Which of the following is a common clinical sign of anemia in fish?

Gill pallor is recognized as a common clinical sign of anemia in fish. Anemia occurs when there is a reduced number of red blood cells or hemoglobin, leading to a decreased oxygen-carrying capacity of the blood. As a result, one can observe a lack of red coloration in the gills; they may appear paler than normal. This is attributed to the decreased amount of hemoglobin in the blood, which is responsible for the characteristic red color of the gills.

In addition to gill pallor, fish may exhibit other systemic signs of anemia, such as lethargy or weakness, as their ability to transport oxygen is compromised. Recognizing gill pallor can prompt further investigation to identify underlying causes of anemia, such as nutritional deficiencies, parasitic infections, or environmental stressors.

Other signs listed, such as bright coloration of the gills, increased swimming activity, and swollen abdomen, may not directly indicate anemia and can instead suggest other health issues or stressors in the aquatic environment.