Today, more than half of the fish we eat globally are raised in fish farms and the proportion originating from farming continue to increase. Unfortunately, there are some problems associated with fish farming. One of these is that it is difficult to assess the animal welfare of fish as they lack many of the behaviours that signal discomfort and disease in other animal species.
Read the article in SLUs Knowledge bank (the content is in Swedish).
https://www.sciencedirect.com/science/article/abs/pii/S004484861830125X
Brijs, J., Sandblom, E., Axelsson, M., Sundell, K., Sundh, H., Huyben, D., Broström, R., Kiesssling, A., Berg, C. & Gräns, A. (2018). The final countdown: Continuous physiological welfare evaluation of farmed fish during common aquaculture practices before and during harvest. Aquaculture, 495, 903-911.