Assessment of sex and maturation

It may be difficult to distinguish between the sexes of charr from appearance alone, except at the period of spawning. This is particularly true of fish recently mature. Older broodfish have more distinct differences.

The signs of sexual maturation become more apparent when the spawning period draws nearer, and it becomes easier to differentiate between the sexes. Many farmers sort out the males from the females to ease the monitoring of the impending roe discharge.It should be pointed out though, that the sperm production in the males, which takes place a little bit earlier than the roe production, may have a stimulating and concerting effect on the females in the group.

The changes in appearance that take place at maturation can be very different, depending on type/stock. Usually both sexes grow darker on the back and assume a reddish hue on the belly or abdomen. The males are usually of brighter and sharper colours than the females. They are slimmer, thinner and more streamlined with a larger and coarser head. The females are of larger belly and with softer, fuller contours. The gill cover/operculum may be have sharp edges in the males, but rounded in the females. On the lower mandible of the males there appears a more distinct hook than on the females.In anadromous stocks the fish is most often brightly red on the belly, with white stripes on the fins. Some of the freshwater species on the other hand are of yellowish colour on the belly. The colour differences can be greatly variable between breeds.

 

Figure: Female (left) can be distinguished from male (right) by head shape. The male usually has a hooked jaw and female has a more curved operculum.

When the period of spawning approaches, the females have to be checked every 4-5 days for roe maturation. When the roe are fully mature, they slip from the roe sac into the abdominal cavity ready to be discharged. When a female is ready for stripping, the roe can be seen to slide to the front forwards, when the fish is taken by the tail. The cloaca also swells and roe are easily released if the belly is softly stroked backwards.

When checking the female group, it may be sorted depending on maturation, and, it may be placed in a separate tank after stripping. This is, of course, dependant on space on the farm and the volume of the roe production.

It is best to strip the female within four days from the release of the roe into the abdominal cavity. If this is done earlier or too early, the roe are hardly fully mature. Done later, they may have become overmature. Whichever the case, immature or overmature roe will lead to inferior results as they are less fertile.

 

Figure: A female is ready for stripping when the eggs shift forwards when the fish is held by its tail.