Behavioural disturbances and underlying neurophysiological mechanisms during ocean acidification and warming in Gadus morhua and Boreogadus saida

Ocean acidification as projected for the end of the 21st century has the potential to cause behavioural alterations in fish with unclear consequences for affected species and ecosystems, both in the short and long term. Recent findings indicate that a change in functionality of A A aminobutyric acid receptors type A (GABAA-receptors) in the brain of fish due to acid-base regulatory processes may be the mechanism underpinning these behavioural disruptions. So far, studies have focused on the effects of CO2 on tropical and temperate species with no information about the relevance of these observations for polar species. The role of environmental temperature for CO2-induced behavioural changes is largely unknown, but highly important, as acidification and warming will occur simultaneously in marine ecosystems. In this thesis, behavioural effects of future CO2 conditions, the role of environmental temperature and the respective physiological background were analyzed in two cold water adapted fish species collected around Svalbard. Atlantic cod, Gadus morhua, is an invasive species currently shifting its distribution northward into colder waters where Polar cod, Boreogadus saida, is a native key species in the local food web. Shifting predator prey interactions and the differing potential of species to acclimate and adapt to future temperature and CO2 conditions will shape the future abundance of each species with concomitant impacts on the polar ecosystem. In manuscript I it is shown that the behaviour of B. saida is more sensitive to future environmental CO2 conditions than the behaviour of G. morhua. Nevertheless, the potential for behavioural resilience of G. morhua under high CO2 conditions may be dependent on the experienced environmental temperature and greatest under optimum temperature conditions. In manuscript II, metabolic changes are illustrated, which indicate CO2-dependent energy limitation in the brain of B. saida at 8 AAdegreeC, but not in G. morhua. However, in G. morhua, temperature and CO2-dependent alterations in GABA-metabolism were found potentially increasing this speciesa behavioural resistance against higher environmental CO2 partial pressures. In manuscript III, maintenance of intracellular pH in the brain of acutely CO2-exposed B. saida indicating sufficient acid-base regulatory processes is reported. However, long-lasting effects of CO2 on blood circulation were also observed with unclear relevance for the fitness of this species under expected ocean acidification scenarios. The results of this thesis indicate that G. morhua will be more capable to survive in warmer, more acidified waters around Svalbard than B. saida. Furthermore, the potential interplay between behaviour, GABA-metabolism and acid-base physiology with respect to their contribution to metabolic and behavioural resistance against environmental hypercapnia is discussed.