What environmental condition has been linked to higher morbidity and mortality in fish affected by EUS?

The correct response highlights the relationship between low temperatures and the increased morbidity and mortality seen in fish afflicted by EUS (Epizootic Ulcerative Syndrome). In many aquatic species, cooler water temperatures can compromise immune function, making fish more susceptible to stress and opportunistic infections. EUS is exacerbated under these conditions, as cooler temperatures may impair the ability of fish to mount an effective immune response against pathogens.

In environments with low temperatures, fish are also often less active, which can lead to further stress and a decline in overall health. Additionally, low temperatures can promote the development of certain pathogens associated with EUS and reduce the rate of recovery in affected fish. This response is a crucial aspect of understanding the epidemiology of EUS and the conditions that heighten the risk for outbreaks in fish populations.

The other choices, while they may have their own effects on fish health, do not have the same direct association with EUS in the context of increased morbidity and mortality. For instance, high ammonia levels can indeed be detrimental to fish health, but its relationship with EUS is not as clearly defined as that of low temperatures.