Analysis of sexually dimorphic growth in captive reared cobia (Rachycentron canadum) and the occurrence of intersex individuals

The growth of three cohorts of captive reared cobia, grown in a combination of flow-though and recirculating aquaculture systems, was progressively measured to determine the existence and extent of sexually dimorphic growth in cobia. Approximately 100 fish from each cohort were individually identified and regularly weighed until the average weight of the fish was approximately 5 kg. The sex of individuals was determined through gonadal observations at the conclusion of each trial and the gender fitted retrospectively to the growth data set. Intersex gonads were observed in the first two cohorts of cobia, with 16.9% incidence in cohort 1 and 6.8% in cohort 2. Cobia is considered a gonochoristic species. This was the first reported observation of intersex gonads in cobia and the first reported occurrence of intersex gonads from a gonochoristic fish species from Australian waters. Only one fish out of the 182 examined in the third cohort was identified as intersex. There was no sexually dimorphic growth in cobia when there was a relatively high incidence of the intersex anomaly, as seen in the first two cohorts of fish. In the relative absence of the intersex condition, female cobia was significantly larger than males from 2 kg onwards. The weight of female cobia was almost 30% more than that of males at 17 months of age when average weight of the cohort was 4.6 kg. It is likely that the first two cobia cohorts were exposed to endocrine disruption in some form, and the possible sources are discussed. Statement of relevance This study demonstrated that female cobia grow significantly faster than male fish and that investigations into monosex culture could lead to significant productivity gains for cobia aquaculture. It also demonstrated that cohorts containing intersex fish did not exhibit sexually dimorphic growth. It is likely that the reproductive anomaly is the result of disruption to the endocrine system.

Parental contribution to progeny during experimental spawning of jungle perch, Kuhlia rupestris

When releasing captive-bred animals into wild populations, it is essential to maintain the capacity for adaptation and resilience by minimising the effect on population genetic diversity. Populations of the jungle perch (Kuhlia rupestris) have become reduced or locally extinct along the Queensland coast; thus, captive breeding of K. rupestris for restocking is presently underway. Currently, multiple individuals are placed in a tank to produce larvae, yet the number of adults contributing to larval production is unknown. We performed a power analysis on pre-existing microsatellite loci to determine the minimum number of loci and larvae required to achieve accurate assignment of parentage. These loci were then used to determine the number of contributing participants during a series of four spawning events through the summer breeding season in 2012-2013. Not all fish contributed to larval production and no relationship was found between male body size and parentage success. In most cases, there was a high skew of offspring to one mating pair (62% was the average contribution of the most successful pair per tank). This has significant implications for the aquaculture, restocking and conservation of K. rupestris.

Alphaherpesvirus-associated disease in greater bilbies (Macrotis lagotis)

Case report: A captive breeding colony of 9 greater bilbies (Macrotis lagotis) exhibited mild upper respiratory signs and sudden deaths with 100% mortality over a 2-week period. Histologically, acute necrotising and erosive epithelial lesions throughout the upper respiratory system and bronchi were associated with eosinophilic intranuclear inclusion bodies. Inclusions were also present in hepatocytes and adrenocortical cells, but were not always associated with necrosis. Transmission electron microscopy of lung sections revealed nucleocapsids forming arrays within some nuclei. A pan-herpesvirus PCR yielded a 440-bp product, with sequencing confirming homology with the alphaherpesviruses. Viral culture in a marsupial cell line resulted in cytopathic effect consistent with an alphaherpesvirus. Conclusion: This is the first report of a herpesvirus-associated disease in greater bilbies. © 2016 Australian Veterinary Association.