Some observations on the littoral barnacle populations of North Wales

Marine Enrironmental Research 5 (1981) 227-240 SOME OBSERVATIONS ON THE LITTORAL BARNACLE POPULATIONS OF NORTH WALES SHEILA J. BENNELL Coastal Surveillance Unit, University College of North Wales, Marine Science Laboratories, Menai Bridge, Gwynedd LL59 5EH, Great Britain (Revised version received: 3 October, 1980) ABSTRACT Fluctuations in the littoral barnacle populations of North Wales from 1974 to 1979 are described. Sites with high barnacle cover had regular seasonal fluctuations in cover. A detailed study of Balanus balanoides in 1979 showed that settlement was heaviest when the wind was offshore and the sea calm, and least along the north coast of Anglesey, where littoral larvae must be more dispersed. Site to site variations in timing differed from year to year. It was suggested that both wind and diffi~sion patterns in the sea could affect timing and distribution of settlement. INTRODUCTION Five species of barnacle are commonly found on the rocky shores of North Wales-- Balanus balanoides L., Balanus crenatus Bruguiere, Elminius modestus Darwin, Chthamalus stellatus (Poli) and Chthamalus montagui Southward. B. balanoides occurs on all rocky shores; B. crenatus is found low on a few shores; E. modestus is most common in sheltered areas, particularly in the Menai Strait; Chthamalus spp. are found only at a few Anglesey west coast sites and on the Lleyn Peninsula and in considerably smaller numbers than B. balanoides. C. montagui is the more common chthamalid species, C. stellatus occurring only on exposed headlands. The Coastal Surveillance Unit of the University College of North Wales has been carrying out a surveillance programme on the shores of North Wales (in particular Anglesey) since 1974 (Jones et al., 1979a). As part of this programme barnacle cover is estimated at monthly intervals on fixed transects at seven sites around Anglesey 227 Marine Environ. Res. 0141-1136/81/0005-0227/$02.50 © Applied Science Publishers Ltd, England, 1981 Printed in Great Britain 228 SHEILA J. BENNELL (Fig. 4). Other sites are visited at less frequent intervals. The sites investigated include some with high barnacle cover and low algal cover (Ynysoedd Duon, Wylfa and Porth Swtan) and others less exposed with lower barnacle cover and high algal cover (Bull Bay East, Amlwch, Porth yr Ysgaw and Menai Bridge). Fluctuations in barnacle cover which were observed by the transect survey suggested a need for more detailed investigation, particularly of Balanus balanoides. This is a boreo-arctic species which approaches its southern limit of distribution in the south of England and is the dominant littoral barnacle species in North Wales. Its cyprids settle during a few weeks in April and May in North Wales. Settlement is known to occur earlier in the south of England and Ireland and later (May-July) on the north coast of Scotland and east coast of England (Crisp, 1959). The results of the transect survey revealed that there may also be differences in both the amount and timing of settlement over a small geographical range. In order to investigate this in more detail a concentrated survey of B. balanoides settlement was undertaken in the spring of 1979. The day to day settlement ofcyprids was monitored at two sites, one at Porth yr Ysgaw on the north coast of Anglesey and the other at Porth Trecastell on the southwest coast of Anglesey. Spot checks of settlement were also made at several other sites around North Wales during the week following the end of settlement. It is proposed to present in the first Section, barnacle fluctuations as recorded from the transect surveys and, in the second Section, the results of the detailed study of B. balanoides settlement during the spring of 1979. I. FLUCTUATIONS IN POPULATIONS OF BARNACLES FROM APRIL 1974 TO JUNE 1979 (TRANSECT SURVEY RESULTS) Methods In this section the data are derived from the stored records of the Coastal Surveillance Unit and have been collected by various members of the Unit whose help is gratefully acknowledged. The data were collected by methods described in detail in Jones et al. (1979b). Briefly, the method comprises a fixed belt transect 50 cm wide stretching from about Mean High Water Springs to Extreme Low Water Springs. 50 x 50 cm quadrats are laid edge to edge down the transect and barnacle cover recorded by eye as a percentage of the area of each quadrat. The results are presented mainly in the form of 'summed' graphs in which the mean cover value of the barnacle within its zone on the shore is plotted against time. In 1974 and 1975 juveniles were recorded as cover for the first few months after settlement. Since 1976 juveniles have been recorded as density over a certain area in each quadrat. To enable these results to be comparable the juvenile values for 1976-79 have been converted to cover by taking the maximum juvenile density over the whole quadrat as 100~ and calculating the actual cover as a proportion of this. OBSERVATIONS ON BARNACLE POPULATIONS 229 ~o • ' , o . u~ < ~.~ e~ ~ • ~ 230 SHEILA J. BENNELL Results Figure 1 shows the changes in cover of B. balanoides at seven sites during the period 1974-1979. The dotted line represents the cover of juveniles in the two months after settlement. In subsequent months such juveniles are included in the adult cover figures. A graph of E. modestus abundance for Menai Bridge is also included. Recruitment levels ofB. balanoides, which varied from site to site and from year to year, are shown in Table 1 along with the mean adult density and cover at each site. Figure 4 shows the location of the sites. TABLE 1 RECRUITMENT OF B. balanoides IN THE MONTH FOLLOWING SETTLEMENT. MEAN ADULT DENSITY AND COVER ARE ALSO SHOWN Site No. juveniles/cm 2 Mean adult Mean adult density cover 1976 1977 1978 (No./cm 2) (%) 1976-78 1976-78 Porth Dinllaen 4 3 3 2-3 30 Ynysoedd Duon 13 9 7 4-5 55 Porth Dafarch 5 7 6 4-5 50 Porth Swtan 3 4 2 2-3 30-35 Wylfa 2 2 2 3--4 45 Bull Bay East 2 2 1 < 1 8 Amlwch 1 2 2 < 1 10 Porth yr Ysgaw 2 2 2 I 10 Menai Bridge 9 9 3 2-3 30 (a) Balanus balanoides: Mean cover was highest at sites with low fucoid cover (Ynysoedd Duon, Wylfa and Porth Swtan) and low at fucoid-covered sites (Amlwch, Bull Bay East, Porth yr Ysgaw and Menai Bridge). In general, sites with high barnacle cover had large seasonal fluctuations in cover which were quite regular from year to year (Ynysoedd Duon and Wylfa) (see Fig. 1 for details). The remaining sites, with lower cover, had more irregular seasonal fluctuations which, in some cases, resulted in longer term change. The amount of settlement varied from place to place and from year to year. Ynysoedd Duon, on the southwest coast of Anglesey, consistently had the highest recruitment, followed by sites at Menai Bridge and Porth Dafarch (in the Menai Strait and on the west coast of Anglesey respectively) and by Porth Swtan and Porth Dinllaen (on the north west coast of Anglesey and Lleyn Peninsula respectively). Wylfa, Bull Bay East, Amlwch and Porth yr Ysgaw, all on the north coast of Anglesey, had very small settlements each year. In 1975 settlement was relatively small at all sites and again was small in 1978 (Table 1). The level of adult cover was not directly related to the amount of settlement. For example, sites with high mean cover did not necessarily have large settlement. This OBSERVATIONS ON BARNACLE POPULATIONS 231 can be illustrated by the graphs for Ynys0edd Duon and Wylfa which had large and small settlements respectively. Both sites had a similar pattern of high cover, summer increases and winter declines, but this pattern was achieved in different ways at each site. At Ynysoedd Duon the first seasonal peak occurred each June, followed by an immediate decrease in numbers owing to growth and consequent competition for space. At Wylfa the peak of abundance occurred later, in August and September, when the less densely settled juveniles had grown to near adult size and occupied a greater area. A second peak occurred at the former site at this time. That sites with low settlement and low cover often have the potential for an increase in cover is well illustrated by the graphs for Bull Bay East, Porth yr Ysgaw and Amlwch. Recolonisation has been taking place at Amlwch since the surface rock was removed in a barge accident in December 1974. Recruitment levels at Amlwch in 1977 and 1978 were very similar to those at the adjacent sites. Settlement at Amlwch was followed by an increase in adult cover whereas no such increase occurred at either Bull Bay East or Porth yr Ysgaw. Thus the survival of juveniles is more important to the resulting adult cover than the number that settle. In a year like 1975, when recruitment at all sites was low, only those sites where settlement was relatively higher were able to maintain their cover. Those with small settlements suffered large losses owing to adult mortality outweighing juvenile input. At any one site the cause of juvenile mortality may vary from year to year with the same final result. In August 1976, many young Balanus died at the top of the shore at Ynysoedd Duon. In contrast, in the other years the reduction in juvenile numbers took place later in the year and at varying levels of the shore. Times of settlement varied yearly and from site to site. In 1974 settlement was first recorded at the beginning of April, whereas in 1975, 1976, 1977, 1978 and 1979, settlement did not begin until the second week in April and the peak of settlement did not occur until the end of that month. In 1974, peak settlement occurred earlier at the northern end of the Menai Strait; from 1975 to 1978, however, peak settlement appeared to be earlier at the southern end of the Strait and on the west coast of Anglesey; in 1979 peak settlement occurred first at the southern end of the Strait, then on the north coast of Anglesey and finally on the west coast. Possible reasons for such differences in timing will be discussed in Section II. (b) Balanus crenatus: This low shore and subtidal barnacle is only found in any numbers at Menai Bridge and Penmon at both of which sites it extends higher on the shore than at the other sites. It is difficult to distinguish from B. balanoides when alive and in mixed populations. Its presence is noted by the calcareous scars left by the individuals which have been removed. (c) Elminius modestus: Although this species can be found all around Anglesey it is only in the Menai Strait that it makes a significant contribution to barnacle cover (Fig. 1). Juveniles settle all around the coasts but only a few survive to adulthood. At Menai Bridge peaks of Elminius abundance occurred at times when Balanus cover was relatively low. This may have resulted from straightforward competition for 232 SHEILA J. BENNELL space. Balanus cyprids settle in very large numbers over a short period of time in April and May but Elminiuscan be found in smaller numbers in the plankton and on the shore at most times of the year, perhaps in greater abundance in late summer and autumn. Since Elminius has, therefore, fewer cyprids settling at any one time, it is likely that those settling at the same time as Balanus is settling would be swamped by the latter's large numbers of newly settled, rapidly growing juveniles. Thus Elminus cover can usually increase only at times when Balanus is decreasing, and settlement all year round is thus to its advantage. One fact that has emerged from the survey is that although EIminius can settle at all times of the year.the numbers settling are usually greater at one time of the year and this time varies quite substantially from year to year. In 1974, 1976, 1978 and 1979 juvenile Elminius were found in greatest numbers during the months July to September but in 1975 and 1977 there was a heavy settlement all around Anglesey between October and December. Barnett (1979) found, from laboratory experiments, that Nucella lapillus fed upon Balanus in preference to Elminius. Thus, removal of adult Balanus each summer would leave space for Elminius to colonise. However some other factor must be capable of reducing Elminius numbers later in the year to leave space for more Balanus to settle. It will be of interest to find out whether, when Nucella is exposed to such mixed populations of EIminius and Balanus as occur in the Menai Strait, it still chooses Balanus at all times. It is possible that the preference for Balanus occurs since that species is the more common during the summer feeding time and Nucella becomes conditioned to it (Morgan, 1972). (d) Chthamalus spp.: Within our area of study the Chthamalid species occur on the Lleyn Peninsula and from Llanddwyn Island to Porth Swtan on the west coast of Anglesey. Only occasional individuals have been found east of Carmel Head (Fig. 4). Since Lewis (1953) studied the Anglesey shores Chthamalus has increased in numbers at Porth Swtan from a total of four to around 15 ~ cover at high shore levels but no increase has occurred at Porth y Pistyll which lies to the east of Carmel Head. Settlement of C. montagui has been very low since the start of the survey. In 1974 and 1975 no settlement of this species was recorded on Anglesey or the north coast of the Lleyn Peninsula; in later years a few settled each September (< 1/cm2). It is notable that on the southern side of the Lleyn Peninsula (outside the survey area) C. montagui juveniles were found in large numbers most years. C. stellatus juveniles have not been found at any time since the Unit recording began. The Chthamalid species show a distribution on Anglesey typical of the genus near its northern limit, with low total numbers confined to a narrow band at the top of the shore and with C. montagui more common than C. stellatus. (e) The transect suroey results: These posed several questions: 1. The amount of B. balanoides settlement varied consistently from place to place, being higher on the west coast of Anglesey and in the Menai Strait. OBSERVATIONS ON BARNACLE POPULATIONS 233 Was this due to the differing conditions on the transects or did the number of larvae settling on similar areas at each locality also vary? 2. At sites with low settlement were there also low numbers of larvae in the plankton? 3. If settlement was lower on some stretches of coast, could the locality where the division between high and low settlement occurred be pinpointed? 4. The timing of settlement varied from place to place and from year to year; what was the cause and extent of this variation? The study in Section II was undertaken to try and answer some of these questions. I I . SETTLEMENT OF Balanus balanoides AROUND NORTH WALES IN SPRING, 1979 The detailed investigations of settlement of this species were concentrated on Anglesey shores with supporting observations from further afield. (a) Daily settlement on Anglesey Methods: Two sites were chosen, one at Porth Trecastell on the south west coast of Anglesey and the other at Porth yr Ysgaw, near Point Lynas on the north coast of Anglesey (Fig. 4). At each site four 20 × 20cm areas in the centre of a dense barnacle covered area, at a mid-shore level and on a seaward facing slope, were scraped clean of adult barnacles about three weeks before settlement began. All limpets in the surrounding areas were also removed. The squares were scraped again at approximately weekly intervals up to the time of settlement. Each day, throughout the settlement period, the number of cyprids which had settled in a 15 × 15 cm quadrat within the cleared areas was counted. After counting, the cyprids were removed by brushing with a wire brush and the squares washed down with sea water. At both sites a fifth square was cleared on 22 May and settlement which occurred was allowed to accumulate in order to find the final density. Spot checks were taken of cyprid and recently settled barnacle density in the surrounding barnacle areas during the first few days of settlement to ascertain whether there was a difference between 'artificial' and 'natural' settlement. Since no noticeable differences were found it was assumed that the settlement squares were giving a reasonable representation of available cyprid settlement. Results: The results from the daily cleared squares are presented in Fig. 2. The mean density from the four squares at each site has been plotted since the daily trend in each of the four squares was similar. Actual densities varied from square to square, probably because of slight variations in topography. Settlement began at about the same time at both sites but the peak of settlement at Porth yr Ysgaw occurred on 21 April whereas it did not occur until 26 April at Porth Trecastell. By 25 April settlement at Porth yr Ysgaw had dropped to a low level. At Porth Trecastell it continued at relatively high levels until 4 May. It is also clear that 234 SHEILA J. BENNELL PORTH TREC.ASTELL 15' / /~ . . . . PORTII YR ~C~ t c~s, Hoct h east I South *es! I / / \ North west I South west n~th co~st winds l WII~s winds winds 10. • ~ ' / ' ~ ~ ~/- o , . . . . . . . . . . . : .', ':,-',-?:',, , , " - ~ II 12 13 1/. 15 15 17 18 19 20 21 22 23 ~J. 25 26 27 28 29 30 1 2 3 i. 5 6 7 8 9 10 II 12 13 1/. 15 APRIL MAY 1979 Fig. 2. Mean density of Balanus balanoides cyprids which settled each day on 15 x 15 cm squares (cleared daily) at Porth Trecastell and Porth yr Ysgaw during April and May 1979 (mean of 4 squares). The predominant wind directions during the settlement period are indicated. many more cyprids settled at Porth Trecastell than at Porth yr Ysgaw. This was borne out by the spot checks on the rest of the shore: at Porth Trecastell every available space was covered by cyprids whereas there was much available space clear of cyprids at Porth yr Ysgaw. There was no doubt that the timing of peak settlement was different at the two sites. Since this timing difference was known to vary from year to year it was thought that wind speed and direction could be influencing factors. Thus Fig. 2 also shows the mean wind directions for each period and Fig. 3 shows the wind index for each site on each day of the settlement period. The wind index was calculated by multiplying the wind force (Beaufort) by the direction of the wind (scaled 1-5, where 30" t .FORTH TRECASTI:LL • . . . . . . e PORTH YR YSGAW ", / 2o. 7 -~t ', ; ', ,~'/', A lo / " , . . .~ 0 APRIl. MAY 1979 Fig. 3. Wi•dindicesf•rP•rt•Trecaste••andP•rt•yrYsgawduringth•peri•d•fsett••m•nt•fBa•anus balanoides in April and May 1979. [Wind index = wind force (Beaufort) x direction ofwind (scaled 1-5) for each site, where 5 represents a directly onshore wind.] OBSERVATIONS ON BARNACLE POPULATIONS 235 TABLE 2 FINAL SETTLEMENT NUMBERS FOR PORTH TRECASTELL AND PORTH YR YSGAW Site 15 x 15 cm square cleared daily Sum of daily totals recorded (Mean of four squares) From beginning to After 22 May end of settlement 15 x 15cm square cleared once only on 22 May Total at end of settlement P. TrecasteU 26750 24275 7750 P. yr Ysgaw 10148 5051 1849 5 = onshore for each site). Thus a value of 30 could represent an onshore force 6 wind. During the peak of settlement at Porth yr Ysgaw the winds were strong south- westerly (offshore) and during the peak at Porth Trecastell the winds were strong north-westerly (again offshore). At each site the sea surface was very calm during offshore winds. The final settlement numbers for the two sites are shown in Table 2. From the Table, it can be seen that there were at least 2.6 times as many cyprids available to settle at Porth Trecastell than at Porth yr Ysgaw. From the results of the uncleared squares, it is seen that at both sites only ½ of the available cyprids settled or survived. This is understandable for Porth Trecastell where the final density of 7750 was about the highest possible density allowed by the space and this was achieved soon after clearance. However, at Porth yr Ysgaw a much higher final density could have been attained, i.e. space was still available. Since rock type and surrounding conditions were similar at both sites, it is possible that the less dense concentrations ofcyprids at Porth yr Ysgaw were less effective in gregariously attracting their successors and this could have been important in governing the final density. The other alternative is that one third of all settling cyprids suffer immediate mortality. Whichever possibility is correct, it appears from the results that Porth Trecastell received a greater input of larvae than Porth yr Ysgaw in Spring 1979. (b) Settlement around North Wales, Spring 1979 Methods: High barnacle cover areas were inspected at eighteen sites around the North Wales coast in the week immediately following the end of settlement. At each site between twenty and thirty areas, 3 x 3 cm in size, were surveyed. In each case the overall area was sited on a densely barnacle covered vertical rock face. To avoid distortion of the results because of different degrees of attraction of jfiveniles by adults, all squares were selected with similar numbers of adult barnacles (approximately half cover of adults). The ratios of numbers of juveniles to the number of adults were calculated. Results: The results are presented in Fig. 4. High ratios (representing heavy settlement) occurred on the southwest and Menai Strait shores of Anglesey and 236 SHEILA J , BENNELL / / / LOW SETTLE~,'EN'f / / HIGH SETTLEMENT / '~]]1 h.1 / F~nt Lynos / ~..-~ Pot I h yr Ysgow Carmel ~.i ~ ~'0 I/-~- / Head ~ ~ ~rvn/z.9 ~ "%% / / / _~, V" M~,'wa. B,,dge / ~..5 S ,, l=~'th" \ y] ~/t .O / Trecastel[ / / Duon ~J / / CAER~RFON Duon l LL~YN PENI NSULA Fig. 4. Map of Anglesey and the adjacent coast showing sites visited during the barnacle surveys and the ratio of juvenile: adult Balanus balanoides at sixteen locations during the week following the end of settlement in 1979 (see text). The dotted line separates areas of high and low settlement. along the North Wales coast to the Great Orme. Medium ratios occurred at Moelfre on the northeast coast of Anglesey and at Rhoscolyn on the west coast. Low ratios were recorded from Porth Dafarch on Holy Island around the north coast of Anglesey to Porth yr Ysgaw, and at Porth Dinllaen on the Lleyn Peninsula. (c) General observations on settlement Some light has now been shed on the four questions posed at the end of Section I. Thus, there is a marked variation in numbers of larvae settling around the North Wales coast, even where the substrates and surrounding conditions are similar (Question 1). In answer to Question 3, it can be said that the division between high and low settlement areas occurs around a line drawn from Rhoscolyn on the west coast of Anglesey to Point Lynas on the north coast with high settlement sites lying to the west and low settlement sites to the east of that line (Fig. 4). The Lleyn Peninsula was an exception lying on the 'wrong' side of the line. These results agreed with the results from the transect surveys. As for the cause and extent of settlement variation (Question 4), timing of peak settlement was found to vary from site to site, being earlier (in 1979) on the north coast than on the west coast. Settlement was highest during calm sea conditions, when offshore winds were blowing. Question 2 OBSERVATIONS ON BARNACLE POPULATIONS 237 can only be properly answered by carrying out an extensive series of plankton hauls on both sides of the island, but the present results suggest that there are fewer larvae in the inshore waters, at least, on the north coast of Anglesey. DISCUSSION Planktonic stages Although it has been shown that B. balanoides settlement and juvenile survival varies from year to year the causes are not yet clear. Low numbers of larvae in the inshore plankton may be related to adult condition or to factors affecting the planktonic stages; fertilisation of the eggs may fail, the adults may fail to release their larvae or the larvae may fail to survive to the settlement stage. Adults are sensitive to cold shock in the spring when their cold tolerance is lessening (Crisp & Ritz, 1967) and they are to some extent dependent on the timing and amounts of the spring phytoplankton bloom in order to release their larvae (Crisp & Spencer, 1958). Barnes (1956) found that in 'failure' years the early March phytoplankton bloom lasted for a shorter time than usual. Since the time spent in the plankton by the larval stages is variable it is difficult to relate numbers settling to factors affecting larval release. For example, late larval release may result in a shorter planktonic stage. Without a detailed survey of all planktonic stages and adult condition every year, it is impossible to pinpoint the effects of those factors. Before this survey was begun, it had been suggested that the division between high and low settlement areas on Anglesey might occur in the region of Carmel Head. It was here that Crisp & Knight-Jones (1954) found discontinuities in the distribution of some littoral marine organisms, in particular that Chthamalus sp. occurred only to the south and west of Carmel Head and was virtually absent from the north coast of Anglesey. They suggested that larvae from the south were swept offshore at Carmel Head. In the present study, however, the divisions between dense and sparse Balanus settlement occurred at Rhoscolyn and Point Lynas (Fig. 4). It was particularly noticeable that Porth Dafarch on Holy Island had variable amounts of settlement from year to year (Table 1) some years having heavy settlement similar to that at sites to the south east and in other years resembling, in its sparser settlement, sites to the north. Strong tidal currents flow around Anglesey. The areas on the inner edge of Caernarfon Bay and over the Lavan Sands at the northern end of the Strait lie in the 'shadow' of these currents. The pattern of variation in amounts of settlement could perhaps be explained by the diffusion or dispersion concept (Weidemann & Sendner, 1970; Okubi, 1971) where Anglesey could be considered as a large headland. On the open north coast of Anglesey greater diffusion would occur due to the stronger tidal currents and the greater area of open sea offshore. In the more sheltered inshore waters less diffusion would occur. With less diffusion the larvae would be concentrated in inshore waters, whereas where diffusion was greater, 238 SHEILA J. BENNELL larvae could either be swept past the coast by currents or diluted into a greater water mass offshore. Thus fewer larvae would be available to settle. Strong onshore winds can be an aid to diffusion and it is possible that this theory could also explain the differences in timing on either side of the island. With calm conditions, produced by offshore winds, the larvae might be concentrated inshore. There is however, also the possibility of an offshore wind causing upwelling of larvae at the coast (Lewis, personal communication). However, Barnes (1956) found that, in the Firth of Clyde, the numbers of larvae in the plankton decreased when the winds were offshore. It is clear that more observations are required before any firm conclusions about the effects of wind can be made. Certainly the timing of settlement is variable from year to year but the geographical variations in amount of settlement are consistent from year to year. Littoral stages Many cyprids must fail to survive initial settlement. At sites with a high density of cyprids there must be a spatial limit to the numbers settling and surviving and, as the results of Section II revealed, even when space was available for settlement, the density did not increase even though new cyprids were appearing each day. Connell (1961) concluded that heavy wave action caused by gales was responsible for a high proportion of juvenile Balanus death. It is also possible that wave action can dislodge the very recently settled cyprids, especially those which have not attached properly. Limpet grazing (Connell, 1961) can also account for large mortalities both at cyprid and juvenile stages. This could not account, however, for mortality in the experimental squares, as all limpets in the vicinity had been removed. As the young barnacles grow, particularly in areas of high settlement, mortality must result, in part, from overcrowding. The death of many young barnacles at high shore levels at Ynysoedd Duon in August 1976 followed a very long, hot, dry spell. This hot spell may also have been a factor in the low survival rate of juveniles at Porth Swtan and Wylfa where the shores are exposed to intense heat and sunlight. The other north coast sites are predominantly covered in Fucus, and under this shade, extreme heat is less likely to have been important. The effects of Fucus on settlement and survival are still under investigation (S. Hawkins, personal communication). It is certainly clear that less settlement occurs beneath Fucus than on adjacent clear areas and the low barnacle cover at Porth yr Ysgaw and Bull Bay East can be partly explained by the algal presence. However, in very sheltered areas, such as the Menai Strait, large populations of fucoids and barnacles co-exist. Perhaps the lack of wave action in the Strait allows barnacles to settle without being brushed by the waving fronds, or it is possible that the Ascophyllum nodosum, which is more common in sheltered areas, and, being a long buoyant alga, causes less disturbance at the water-rock interface and allows the cyprids to settle more freely. OBSERVATIONS ON BARNACLE POPULATIONS 239 Large mortalities of adults occurred at several sites in the spring and summer of 1975, following small settlements (Bull Bay East, Porth yr Ysgaw and Wylfa). It is not clear whether these mortalities occur most years and became noticeable in that year due to low recruitment or whether the adults were already in poor condition in the early spring and were unable to produce larvae in sufficient numbers, so that settlement was, in turn, poor. Nucella lapillus feeds extensively on barnacles, particularly in the summer months (Feare, 1970). Connell (1961) found that Nucella chose the larger barnacles as prey and this would account for a high proportion of the mortality of adult barnacles. However, numbers of Nucella were no higher in the summer of 1975 than in previous or later years. A longer period of feeding by Nucella during that summer cannot be ruled out. Certainly, when large-scale deaths of adults do occur, by whatever means, the extent of competition from other organisms becomes very apparent. The shell plates of individuals on the lower shore at Wylfa were rapidly covered by encrusting coralline algae in 1975. Since juvenile barnacles rarely settle on this type of alga, there has been no recolonisation by barnacles. A more detailed investigation of causes of mortality, particularly in relation to weather conditions, is at present being undertaken. ACKNOWLEDGEMENTS I wish to acknowledge the work of all past and present members of the Coastal Surveillance Unit, and especially that of Dr K. Hiscock, Dr J. Roberts and Mr J. Mitchell who collected some of the barnacle transect data. I thank Dr W. E. Jones and Dr G. Walker for helpful guidance and critical reading of the manuscript and Prof D. J. Crisp and Dr J. Simpson for helpful discussion. This work would not have been possible without the generous support of Shell UK Ltd. REFERENCES BARNES, H. (1956). Balanus balanoides L. in the Firth of Clyde: The development and annual variation of the larval population, and the causative factors. J. Anita. Ecol., 25, 72-84. B^Rr~ETT, B. E. (1979). A laboratory study of predation by the dog-whelk Nucella lapillus on the barnacles Elminius modestus and Balanus balanoides, j . mar. biol. Ass. U.K., 59, 299-306. CONNELL, J. H. (1961). Effects of competition, predation, by Thais lapillus and other factors on natural populations of the barnacle Balanus balanoides. Ecol. Monographs, 31, 61-105. CRISP, D. J. (1959). The rate of development of Balanus balanoides L. embryos in vitro. J. Anita. Ecol., 28, 119-32. CRISP, D. J. & KNXGItT-Jo~ES, E. W. (1954). Discontinuities in the distribution of shore animals in N. Wales. Bardsey Observatory 1954 Report. CRISP, D. J. & RITZ, D. A. (1967). Temperature acclimation in barnacles. J. e.xp. mar. Biol. Ecol., 1, 236-56. CRisP, D. J. & SPENCER, C. P. (1958). The control of the hatching process in barnacles. Proc. Roy. Soc. B., 148, 278-99. 240 SHEILA J. BENNELL FEAI~, C. J. (1970). Aspects of the ecology of an exposed shore population of dogwhelk Nucella lapillus L. Oecologia, Berlin, 5, i-18. JONES, W. E., FLETCHER, A., BENNELL, S. J., MCCONNELL, B. J., RICHARDS, A. V. L. & MACK SMITH, S. (1979a). Intertidal surveillance. In Monitoring the marine environment, ed. by Nichols, D., Symposium of the Institute of Biology, Vol. 24, !-23. JONES, W. E., BENNELL, S., BEVERIDGE, C., FLETCHER, A., MCCONNELL, B., MACK SMITH, S. ,g' MITCHELL, J. (1979b). Methods of data collection and processing in rocky intertidal monitoring. In The shore environment: Methods and ecosystems, Systematics Society. LEWIS, J. R. (1953). The ecology of rocky shores around Anglesey. Proc. Zool. Soc. Lond., 123, 481-549. MORGAN, P. R. (1972). The influence of prey availability on the distribution and predatory behaviour of Nucella lapillus L. J. Anim. Ecol., 41,257-74. Ogunl, A. (1971). Oceanic diffusion diagrams. Deep Sea Res., 18, 780-802. WEIDMANN, H. & SENDNER, H. (1970). Dilution and dispersion of pollutants by physical processes. FA 0 technical conference on marine pollution and its effects on living resources and fishing, Rome, FAO Publications.