What's best for mussels?
Science Centres: Fisheries
Mussel growth and condition rely on the availability of naturally occurring food, mainly phytoplankton. In the Marlborough Sounds, the main mussel farming area, diatoms and flagellates are the most important food organisms for mussels, but the composition of the phytoplankton biomass is highly variable. There is, therefore, potential for great variability in growth and condition of mussels. Our recent study shows that the efficiency with which mussels use particular phytoplankton species also varies greatly.
Diatoms – Chaetoceros spp.
Dinoflagellate Procentrum mucans.
Chlorophyll a pigment concentration is often used to estimate phytoplankton biomass in field monitoring and, therefore, can be an indicator of mussel growth conditions. Over the last few years, the chlorophyll concentration in Pelorus Sound has generally decreased and there have been concurrent decreases in both the condition and growth rate of the mussels. Although these trends appear to be broadly related, there have been discrepancies, such as poor mussel growth when chlorophyll levels have been moderate. These discrepancies may relate to the utilisation of different phytoplankton species or groups.
Recently, we have used a C14 radiotracer technique to measure the assimilation efficiencies of greenshell mussels feeding on various phytoplankton species. In the Marlborough Sounds, diatoms and dinoflagellates dominate the phytoplankton biomass. In our experiments we used several diatom and flagellate species from the NIWA microalgae collection at Mahanga Bay. The cultured diatoms were species that are abundant in the Marlborough Sounds; the flagellates were species readily available from Mahanga Bay.
The mussels assimilated flagellates with efficiencies ranging from 76 to 84% and diatoms with efficiencies of 53 to 70%. This means that in the natural phytoplankton biomass in the Sounds, for a given chlorophyll concentration, the energy available for mussel growth could vary by as much as 22%, depending on the phytoplankton species composition. Growth simulations, using a mussel energetics model that we have developed, indicate that persistent differences in assimilation efficiencies of that magnitude would result in significant differences in dry flesh weigh of the mussels. These findings have potential application in the management of mussel farming.
Chlorophyll a is a useful indicator of available food, but with additional data on species composition we can improve our understanding of the environmental controls on mussel growth and condition. Further experiments are planned to better understand mussel selectivity in feeding on different types of phytoplankton, particularly the dinoflagellates, and their energy values to mussels. Other members of the NIWA research team are looking at the utilisation by mussels of small phytoplankton (picophytoplankton), as well as both micro- and meso-zooplankton. Combining the information from all these sources is enabling us to gain a better understanding of the environmental effects of mussel farming, and its implications for the sustainable carrying capacity of the whole ecosystem.