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fin ray

The segmented structures in a fin that provide support.

fingerling

A young or small fish, especially a young salmon or trout.

fluvial

A fish that hatches in a small tributary, rears there for 1-4 years, and then migrates out to a larger river to mature.  The fish then migrates back to the tributary to spawn.

frazil ice

Ice formed in the water column in very cold temperatures. Frazil ice typically forms in shallow riffles where most of the heat in a stream is lost.

fry

Small fish, especially young, recently hatched fish.

fusiform

A streamlined body shape typical of most salmonines (trout, salmon and whitefish) which consists of a body which is taller than it is wide. This "torpedo" shape reduces resistance in the current allowing trout to hold position in a stream. An example of a fish with a non-fusiform body shape is the largescale sucker.