No abstract

Recruitment of Semibalanus balanoides at Robin Hood Bay, North Yorkshire, between 1969 and 1981 ranged from almost total failure to ca. 30 spat/cm2. A similar annual pattern over 100 km of adjacent coast appears to implicate climatic or hydrographic factors. Adult fecundity did not vary greatly, but as there was substantial variation in the number of cyprids reaching the shore, events during the planktonic phase (particularly wind-driven larval dispersion) appeared to be of prime importance.Thereafter, settlement density was modified by cyprid behaviour, except when larvae were very abundant, by the availability of space and the temporal pattern of the settlement. Mortality during early shore life was highly variable, photographic studies in 1981 showing between 1% and 92% of settlers dying within ten days. Under highly desiccating conditions 70% of a single day's input to the upper shore died in the first 24h. Such severe mortality could be compensated for by later inputs as settlement extended over 4 weeks or more, primarily as a result of unexpectedly long and asynchronous release.

No abstract

Fluctuations in the annual recruitment of Patella vulgata at Robin Hood's Bay over 7 years have been analysed in relation to annual variations in gonad cycles and environmental conditions. Evidence to date suggests that short periods of low air temperatures during the first few weeks after settlement of spat in the late autumn exert the greatest influence upon the level of recruitment. Not only are the time and severity of autumnal frosts highly variable from one year to another, but gonad ripening and first spawning, determined respectively by summer temperatures and rough seas, can also vary independently by up to 7 weeks. Although an early reproductive season is more likely than not to lead to successful recruitment, success or failure appears to depend less upon the time of spawning than on the chance occurrence of frost within 4-5 weeks of this variable event. Thereafter the severity of mid-winter conditions appears to be irrelevant.Regional data from the northern half of the British Isles are less detailed than those from Robin Hood's Bay, especially about the exact spawningtime in some areas. Nevertheless there has been no instance of poor recruitment that does not appear to accord with a frost-control hypothesis. Continuing work in these areas suggests that lack of spawning synchrony and a tendency to re-ripen could complicate future interpretation.In sub-habitats where limpet life-spans are long, several years of above-average recruitment at Robin Hood's Bay have resulted in high adult densities persisting during years of poor recruitment. Where longevity is less, the adult densities respond more closely or irregularly to recruitment fluctuations.On the basis of the frost-controlled fluctuations, of preliminary data on gonad cycles in northern Norway and Portugal, and on the local distribution towards the species' southern limit in Portugal it is tentatively suggested that geographical limits are set, to the north, by spat intolerance of low temperature and, to the south, by inability of juveniles to withstand heat/desiccation during their first summer after settlement. I N T R O D U C T I O NIn an attempt to identify the causes of the natural population fluctuations on British rocky coasts, an exploratory programme on the recruitment fluctuations of certain 'key species' has been operating for several years. The choice of species and the emphasis upon their recruitment have been discussed elsewhere (Lewis, 1976) and were justified as the most economical means of gaining the ability to (a) 'explain' major changes in the general character of rocky shore communities, and (b) make predictions about natural events against which to detect regional or temporal consequences of man-made changes in water quality. Mussels, barnacles and limpets are being studied but it is the last with which most interpretative progress appears to have been made, and this paper is concerned solely with Patella vulgata L. It is, however, essentially a preliminary report in a continuing study and it illustrates the practical and int...