You keep your dirty fish to yourself, SIR

 They're invasive species in our island, we do not want the foreign species and the microbiota that they bring within. It's been shown that at death they can be poisonous to local predator population, thus potentially causing a total collapse of local ecosystem with introduction of an otherwise small population of the fish in question. The threats to the ecosystem are not one-directional, it is projected to affect the both ends: these fish have been discovered to devour the local flora, thus outcompeting the local fish at the bottom of the food chain. By attacking both the top and bottom ends of the food pyramid, their destruction of the ecosystem is accelerated, and takes significant efforts to re-establish populations and bring the interactions to a balance.