I need to estimate the quality of fish larvae by estimating their stress resistance OR their resistance to the lack of oxygen. However due to ethical issues I need to use non-violent methods (i.e. depletion of oxygen). Any suggestions ?
It would be nice to know more about the species and the stresses you want to test. You have to do it on the hard way; instead of an experiment, you do observations (better to say something between experiment and observation), and from the observation results you will get the correlations and regressions. Install proper fish tanks (3-5) in different natural environments and monitor the stress factors (temperature, oxygen, NH3, etc.) and the larvae parallel. I think even a hostile environment where the fish larvae naturally present cannot be unethical.
You need to choose diverse environments and collect way more data compared to a lab experiment. The good thing is that some factors you close out in the lab will be present in the nature, and maybe you discover something important (like a shorter term oxygen deficit different from longer term or the light affects something).
I do social research, and most of the times the only ethical way is the observation :)
Behavioral responses can be an excellent way to measure fitness in larval fish and fitness can be a proxy for stress responses. Taking this approach you could avoid ethical issues. Some approaches could include monitoring startling response (e.g. to light), responses to feed addition, or overall activity levels. Behavioral quantification can be done with video cameras or direct observation.
The following reference might be helpful:
Fuiman, Lee A.; Ojanguren, Alfredo F. 2011. Fatty acid content of eggs determines antipredator performance of fish larvae. Journal of Experimental Marine Biology and Ecology 407(2):155-165DOI10.1016/j.jembe.2011.06.004