The response of a protandrous species to exploitation, and the implications for management : a case study with patellid limpets
A zygote production model for the patellid limpet Patella vulgata has been developed to examine the effect of exploitation on the reproductive output of a protandrous (male to female sequential) hermaphrodite. Patellid limpets are broadcast spawners which can have specific implications for the effect of exploitation on reproductive output, due to sperm limitation. The combined zygote production model was made of three component sub-models; a population fecundity model, a gamete dispersal model, and a fertilisation model. The model makes explicit account of sperm limitation, and is based upon data collected through field and laboratory investigations conducted as part of this thesis. The model was used to examine the relationship between egg and zygote production, and spawning stock biomass (SSB) and fishery yield for a range of P. vulgata populations across a wave exposure gradient. The effect of different management strategies, minimum landing size or marine protected areas, on the relationship between reproductive output and yield was also examined. Protandry lead to a decoupling between SSB and zygote production as the populations were exposed to the simulated fishery. There was a five-fold variation in zygote production per unit area across a wave exposure gradient. Comparison of different management strategies indicates that the fishery yield could vary by up to three-fold depending on the management strategy used, whilst still protecting the same level of population reproductive output. The genetic population structure of the Azorean Patella candei population was also examined to determine the scale of larval dispersal to allow the management recommendations of the zygote production model to be examined in a wider ecological context. Due to evidence of a recent population bottleneck in the Azorean P. candei population no firm conclusions could be drawn from this study as to the scale of larval dispersal.