Predicting the dispersion of invasive marine species
The manila clam was introduced to Poole Harbour to create a fishery for this commercially important foreign shellfish. The clams are seeded and grown to a juvenile state on land before being placed within the fishery’s boundaries to complete their growth prior to harvesting. This method was employed to prevent spawning of the clam within the harbour and thus the potential spread of the species outside of the designated fishery. However, beds of manila clam have appeared all around the harbour, including low salinity areas which were previously thought to be lethal to the clam, and also in Southampton Water. HR Wallingford modelled the movement of the clam larvae to investigate whether the larvae could travel to Southampton Water and also how the larvae could survive in lower salinities than it was previously thought they could tolerate.
Larvae behaviour studies carried out at Bournemouth University, part-funded by HR Wallingford, indicated that the larvae of the manila clam are able to swim vertically through the water column and that they do so in conjunction with the state of the tide i.e. as the tide comes in they swim up, and as it goes out they swim down. This behaviour allowed them to move through the harbour (rather than stay where placed as was previously believed). We combined our existing hydrodynamic model of Poole Harbour with an ecological model of larval movement to consider whether this new mechanism would result in dispersion of the larvae to Southampton Water. Particles were added to the model and coded with the behavioural characteristics of the clam larvae. The model indicated that the larvae were able to move out of the mouth of Poole Harbour into open water and, once out of the harbour, the prevailing currents and tides eventually swept the larvae to Southampton Water.
The second question, of how the clam was able to colonise low salinity areas which were believed to be unsuitable for it, was also answered through a combination of research at Bournemouth University and the use of HR Wallingford’s hydrodynamic model. Genetic work showed that the larvae developed with a preference for water of either 17 or 21 salinity and that they would actively move towards the preferred salinity as well as vertically within the water column. The ecological model showing the larval movement indicated areas of Poole Harbour where colonies of the clam should be found. On comparison with local records these model predictions were found to be accurate. This work has improved our understanding of how an invasive species can spread in UK waters.
This work has improved our understanding of how an invasive species can spread in UK waters.