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Morten Bo Søndergaard Svendsen:
Effects of harmful algae on the physiology of fishes

Date: 31-12-2016    Supervisor: John Fleng Steffensen

Blooms of harmful planktonic algae causing adverse effects in aquatic environments are a global problem, causing both human morbidity and killing aquatic lifeforms worldwide. Focusing on fish kills, it is largely unknown what mechanisms of the fish’s physiology are affected during exposure to harmful algae. It is demonstrated that for an alga with a known mode of action, Prymnesium parvum affecting the gills, conventional readily available methods in fish physiology can be used to establish an adverse outcome pathway. More specifically, intermittent flow respirometry and observing ventilatory movements and gill clearing frequency can be used to determine the action of Prymnesium parvum as a gill irritant, causing mortality of fish via reduced oxygen uptake capability, a pathway likely initiated by the alleged toxin Prymnesin. The hypothesis that fish can recover after Prymnesium parvum exposure is deemed unlikely on basis of rainbow trout (Onchorhyncus mykiss) not recovering.

The specific toxicity of Pseudochattonella spp. is unknown, but by studying the effects of Pseudochattonella spp. on fish during a natural bloom occurring at a trout farm an adverse outcome could be created. The adverse outcome pathway indicate that fish mortality arise as a result of winter stress syndrome; the syndrome being an interaction of low temperature and an environmental stressor. This is concluded on the basis of sorting affected fish on according to their reflex impairment score combined with an anatomical health assessment index, in turn forming similar anatomical appearance as fish affected by the winter stress syndrome. The conclusion is supported by modelling the aerobic scope of the fish during the bloom based on previously reported data for salmonids, and available environmental data.

Even though Alexandrium monilatum has been studied intensively the effects of Alexandrium monilatum on fish is largely unknown. In the Chesapeake Bay, Eastern U.S.A., fishes are further challenged in late summer by an oxygen squeeze from deep part of the water column, limiting their utilizable habitat to mid-waters having enough oxygen to sustain life, but not too warm like the surface water layer. The proposed adverse pathway, being gill destruction, for fish exposed to Alexandrium monilatum suggests that co-occurring events of Alexandrium monilatum and oxygen squeeze events will tighten the oxygen limitation squeeze on striped bass (Morone saxatilis).

In order to ensure continuous progress in research in physiology and toxicology of fishes, analytical approaches to environmental stressors are needed. One such approach is the Dynamic Energy Budget models; however, a recent paper called for solution to a problem of a weird theoretical animal challenging the assumptions of the model. It is argued based on literature review, that the maintenance assumptions of the weird animal are similar to those of fishes, where challenges to homoeostasis are related to gill area. It is further emphasized that the assumptions of a model should correspond to what one can observe on live animals. It is emphasized that homoeostasis and challenges to it are continuously changing, and thus cannot be assumed constant.

The last study of the thesis demonstrates that for Prymnesium parvum as well as for Alexandrium monilatum the oxygen tolerance of fish decrease when exposed for a short time. Based on the analytical derivation, two thresholds for oxygen limitation are proposed. It is further concluded that a measure with concentration units, KB, can be used for describing changes of hypoxia tolerance in fishes.

Work surrounding the core subject of the thesis primarily concerned intermittent flow respirometry. Intermittent flow respirometry is a common used method, and a key method of this thesis. A review on how to design and setup an experiment using intermittent flow respirometry was conducted in the beginning of the thesis. Likewise, the development of a free open-source software, AquaResp 2 (and now 3), for use in automating intermittent flow respirometry experiments. By using the open source software, it was also established prior to harmful algae experimentation that the observed variation in respirometry experiments is largely caused by the fish and to a lesser extent the experimental setup.

Before this thesis, systematic studies of fish physiology under the influence of harmful algae consisted of one algae species, Chattonella marina. Now there are a total of 4 species studied. Lastly, during the PhDproject participation in three EU Cost Action conferences resulted in the creation of two physiological databases; one on acute thermal tolerance of aquatic ectotherms, and one on the aerobic scope of fishes. Likewise work carried out concerning maximum swimming speeds of fishes was presented at the Society of Experimental Biology annual conference in 2015. Finally I have refereed 6 manuscripts during my thesis employment.