Evaluation of the effect of larval stocking density, salinity, and temperature on stress response and sex differentiation in the Dwarf Gourami and Rosy Barb

Abstract

There is a variety of sexual determination and differentiation mechanisms among teleosts. The phenotypic physiology may be influenced by environmental stress during development as well as genetic factors. In these species, increased levels of the stress hormone cortisol during the undifferentiated labile period correlates with masculinization. In ornamental aquaculture, production of monosex male cohorts is desirable in several species, including the Rosy Barb Pethia conchonius and Dwarf Gourami Trichogaster lalius, which exhibit sexual dichromatism, where the males are more vibrantly colored and therefore more commercially valuable than female conspecifics. Internationally, the use of the synthetic androgen 17α-methyltestosterone (MT) to masculinize cohorts of ornamental fish is common. However, in the U.S., it is currently prohibited to use MT in ornamental aquaculture, putting domestic producers at an economic disadvantage. The current study explored the feasibility and efficacy of using environmental stressors during early larval rearing to influence sexual differentiation and skew populations toward a male phenotype.A suite of experiments were conducted during the first 30 days post hatch with both Rosy Barbs and Dwarf Gouramis. The aim of each experiment was to elevate endogenous cortisol levels in developing larvae using environmental stressors, thus potentially inducing masculinization. Stocking density, salinity, and temperature were manipulated to elicit a natural increase in circulating cortisol. Whole body cortisol levels, survival, growth, and sex ratios of fish in replicate tanks were determined for each experimental treatment. In both species, experimental treatments resulted in differences in growth, survival, and whole-body cortisol levels in specific experiments, but only one experiment resulted in modified sex ratios. The first potential evidence of temperature influence on sex determination was observed in Dwarf Gouramis with a significantly reduced proportion of males found after larval rearing in 30 °C conditions compared to 25 °C. These results provide novel insights into the larval stress physiology of these two species, but also suggest limited sexual plasticity perhaps due to a strong genetic sex determination system.