1. Bacteriological changes

2. Native bacterial flora of fishes
The flesh and body fluids of live healthy fish are generally free from bacteria. Bacteria, which are naturally present on fishes, are called the native bacterial flora of fish.
Bacterial counts in the intestine are the highest.
Bacterial populations exhibit seasonal variations.
Majority of the bacterial flora of marine fishes are gram negative.
Fresh water fishes predominantly gram positive.
Brackish water fishes are evenly composed of both gram +ives and gram –ives.

3. Factors that influence the growth of microorganisms
The bacteria on temperate water fish are all classified as either psychrotrophs or psychrophiles.