Abstract
Oyster populations play an important ecological role in coastal ecosystems for numerous reasons such as habitat structure, carbon sequestration, and water filtering. Since the identification of the protozoan parasite Bonamia ostreae in 2015 the native flat oyster, Ostrea chilensis has experienced declines due to disease in commercial areas such as the Foveaux Strait, Southland. Despite reports of O. chilensis being harvested in Otago in the early 1860s, limited research has been done on the population dynamics of flat oysters around Otago. With increasing environmental pressures and increasing interest in the development of oyster aquaculture, it is necessary to understand how flat oyster populations are performing within Otago. Despite increasing information on marine disease prevalence globally and nationally, there is a lack of literature on disease prevalence within Otago. Population dynamics can have a direct impact on how a disease presents within a population. High density can facilitate faster transmission while age structure can influence a populations vulnerability. This study investigated population dynamics influencing O. chilensis populations across multiple sites in the Otago region. Transect surveys were conducted at five locations - Rabbit Island, Doctors Point, Pūrākanui, Dowling Bay and Papanui, to quantify oyster density (oyster per m2), size structure, substrate composition (%), temperature (ºC), and spat recruitment. Oyster density and shell height measurements were used to assess population structure, while substrate composition was quantified to evaluate habitat characteristics. Spat collectors were also used to determine settlement within three of the populations, Doctors Point, Dowling Bay, and Papanui. Results showed that O. chilensis were found in greater density in habitats where the substrate was mostly boulder and shell hash. On average O. chilensis were greater in height in habitats where substrate was predominantly boulder, showing favourable environmental conditions for growth. The largest oysters were found at Pūrākanui, and the smallest were found at Dowling Bay suggesting possible environmental or disease pressures for this population. Density did not differ significantly within sites. Oyster settlement peaked January to February, indicating potential delayed settlement and brooding period compared to O. chilensis from Foveaux strait.
Thirty oysters from six sites within Otago were dissected and DNA was extracted from the mantle, gills, and heart. Oysters went through diagnostic testing via qPCR and results confirmed a 93.3% infection rate at Dowling Bay and was not detected at any other site tested. Diagnostic species testing on oysters from Dowling Bay via conventional PCR confirmed the parasite was Bonamia exitiosa. None tested positive for B. ostreae. The presence of B. exitiosa at Dowling Bay could explain population characteristics observed in transect surveys such as reduced shell height as well as low spat recruitment. The presence of B. exitiosa in Otago Harbour is its first identification that it has moved from previously infected southern areas. The movement of B. exitiosa is potentially from ship and brooding stock translocation as this is a known movement vessel seen by this parasite before. Positive identification shows the importance for routine surveillance within the Otago harbour as well as neighbouring bays and other coastal sites to identify/control spread.
Overall, this research provides insight into the ecological and environmental factors influencing oyster population dynamics in coastal Otago. It confirms disease presence of B. exitiosa within the Otago harbour which could be impacting populations. Understanding how habitat characteristics, settlement processes, and disease presence interact is essential for informing future management and aquaculture development. These findings contribute to a growing body of knowledge supporting the protection and potential restoration of native flat oyster populations in New Zealand.