I have loose bowel move after eating fish. what causes this?

Problem eating fish: Dietary triggers for GI symptoms are individualized; it's worthwhile keeping a food diary to see if correlates with certain foods. In general, gut stimulants (caffeine) &dietary fats often precipitate IBS-like complaints. About a third of IBS patients tolerate poorly dietary fiber sources. How your fish is prepared may be more important from the perspective of symptoms than that it is fish.

Avoidance: GI symptoms are common among food-allergic patients regardless of the cause but usually occur within 2 hours and rarely 4 hr after eating. However over 90% of the cases are associated with skin symptoms as well in food allergy. Rarely the fish protein can be toxic to some people . Regardless of the cause , avoidance is the answer.