MEPS 153:153-165 (1997)  -  doi:10.3354/meps153153

Winter growth of mussels Mytilus edulis as a possible counter to food depletion by oystercatchers Haematopus ostralegus

McGrorty S

The growth of mussels in temperate waters is generally described as fast in summer and slow or absent in winter. Two experiments are described which investigated whether adult mussels Mytilus edulis in the Exe estuary, Devon, UK, grew over the winters of 1994-95 and 1995-96 and, if so, whether this process countered depletion by, and substantially increased the food available for, their principal predator, the oystercatcher Haematopus ostralegus. Mussels grew in length over both winters (September to March). In 1995-96, for example, there was a significant mean incremental growth for 25 to 45 mm mussels at 75% of sites. Overall, 71% of individually labelled mussels grew by >1 mm and the largest individual increment was 9.6 mm. Thus, some small mussels could have grown into the oystercatchers' preferred prey size range (lower limit ca 40 mm) on all of the beds; but, given the rapid decline in growth with increasing length, it was unlikely that any grew out of it (upper limit ca 65 mm). Growth varied between mussel beds but, contrary to expectation, there was no evidence that it varied with mussel clump density within beds. At the bed scale, variation in growth increment was not strongly related to sediment type (sand/mud; including sediment with and sediment without naturally occurring mussel populations), suggesting that the location of the bed within the estuary was important. Position along the exposure/submergence (upshore) gradient explained (p < 0.0001) the largest proportion (44%) of the variance in growth increment for <40 mm mussels. In contrast, the density of adult mussels on the beds explained (p < 0.0001) only an additional 1.2% of the variance after the effect of exposure and initial length had been taken into account. Increasing density across the range of values encountered on the mussel beds reduced the growth increment by 33%, compared with a 99% reduction across the range of exposure values. Thus, the density-dependent effect on winter growth was small compared to the effect of bed location. Nonetheless, by reducing the density of the larger mussels, oystercatcher predation could have some effect on the growth of smaller ones. Certainly, growth could counter depletion to some extent on all of the beds. Even on those high-shore beds with <10% increase in prey density due to the growth of small adult mussels, this could be important to the birds. On the 3 large, low-shore beds with the largest increases (>20%) in prey due to growth, equivalent to >90% of the estimated depletion, this must have had a substantial effect on the birds' food supply over the winter period.

Winter growth · Mussels · Oystercatchers

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