Reproductive changes in Foveaux Strait Ostrea chilensis, Southern New Zealand, after Bonamia exitiosa epidemics

The commercially significant Foveaux Strait Ostrea chilensis fishery has been typified by consistently low brooding rates and recruitment failures since recurrent mass mortalities from Bonamia exitiosa infection. The present study examined the gametogenic cycle of Foveaux Strait O. chilensis monthly, from July 2017 to September 2018, using histological examination of the gonad and condition indices to determine resulting changes to gametogenesis, spawning, brooding, and spatial variation, compared to an initial study of reproduction before B. exitiosa epidemics. The seasonal gametogenic cycle and brooding period were similar to previous descriptions. However, there was evidence for a more female skewed sex ratio, and the development and release of ova into the mantle cavity at smaller sizes. Despite similarities in gametogenesis, sex ratios, temperature, and chlorophyll-a concentrations between five sites across Foveaux Strait, the western-most site had no brooders and consistently lower condition indices, compared to the eastern-most site. Oysters may need to be in sufficient condition to initiate brooding, and reduced male densities may contribute to sperm limitation, which, along with a disconnect between the peak release of gametes and brooding, may have contributed to lower brooding rates.

Automated summary

The study investigated the gametogenic cycle and spatial variation of Foveaux Strait Ostrea chilensis. The findings suggest that reduced brooding rates may be due to poor condition of oysters and decreased male densities.