High Mussel Densities Research Articles

The effects of a major flood on an endangered freshwater mussel population

During February 1998, a 100-year return flood occurred in the River Kerry, north-western Scotland. A study was undertaken in order to assess the impact of this event on a large, internationally important freshwater pearl mussel ( Margaritifera margaritifera L.) population. A conservative estimate of 50,000 mussels killed by the flood was made. This represents 4–8% of the total population. Significant channel reformation and large-scale movements of substrata occurred in some reaches, whereas others appeared to be unchanged. Some mussel beds disappeared or were largely depleted whereas others remained intact. Boulder-dominated substrata appear to have provided the most protection from scouring. The relatively high mussel densities and recruitment levels found in the middle reaches are probably due to the fact that this part of the river is hydrologically stable. It is likely that the intact mussel beds in the middle reaches will be the main source of regeneration in other parts of the river as the population recovers. As a result of recent changes in the hydrological behaviour of Scottish rivers, several M. margaritifera populations may now be more at risk from these catastrophic events. Therefore, it is important that conservation managers are aware of this potential threat.

Density variation of the zebra mussel Dreissena polymorpha in Lake Zürich, from 1976 to 1988

A population explosion of the zebra mussel Dreissena polymorpha in Lake Zurich in the early seventies was followed by a decline, most noticeably in the littoral zone (Burla and Lubini-Ferlin, 1976). To check for further density changes, zebra mussels were counted every winter from 1978 to 1988, and larval settlement was recorded every summer from 1976 to 1988. During this period, mussel densities ranged from 11 to 1500 ind. m−2; juvenile densities from 11 to 108 ind. cm−2. Counts of newly settled juveniles and counts of established mussels were positively correlated between years. This suggests that the number of larvae in any given year depends on the size of the breeding population of mussels. Juvenile counts from different sites were more similar than mussel counts in any given year, indicating that within-lake dispersal of larvae is high. In both newly settled juveniles and established mussels, density variation occurred in what appeared to be two full cycles of 5 years. Mussel densities were positively correlated with total bird counts of three known predators of D. polymorpha, the Coot (Fulica atra), Tufted duck (Aythya fuligula), and Pochard (A. ferina). Higher mussel densities were observed near Zollikon and Goldbach than at other sampling sites. This may be a consequence of a steeper, stonier slope of the lake bed at these sites. Twenty-eight years after it was first recorded in Lake Zurich, D. polymorpha appears to be firmly established in the lake.

The availability and quality of the mussel prey ( Mytilus edulis) of oystercatchers ( Haematopus ostralegus)

Spatial variations in the availability and quality of the mussel Mytilus edulis food supply of Oystercatchers Haematopus ostralegus on the Exe estuary, England, are described. Oystercatchers open mussels by stabbing into gaping mussels (or prising open closed ones) or by hammering a hole in either the dorsal or ventral shells. Spatial variations in the food supply are considered at four scales. In decreasing order of size, these are (i) whole mussel beds, (ii) zones within a mussel bed, (iii) different places within one zone, and (iv) different places within one clump of mussels. The first two scales are clearly related to exposure time. Both between and within the 12 main mussel beds of the estuary, most upshore mussels are up to 10% less likely than downshore mussels to be hidden under mud. However, upshore mussels of a given length contain less flesh, have thicker ventral shells and, except on high-level beds subject to wave erosion, have thicker dorsal shells than downshore mussels. Mussels at the top of the shore also contain the highest infestations of the helminth parasite of Oystercatchers, Psilostomum brevicolle. At a particular shore level, mussels of a given length have less flesh and thicker shells, though only on the dorsal side, in areas of high mussel density. Within one clump, mussels of a given length with thick dorsal shells have more flesh than those with thin shells. In contrast, flesh content is slightly higher in mussels that are thin on the ventral side. Flesh content and shell thickness on both sides are unaffected by whether a mussel is visible at the edge of a clump or hidden inside. Simulations with a model of foraging Oystercatchers suggested that variations in mussel availability (visibility and shell thickness) and flesh content at all four scales could sometimes have an important influence on intake rate. Most published values of intake rates of Oystercatchers eating surface-dwelling mussels may be biassed (and often considerably over-estimated) by the erroneous assumption that prey of average flesh content are taken.

Population dynamics of the ribbed mussel, Geukensia demissa: The costs and benefits of an aggregated distribution.

Although mussel beds are common in many intertidal habitats, the ecological significance of the aggregated distribution of mussels has not been examined. The ribbed mussel, Geukensia demissa, is found in dense aggregations on the seaward margin of many salt marshes in New England. Here, we examine the population structure of G. demissa in a New England salt marsh and investigate experimentally the costs and benefits of aggregation.Size, growth rate, and settlement rates of mussels decrease with increasing tidal height, whereas survivorship and longevity increase with increasing tidal height. Winter ice dislodges mussels from the substratum, resulting in mortality over all size classes, whereas crab predation results in the mortality of smaller mussels. The intensity of each of these mortality agents decreases with increasing tidal height. Effects of intraspecific competition on individual growth and mortality also decrease with increasing tidal height.At high densities, individual growth rates were reduced, with depression of growth rates most pronounced on smaller individuals. Mortality from sources other than intraspecific crowding, however, was reduced at high mussel densities, including mortality due to winter ice and crab predators. As a result, our data suggest that the mussel population at our study site would be reduced by 90% in only five years and no juveniles would survive through their second year without an aggregated distribution.Juveniles settle gregariously with or without adults present. The aggregated distribution of settlers and the postsettlement movement of smaller mussels to favorable microhabitats result in size and age class segregation within the population. This probably reduces intraspecific competition for food, while maintaining the survivorship advantages of an aggregated distribution.