Development and
evaluation of a stress challenge testing methodology for assessment of Nile
tilapia (Oreochromis niloticus, Linn.) fry quality
A.M. MacNiven, D.C. Little-2001
Aquaculture Research, 32(9): 671-679
(from Current Contents)
Abstract :
A decline in the quality of the freshwater fish seed
available to small-scale farmers in Asia has been identified 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 field, to
develop and evaluate a stress challenge test for Nile tilapia. (Oreochromis
niloticus, Linn.) fry as the effects of stress on fish performance have been
well documented and similar tests are used elsewhere in the sector. Formalin
and iodized salt were used as reference toxicants. LC50 values were
estimated using mortality figures 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 significant co-variable;
adjusted means showed no significant 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 modified,
but that it was vulnerable to unquantifiable and uncontrollable factors
arising from management and the environment, which may have altered
responses to the stressor.
(Asian Inst Technol, SERD, POB 4 Klong Luang, Pathum
Thani 12120, Thailand, e-mail: angusm@ait.ac.th)