Skin health in king salmon (Oncorhynchus tshawytscha) farmed in New Zealand

Skin health is a primary indicator of fish general health and welfare in aquaculture, and it often correlates with fish growth and performance. Summer is the most stressful period for the skin health of king salmon (Oncorhynchus tshawytscha) farmed in Marlborough Sounds New Zealand, with some fish affected by seasonal development of skin lesions. Growth and skin health were investigated during a summer feeding trial. The first part of the study is a comprehensive evaluation of sampling methods and gross scoring system employed by the local industry to assess skin health in farm-based trials. Sampling bias was found in each of the sampling techniques assessed. However, depending on the method, samples were biased either in favour of fish with better skin health and higher weight (+ 0.20 kg), or towards fish with poorer skin health and lower weight (- 0.14 kg) than the pen population average. Basic histological assessment was performed to determine if the gross scoring of lesion severity could be objectively confirmed at the histological level based on the perforation of lower dermis (stratum compactum). Gross scoring-histology agreement on average was good, but only moderate for the most severe skin lesion category. Because the same sampling and scoring methods are used in routine surveillance of commercial pens, these results should be considered in future research involving analysis of farm data, including epidemiological studies. The second part of the thesis focusses on growth and skin health differences in post-smolts fed 3 different commercial diets over the summer. Diet had a significant effect on weight, growth and general health as suggested by haematology and plasma biochemistry indicators. Most ulcers were healed or at an advanced healing stage by the end of the summer. The prevalence of fish with large healed ulcers and fish without lesions differed among diet treatments, suggesting a possible influence of the diet on skin ulceration. However, evidence of the effects of diet was conclusive only for growth and general health. In addition to the direct contribution to our knowledge of farmed salmon health and welfare management, results of this work highlight research challenges relevant to the entire field of skin health investigation in cultured fish.