Trying to explain scale loss mortality: A continuing puzzle

Abstract

Abstract Beginning in the 1920s, Homer Smith and co‐workers described kidney functions in marine fish. He also described fluid loss via the kidney (diuresis) associated with scale loss, which usually killed fish in 24 h or less. Pacific salmon also experienced diuresis following stress, but only for an hour or two, and rarely died. Marine juvenile salmonids died following loss of >30% of their scales. Death followed a period of increasing lethargy, decreased circulatory output, and decreased weight and length. At death, the fish were limp (flaccid paralysis) and rigor mortis was delayed. Immersion of descaled fish in freshwater or dilute seawater or infusion of physiological saline had little effect. Measurements of plasma ions in descaled fish provided only a few clues as to the cause of death. Early experiments suggested that increased plasma magnesium ion levels may explain the lethargy and flaccid paralysis, but later experiments did not support this hypothesis. Loss of sufficient body fluids to account for the severe weight losses probably occurred across the gill surfaces and not through descaled skin. More recent experiments in other labs have confirmed that decreased weight and length may result from several stressful circumstances, not just scale loss. Increased plasma adrenaline and decreased plasma calcium ion may be fruitful areas for further research on the mechanisms of scale loss mortality. Key words: Scale lossosmoregulationsecondary circulatory systemplasma magnesium ionplasma calcium ionmarine salmonidsweight lossshrinkageflaccid paralysissmoltificationseawater entryrigor mortisdiuresis