West Coast Groundfish Observer Program Manual, Part2 - NOAA

CHAPTER 6
Sexing Flatfish
Flatfish gonads are paired, are located posterior to the
visceral cavity and extend just under the flesh on both sides
of the fish. If the flatfish has an anal spine, the gonads will
begin just behind this spine. On the eyeless side of the fish,
cut from the anal spine back toward the tail of the fish.
Lift the skin flap and check for a triangular shaped gonad.
Female flatfishes have elongate triangular ovaries that
extend from behind the anal spine almost to the tail when
mature. When immature, the ovaries are shaped like
equilateral triangles with one corner shaped like a smoothly
rounded tube extending slightly back toward the tail (the
triangle looks like a funnel in shape). The color will be pink
(spent, immature) or orange (ready to spawn). Ovaries
always have rounded edges on the triangular gonad (See
Figure 6-6).
Male flatfishes have a white, equilateral triangle shaped
gonads. Unlike the female fish, the male’s gonad is not
elongated towards the posterior end. Immature males have
a small crescent moon shaped, tan colored gonad lying
directly at or slightly behind the anal spine. All male
flatfishes have sharp edges on the triangular gonad (See
Figure 6-6). Lift the gonad with a knife or scalpel and
examine the sides of the triangular gonad to distinguish the
sharp edges (male) or rounded sides (female). This
technique works even on an immature flatfish.
1/30/2004 6-21