Annual fluctuations in the recruitment of patella vulgata l.

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. © 1977, Marine Biological Association of the United Kingdom. All rights reserved.