Assessing the morphological impacts of long-term harvesting in intertidal gastropods using historical data and morphometric tools

Size-selective harvesting of intertidal molluscs is a common practice. However, the effects of long-term traditional harvesting remain unclear. Changes in mean shell size are generally taken as evidence of changes in harvesting intensity. However, mean shell size is also influenced by environmental pressures, which may confound the analysis of size variation over time. In this study, we apply geometric morphometrics to historical data from two shell middens, in an attempt to classify Littorina littorea shells to their environmental origins, prior to carrying out shell size analysis. Using this method, shell shape was found to be consistent within and between the midden sites. Based on comparison with modern populations from shores of known wave exposure, the midden shells were found to be more consistent with sheltered shores, and to differ most from the very exposed shore sites, the latter of which are located adjacent to the midden remains. The mean shell size was significantly smaller in the more recent midden site. We hypothesize this reduction is caused by an increase in harvesting intensity over this period. It is also possible, given the very slight reduction in shell size, coupled with certain life-history traits of L. littorea, that the change in shell size was caused by slight differences in environmental conditions. The use of midden shells and morphometrics has the potential to provide an insight into previous environmental conditions and past harvesting practices, which may be used to inform current harvesting practices.