The global impact of climate change on fish

Abstract

Climate change is a global issue and the effects on fish populations remain largely unknown. It is thought that climate change could affect fish at all levels of biological organisation, from cellular, individual, population and community. This thesis has taken a holistic approach to examine the ways in which climate change could affect fish from both tropical, marine ecosystems (Great Barrier Reef, Australia) and temperate, freshwater ecosystems (non-tidal River Thames, Britain). Aerobic scope of coral reef fish tested on the Great Barrier Reef was significantly reduced by just a 2°C rise in water temperature (31, 32 and 33°C, compared to the current summer mean of 29°C) due to increased resting oxygen consumption and an inability to increase the maximal oxygen uptake. A 0.3 unit decline in pH, representative of ocean acidification, caused the same percentage loss in aerobic scope as did a 3°C warming. Interfamilial differences in ability to cope aerobically with warming waters will likely lead to changes in the community structure on coral reefs with damselfish replacing cardinalfish. Concerning Britain, there is evidence of gradual warming and increased rainfall in winter months over a 150 year period, suggesting that British fish are already experiencing climate change. It was evident from an analysis of a 15 year dataset on fish populations in the River Thames, that cyprinid species displayed a different pattern in biomass and density to all the non-cyprinid fish population, suggesting that there will be interfamilial differences in responses to climate change. Using a Biological Indicator Approach on the three-spined stickleback, Gasterosteus aculeatus, a 2°C rise in water temperature resulted in a stress response at the cellular and whole organism level. A 6°C rise in temperature resulted in a stress response at the biochemical level (higher cortisol and glucose concentrations), cellular level (higher neutrophil: lymphocyte ratio) and whole organism level (higher ventilation rate and lowered condition factor, hepatosomatic index and growth). G. aculeatus is considered to be temperature tolerant; therefore these results indicate that climate change may prove to be stressful for more temperature-sensitive species. This study has demonstrated that climate change will have direct effects on fish populations, whether they are in temperate regions such as Britain or in tropical coral reefs, but with strong interfamilial differences in those responses.