| Summary: | Crassostera gigas (Thunberg 1793), commonly known as the Pacific oyster originates from Japan but can now be found in many parts of the world due to it's extensive use in aquaculture. C. gigas was introduced to Europe in the 60's and 70's for aquaculture but has spread into the wild and is now considered a bio-invader in many parts for Europe. For the past 20 years, mass mortality events have been recorded in C. gigas populations globally that have become known as 'summer mortality' events. Most studies have focused on Herpes virus OsHV-1 and Vibrio bacteria, however the latest research indicates that other factors may be involved (Petton et al. 2015). Using samples taken from wild C. gigas populations affected by high mortalities during autumn 2014, this thesis uses oxidative stress as an indicator of environmental factors in summer mortality events. As little previous research exists on measuring for oxidative stress in C. gigas, the first objective was to optimize pH antioxidant enzyme measurement assays for glutathione S-transferase (GST), glutathione reductase (GR) and Catalase. The second objective was to determine whether environmental factors are contributing to summer mortality events by measuring for oxidative stress in the form of antioxidant activity measurements for GST, GR, Catalase and Superoxide dismutase (SOD). As oxidative stress can be caused by stressors other than environmental, these were also evaluated in this thesis. Results for the pH optimisation measurements showed that for GST and GR, the standard pH level of 7.5 is unsuitable for C. gigas with 7.2 being optimal. For Catalase the result was inconclusive and the standard pH 6.5 was used in this thesis. It should be noted that not all pH levels were measured for catalase and possible future pH optimisation cannot be excluded. Results from measuring the samples for oxidative stress, although varying across the samples, showed a general trend confirming oxidative stress. This, in combination with a review of previous research on the ...
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