Abstract: Underwater fish grading may be done in a fish pen defined by a rectangular floating walkway. A flexible net in the pen is supported at its perimeter by the walkway or by support members thereon, and a pair of spaced-apart rigid members bridge the pen, each end resting on suitable support means. Below that, a belly rope is passed below the net. A grading barrier, suspended from the rigid members, has a horizontal cross-member held well above water level. An arcuate rim secured to both ends of the cross-member extends down in a vertical plane, and a series of vertical rods extend between and are secured to both the rim and the cross-member, the rods being evenly spaced apart to define grading passages between successive rods. In cross section, the rim defines an exterior channel, and the flexible net is brought into snug contact with the exterior channel by means of the belly rope and is then anchored near the ends of the rim.