What causes the salmon to stress during summer?

This is a question asked of me by a colleague recently. What happens when the water temperature rises? Is it due to reduced dissolved oxygen levels or anoxia (link to article)? Is it due to impaired cardiovascular function (link to article)? Is it due to the increased presence of dangers such as jellyfish or algae? Is it immunosuppressive? Maybe reovirus might be playing a role?

When working at the labs in Tasmania as a veterinary fish pathologist, I did see quite a number of fish hearts which we described as having a “mural reaction” on microscopic examination. In simpler terms, there was a fuzziness to the look of the heart, indicating inflammation. Since such findings are not specific to any particular disease, there was also testing to look for the presence or absence of various pathogens, bacterial or viral. A thorough study would be indicated to correlate these findings with the water temperature and presence of pathogens.

Have a read of the articles below and see if it adds to more information.

Journal of Fish Diseases
Volume 36, Number 5 (May 2013)
	Piscine reovirus (PRV) in wild Atlantic salmon,Salmo salarL., and sea-trout, Salmo truttaL., in Norway
	Authors:	Å. H. Garseth, C. Fritsvold, M. Opheim, E. Skjerve, E. Biering
	Author Affiliations:	no affiliations available
	Source:	Journal of Fish Diseases, Volume 36, Number 5 (May 2013)
	Page Numbers:	483 – 493
	Available Full Text:	Full Text: Subscription Required to view full text Format: PDF Size: unknown Location: Publisher’s Site Authentication: EBSCOhost EJS
	Abstract:	This is the first comprehensive study on the occurrence and distribution of piscine reovirus (PRV) in Atlantic salmon, Salmo salarL., caught in Norwegian rivers. PRV is a newly discovered reovirus associated with heart and skeletal muscle inflammation (HSMI), a serious and commercially important disease affecting farmed Atlantic salmon in Norway. A cross-sectional survey based on real-time RT-PCRscreening of head kidney samples from wild, cultivated and escaped farmed Atlantic salmon caught from 2007 to 2009 in Norwegian rivers has been conducted. In addition, anadromous trout (sea-trout), Salmo truttaL., caught from 2007 to 2010, and anadromous Arctic char, Salvelinus alpinus(L.), caught from 2007 to 2009, were tested. PRVwas detected in Atlantic salmon from all counties included in the study and in 31 of 36 examined rivers. PRVwas also detected in sea-trout but not in anadromous Arctic char. In this study, the mean proportion of PRVpositives was 13.4% in wild Atlantic salmon, 24.0% in salmon released for stock enhancement purposes and 55.2% in escaped farmed salmon. Histopathological examination of hearts from 21 PRV-positive wild and one cultivated salmon (Ct values ranging from 17.0 to 39.8) revealed no HSMI-related lesions. Thus, it seems that PRVis widespread in Atlantic salmon returning to Norwegian rivers, and that the virus can be present in high titres without causing lesions traditionally associated with HSMI.
	Citation:	Å. H. Garseth, C. Fritsvold, M. Opheim, E. Skjerve, E. Biering . Piscine reovirus (PRV) in wild Atlantic salmon,Salmo salarL., and sea-trout, Salmo truttaL., in Norway. Journal of Fish Diseases, Volume 36, Number 5 (May 2013), pp. 483-493, <http://ejournals.ebsco.com/direct.asp?ArticleID=4F948FD160688861678E&gt;
	URL:	http://ejournals.ebsco.com/direct.asp?ArticleID=4F948FD160688861678E