Athauda, A.R. Saman Bandara (2014) Effect of culture environmental conditions on sex inversion of Asian seabass (barramundi), Lates calcarifer (Bloch). PhD thesis, James Cook University.

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Abstract

Asian seabass, Lates calcarifer, is a protandrous catadromous fish species cultured worldwide for commercial aquaculture. Farmed Asian seabass exhibit precocious sex inversion before two years of age and this phenomenon is the major impediment to maintaining broodstock in a hatchery. In the wild, age, seasonal temperatures and movement of the fish to a brackish or saline environment apparently affect spawning and sex inversion of Asian seabass. This series of experiments investigated the role and relationships between age and the hatchery conditions which appear to induce sex inversion in captivity. Asian seabass grown in freshwater under natural conditions were transported to the research facility at James Cook University, Australia to conduct four experiments as follows.

In the first experiment investigating age effects, Asian seabass of four different sizes grown in freshwater were transferred to salinewater (32 g L⁻¹) at 28 °C and 14: 10 L:D photoperiod and fed to satiety twice daily. Eight groups of Asian seabass (n=8/group, two groups per each size) according to their body weight were allocated to 3000 L tanks in a single enclosed room. In the rest of the experiments, 14 month old seabass grown in freshwater under natural temperature in the same farm were transferred to the research facility and held in freshwater at 28 °C until acclimatized to the experimental conditions at different salinities (0 g L⁻¹, 20 g L⁻¹or 30 g L⁻¹ salinity), different temperatures (22 °C, 25 °C, 28 °C, 31 °C and 34 °C) and both salinity versus temperature (24 °C, 29 °C or 34 °C at each of 0 g L⁻¹ or 30 -32 g L⁻¹) in experiments two, three and four, respectively. Each experiment additional rearing conditions were similar, except the variable environmental parameter tested. Fish were anaesthetized to collect the blood samples for plasma steroids assays at the beginning and at the end of the experimental period. Upon collection of blood, the fish were sacrificed; brain and gonad were removed aseptically, and labeled vials were placed in liquid N₂ at ⁻80 °C for aromatase assay for all experimental fish, while histological analyses were conducted for last two experiments.

Brain aromatase activity appeared to respond to age/size rather than environmental conditions, while gonadal aromatase was detectable only in the 700 – 1000 g fish group, plasma T increased in response to the environmental change in fish groups of 300 – 500 g and 700 – 1000 g while the 50 – 100 g and 2.5 – 4 kg fish had no increases (P˃0.05). Plasma E₂ increased significantly in all groups of fish in experiment one, while 11 KT was detected in the 700 – 1000 g and 2.5 – 4 kg fish and was significantly different (P˂0.05). Results indicated that the hormonal conditions are pre-requisite for inducing sex change in captive Asian seabass from 435 ± 27 g body weight.

The results of the second experiment indicated that there were no differences (p˃0.05) between the aromatase activities in the brains of fish held at 0 g L⁻¹, 20 g L⁻¹ or 30 g L⁻¹ salinity, while no differences (p ˃ 0.05) between the gonadal aromatase activities were also observed in any of the treatment groups except for fish held in 10 g L⁻¹ and 30 g L⁻¹ salinities, respectively. The highest gonadal aromatase level was recorded in fish held in 0 g L⁻¹and 20 g L⁻¹. Plasma T concentration in fish in all treatments at the end were not different (P ˃ 0.5), while the highest E₂ level was recorded in fish held at 0 g L⁻¹ followed by fish held at 10 g L⁻¹ and 30 g L⁻¹, respectively. However, no measurable amount of 11- KT was detected in any salinity group of fish in this experiment.

Results of the third experiment which examined the effects of temperature on sex change indicated that there was an increase in plasma E₂ levels with increasing temperature from 25 °C, while no significant difference was observed among all treatment temperatures except at 25 °C. However, fish held at 22 °C expressed higher E₂ level than at either 25 °C and 28 °C. Significantly higher plasma T levels were detected in fish held at 31 °C and 34 °C, while a reducing trend was observed towards lower temperature regimes. Fish held at 22 °C had significantly lower plasma T than all others as well as those sampled at the beginning. The plasma 11- KT was at non-detectable levels in all experimental temperatures as shown in the initial fish sampled. The average aromatase activity in the brain was highest at 28 °C among all temperatures, but no significant differences were observed. The average aromatase activity in the gonad was higher at 31 °C, followed by 34 °C and 28 °C. No or very low levels of gonad aromatase activity was recorded in fish sacrificed prior to treatment. The aromatase activity was greater in brain than in gonad suggesting that the initial responses to changes in environmental temperatures occur in aromatase produced in the brain.

The results of the final experiment indicated that there was an increase in plasma E₂ level with temperature in fish held at 34 °C, whereas no significant difference was observed at 24 °C and 29 °C, although the highest plasma T level was detected in fish at 34 °C which, except for those fish held at 24 °C in freshwater, had significantly lower levels than at the beginning. Plasma 11- KT was significantly greater in fish held at 24 °C compared with 29 °C or 34 °C, which was opposite to that of E₂. Aromatase activity in the brain was higher at 29 °C than at either 24 °C or 34 °C, whereas gonadal aromatase was recorded the highest at 34 °C. It is apparent from the data presented in the final experiment that there is a relationship between culture water temperature, independent of salinity, and induction of sex change as demonstrated by histological staging and measured through changes in the concentrations of aromatase and reproductive hormones.

Item ID:	45404
Item Type:	Thesis (PhD)
Keywords:	aromatase activity, Asian seabass, fisheries, giant perch, induced spawning, Lates calcarifer, ontogeny, reproduction, salinity, sex change, sex inversion, sex steroids, sexual behaviour, temperature
Related URLs:	http://researchonline.jcu.edu.au/20838/ http://researchonline.jcu.edu.au/38851/
Additional Information:	Publications arising from this thesis are available from the Related URLs field. The publications are: Chapter 5: Athauda, Saman, Anderson, Trevor, and de Nys, Rocky (2012) Effect of rearing water temperature on protandrous sex inversion in cultured Asian Seabass (Lates calcarifer). General and Comparative Endocrinology, 175 (3). pp. 416-423. Chapter 6: Athauda, Saman, and Anderson, Trevor (2014) Effect of temperature and salinity on sex inversion in Asian Seabass (Lates calcarifer): relationship with plasma sex steroids concentration and aromatase activity of gonad and brain. Aquaculture Research, 45. pp. 787-797.
Date Deposited:	01 Feb 2017 04:27
FoR Codes:	07 AGRICULTURAL AND VETERINARY SCIENCES > 0704 Fisheries Sciences > 070401 Aquaculture @ 100%
SEO Codes:	83 ANIMAL PRODUCTION AND ANIMAL PRIMARY PRODUCTS > 8301 Fisheries - Aquaculture > 830102 Aquaculture Fin Fish (excl. Tuna) @ 100%
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