Frommel, Andrea (2012) Effect of ocean acidification on the growth and development of cod larvae (Gadus morhua). (PhD/ Doctoral thesis), Christian-Albrechts-Universität Kiel, Kiel, Germany, 107 pp.

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Abstract

Ocean acidification is presently considered to be one of the most complex and critical anthropogenic threats to marine life. It is caused by the absorption of excess atmospheric CO2 by the oceans, reducing the pH of surface seawater. Ocean acidification has been found to affect a wide array of organisms from single-celled algae to complex invertebrates. While calcifiers are particularly challenged by acidified waters, fish are considered less vulnerable as they have a well developed acid-base regulatory system in the gills. However, early life stages that hatch without functional gills may be more vulnerable to high CO2 concentrations. In the present thesis, the sensitivity of early life stages of Atlantic cod to ocean acidification was investigated in two distinct populations adapted to very different abiotic conditions. By examining the effect of elevated CO2 concentrations from gametes to juveniles, vulnerable stages could be identified that may act as bottlenecks. Analyses of morphometric and biochemical indicators of growth and condition showed that gametes and yolk-sac larvae seem to be robust to very high levels of CO2. However, the transition from the larval to the juvenile phase was heavily impacted by CO2. Histological analyses revealed severe damage to internal organs in larvae undergoing a phase of energy costly organ restructuring. Most importantly, it seems they outgrew the critical surface to volume ratio for effective cutaneous respiration without having functional gills. After this bottleneck phase, those individuals that were able to develop gills successfully, could switch to branchial respiration and acid-base regulation and thus overcome the CO2 stress. The sensitivity to ocean acidification was found in Atlantic cod larvae from a population that spawns along the Norwegian coast in full saline, temperate and oxygenated waters. Its Baltic Sea counterpart spawns in sub-optimal conditions of low salinity, high temperature and depleted oxygen layers with high levels of CO2 and was found to be robust to ocean acidification in the early larval stage. The adaptation of Baltic cod to these conditions may allow its larvae to cope with future ocean acidification better than other populations. However, multiple environmental stressors along with overfishing may pose a problem for the population in the long run. Since the recruitment process is determined during the early life stages, effects of ocean acidification on the survival of these stages must be further analyzed in combination with other relevant climate change variables to ensure a better projection on the effects of OA on fish populations and their resilience.

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