I don't really get the question. Fish are cold blooded and hence their body temperature is function of the environment they live in. Maybe you can measure some tiny differences due to energy conversion, but I doubt if you would find any statistically significant correlation. I don't even see how you would measure brain temperature and behavior in the same fish.

Some species have rete mirabilia behind the eyes (e.g. scombrids, xiphiids) and thus might "heat" the brain in this fashion. But generally, as Freddy pointed out, the brain will be the same temp as the water they occupy.

Dear colleagues

The relationship between brain temperature and some aspects of life of mammalian species such as locomotor activation, reproduction, etc., have been reported.

In my opinion, being poikilotherm cannot be a good reason for the lack of relationship between behavior and brain temperature in fish.

Thank you very much