POPULATION DYNAMICS OF THE COCKLE CERASTODERMA EDULE IN THE STOUR ESTUARY P.
N.
SHARPE
The cockle Cerastoderma edule (L.) inhabits a wide ränge of sediments from soft mud to coarse sand. Its geographical distribution extends from the Baltic and Barents Sea to the West African coast of Senegal (Tebble, 1966). The species is common on most intertidal mud-flats and is the dominant bivalve on the Stour estuary at Wrabness. A number of studies on the growth rate and population structure of C. edule have been carried out, although none have been made on the Stour estuary. In addition, most studies have been made over a relatively short period, whilst the present investigation was made over a period of approximately three years. Methods The site at Wrabness (Grid reference TM 163323) was visited at approximately two monthly intervals between March 1971 and April 1973. A Square galvanised steel quadrat frame with sides 25cm in length and 30 cm deep was pushed vertically into the sediment at mid-tide level and the enclosed sediment was removed. When larger samples were required the frame was withdrawn and the above method was repeated. The sediment was passed through a sieve with a 1mm square mesh and the cockles were extracted. A 1mm mesh was chosen to give results comparable with previous studies by other workers. The length of each shell was measured with a pair of vernier gauge calipers, although for very small individuals a calibrated graticule eyepiece in a low power microscope was used instead. The length of each annual ring, when visible, was also measured as this represented the amount of growth, in length, of the previous year. The age of most individuals could therefore be determined by interpretation of the growth rings on the shells. A few individuals did not produce an annual ring every winter. Their age was determined by counting the internal daily growth lines of the shell. Any cockle having either no annual rings, or too few for its length when compared with other individuals from that sample, was treated in this manner. The technique used was a slight modification of that described by Farrow (1971). Disturbance rings were also present on some of the shells although these could be
336
Suffolk
Natural History, Vol. 17, Part 4
<x
Z3
o
nO-
•O
o
2
"-fW^i
in
(—
o 00
Z
<f)
I
•«
1
h-f—i
11 l—«—t
(UJW)l|j2u3| ||3l|S UE3W
D Y N A M I C S O F T H E COCKLE IN T H E STOUR ESTUARY
337
distinguished from the annual rings as the former were Iess well defined and often accompanied by a marked change in the shell outline. On determining its age, each animal was placed into a year-group, the youngest being the O-group. When spatfall occurred, the new animals became the O-group and the other year-groups advanced by one. To investigate the growth rate, the mean shell length and the ränge of lengths of each year-group was plotted against the time of year, irrespective of the year in which the sample was taken (Fig 1). Recruitment of Cerastoderma edule Cerastoderma edule has a planktonic larva which takes approximately one month to metamorphose (Orton, 1926). It then settles as a spat, resembling the adult cockle, on the inter-tidal sediment. The earliest time of year in which the newly settled spat was found was taken in early August 1972. The animals formed a distinct group with shell lengths ranging from 2.9mm to 7.3mm (n = 61, x = 5.1mm, Ü = 1.13) (Fig 1). By early October 1972 the O-group animals ranged in length from 3.6mm to 14.6mm (n = 82, x = 10.88mm, cf = 2.67). These results indicate that spawning probably took place in late spring and extended over several months into the summer. In late November 1971 a distinct sub-group of O-group animals, all less than 5mm in length was found, whilst in mid-April 1973 a similar sub-group, some as small as 2mm in length, was present. The small size of these animals indicated a second peak of spawning in the late summer and early autumn of both 1971 and 1972. Growth rate Figure 1 shows the growth rate of C. edule at Wrabness. After settlement, growth of the spat was rapid. It slowed during the autumn and ceased altogether in most of the O-group animals during the winter. Cessation of growth was indicated by the formation of a distinct black line on the shell margin. The time of resumption of growth was variable. In mid-March 1972, a number of O-group animals had growth check lines on the shell margins, so growth had only recently resumed. By contrast, in late March 1971 the O-group had increased its mean length by 8.6mm (n = 48, x = 15.1mm, d = 0.84) from the mean length of the first annual ring, whilst in
Suffolk
338
Natural History,
Vol. 17, Part 4
mid-April 1973 the mean length of the O-group had increased by only 0.3mm (n = 69, x = 10.9mm, a = 0.26) since the winter growth check. Younger animals generally resumed growth before the older year-groups. A fast growth rate continued throughout the animals' second summer and slowed during the autumn. Most individuals ceased growth for some time during the winter. Growth during subsequent years showed a similar pattern to that previously described, except that the rate slowed considerably. Table 1 shows the mean shell length at which the annual rings were formed. Animals from the 3- and 4-groups were only found infrequently, the largest individual being 31.7mm in length. Table 1.
Mean length (mm) of annual rings of Cerastoderma edule at Wrabness. First ring (mm)
Second ring (mm)
1970/1971
7.2 (n=90)
19.8 (n=31)
1971/1972
9.5 (n=57)
21.4 (n=38)
1972/1973
10.6 (n = 64)
20.8 (n=13)
Year
Density and age structure The total density and density of each year-group was calculated for all the samples. The results are shown in figure 2. The numbers of animals from each year-group are also expressed as a percentage of the total population (Fig 3). The O-group was numerically important throughout the period of the study despite the great variability in spatfall. A f t e r settlement, the density of a year-group decreased steadily and after two years it would comprise less than 20 per cent of the population. Animals older than the 3-group were rarely found and never had a density of greater than 10 per m2 or formed more than 5 per cent of the population. However, when biomass was considered, the older year-groups were still very important. For example, in late November 1971, the O-group formed 48 per cent of the population and had a total dry flesh weight of approximately 0.4g per m 2 , whilst the 3-group which comprised only 4 per cent of the population weighed4.1 g p e r m 2 .
DYNAMICS OF THE COCKLE IN THE STOUR ESTUARY
339
Mortality Mortality was continuous throughout the year although during spatfall this was masked. It was particularly heavy in the 0- and 1-groups and by the animals' second winter the densities were relatively low (Fig 2). Further decreases in the density were more gradual than in the previous years and some individuals could live for at least four years. Discussion T h e time and duration of spatfall of Cerastoderma edule at Wrabness was s i m i l a r to that at other sites reported in the literature. On the coast of Denmark the species spawned mainly in the spring and early suramer, although a few planktonic veliger larvae were found in October (J0rgensen, 1946). Off the South West coast of England, Lebour (1938) found veligers in the spring and summer, whilst Orton (1926) found eggs in the plankton between mid-April and midO c t o b e r . O r t o n considered that C. edule spawned continously during the summer and the results of the present study also suggest this. A second peak in spatfall, during the a u t u m n , has also been reported by Hancock (1967) on the Llanrhidian sands although it was earlier than at Wrabness. H o w e v e r , none of these reports showed the considerable V a r i a t i o n which could exist irr spatfall at one site in different years, as exemplified by the present study. A f t e r settlement, growth of the spat was rapid and continued to be so until the autumn. The rate slowed during the winter and most of the 0-group animals ceased growth for a time, as shown by the winter growth rings. Similar patterns of growth were reported by Orton (1926), Kreger (1940), Kristensen (1957) and House & Farrow (1968). A number of authors including Stephen (1931), Cole (1956) and Hancock (1967) also reported the presence of growth rings on the shells. Considerable Variation was found in the literature in the length attained at the winter growth checks. Cole (1956) and H a n c o c k (1967) found similar mean lengths of the winter growth checks to those at Wrabness for C. edule from Poole H a r b o u r and the Llanrhidian sands respectively. At Plymouth ( O r t o n , 1926) and on the Dutch Waddensea (Kreger, 1940) much greater lengths were attained, whilst in the Clyde estuary much smaller mean shell lengths were recorded (Stephen, 1931). Further, considerable annual Variation in the
340
Suffolk
Natural History,
Vol. 17, Part 4
Density per
M A M J
J A S O N D J F M A M J 1971
J A S O N D J F M A M 1972
1973
Density of Cerastoderma edule at Wrabness. • 2-group, A 1-group, • 0 - g r o u p , • t o t a l .
Fig 2.
Key:
03-group,
Percentage of population
60
M A M J
J A S O N D J F M A M J 1971
Fig 3.
J
A S O N D J
1972
Percentage composition of each year group of Cerastoderma Wrabness. Key: 0 3-group, • 2-group, A 1-group, # 0 - g r o u p .
FMA 1973
edule at
D Y N A M I C S O F T H E COCKLE IN T H E STOUR ESTUARY
341
length at which the first winter ring was formed existed at Wrabness. This reflected annual differences in the time and duration of spatfall, and subsequent development of the 0-group animals. Orton (1926) and Kristensen (1957) also found the growth rate showed considerable variations between one year and the next. Growth resumed in the early spring, although the time was variable. Annual differences in the environmental temperatures could have influenced the times of resumption of growth. Such temperatures could not be recorded so their influence is unknown. On the Dutch Waddensea, the 0-group also re-commenced growth in the early spring (Kreger, 1940). A fast growth rate in the animals' second summer, followed by a slowing in the autumn, was recorded in the present study. Similar situations were reported by Orton (1926), Kristensen (1957) and House & Farrow (1968). A decreasing growth rate in subsequent years was observed at Wrabness and compared with results recorded by Orton (1926) and Kristensen (1957). With increasing age the shell length r채nge of C. edule from Wrabness decreased. This was also observed by Kristensen (1957) for C. edule and by Gibson (1956) for Pecten maximus. The narrowing of the r채nge of a year-group was probably due to the 'catching-up' phenomenon, discussed in detail by Lammens (1967) in relation to Macoma balthica. After a heavy spatfall a particular year-group may remain dominant for several years (Stephen, 1932; Kristensen, 1957; K체hl, 1972). Probably as a reflection of light spatfalls at Wrabness a year-group did not remain dominant for more than P/2 years during the study period. The life span of C. edule was similar to that at other sites (Brady, 1943; Cole, 1956; Hancock, 1967). The density of C. edule at Wrabness showed seasonal and annual fluctuations, and was mainly influenced by the abundance of 0-group animals. An 'average' figure for the density of C. edule at Wrabness is therefore of little value although generally the density was similar to those given by a number of authors (Fr채ser, 1932; Spooner & Moore, 1940; Holme, 1949; Davis, 1967). Far higher densities have also been recorded (Kreger, 1940; Stopford, 1951; Smidt, 1951). These high densities were due to the abundance of spat and mass mortality followed, through overcrowding. Mass mortalities have also been caused by environmental extremes (Orton, 1929; Smidt, 1951; Kristensen, 1957; Crisp, 1964). During the present study, environmental extremes did not occur and mass
342
Suffolk
Natural History,
Vol. 17, Part 4
mortalities did not take place. However, the density of the 0-group feil noticeably in each year of the study, and by the animals' second winter it was relatively low. A n u m b e r of factors may have contributed to the changes in population structure and density. C. edule, especially the young animals, formed the food of several vertebrates including plaice (Pleuronectes platessa), flounder (P. flessus), oystercatchers ( H a e m a t o p u s ostralegus) and herring gulls (Larus argentatus) (Kreger, 1940). In Morecambe Bay, H. ostralegus fed heavily on young cockles (Davidson, 1967). D r i n n a n (1957) found each bird could eat up to 315 cockles per day and the flocks took about 22 per cent of the cockle population. These birds, together with other species, were commonly observed at Wrabness, so predation may have been one of the most important factors. High densities of bivalves, either of the same or different species may adversely affect the settlement and subsequent growth of spat (Green, 1957). Being drawn into the inhalent siphons of the adults and later rejected is sufficient to cause the death of the spat of C. edule (Kristensen, 1957). When the spat settle, competition from the adults for food and space can prevent many of the spat reaching maturity (Kreger, 1940). At W r a b n e s s the densities of adults of C. edule or any other species were not very high and it is doubtful if there was any great destruction of the spat by them. Inter- and intra-specific competition for food and space may reduce the density of a species, although only when the density is initially relatively high. Mytilus and Mya both feed on suspended food particles and could compete with C. edule for f o o d . These species were poorly r^presented at Wrabness and would have had little effect on the density of C. edule. Similarly, high densities of C. edule may reduce the growth rate (Cole, 1956; Farrow, 1971). A t Wrabness the densities w e r e too low for this to have been important. T h e nature of the sediment may influence both the growth rate and the density of C. edule, although reports in the literature are conflicting. In the present study, sampling was carried out in the same area throughout and although continuous monitoring of the nature of the sediment was not u n d e r t a k e n , there was no evidence to suggest that there were sufficient changes to cause the fluctuations in the population structure or the growth rate. T h e salinity of an estuary is never constant and in extreme situations, reduced salinities may have an effect on the growth
D Y N A M I C S O F T H E COCKLE IN T H E STOUR ESTUARY
343
rate or density of an animal. However, C. edule inhabits sediments through which there is little movement of the overlying water (Reid, 1930, 1932; Smith, 1955). Periodic covering of the mud-flats by low salinity water would have had a negligible effect on either the density or the growth rate. The waters of an estuary undergo diurnal and annual fluctuations in temperature, whilst the surface of the mud-flats have an even greater ränge. Environmental temperature change was probably the most important factor in causing the changes in the growth rate during the year, although as Farrow (1971) has shown, the growth rate does not exactly parallel changes in environmental temperature. The abundance of the 0-group had a great effect on the density of the species. Its abundance in turn depended on the numbers of spat which settled out from the plankton. During the period of the study, the numbers of spat were never very large and hence the density of C. edule remained relatively low. Although each environmental factor exerted a varying degree of influence on the population dynamics of C. edule, no Single factor acted in isolation. Summary The growth rate of C. edule was studied over a three year period. It was shown not to be constant with age of the animals or between animals of the same age that had been spawned in different years. The population structure and density was shown to fluctuate both during a year and from one year to the next. The time of spawning at Wrabness was similar to that reported in the literature although the Variation in timing and duration which existed has not been demonstrated previously. Acknowledgements This work formed part of a Ph.D. thesis submitted to the University of London, and was carried out under the supervision of Dr. W. A . M. Courtney of Westfield College, whose advice and criticism is gratefully acknowledged. The work was supported by a grant from the Natural Environment Research Council. References Brady, F. (1943). The distribution of the fauna of some intertidal sands and muds on the Northumberland coast. J. Anim. Ecol. 12: 27-41.
344
Suffolk
Natural History, Vol. 17, Part 4
Cole, H. A . (1956). A preliminary study of growth-rate in cockles (Cardium edule L.) in relation to commercial exploitation. J. Cons. perm. int. Explor. Mer. 22: 77-90. Crisp, D . J. (1964) (Ed.). The effects of the severe winter of 1962-63 on marine life in Britain. J. Anim. Ecol. 33: 165-210. Davidson, P. E. (1967). The oystercatcher as a predator of commercial shell fisheries. Ibis 109: 473-474. Davis, D. S. (1967). The marine fauna of the Blackwater estuary and adjacent waters, Essex. Essex Nat. 32: 1-61. Drinr.an, R. E. (1957). The winter feeding of the oystercatcher (Haematopus ostralegus) on the edible cockle (Cardium edule). J. Anim. Ecol. 26: 441-469. Farrow, G. E. (1971). Periodicity structures in the bivalve shell: Experiments to establish growth controls in Cerastoderma edule from the Thames estuary. Palaeontology, London 14: 571-588. Fräser, J. H. (1932). Observations on the fauna and constituents of an estuarine mud in a polluted area. J. mar. biol. Ass. U.K. 18: 69-85. Gibson, F. A . (1956). Escallops (Pecten maximus) in Irish waters. Sei. Proc. Roy. Dublin Soc. 27: 253-270. G r e e n , J. (1957). The growth of Scrobicularia plana (da Costa) in the Gwendraeth estuary. J. mar. biol. Ass. U. K. 36: 41-47. Hancock, D. A. (1967). Growth and mesh selection in the edible cockle (Cardium edule L.). J. appl. Ecol. 4: 137-157. H o l m e , N. A . (1949). The fauna of sand and mud banks near the mouth of the Exe estuary. J. mar. biol. Ass. U.K. 28: 189-237. House, M. R. & Farrow, G. E. (1968). Daily growth banding in the shell of the cockle, Cardium edule. Nature, Lond. 219: 1384-1386. J0rgensen, C. B. (1946). Lamellibranchia, in Thorson, G . , 'Reproduction and larval development of Danish marine bottom invertebrates, with special reference to the planktonic larvae in the Sound (0resund)'. Medd. Komm. Danmarks Fisk. Havundersog (Plankton). 4 (1): 277-311. Kreger, D. (1940). On the ecology of Cardium edule L. Arch. neerl. Zool. 4: 157-200. Kristensen, I. (1957). Differences in density and growth in a cockle population in the Dutch Wadden Sea. Arch. neerl. Zool. 12: 351-453.
D Y N A M I C S O F T H E COCKLE IN T H E STOUR ESTUARY
345
KĂźhl, H. (1972). Hydrography and biology of the Elbe estuary. Oceanogr. Mar. Biol. Ann. Rev. 10; 225-309. L a m m e n s , J. J. (1967). Growth and reproduction in a tidal flat population of Macoma balthica (L..). Neth. J. Sea Res. 3 (3): 315-382. L e b o u r , M. V. (1938). Notes on the breeding of some lamellibranchs from Plymouth and their larvae. J. mar. biol. Ass. U.K. 23: 119-145. O r t o n , J. H. (1926). O n the rate of growth of Cardium edule. Part 1. Experimental observations. J. mar. biol. Ass. U.K. 14: 239-280. O r t o n , J. H . (1929). Severe environmental mortality among Abra (Syndosmya) alba, Donax vitatus, and other organisms off the Lancashire coast. Nature, Lond. 124: 911. Reid, D. M. (1930). Salinity interchange between sea water in sand and overflowing fresh-water at low tide. J. mar. biol.' Ass. U.K. 16: 609-614. Reid, D. M. (1932). Salinity interchange between salt water in sand and overflowing fresh-water at low tide II. J. mar. biol. Ass. U.K. 18: 299-306. Smidt, E. L. B. (1951). Animal production in the Danish W a d d e n s e a . Medd. Komm. Denmarks Fiskeri-og Havunders. 11 (6): 1-151. Smith, R. I. (1955). Salinity Variation in interstitial water at K a m e s Bay, Millport, with reference to the distribution of Nereis diversicolor. J. mar. biol. Ass. U.K. 34: 33-46. S p o o n e r , G. M. & Moore, H. B. (1940). The ecology of the T a m a r estuary. VI. A n account of the macrofauna of the intertidal muds. J. mar. biol. Ass. U.K. 24: 283-330. S t e p h e n , A . C. (1931). Notes on the biology of certain lamellibranchs of the Scottish coast. J. mar. biol. Ass. U.K. 17: 277-300. Stephen, A . C. (1932). Notes on the biology of some lamellibranchs in the Clyde area. J. mar. biol. Ass. U.K. 18: 51-68. Stopford, S. C. (1951). A n ecological survey of the Cheshire foreshore of the Dee estuary. J. Anim. Ecol. 20: 103-122. Tebble, N. (1966). British bivalve seashells. London: British M u s e u m (Natural History). P. N. Sharpe, Ph.D. 168 Thorpe Road, Kirby Cross, Frinton-on-Sea,
Essex.