The influence of environment and exploitation on sex change limpets

Abstract

Patellid limpets are harvested around the world and many stocks are currently overexploited. This study investigated the influence of environment and harvesting upon the biology of target species (Patella spp.) and upon the macro-community structure of non-target species. A broad-scale survey of Patella spp. across the British Isles and Portugal revealed that non-exploited Patella vulgata populations from England had traits indirectly associated with protandry. Females predominated in larger size classes; cumulative frequency distributions of males and females were different; sex ratios were biased towards the first sex and smallest sizes of males were smaller than the smallest sizes of females. In Portugal, P. vulgata populations did not show these patterns, suggesting that protandry was not occurring in those P. vulgata populations, although alternative explanations include the influence of low recruitment leading to fewer small males. In the England Patella depressa appeared to be gonochorist with a sex ratio of 1:1. In Portugal, however, P. depressa also showed some patterns indicating the possibility of slight protandry. In a manipulative experiment to simulate size-selective harvesting of limpets by humans, changes in sex ratio of P. vulgata in the south-west of England were monitored over an 18-month period of repeated removal of bigger limpets. There was strong evidence that the size at sex change decreased in response to the exploitation treatment, emphasizing its plasticity. Sex change occurred at a bigger size than expected from the overlap in male and female size classes. The limpet exploitation led to establishment of Fucus spp. At the end of the experiment, control plots had lower percentage cover of Fucus spp. than both low- and high-exploitation plots. Univariate and multivariate analyses confirmed that the communities on the two shores responded differently to the same source of disturbance. A comparative survey of populations across a gradient of exploitation in the Canary Islands indicated that the abundance of Patella aspera decreased from 1992 to 1999, while for Patella candei crenata no differences were detected, suggesting that P. aspera was under a higher harvest pressure during that period. The observed differences in catches between islands in 1999 indicated that overexploitation of limpet stocks at that time was not yet evident. The results of the field surveys and the manipulative experiment are discussed with respect to the role of ecological experimentation and aquaculture in resource management, and suggestions are made for key issues in future research and conservation.

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ORCID for C. Patrick Doncaster: orcid.org/0000-0001-9406-0693

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