Cockle Beds Research Articles

How two large-scale “experiments” illustrate the Importance of enrichment and fishery for the functioning of the Wadden Sea ecosystem

In the recent past, two far-reaching changes have taken place in the Dutch part of the Wadden Sea: (1) a drastic increase in the late 1970s of the annual rates of primary production and of chlorophyll concentrations (restricted to the western part of the Dutch Wadden Sea and probably induced by eutrophicated fresh water sluiced from Lake IJssel), and (2) a sudden removal in 1990 of nearly all mussel and cockle beds by commercial fishery all over the Dutch part of the Wadden Sea.

Seston Dynamics and Food Availability on Mussel and Cockle Beds

To provide a better understanding of seston dynamics in relation to food supply to the benthos, a series of 13-h tidal cycle sampling programmes was executed in the Oosterschelde estuary (The Netherlands). Samples were taken near the surface and near the bottom on two subtidal mussel cultivation plots and on two intertidal cockle beds.Long-term annual variablity of seston concentrations was lower than coefficients of variance of the short-term tidal cycle seston data at the intertidal stations, and higher than at the subtidal stations. Near-bottom relative to surface concentrations were highest for suspended particulate matter (SPM), followed by particulate organic carbon (POC), and chlorophyll. There was no food depletion near the bottom but food quality was lower, presumably due to re-suspension of sediment, including low-quality biodeposits. Chlorophyll concentrations were lower in ebb than flood water at all stations, which was ascribed to feeding activity of the bivalves. At the subtidal stations, SPM and POC concentrations were also lower during low water, owing to sedimentation. There was a positive correlation at the intertidal stations of seston quantity with wind speed and wave action. At a wave length exceeding twice the water depth, re-suspension of low quality bottom material was observed and seston quality decreased.It was concluded that food availability for benthic suspension feeders was lower than suggested by routine monitoring data. At the intertidal stations, food quality was further reduced during periods of increased wind velocities and wave action. The low near-bottom food quality can partly be considered as an effect of the feeding activity of the benthic suspension feeders.

Consequences of the Sudden Removal of Nearly All Mussels and Cockles from the Dutch Wadden Sea

Abstract. Three successive years of recruitment failure, intensive fishery, and high rates of natural mortality eliminated all intertidal mussel beds and nearly all cockle beds from the Dutch Wadden Sea in the course of 1990. In late 1990 and early 1991, zoobenthic biomass and particularly bivalve stocks were unprecedentedly low. The following unusual events were noted: Relatively high diatom and chlorophyll concentrations in the autumn of 1990 and an exceptionally early and dense spring bloom of diatoms in March 1991; Exceptionally early onset of rapid growth in the bivalve Macoma balthica in March 1991; Above‐average weights of the soft parts of bivalves in the 1990/91 winter; Elevated mortality rates in bivalves that could serve as alternative prey for birds specialised on big bivalves (oystercatchers and eiders); Signs of food shortage in such birds: high mortality rates and emigration to other feeding areas; A rapid recovery of the benthos started in the summer of 1991, accelerated by an exceptionally high recruitment in some of the affected species. It is concluded that the Wadden Sea ecosystem responded in an elastic way to the removal of two of its key species. The events are not seen as merely coincidental but are regarded as a chain of causes and effects.

The Role of the Mussel (Mytilus edulis L.) and Mussel Culture in the Dutch Wadden Sea

Mussel populations (Mytilus edulis) in the Dutch Wadden Sea (intertidal mussel beds, subtidal beds and culture plots), the culture methods, the extent of mussel culture, and the ecology of the mussel are described. Mussels filter suspended matter from the water column and deposit it as feces and pseudofeces. Mussel beds consume large amounts of phytoplankton and speed up the cycle of production and breakdown of organic matter. There are indications that the consumption of phytoplankton can lead to food shortage for several animal groups. Mussels serve as an important food source for a wide range of organisms (e.g., starfish, eider ducks, and oystercatchers). Because mussel culture increased the mussel biomass in the Dutch Wadden Sea, the impact also increased. The most obvious impact of the culture is the dredging of seed mussels. Overexploitation of intertidal mussel and cockle beds and bad spatfall of both mussels and cockles since 1988 had a negative impact on bird populations. The extent of positive and negative aspects of mussel culture depends on natural and human influences. The negative aspects may (partly) be overcome by appropriate measures. *** DIRECT SUPPORT *** A01BY069 00007

Survey of human pathogenic bacteria and viruses in cockle beds at Otakou, Otago Harbour, New Zealand

Abstract The Otakou area on the Otago Peninsula, New Zealand has extensive cockle beds which may be exposed to faecal contamination from several sources. Shellfish and sediments from four sites at Otakou were sampled monthly from March to August 1988 and tested for faecal coliforms, Salmonella, Shigella, Aeromonas hydrophila, Vibrio para‐haemolyticus, and enteroviruses. No pathogenic bacteria were isolated from the shellfish or the sediment samples. Poliovirus Type 1 was isolated from the shellfish on three occasions at three separate sites. Faecal coliform concentrations in the cockles were below 230 MPN/100 g on all but one occasion including the times when viruses were isolated. It was concluded that the Otakou area is subjected to sporadic faecal contamination and that the presence of faecal coliform bacteria did not provide an adequate indication of the presence of viruses.

The importance of some epibenthic predators on the density of juvenile benthic macrofauna in the Danish Wadden Sea

Recruitment of Cerastoderma edule (L.), Mya arenaria L., Mytilus edulis L., and Macoma balthica L.) on an intertidal flat in the Danish Wadden Sea was very successful in a year following a severe winter. Initially, densities of Cerastoderma edule up to 60000 ind · m −2 were observed but shortly after settling declining densities occurred. Among the main epibenthic predators on juvenile animals on the flat, small individuals of the shore crab, Carcinus maenas (L.) were found to be important. Thus, laboratory experiments demonstrated that juvenile crabs in particular were preying on Cerastoderma edule. The juvenile crabs arrived simultaneously with the spat of C. edule. In July and August, the production of juvenile crabs was estimated to account for 26.1% of the elimination of cockles. Later the cockles were protected from predation by crabs by size. Analysis of stomach contents showed that another dominating predator on the flat, Crangon crangon (L), was mainly feeding on meiofauna. We suggest that the juvenile individuals of Carcinus maenas play a key rôle in structuring the benthic community in the Wadden Sea by preventing the development of cockle beds.

Prey depletion and foraging strategy in the Oystercatcher Haematopus ostralegus.

The responses of a population of Oystercatchers to their own depletion of their prey, the edible cockle Cerastoderma edule, have been examined in Strangford Lough, Northern Ireland., Cockle stocks were severely depleted each winter as a result of predation by Oystercatchers and about half the birds present in October in the main study area had left by March: the reduction was greater when the initial population of Oystercatchers was high than when it was low. Oystercatchers were initially widespread around the Lough but tended to aggregate by January into a few good sites. Within each site annual differences in the location of feeding effort were correlated with year to year variation in the location of second winter cockles.Within the feeding areas in a bay the birds apparently hunted in the short-term on the basis of expectation, ceasing to crop, the densest cockle beds once an average yield for the area has been removed; this yield was apparently estimated from the local density of the anvils on which captured cockles were broken open. This effect led to marginal areas being depleted of cockles more rapidly than high density areas, so that the birds gradually concentrated their hunting onto an increasingly restricted area of the bay. The birds initially took only second winter and larger cockles but by late winter cockle densities had fallen so much that smaller cockles were accepted: areas previously abandoned or unused for hunting were then economically viable because of the reduced expectations of the birds and were incorporated into the feeding areas.These results are seen as consistent with Royama's profitability theory.

Observations on some population parameters of exploited stocks of Senilia senilis (=Arca senilis) in Sierra Leone

The West African cockle Senilia senilis (L.) (=Arca senilis L., 1758) from some estuaries and muddy shores of Sierra Leone was studied from September, 1973 to February, 1975. Quantitative samples were obtained along transects using quadrats and sieves; some random hand-picked collections were also made. The length, number of rings and the maximum width of the inter-ring bands along the outer convex surface of the right valve were recorded from samples of ≥25 specimens. All the cockle beds examined were exploited and had stocks with a mean density of ≤9 large (>10 mm) cockles/m2, and at River No. 2 estuary at peak spatfall, of 130 seed cockles/m2. Spat settlement began in November-December after the rainy season, reached a peak in January-February and continued to the early part of the following rainy season (May-June). Seed cockles were absent from August to October. In the length-frequency distributions, modes were clear and associated with certain shell-ring numbers. Rings on shells were formed once a year, during July to September, the period of maximum rainfall, and they were successfully used in ageing the cockles. The first ring was formed at a mean age of 7 months and the subsequent ones annually. The growth period of the inter-ring bands, considered to extend from August to the following July (s. senilis growth year), may actually be of only 10 months duration. Sublittoral cockles from No. 2 estuary showed a higher growth rate than those from the levels exposed at low tide and subjected to greater exploitation. Cockles from the Sierra Leone River Estuary showed even greater growth. s. senilis grows slowly and lives long (up to 8 to 9 years), and its von Bertalanffy parameters are: River No. 2 estuary cockles →k=0.27 to 0.31 and L ∞=99 mm; Sierra Leone River Estuary cockles →k=0.22 and L ∞=145 mm. At No. 2 estuary, cockles enter the fishery at 22 to 25 mm (14 to 15 months old) and exploitation is heavy, resulting in the stocks being dominated by a few young year-classes.

Observations on Arenicola marina

ON July 19 whilst passing across the Dee sands (Cheshire) from Heswall to the cockle beds, many thousands of the common lugworm, Arenicola marina, were seen lying out of their burrows partly or completely exposed on the surface of the dry sand and in pools almost everywhere at half-tide level, while numbers were being washed downstream in the drainings from the sands with little chance of survival. None had been seen on similar visits made on May 27 and 31.

Summer Mortality of Cockles on some Lancashire and Cheshire Dee Beds in 1933

DURING the last two years frequent visits have been made to the cockle beds in the north-west part of Morecambe Bay, and lately also to the beds in the Dee Estuary, Cheshire, in connexion with a study of sex, spawning and spatfall of Cardium edule and its allies.

Strange Spatfall of the Common Mussel on the Common Cockle

AN interesting and unusual spatfall of the common mussel, Mytilus edulis, on the common cockle, Cardium edule, resulting in a related and dual mortality was observed in 1932 in the course of investigations on the extensive cockle beds of Cark sands at the head of Morecambe Bay. In June, heavy falls of mussel spat (5–10 mm. long) were found in several large areas on these beds, an event unprecedented in the experience of local middle-aged fishermen. At low water on these desert-like sands, the areas with mussels loomed in the distance like rocky outcrops. Such an unusual spatfall may be regarded as a natural experiment on the part of Mytilus to establish itself on sandy ground.

Marine Biology at Plymouth

THE latest number of the Journal of the Marine Biological Association (N.S., vol. 14, No. 2, August 1926, 10s. net) is full of good things. Dr. Orton resumes the interrupted publication of his studies on the rate of growth of marine organisms with a paper on the cockle. The investigations were carried out mainly in an experimental box laid down in the estuary of the river Yealm, in which the growing cockles were exposed to practically natural conditions, and the results have been checked by observations on near-by cockle beds. Apart from the definite determination of growth-rate—a matter of some economic importance, though it may be expected that the growth-rate will vary in different localities—the main interest of the paper lies in the study of the growth rings on the shell. It appears that, in the main, the deeply marked rings do indicate the winter checks in growth, and may be used—with caution—to determine the age of the cockle. Dr. Orton has, however, made the interesting observation that the mere removal of the cockles from the box for an hour or so for the purpose of examination suffices to cause the appearance of a “disturbance ring” on the shell. Specially well - defined rings are induced by the technique adopted of marking the shells for identification purposes with a file. Such disturbance rings can also be induced in the mussel. Further, in mild winters the winter-ring may become extended and spread out into several rings, so that accurate determination of the age by means of winter-rings is a matter of some difficulty, especially in the larger individuals. The paper would have been improved by the addition of a summary.