As well as compromising the survival of freshwater organisms, high water temperatures make life more difficult by reducing the solubility of oxygen, increasing oxygen demand and encouraging lake stratification. In the case of fishes, there’s conflicting evidence about the sensitivity of different life stages to low-oxygen conditions. Because fish eggs and embryos have low surface area to volume ratios and lack active methods of ventilation, they might be expected to be more oxygen-sensitive than individuals at the juvenile and adult stages. This prediction was examined by analysing published data on the low-oxygen tolerance of 68 species of fish found in river basins of north-west Europe. Two measures of tolerance were examined: the lowest observed effect concentration (LOEC) and the 100% lethal concentration (LC100). In the case of both measures, the critical effect concentrations for eggs and embryos were significantly higher (means of 6.5 and 3.1 mgO2/l for LOEC and LC100 respectively) than those for juveniles (2.4, 1.3 mgO2/l ) or adults (3.8, 1.9 mgO2/l ). The LOEC means were significantly lower for juveniles than for adults, possibly because juvenile fish tend to be forced into less optimal, more poorly oxygenated habitats as a result of competition and predation. More information is needed on the combined effects of high temperature and low oxygen concentrations, especially with regard to sensitive eggs and embryos.
Reference: Elshout, P.M.F. et al. 2013. Low oxygen tolerance of different life stages of temperate freshwater fish species. Journal of Fish Biology 83, 190–206. http://onlinelibrary.wiley.com/doi/10.1111/jfb.12167/pdf
Freshwater Research News 
Posted on October 1, 2014