Four-year decline in Ostrea chilensis recruits per spawner in Foveaux Strait, New Zealand, suggests a diminishing stock-recruitment relationship

Rebuilding and maintaining sufficient spawning stock to ensure recruitment is a key strategy for fisheries management and ecological restoration. We evaluated variation in Ostrea chilensis recruitment across seasons and sites over 4-6 yr in Foveaux Strait (New Zealand) to infer the relative importance of determinants of population recruitment. Recruitment varied significantly between seasons (p < 0.001). Most recruitment in any given year (97.8 +/- 0.9%, mean +/- SE) occurred in the austral spring and summer (November to February). Recruitment also varied significantly between years (p < 0.001). In a separate fishery-wide study, we investigated the effect of spawner densities on recruitment, relative to other climatic and biological factors. We deployed spat collectors at 6 sites across 3 discrete fishery areas, and estimated densities of spawning-sized oysters from dredge samples. We modelled counts of oyster spat and spawners with a negative binomial regression to evaluate the stock-recruitment relationship. Recruitment varied between years (50.8% of the deviance explained), spawner densities (13.8%), and areas (11.6%), with further 2-way interactions among these factors. Importantly, our analysis showed a continued decline in recruits per spawner, despite similar or increasing densities of spawning-sized oysters. Average recruitment for 2010-11 when spawner densities were highest was 4.6% of the level observed in 2007-08. Our data suggest that factors other than densities of oysters play a major role in the numbers of competent larvae available for settlement. Managing oyster fisheries as a single stock and maintaining oyster densities above management reference points alone may not be sufficient to ensure recruitment to rebuild populations.