If using large fish, I can use its blood to assess osmolality, electrolyte content, stress indicator (such as blood glucose), hormone, etc, what if i use small fish? is whole body measurement reliable enough?
I think whole body measurements are reliable specially whole body cortisol. Whole body cortisol measurements are very important to measure stress of small fish like zebrafish.
Much better to use water release rate method and not have to sacrifice all your fish. It works well for zebrafish cortisol measurement. See attached review for latest commentary and also look at Sebire, M., Katsiadaki, I. and Scott, A. P. 2007. Non-invasive measurement of 11-ketotestosterone, androstenedione and cortisol in male three-spined stickleback (Gasterosteus aculeatus). General and Comparative Endocrinology152, 30-38. and paper by Felix et al http://online.liebertpub.com/doi/abs/10.1089/zeb.2012.0792
You cannot use whole body values instead of blood values for many hormones, since their targets in the body can be widely varying in the tissues, and because blood volume is only 3-4 % of the total fish volume. So, even if you in large fish, where blood sampling succeeds easily, see a significant change in a hormone concentration of blood, if hormone binding occurs only in a small percentage of the rest of the body, any change may remain undetected in whole body measurements because of limits of resolution of the methods. Where the hormones are secreted to water (only some of them are), the water measurements can be a useful measure (of changes).
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