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

ABSTRACT

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.

KEYWORDS:

• Ostrea chilensis
• flat oyster
• reproduction
• gametogenesis
• Bonamia exitiosa
• New Zealand

Acknowledgements

We thank Mathew Downes, Jo Ward, Erin Damsteegt, Department of Zoology, as well as Chris Hepburn, Pete Russell, and the Portobello Marine Laboratory staff, Department of Marine Science, the University of Otago. Thanks to Andrew Jeffs, University of Auckland, and Henry Lane, Ministry for Primary Industries, for their assistance with the project. We would like to thank the providers of the Vision Mātauranga – Deep South Challenge Masters Scholarship for supporting this work. Additional thanks to the skippers and crew of the RV Polaris II and The Golden Quest and Graeme Wright, Barnes Oysters for their help obtaining samples.

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No potential conflict of interest was reported by the author(s).

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Funding

The author(s) reported there is no funding associated with the work featured in this article.