Recruitment of Ostrea chilensis (Philippi, 1844) in Foveaux Strait, Southern New Zealand

This research evaluates fundamental ecological processes to facilitate an understanding of recruitment in Ostrea chilensis from Foveaux Strait, southern New Zealand. Foveaux Strait represents an extreme habitat for oysters that differs to the sheltered nearshore, muddy habitats of most other oyster populations. O. chilensis exhibits the extreme end of brooding strategies in Ostreinae, does not form extensive reefs, and comprises putative self-recruiting populations. The Foveaux Strait oyster fishery is nationally important. Recurrent disease mortality in these populations has put greater onus on understanding recruitment. To evaluate the strength of a recruit-per-spawner relationship in oysters, seasonality in the settlement of larvae was determined. Most recruitment in any given year, over a 6-year period, occurred in the austral spring and summer (November to February). Fishery-wide, recruitment varied significantly between years, with most variation (50.8%) explained by a year effect that represents the combined influences of climatic and biological conditions. Spawner densities and fishery areas explain further variation (13.8% and 11.6%, respectively), with further 2-way interactions between these factors. Recruits-per-spawner declined serially over time, despite similar or increasing densities of spawning-sized oysters. Average recruitment was lowest when spawner densities were highest; this suggests a more complex relationship between recruitment and density that has implications for management of this oyster fishery. Recruitment to the O. chilensis fishery declined abruptly to low levels in 2010 and remained low until 2017. Relatively high spawning-stock sizes over this period had previously supported high recruitment. Density and oyster mortality from Bonamia exitiosa (a proxy for one or more infections) and their two-way interaction were the main determinants of recruitment. The highest recruitment occurred at times of low mortality and low density, suggesting reduced effects of disease on gametogenesis and reduced disease transmission. The contributions of climate factors were minor; however, a 3-way interaction between oyster density, mortality, and climate is likely to drive variation in recruitment. Pathobiomes (multiple infections in populations) may be important determinants of shellfish recruitment and population dynamics. This research evaluates the hypothesis of self-recruitment from distributions of recruit densities around an isolated natal population, and from the relationship between recruitment and brooding-sized oyster densities. Distance from the natal population, direction along or across the tidal current, or brooders did not predict recruit densities. Recruit distributions imply greater dispersal and larval mixing than previously reported. The swift tidal currents and possibility of more variable pelagic larval durations may enhance mixing and connectivity between populations in Foveaux Strait. Post-settlement mortality is the primary determinant of spatial structure in Foveaux Strait oysters. Productive fishery areas comprise mostly stable substrates of shells, sand, and gravel, with no or little other epifauna. Most (66.8%) post-settlement survivors were on the heavy shells of both live and dead O. chilensis, which suggests an unusual recruit-adult relationship based on survival rather than settlement. Recruits and 1+ year spat grew larger and had lower mortality at eastern sites with the lowest exposure to oceanic swells and putative lowest sediment movement. Moreover, recruits on spat collectors also grew larger and had lower mortality at heights ≥ 12 cm than those 2 cm off the seabed. This research suggests the effects of disease on brooding percentages and thereby larval supply may be the main determinant of the variation in recruitment in O. chilenesis, and the spatial structure of oyster populations in Foveaux Strait shaped by abiotic as well as biotic post-settlement mortality.