A model of water temperature, dissolved oxygen (DO), and fish growth in
stratified fish culture ponds has been developed. The model uses
stochastically-generated weather parameters as inputs. A Monte Carlo technique
is used in generating hourly values for solar radiation, wind speed, and wind
direction based on probabilistic functions obtained from historical weather
records for the pond site. The model can be executed for a maximum of
32,768 steps, corresponding to an 85 day simulation with a time step of
0.0625 h. Temperature and dissolved oxygen predictions match well with measured
values, but fish growth and chlorophyll-a concentrations are
reversal of newly hatched tilapia generally is accomplished via oral
administration of 17a methyltestosterone (MT), which has been
incorporated into a starter fish feed at 60 mg MT/kg feed. While use of the
60 mg MT/kg feed dose consistently yields populations comprised of less than 5%
females (i.e., > 95% males), this has not been shown to be the optimal
dose. Other investigators have reported sex reversal of tilapia at dose rates
less than 60 mg MT/kg feed; however results from some of these studies are
inconsistent, and it is difficult to separate treatment environment effects.
Naturally occurring sources of testosterone may be an alternative to using a
synthetic androgen, which also is an anabolic steroid, for tilapia sex
reversal. Bull testes are a by-product of beef industry in the US, and are
a potential source of dietary testosterone for tilapia sex reversal. The
objectives of this research were to determine the efficacy of sex reversal of
different dosage rates of MT to fish treated in different environments, and to
evaluate the potential of freeze-dried bull testes as a dietary source of
testosterone for tilapia sex reversal.
Newly hatched Nile tilapia (Oreochromis niloticus) were stocked at 8 fry/L into 80-L glass aquaria located inside a hatchery building or into hapas (45-L volume) suspended in 20-m 3 outdoor concrete tanks. Trout chow (42% protein) was the carrier for MT, which was incorporated into the feed at 0, 15, 30, 45 or 60 mg MT/kg of feed. Fry in each treatment were fed at 20% body weight during week 1; the daily ration was divided into four meals. Feed rate was decreased by 2.5%/wk during weeks 2-4. Treatment duration was 28 days.
Frozen bull testes, obtained from a meat packing plant, were freeze-dried and ground, and mixed with trout chow either in a 1:1 or 1:3 freeze-dried testes:trout chow ratio. Mixed feed was offered as described above.
Preliminary data is available now; data collection and analysis continues through December 1995. After the 28-d MT treatment period, fry total length ranged from 32.8-39.6 mm and 40.7-44.3 mm for fry treated in aquaria (indoors) and hapas (outdoors), respectively. Average respective final weight ranges were 0.7-1.0 and 1.2-1.9 g/fry. Fry survival in both environments was low and ranged from 16.7-27.7% and 25.7-43.6% in aquaria (indoors) and hapas (outdoors), respectively.
Fry fed feed containing bull testes were 55.6 and 59.7 mm total length for 1:1 and 1:3 ratio feeds, respectively, following the 28-d treatment period. Mean final weights were 2.0 and 0.7 g/fry for 1:1 and 1:3 ratio feeds, respectively, which undoubtedly reflected the difference in respective survival during treatment (28.3% versus 69.2%).