This thesis examines the impact of the common starfish, Asterias rubens, on a
commercial mussel Mytilus edulis, bed in the Menai Strait, Anglesey, North Wales. Asterias rubens is a common and voracious predator of mussels in Northern Europe and is believed to be a major source of mortality within the commercial bottom-grown mussel fishery in the Menai Strait. However, the impact of their feeding activities and the level of financial losses incurred by local mussel fishermen remain unquantified. The current study was conducted in the Menai Strait on mussel lays leased by the commercial mussel company Extramussel Ltd, and in the surrounding areas ( ~ 4 km²) . Seasonal changes in the abundance and size distribution of starfish and mussels were examined along with the annual reproductive cycle and growth rates of starfish, utilising a combination of proven and novel techniques. Several laboratory studies were conducted to assess the foraging behaviour of A. rubens feeding on mussels.
Starfish and mussels followed seasonal patterns of abundance and growth and
were most abundant on the subtidal mussel beds attaining densities of~ 71,000
starfish.ha-¹ and ~ 40 percent coverage respectively, whilst starfish were
generally absent from intertidal mussel lays. Starfish growth was non-seasonal,
and a peak in reproductive condition and spawning occurred annually between
April and July in adult individuals (>100mm arm length). Seasonal patterns of
starfish migration and abundance were observed and were deemed to be in
response to the seasonal harvesting and transplantation of mussels during the
autumn/winter (October to January) and early spring (March to May); starfish
moved into areas where mussels were being harvested and then were removed
together with the mussels during harvesting, resulting in a reduction in starfish
numbers over the winter.
Adult starfish (>I00mm) selected and consumed a wide size range (40-60mm) of mussels offered in laboratory experiments, and although mean size of mussel
chosen increased with starfish diameter, feeding rates and size selection of mussels were found to be independent of seawater temperature. Starfish consumed mussels in the proportions that they were encountered, irrespective of
their size, with adult starfish consuming between 0.3 to 1 mussels.day–¹• The data generated from this research were used to estimate the annual loss (April through March the following year) of mussels to starfish predation and to produce a mathematical model that could be used to predict the annual impact of starfish predation on mussel production under a range of environmental scenarios (e.g. rising seawater temperatures and size at transplantation). Starfish were estimated to have consumed 1148 ±425 tonnes of marketable mussels annually between 2007 and 2008 equating to a mean loss of £574,264, or 11 to 32 % of all marketable mussels to Extramussel Ltd.