6. RELATIVE POPULATIONS

6.1 COMMERCIAL CATCH

The importance of Tilapia in the commercial catch is indicated by a subjective estimate of groups of fish seen at the Shagunu fish market, provided through the kindness of Mr. N. Willoughby (Table 6). For this purpose a tour of the fish available for sale was made on most market days from the end of November 1971 to mid-October 1972 between 12.00 hours and 13.00 hours. Three later observations (November 1972 and February 1973) were made by the expert.

The observations indicate a progressive increase in the proportion of Tilapia in the market fish from late 1971 to a place of prominence in late 1972, with no suggestion of a a decline in importance. A count of individual Tilapia seen by species in February 1973 showed that T. galilaea was about ten times more abundant than T. nilotica and 18 times more abundant than T. zillii.

The increased importance of Tilapia in the commercial catches has been noted by personal observations of survey fishermen and the catch assessment survey crew on Kainji Lake.

6.2 GILLNET SAMPLING

Stationary survey gillnets set throughout the lake between 1969 and 1972 did not catch large numbers of Tilapia and generally do not show any trend in the Tilapia populations. The catch in a conservatively estimated 200 000 metres of gillnet set in this period was as follows:

Thus T. galilaea was nearly eight times as abundant as T. nilotica and 29 times as abundant as T. zillii in the catches.

6.3 SHORELINE SAMPLING

6.3.1 Electrofishing

The relative importance of species of Tilapia in the electrofishing rounds is recorded below:

Although electrofishing is a very subjective method of sampling and depends largely on the skill of the sampling crew, the increase in numbers of Tilapia per fishing round indicates an increase in the Tilapia population from late 1970 to late 1972.

The proportion of each species in the catch corroborates other information on the dominance of T. galilaea among the Tilapias. These were 6.5 tines more abundant than T. nilotica and 7.5 times as abundant as T. zillii. Among these samples T. zillii closely approached the abundance of T. nilotica, and relatively large numbers of T. dageti were captured. The January 1973 catch of T. nilotica suggests that these may be increasing in relative abundance.

6.3.2 Castnet samples

Castnet catches (½ to 3-inch stretched mesh) by research crews and commercial fisher­men from September 1972 to February 1973 resulted in the following:

Again the dominance of T. galilaea is notable in the catch.

6.3.3 Dipnet samples

Dipnet samples taken in November 1972 and February 1973 (Tables 4 and 5) showed the following composition:

As noted previously no T. zillii or T. dageti were captured in shallow water with young T. galilaea and T. nilotica.

6.4 SUMMARY OR RELATIVE ABUNDANCE

A review of the percent composition of Tilapia spp. by the various sampling methods is as follows:

The dominance of T. galilaea among the Tilapia in Kainji Lake is very marked at present. Banks, Holden and McConnell (1966) predicted that T. nilotica would become the dominant species of fish in Kainji Lake, basing this on the assumption that the shallow flooded Foge Island area would provide ideal habitat for this species; the prediction of Tilapia dominance has been questioned (FAO, 1970a p. 45; Imevbore, 1969). The final position of Tilapia among the fish populations in the lake may not be fully revealed for some years.