Reproductive Biology and Controlled Reproductive Development of Captive Cobia (Rachycentron canadum)

Abstract

Cobia aquaculture began in Australia in 2007; however, expansion of commercial production has been limited, due in part to low and inconsistent supply of seed stock for ongrowing. This study aimed to address the constraints of reproductive performance of cobia in captive conditions and investigate strategies to improve the efficiencies of broodstock management in commercial and research facilities.
In a study evaluating the growth of three cohorts of captive reared cobia, to determine the existence and extent of sexually dimorphic growth, intersex development in cobia was identified and recorded for the first time. There was no sexually dimorphic growth in the first two cohorts of fish. In contrast, females from third cohort were significantly larger than males once mean body weight exceeded 2 kg. It is proposed that the observed variations in growth and gonad malformations observed in cohorts 1 and 2 were the result of exposure to endocrine disruptors, the type and source of which remains unknown.
In order to quantify the ovarian development of cobia, there was a need to develop an accurate method of assessing and quantifying ovarian maturation in the presence of asynchronous development. A simple, commercially applicable methodology was devised that used the proportional distribution of different oocyte stages to describe the developmental state of the ovary.
Two trials were conducted to examine the effect of repeated injections of luteinising hormone releasing hormone analogue (LHRHa) and human chorionic gonadotropin (hCG) on stimulating vitellogenesis, supporting ovarian maturation and levels of circulating 17βestradiol (E2). Ovarian development was highly variable within treatments and those fish that initiated vitellogenesis continued to develop regardless of treatment with exogenous hormones. There was no change in E2 concentrations as a result of LHRHa injections and limited evidence to suggest a change in plasma E2 in response to hCG injection. While iv hormonal therapy is effective in inducing spawning in cobia, the results suggest that hormone therapy is not an effective approach to initiating or supporting early stage ovarian development in cobia.
Two cohorts of cobia broodstock were assessed to examine ovarian development and circulating E2 in response to photothermal manipulation. In each study, broodstock were subjected to either a temporally compressed regime or an ambient regime. In the first study ovarian development was generally limited, irrespective of the phototherm regime and there was no significant difference in development between treatments. At the completion of the second trial there was no significant difference in ovarian development between the compressed and ambient phototherm; however, fish in the compressed phototherm were found to develop earlier in the season than those in ambient conditions. Fish in the first trial showed sporadic development in which ovarian samples contained low numbers of late stage oocytes amongst a large percentage of previtellogenic oocytes, possibly due to exposure to endocrine disruption in the early life history of the cohort.
Two cohorts of captive reared cobia were progressively examined as pre- and postpubescent fish to examine the suitability of identifying gender by analysing the androgen 11-ketotestosterone (11KT) in blood and fin clip samples. The gender of individual cobia could be identified by analysing plasma 11KT between the months of October and March, provided the mean population weight was 2 kg or above. The measurement of 11KT concentrations in fin clip samples did not provide an accurate indication of plasma 11KT and as such, was not suitable for predicting gender in cobia. Overall the relative cost, infrastructure and equipment required to conduct steroid analysis limits the application of this methodology in commercial cobia production in comparison to the traditional method of gonadal biopsy.ref:68ga4