T.J. Lam

Development of digestive tract and proteolytic enzyme activity in seabass (Lates calcarifer) larvae and juveniles

Abstract The development of the digestive tract and changes in activity of proteolytic enzymes were studied in seabass larvae and juveniles. The stomach and pyloric sphincter did not start to be formed until 13 days after hatching (day 13) and were not completely formed until day 17. Prior to this, a high level of pinocytotic activity was found in the rectal cells of 6-day-old and 14-day-old seabass larvae indicating that protein macromolecules were being absorbed by these cells. The pH of the anterior gut (the presumptive stomach before the stomach was formed) in early larvae was alkaline (pH 7.7 on day 8); on day 17, the pH of the stomach had become acidic (pH 5.0) and pepsin-type enzyme activity in the larvae had increased from an initial basal level. By day 22, the acidity of the stomach had become more pronounced (pH 3.7) and the pepsin-type enzyme activity had become well established. Digestion of dietary proteins bv the larvae and the possible contribution of live food to the process are discussed.

Serum cortisol response to handling stress and the effect of cortisol implantation on testosterone level in the tilapia, Oreochromis mossambicus

A tropical fish species (Oreochromis mossambicus) was used to study cortisol response to handling stress. Unstressed fish were found to have very low (<10 ng/ml) serum cortisol levels. Stressed fish showed a rapid and significant rise in cortisol levels within 4 min. The peak level, reached within 30 min of stress, was dependent on the severity of the stressor, being lower (about 64 ng/ml) in fish disturbed by netting, and higher (about 119 ng/ml) in fish captured and confined. Removal of the stressor resulted in a rapid return of cortisol to a normal level. Cortisol cocoa-butter implants were successfully used to elevate and maintain cortisol at a level characteristic of a stressed fish for 19 days. Cortisol implants of 1.5 and 3.0 mg/g body weight were found to depress testosterone levels significantly over the study period.

Effects of salinity and thyroxine on larval survival, growth and development in the carp, Cyprinus carpio

Abstract The survival, growth and development of carp larvae (Cyprinus carpio) increased with increasing salinity of the ambient medium from fresh water to 10% sea water (3‰). The salinities investigated (1, 5 and 10% SW or 0.3, 1.5 and 3‰ respectively also appeared to enhance the viability and hatchability of the eggs compared with fresh water, although not increasingly. Thyroxine (Eltroxin, Glaxo) further improved egg viability and hatchability, and promoted larval survival, growth and development, but the relative efficacy of the different concentrations of thyroxine in the ambient medium (0.01, 0.05 and 0.1 ppm) depended on the salinity of the medium. The results may find application in the improvement of carp larval rearing.

Siganids: Their biology and mariculture potential

Abstract Siganids (rabbitfishes), which are widely distributed in the Indo-Pacific, have recently attracted the attention of mariculturists in the Pacific and Israel. Research into siganid mariculture has been initiated in several countries. This paper lists the reasons behind the interest in siganid mariculture, and reviews: (1) what is currently known about the biology of siganids considered under (a) species identification, (b) fisheries, (c) ecology, (d) reproduction and early development and (e) behaviour; (2) problems requiring solutions before large-scale siganid mariculture can be realized; and (3) progress made to date in each area.

Thyroxine injection of female rabbitfish (Siganus guttatus) broodstock: changes in thyroid hormone levels in plasma, eggs, and yolk-sac larvae, and its effect on larval growth and survival

Abstract The thyroid hormone levels in female rabbitfish, Siganus guttatus , plasma, eggs, and yolk-sac larvae were measured before and after thyroxine (T 4 ) injection in female spawners at doses of 1, 10, and 100 μ g T 4 /g body weight (BW) fish. T 4 and triiodothyronine (T 3 ) levels in maternal plasma, eggs, and yolk-sac larvae were elevated following T 4 administration. Apparently, there is conversion of T 4 into T 3 in the broodfish which suggests the presence of the enzyme, 5′-monodeiodinase, in rabbitfish. T 4 and T 3 in maternal circulation were easily transferred into the oocytes and subsequently into the larvae. Larvae from spawners treated with 10 and 100 μ g T 4 /g BW tended to be longer and showed sornewhat better survival compared to the control and those injected with 1 μ g T 4 /g BW 7 days after hatching. These findings suggest that thyroid hormones may play an important role during early larval development of rabbitfish.

Replacing live foods with microencapsulated diets in the rearing of seabass (Lates calcarifer) larvae: do the larvae ingest and digest protein-membrane microcapsules?

Abstract Seabass larvae were fed all-protein-membrane microcapsules from first feeding, either alone or together with rotifers. To investigate ingestion and digestion, anaesthetized larvae that had been fed fluorescent microcapsules were examined under a confocal scanning laser-microscope. The passage of the microcapsules could be followed under continuous scan. Microcapsules were ingested by the larvae from first feeding. Although microcapsules from 15 μ m to 150 μ m were provided to the larvae, those ingested were 40–60 μ m and took less than 60 min to pass through the gut. When only larger size microcapsules (80–150 μ m) were provided, passage time was extended to more than 2 h because the microcapsules passed through the intestino-rectal valve with difficulty. All larvae fed microcapsules alone had died by the 10th day after hatching. The fluorescent protein wall of the microcapsules was not broken down in the larval intestine and the microcapsules passed through the rectum intact. Larvae fed microcapsules together with rotifers for 5 days and then microcapsules alone for 1 week had a mean survival rate of 2.4%. There was evidence that the protein wall of the microcapsules was broken down in the larval intestine when rotifers were present and that the protein membrane was absorbed.

Thyroxine enhances larval development and survival in Sarotherodon (Tilapia) mossambicus Ruppell

Abstract Yolk-sac larvae were obtained from the mouth of brooding female tilapia (Sarotherodon mossambicus Ruppell). Treatment of these larvae with l -thyroxine-sodium (Eltroxin, Glaxo) by immersion in 0.1 ppm solution (changed daily) markedly accelerated their development and enhanced their survival. This effect of thyroxine, if confirmed in other species, may find applications in the improvement of larval rearing.

Development of a tropical marine fish cell line from Asian seabass (Lates calcarifer) for virus isolation

A tropical marine fish cell line (SF) was established from Asian seabass (Lates calcarifer) fry. The cell line was maintained in Earles minimum essential medium (EMEM) supplemented with foetal calf serum and incubated at 25°C. The susceptibility of the cell-line at the 16th, 35th and 46th subculture to several iridoviruses, birnaviruses, reoviruses, a rhabdovirus, and a nodavirus was studied. Cytopathic effect (CPE) was observed daily after virus inoculation and viral replication efficiency was determined for six viruses. SF cell cultures infected with an iridovirus, a nodavirus and a reovirus were further elucidated by electron microscopy. All the viruses tested were shown to induce CPE on SF cells. This suggests that the SF cell line has good potential for the isolation of various fish viruses. The SF cell line consists predominantly of epithelial-like cells and has been subcultured 85 times over a period of 24 months.

Effect of dietary protein level on puberty, oocyte growth and egg chemical composition in the tilapia, Oreochromis niloticus (L.)

Abstract Oreochromis niloticus were maintained on 5 isocaloric diets of different dietary protein levels (10, 17, 25, 32 and 40%). In Expt. 1, O. niloticus (initial mean weight approximately 0.3 g) maintained on diets of 10 and 17% protein levels showed lower growth rates compared to those on the higher protein levels and none reached puberty. In Expt. 2 (initial mean weight approximately 3 g), only those receiving 10% dietary protein level gave similar results; fish fed 17% reached puberty. Fish fed higher protein levels (32 and 40%) reached puberty at an earlier age, with oocytes growing and maturing faster, than those fed lower levels (≤ 25%). The former also showed higher growth rates, lower food conversion ratios and higher survival rates. When the onset of puberty and the rate of oocyte maturation were evaluated in relation to size (regardless of age), there were no significant differences among the treatments, suggesting that dietary protein may have influenced oocyte growth and puberty by its effect on growth. The chemical composition of post-vitellogenic oocytes, expressed as percent protein, lipid, moisture, and essential amino acids (mg in 100 mg protein), was not significantly different among the different dietary treatments.

Retardation of ovarian growth and depression of serum steroid levels in the tilapia, Oreochromis mossambicus, by cortisol implantation

Abstract Female Oreochromis mossambicus were implanted with cortisol suspended in cocoa-butter for the evaluation of the chronic effects of cortisol on reproduction. Implant doses of 1.5 and 3.0 mg/g body weight, mimicking the effects of long-term chronic stress, suppressed various reproductive parameters over the 18-day treatment period. Most fish treated with cortisol showed reductions in body weight. Oocyte size and gonadosomatic index of treated fish became significantly lower than the control after 9 and 12 days of treatment, respectively. The retardation of oocyte growth was accompanied by depression of serum testosterone and oestradiol-17β levels.