Stringer -
They are leeches -
They can be various colors ranging from black to graysih-white. Typically they are found on the gills, pectoral and anal fins.

As they are just on the ouside of the fish they don't affect the quality of the meat much like sea-lice. However the fish don't usually pick them up until they have been in the river a while.

Tight lines
Smalma