An aquatic crustacean which is a parasite of fish, typically attached to the skin or gills.
（寄生在鱼皮或鱼鳃上的）鱼虱
a free-swimming crustacean with a shieldlike carapace and a pair of suckers (class Branchiura: several genera, in particular Argulus).
an elongated crustacean that becomes permanently attached to the host and typically highly modified (class Copepoda: several orders and numerous species).
Definition of fish louse in US English:
fish louse
noun
An aquatic crustacean which is a parasite of fish, typically attached to the skin or gills.
（寄生在鱼皮或鱼鳃上的）鱼虱
an elongated crustacean that becomes permanently attached to the host and typically highly modified (class Copepoda: several orders and numerous species)
a free-swimming crustacean with a shieldlike carapace and a pair of suckers (class Branchiura: several genera, in particular Argulus)