Whether they are salmon or snapper, trout or tilapia, all teleost (bony) finfish typically exhibit clinical signs of disease well before mass mortality events set in. Learning to recognize early warning signs can better equip you to improve outcomes and manage your risk.

All farms monitor mortality, but many fail to keep track of morbidity.  Some diseases – much like the flu in humans – may cause relatively few deaths, but significant reductions to productivity and weight gain, while also making fish more vulnerable to other opportunistic pathogens at times of stress.  For this reason, our most successful clients carefully track and monitor the clinical health of their stock at all stages of the production cycle.

The list below is only a partial list of common symptoms of disease in finfish.  While the issues below are often signs of disease, it is also important to note that many infections may go for months or years without detection, causing no easily visible symptoms. Rather, they lurk just below the surface, weakening the immune system and waiting for a period of stress or other opportunistic pathogen to cause mass mortality and loss. Successful producers understand this and manage risk by conducting regular biosurveillance, to keep small problems from becoming crises.

Looking to manage your risks better, but not sure where to start? Contact us and we can help you put together a biosurveillance plan and fish health strategy that works best for you.

 

Behavioral Signs:

 

  • Lethargy; weak or slow swimming
  • Reduced or absent feeding
  • Erratic swimming (spiraling, flashing, darting)
  • Surface gasping; hanging at water inlet/outlet
  • Hyperactivity or agitation
  • Loss of schooling cohesion; isolating
  • Loss of equilibrium; sinking, floating, rolling
  • Rubbing/scratching against tank walls or substrate
  • Increased or decreased aggression
  • Frequent surfacing or “piping”

 

External Physical Signs: 

 

Body Condition & Coloration

  • Emaciation
  • Distended or sunken abdomen
  • Darkened body coloration
  • Pale or blotchy coloration
  • Excessive mucus or loss of protective slime
  • Dehydration (in freshwater species under stress)
  • Exophthalmia (pop-eye) or sunken eyes
  • Deformed spine or body curvature
  • Scale raising (pinecone appearance)

Skin, Scales, and Fins

  • Ulcers; open sores; erosions
  • Hemorrhages (petechiae, ecchymoses)
  • Scale loss or scale lifting
  • Fin rot, fraying, or splitting
  • Nodules, cysts, or granulomas
  • White spots or salt-like granules
  • Cotton-like growths (fungal plaques)
  • Gill cover deformities or swelling
  • Prolapsed vent; reddened vent
  • Visible parasites or lice on the skin

Gill Appearance (External View)

  • Pale, dark, or mottled gills
  • Gill congestion (reddening)
  • Excessive mucus
  • Gill necrosis or erosion
  • Flaring or clamped operculum

Physiological Signs:

 

  • Rapid or labored breathing
  • Irregular heartbeat (in species where observable)
  • Edema (fluid retention)
  • Buoyancy disorders (positive/negative buoyancy)
  • Osmoregulatory stress signs (e.g., dehydration, swelling)
  • Gas bubble trauma signs (bubbles under skin)

Necropsy Findings (Internal Signs):

(Visible during internal examination of deceased or euthanized fish)

 

External-at-Necropsy

  • Foul odor
  • Thickened slime layer
  • Abnormal rigor or soft musculature

Body Cavity

  • Ascites (clear, cloudy, or bloody fluid)
  • Adhesions between organs
  • Gas bubbles in cavity (gas emboli)
  • Hemorrhage in musculature or cavity
  • Visceral prolapse

Liver

  • Enlarged (hepatomegaly) or shrunken
  • Pale, dark, mottled, or yellow coloration
  • Fatty liver appearance
  • Nodules, granulomas, or necrotic areas

Kidney & Spleen

  • Swollen or pale kidney
  • Spleen enlargement (splenomegaly)
  • Kidney congestion, necrosis, or atrophy
  • Granulomas or cysts within organs

Gastrointestinal Tract

  • Inflammation of gut mucosa
  • Hemorrhagic gut contents
  • Pale or empty GI tract despite recent feeding
  • Mucus accumulation in intestines
  • Parasites visible in lumen or wall
  • Stomach/intestinal ulcers

Heart

  • Pericardial effusion
  • Pale or hemorrhagic myocardium
  • Enlargement or atrophy

Gonads

  • Underdeveloped or regressed gonads
  • Hemorrhagic or necrotic tissue
  • Parasitic cysts

Swim Bladder

  • Overinflation or collapse
  • Fluid-filled swim bladder
  • Thickened walls or hemorrhage
  • Parasites within or around swim bladder

Gills (Internal Examination)

  • Necrotic lamellae
  • Lamellar fusion
  • Excess mucus or plaques
  • Visible parasites between lamellae

Muscle & Subcutaneous Tissue

  • Hemorrhages or bruising
  • Soft, pale, or liquefied muscle (myolysis)
  • Cysts or parasites
  • Gas pockets in tissue

Other Internal Findings

  • Brain swelling or hemorrhage
  • Spinal deformities
  • Enlarged gallbladder
  • Bile discoloration
  • Abnormal fat deposits or complete depletion