Passage: Although originally a mild pathogen of Salmo trutta in central Europe and other salmonids in northeast Asia, the introduction of the rainbow trout (Oncorhynchus mykiss) has greatly increased the impact of this parasite. Having no innate immunity to M. cerebralis, rainbow trout are particularly susceptible, and can release so many spores that even more resistant species in the same area, such as S. trutta, can become overloaded with parasites and incur 80%–90% mortalities.  Where M. cerebralis has become well-established, it has caused decline or even elimination of whole cohorts of fish.

Question: What has caused a decreased in the amount of whole groups of fish?
M. cerebralis