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Did you know that the West coast seabream is protandrous?

Avatar of inke inke 05. October 2017 - Environment, Discover Namibia

Dr Hannes Holtzhausen is seen here with a big female west coast seabream in his left hand and a small male in his right hand, which will become a female in a few years’ time.

One of the few line-fish species sought after by anglers on our coast, is the west coast seabream (Lithognathus aureti), better known as Steenbras. The bigger the fish the better, but that had tremendous effects on the species. In the 90s Dr Hannes Holtzhausen, researcher at the Ministry of Fisheries and Marine Resources, found out that the west coast seabream is one of the few species which is protandrous. That means that the hatchlings start developing into males. They reach sexual maturity at a certain length and after having grown a few centimetres more, the male sex organs start to disappear while female sex organs develop. At a certain length, all west coast seabreams are fully sexually mature females for the rest of their lives. The development and growth of this species depends on the area they live in and the average water temperature of the Atlantic Ocean.

In the Meob Bay south of Walvis Bay, where the west coast seabreams in that area are busy developing into a subspecies, the young fish are sexless up to 27 cm in length. When 27 to 33 long the west coast seabreams are mature males and can reproduce. After 33 cm the male sex organs start to disappear while the female organs develop. This process takes the same amount of time in which the fish grows to a length of 40 cm. Once at that length and larger, the west coast seabream is a fully developed female for the rest of its life. In the waters along the Skeleton Coast National Park, which are on average a few Celsius warmer that in Meob Bay, the fish grow faster and therefore a male is only mature between 31 and 37 cm in length and between 37 and up to 67 cm the transformation from male to female takes place. In this time, the west coast seabream in that area cannot reproduce. Some females are fully developed at 46 cm others only at a length of 67 cm.

With the help of otoliths, also called, statoconium, otoconium or statolith, which are found in the skull of a fish, can help researchers determine the age of a fish. To do that the otolith is cut into thin slices and under a microscope the year-rings are counted. A 78 cm west coast seabream from Meob Bay is about 47 years old. Specimens of the same species from Rocky Point in the Skeleton Coast National Park at 78 cm length were “only” 22 years old.

Dirk Heinrich

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