as controls the conditions were: 50% increased stocking density, formalin treatment, 40% reduction in feed rate and no supplementary feed. Fry length proved to be a very signi®cant co-variable; adjusted means showed no signi®cant difference between treatments. The importance of only marketing tilapia fry weighing over 0.2 g was emphasized. It was recommended that the challenge testing approach could be modi®ed, but that it was vulnerable to unquanti®able and uncontrollable factors arising from management and the environment, which may have altered responses to the stressor.

No abstract