Spill-over from aquaculture may provide a larval subsidy for the restoration of mussel reefs

Worldwide bivalve aquaculture is expanding rapidly. Simultaneously, there has been a loss of natural bivalve reefs due to anthropogenic activities. As bivalve reefs support several ecosystem functions disproportionate to the area they cover, there is interest in their restoration. The Firth of Thames (FoT) in northern New Zealand once supported dense populations of green lipped mussels Perna canaliculus, which were extirpated by a dredge fishery in the mid-20th century. Efforts to restore these biogenic habitats are underway. The largest standing populations of this species in the area currently exist in aquaculture. This study aimed to determine if larval spill-over from aquaculture can provide a larval subsidy to bivalve reef restoration efforts in the FoT. We used a combination of trace elemental fingerprinting and biophysical modelling techniques to determine patterns of larval dispersal in the area. Results of both approaches indicated that the larval pool in the area is well mixed with larvae produced at aquaculture locations capable of settling throughout the study area. Overall this shows, for the first time, that larval spill-over from aquaculture may provide a subsidy to restoration efforts and assist with establishing sustainable populations. When determining restoration locations, the potential for aquaculture populations to act as a larval source should be explicitly considered. Conversely, when considering the location of new aquaculture sites, the consequences of larval spill-over to surrounding wild populations should be assessed. We recommend that restoration efforts and aquaculture be carefully integrated in a network approach which could provide both ecological and economic benefits.

Spatio-temporal resolution of spawning and larval nursery habitats using otolith microchemistry is element dependent

Otolith chemistry is frequently employed in the reconstruction of fish environmental histories. While some elements have been strongly correlated with environmental factors (e.g. salinity, temperature, water chemistry), others may not indicate exogenous factors and simply add endogenous variability to a data set. Several commonly assessed elements were previously identified as being only present in the proteinaceous fraction of endolymph from black bream Acanthopagrus butcheri, suggesting that the choice of elements in otolith multi-elemental fingerprinting could influence their utility as natural environmental markers. To test this hypothesis, we performed several cluster analyses based on different sets of trace element data extracted from both the core and larval region of otoliths of juvenile black bream. We clustered in 3 different ways: (1) all elements analysed; (2) elements identified as being primarily in the salt fraction of the endolymph (i.e. inorganic); and (3) elements identified as being associated with the protein fraction of the endolymph. We subsequently assessed the power of the resulting clusters to resolve cohort identity and/or collection location based on differences in otolith chemistry using multinomial logistic regression. Our results indicate that clustering based solely on salt-fraction elements is best for resolving spatio-temporal variability in spawning sources and larval nursery habitats of black bream.