Forensic potential of oyster shells: Geochemistry and morphology of Ostrea chilensis shells from New Zealand
McCracken, Ellen M
Control of Bonamia exitiosus epidemics, which pose a continuing threat to Ostrea chilensis (dredge oyster) populations in Foveaux Strait, New Zealand, is limited to restrictions on areas or times O. chilensis can be harvested and/or transported, but policing of this and other illegal harvest is difficult. A method to distinguish between dredge oysters taken from different areas or regions is required. The aim of this study is to determine the forensic potential of O. chilensis shell morphological and geochemical characteristics. Weights and dimensions (height, length, thickness, depth and flaky margin width) of shells from 188 dredge oysters from Foveaux Strait, Tasman Bay, Oyster Bay, Port Underwood, Chalky Inlet and the Chatham Islands were measured, and Ba/Ca, Mg/Ca and Sr/Ca concentration ratios of samples taken from 62 of these shells determined using an Inductively Coupled Plasma Atomic Emission Spectrometer (ICP-AES). Shell thickness to growth layer number relationships in 75 O. chilensis were used to predict relative age. O. chilensis shell weights, dimensions, proportions and relative age, but not Ba/Ca, Mg/Ca or Sr/Ca concentration ratios, varied significantly among study sites. Shell height, length, weight, hinge thickness and left valve inflation were the best shell measures for determination of dredge oyster origin. Age differences caused many of the spatial differences in the above measures. It cannot be guaranteed that O. chilensis will be collected in the same way as those in the current study and dredge oysters will not necessarily be of the same age or have the same shell dimensions, proportions and weights as those measured here. Maximum shell heights and lengths, calcification rates (total shell weight/age), left valve inflation and hinge thickness growth rates (measures independent of age) are more useful. The qualitative O. chilensis ecomorph definition (based on shell appearance) can occasionally be used to elucidate origin where the above shell measures cannot. This ecomorph definition is subjective but a quantitative definition based on shell morphology data from the current study is less useful due to the variability in shell morphology among O. chilensis of the same ecomorph but different locations. Environmental variations caused differences in O. chilensis shell morphology not attributable to age. This is useful in terms of determining the origin of O. chilensis that have not been studied here but that are from similar environments. It is recommended that calcification rate (which is highly spatially variable) is used with maximum shell heights and lengths and the qualitative ecomorph definition to determine O. chilensis origin. These characteristics are relatively easy and cheap to measure and their use allows discrimination between O. chilensis from several areas around New Zealand. A venues for future research include investigation of forensic potential of other O. chilensis shell characteristics (including other elemental ratios, stable isotope composition, degree of infection by parasites or species assemblages adhering to shells), further study of environmental factors causing measured variations in O. chilensis shell morphology and investigation of factors affecting O. chilensis shell elemental concentration ratios.
Advisor: Smith, Abby; Frew, Russell; Bremner, Graeme
Degree Name: Master of Science
Degree Discipline: Environmental Science
Research Type: Thesis