The fish health professional will want to observe the fish while they are in their environment, evaluate their swimming behavior, as well as their feeding response.  Usually sick fish will not eat food; however, they may put the feed in their mouth and then reject it, by rapidly spitting it back out into the water.  Therefore, close observation of the fish during the time they are offered food is critical.  Sick fish may loose their “fear or fright” response and will not move away from a shadow or a hand waved slowly over the tank. Healthy fish will usually respond to this stimulus by rapidly swimming away from the shadows.  However, if the fish are healthy and they are accustomed to being fed by hand, they may actually come to the surface of the water in anticipation of feeding.  Another important aspect of the physical examination is to look for any erratic swimming behaviors.  In a raceway system, the sick fish are usually those that are at the end of the raceway nearest the drainage outflow pipe, as opposed to the healthy fish, which will be swimming against the current closest to the inflow water pipe.  Sick fish may be deeper in the water column and not vigorously swimming. 

Flashing is a common clinical sign of fish that have external parasites.  Flashing denotes the rubbing of the fish against the sides of the tank whereby the underside of the fish is visualized.  The movement of the operculum, which is the covering over the gills is important to evaluate.  Those fish that are having respiratory difficulty will have more rapid operculum movement (“pumping”) than will healthy fish.  With severe respiratory compromise, fish may actually have their heads extending out of the water and may be “piping”.  Piping is the term which characterizes the fish at the water-air interface with flared opercula, that appear to actually be gulping air. 

Other features which will be observed may include any physical abnormalities in the fish, such as curvatures of the spine, which may first be noticed in those fish that have a circular or “whirling” swimming pattern.  Lesions, such as cutaneous lesions such as fraying or loss of fins, ulcerations, or neoplasms may also be observed during the physical examination.  Because fish have several layers of pigments within their skin, pay particular attention to changes in the color of the fish. Sick fish may be darker or paler than healthy fish.  Hemorrhages of the skin, especially around the fins, and accumulation of fluid within the coelomic cavity (ascites) are non-specific lesions, but they are commonly associated with a bacterial septicemia.   Exophathalmos or “bug-eyes” can be unilateral or bilateral and is commonly due to osmotic regulatory failure or gas bubble disease, however, it can occur with many different disease processes. 

Once observations have been noted of the fish within their aquatic environment, a small number of these fish with lesions or clinical findings representative of the current disease problem should be removed from the water and examined.  Latex gloves are recommended when handling the fish, since some bacterial pathogens of fish may be opportunistic pathogens of humans, such as Mycobacterium sp.  A closer examination of the fish removed from the water often will reveal lesions which could not be determined while the fish was within its aquatic environment. If the fish contain lesions, biopsies can be taken following anesthesia (see Biopsy) or the fish may be euthanized for necropsy, by the fish health professional (see Necropsy).