Abstract:
This thesis investigates the reason(s) for an anomalous distribution of mussels on Wellington’s South Coast. Wellington Harbour supports large populations the mussels (Perna canaliculus, Aulacomya maoriana and Mytilus galloprovincialis) however; all these species are absent from the nearby South Coast. The physical difference between Harbour and Coastal sites was confirmed from water samples taken at eight sites inside and outside Wellington Harbour between September 1998 and February 2000. These data showed significant differences between Harbour and Coastal sites in seven of eleven parameters tested. Larval densities were investigated at Harbour and Coastal sites; data showed larval density to be approximately an order of magnitude greater in Harbour waters compared to the Coast. Larval density in Coastal waters reached a maximum of 380 mˉ³ and is therefore not expected to be a major limiting factor. Similar results were found for studies of settlement density on artificial substrates. Settlement at Harbour sites was approximately 10 times that at Coastal sites, however, mussel densities of up to ≈3700 mˉ² at Coastal sites indicate that settlement is also unlikely to be a factor contributing to mussel absence.
Comparisons were made of body condition index between Harbour mussels and those from the Harbour relocated to the Island Bay Marine Laboratory (IBML) on Wellington’s South Coast. All mussel species experienced a significant decrease in condition at IBML when compared with those from the parent population in Wellington Harbour. Levels of mortality were high in P. canaliculus (60%) and M. galloprovincialis (70%) and moderate in A. maoriana (30%). This indicated the possibility of nutrient limitation as a cause of mussel absence. This was corroborated by a similar study using RNA:DNA ratio as a biochemical index of condition. Mussels held at IBML experienced a significant decrease in RNA:DNA compared to those from the parent population within the Harbour.
Further investigation of nutrient limitation was conducted by way of feeding experiments to estimate the scope for growth (SFG) of each mussel species. In P. canaliculus, SFG had a mean (±SD) of 45.6 J gˉ¹ hˉ¹ (±78.9) and only 27% of observations were negative. A. maoriana had lower SFG estimates, mean SFG was 19.1 J gˉ¹ hˉ¹ (±66.7) and negative SFG was recorded 43% of the time. M.galloprovincialis had mean SFG of 1.26 J gˉ¹ hˉ¹ (±39.1), 52% of SFG estimates were negative. While these data do not overwhelming support the hypothesis of nutrient limitation, they do go some way toward corroborating the findings of the condition index experiments.
Models to predict SFG were derived from data in the feeding experiments, these models were derived from a small range of low seston concentrations. A second group of models was derived from studies conducted by Gardner (2001) to give a broader range of seston concentrations. Seston data from the water sampling described above were used to estimate SFG at eight sites inside and outside the Harbour over the period from September 1998 to February 2000. Both models estimated lower SFG at Coastal sites compared to Harbour sites. However, estimates were not consistently negative at Coastal sites. These models had mixed results and would benefit from the combination of data from recent SFG experiments conducted at IBML and Wellington Harbour to derive new, more robust models.
While the findings in this study are not conclusive, the absence of mussels from Wellington’s South Coast is most likely to be the result of nutrient limitation for at least part of the year. The reduced ability of mussels to store nutrients when feeding on Cook Strait water is likely to exacerbate the effects of low seston quality.
