Quantifying population connectivity of marine larvae: Hydrodynamic modelling and shell microchemistry methods to determine larval dispersal of Perna canaliculus

Abstract

Population connectivity plays an important role in population dynamics. In the marine environment many species have a biphasic life history which consists of a pelagic larval phase followed by a relatively sedentary adult phase. Larval dispersal therefore plays an important role in population connectivity. Bivalve aquaculture is expanding worldwide while simultaneously many wild bivalve reefs are being degraded. There is interest in the restoration of these degraded bivalve reefs to which aquaculture may provide a larval subsidy through larval spill-over. The aim of this thesis was therefore to examine larval dispersal of the bivalve mussel Perna canaliculus (mussels). The Firth of Thames (FoT) in Northern New Zealand once supported dense mussel reefs, however currently the largest known populations in the area are in aquaculture. This thesis used a combination of trace elemental fingerprinting (TEF) and biophysical modelling to examine larval dispersal. TEF is based on the trace elemental composition of bivalve shell reflecting the environmental conditions under which it formed. Prior to the application of TEF in the FoT I investigated the role played by pH and the genetic history of an individual on the elemental composition of shell material. I demonstrated that while the genetic history of an individual will affect the composition of shell material, this will not affect the ability to determine the location at which it formed. I then demonstrated that despite pH changes TEF will remain a viable technique to track the dispersal of larvae under possible future conditions of ocean acidification. Both these results are novel findings. In the biophysical modelling study, virtual larval were continually released over a 10 year period and tracked until settlement. This study demonstrated that larvae from aquaculture are capable of settling throughout the FoT. TEF then demonstrated that the larval pool in the FoT is well mixed with larvae predicted to have originated throughout the FoT found at all locations monitored. These results showed that larval spill-over from aquaculture has the potential to contribute to the restoration of bivalve reefs through a larval subsidy. Restoration programmes should therefore carefully consider potential population subsidies from aquaculture and apply a network approach.