When I think of rainbow trout, the first image that comes to mind is usually something like this:
In the wild, they look more like this:
…not as appetizing, but a lot more fascinating.
When salmonid fish like rainbow trout are in environments with limited food or space, they form social hierarchies. In one study of limited food availability, Grobler et al., found that when they placed 4 rainbow trout into a group, they formed hierarchies in which one fish was dominant and the other three became subordinate. Remarkably, the physiology of the subordinates changed. Compared to the subordinates, the dominant fish had greater feeding and growth rates combined with a greater ability to use proteins and to eliminate ammonia waste.
A new study published earlier this month in the American Journal of Physiology – Regulatory, Integrative and Comparative Physiology examined additional physiological differences in the subordinate rainbow trout. In this new study, Gilmour et al., wanted to examine how fasting and higher stress levels of subordinate fish might contribute to their lower growth rates. They chose to examine an enzyme called AMP-activated protein kinase (AMPK), which is important in metabolism. They found that the subordinate fish had higher levels of this enzyme in the liver compared to the dominant fish. Simply fasting a fish had similar effects on liver AMPK levels even if it were not a member of a hierarchy. This suggests that fasting itself may cause the rise in liver AMPK. In muscle tissue, dominant fish had higher levels of AMPK. The findings of this study show that AMPK may play a role in the altered metabolism of subordinate fish living in a social hierarchy.
Grobler JM, Wood CM. The physiology of rainbow trout in social hierarchies: two ways of looking at the same data. Journal of Comparative Physiology – B. 183(6): 787-799, 2013.
Gilmour KM, Craig PM, Dhillon RS, Lau GY, Richards JG. Regulation of energy metabolism during social interactions in rainbow trout: a role for AMP-activated protein kinase. American Journal of Physiology – Regulatory, Integrative and Comparative Physiology. 313(5): R549-R559, 2017.