Development and evaluation of a stress challenge testing methodology for assessment of Nile tilapia (Oreochromis niloticus, Linn.) fry quality
Aquaculture
Research, 32(9): 671-679
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.
(AARM/SERD, Asian Institute of Technology, PO Box 4
Klong Luang, Pathumthani 12120, Thailand, E-mail: angusm@ait.ac.th)