The effects of oxygen and temperature on the physiology of hatching stage Atlantic salmon

Environmental change and its impact on the form, function and adaptive responses of fish, especially in relation to maternal effects or individual genotype, is of major interest to biologists. Oxygen levels and temperature have profound effects on the physiology of fish, and early developmental stages are particularly susceptible to environmental challenges. In this thesis the metabolic, cardiorespiratory and cellular stress responses to acute or chronic changes of these variables in Atlantic salmon (Salmo salar) embryos and yolk-sac alevins were investigated. Eggs and alevins matched their metabolic demand to acute changes in oxygen levels without altering cardiorespiratory function unless oxygen reached critically low levels (5 kPa). In contrast, chronic hypoxia (10.5 kPa for 15 days), but not hyperoxia (28 kPa for 15 days) resulted in functional and structural modifications that enabled metabolic rate to return to the normoxic (21 kPa) pre-exposure level while maintained in hypoxia (10.5 kPa); this indicating a metabolic compensation. On return to acute normoxia, irrespective of the measurement temperature (4, 8, 12°C), metabolic rate was elevated above the pre-hypoxic value observed in normoxia, presumably a result of the structural and/or functional modifications that occured in hypoxia. In addition, differences in metabolism and responses to hypoxia were influenced by maternal factors such as egg size, which in turn are determined by maternal body size. Egg size-dependent differences in metabolic rate of embryos were present and larger embryos from repeat spawning females exhibited increased tolerance to hypoxia, as represented by lower critical oxygen levels for hatching than observed in embryos from smaller eggs from maiden spawners (13 kPa vs. 17 kPa). This result suggests an advantage in embryos from larger eggs whereby the embryo can obtain sufficient oxygen due to the larger surface area of the egg in respect to the embryo’s metabolic rate. Genetic modification for growth (growth hormone transgenesis) or triploidy (three sets of paired chromosomes) increased metabolic rate above a diploid conspecific (both by 8%). This effect was additive in triploid transgenic alevins that also displayed an altered cardiorespiratory response to acute, severe hypoxia (5 kPa). In addition, acute hypoxia did not elicit a cellular stress response, but was associated with differential (reduced) expression of cellular stress proteins (heat shock proteins) that in one case (Hsp90) was dependent on the growth hormone transgenic genotype. Taken together, the above observations demonstrate that extrinsic as well as intrinsic factors have substantial effects on the physiology of developing fish, and that the overall response to extrinsic factors contains a temporal component. Given the associated effects are reflected in changes in metabolism, it is likely that the effects will significantly impact growth, survival and performance of the developing fish