An Investigation of CO2-Induced Behavioural Changes in Freshwater Fish

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The levels of carbon dioxide (CO2) in the atmosphere are continually rising due to anthropogenic industrial activity; they were 280 per million (ppm) in the 18th century and have currently risen to 400 ppm. CO2 levels are predicted to further increase to ~1000 ppm by the year 2100. Atmospheric CO2 dissolves in the ocean, lakes and rivers, reacting with water to form bicarbonate ions as well as protons that acidify the water. CO2 can also move across cell membranes, lowering pH and increasing bicarbonate levels inside cells. Changes in CO2, pH, and bicarbonate levels can potentially have a great effect on enzyme and cell functions. In marine organisms ocean acidification alters calcification processes in phytoplankton and invertebrates, and shifts behavioural lateralization, alters olfactory ability, and impairs learning in fish. We also have evidence that marine fish exposed to high CO2 (~1100 ppm) have increased anxiety. The mechanism is at least in part due to altered GABAA receptor functioning because of a shift in the equilibrium potential for Cl- caused by high CO2. However, to date there have been no studies examining the affect of increased CO2 on freshwater fish. The main objective of this study is to examine the effects of increased CO2, which causes freshwater to become more acidic (acidification), on fish behaviour. Specifically, we will examine changes in individual (anxiety, memory, locomotion) and group (schooling/shoaling) behaviour and explore the neuronal mechanisms involved with various pharmacological agonists and antagonists. We will also investigate specific brain changes in GABAA receptor distribution due to freshwater acidification. Another objective is to examine long durations of freshwater acidification to elucidate any adaptation to high CO2 levels. Zebrafish were chosen as the experimental model organism because of the possibility for genetic manipulations to characterize the cellular mechanisms involved. The study of fear and anxiety in relation to climate change is novel and has not been performed by any other research group. To date, there have been no other studies on the effects of water acidification on freshwater fish behaviour. Since fish are an essential part of the ecosystem and a major food source for humans, any alteration in their existence could be crucial for human populations. Therefore, results from this study will be significant regardless of the outcome.