3. TILAPIA FISHERY

For the first two years after impoundment, catches of Cichlidae were less than one percent of the total fish production of the lake.

Regular shoreline sampling with electro-fishing equipment and fleets of graded gill nets since that time, indicate a substantial and consistent increase in the tilapia population.

The four species of Tilapia presently recognized in Kainji Lake include, T. galilaea T. nilotica. T. zillii and T. dugeti; with T. galilaea predominating. The main methods of harvesting these species is by castnet and gillnet.

Cast nets at present account for the largest proportion of tilapia landings. All the castnets observed were constructed of nylon twine and in the hands of experienced fishermen are quite efficient in shallow waters, especially where clear spaces can be found among the floating grasses. Because of the small area required for each cast this method is also used successfully among rocks and tree stumps.

Results from various, earlier gill net programmes suggest that tilapia are not easily caught in gill nets unless driven.

The driving or beating method is already known and has proved to be very effective in Kainji Lake but unfortunately has caused a great deal of friction between the fishermen operating this method and the owners of static gill nets. In many cases disputes have been taken to the local authorities who, in general, have disapproved of the practice of active gill netting. This method was not included in the tilapia fishing programme.

No seine nets were observed on the lake despite the fact that the method is used in the the Niger River system.

3.1 GILL NETS

Comparative trials to determine the relative efficiency of monofilament and multifilament gill nets were carried out. For this purpose three monofilament nets and two multifilament nets were constructed. Each net was mounted by halves (50 percent slack) to 5 mm diameter braided nylon ropes of 100 metres length. Plastic floats were attached to the headrope at three metre intervals and lead weights were used to sink the footrope.

Specification of monofilament nets were as follows:

Multifilament nets were of similar mesh sizes, all of 210/3 twine. Nets were set in the late afternoon and hauled the following morning.

Results so far indicate the superiority of monofilament nets, which are outfishing multifilament in the ratio of 5:1 in the case of the smaller 112 mm mesh size and between 2:1 and 3:1 when comparing the 128 mm sizes.

Delivery of the monofilament netting was delayed and conclusions therefore are based on only 23 settings. The experiment should continue until the nets are worn out in order to assess the relative durability of these types of netting. At the time of reporting the nets were still in good order and it may be sufficient to compare the costs of the two types of net with the relative catches to date to produce some conclusions regarding their commercial potential.

3.2 SEINE NETTING

The absence of seine nets on Kainji Lake is explained by the predominantly rocky or tree studded shoreline, where this method of fishing might be used.

As this same littoral zone is the favoured habitat of tilapia, beach seining was considered to hold great potential as a harvesting method for this species.

A number of suitable sites as selected and clearing operations were commenced at the time of lowest water level. A programme of regular seine net sampling will be carried out as soon as the water level rises sufficiently and these areas become flooded (Fig. 10 and 11).

Because of the difficulty of removing tree stumps and other obstruction from below low water level, the effectiveness of cleared sites reduces as the lake approaches its lowest level. At this time however large areas of Foge Island are being exposed in the form of sand banks, providing beaches which, although not completely clear of tree stumps, may be used for seining as the surrounding water is shallow and the footrope of the net can be lifted carefully over the few obstructions encountered.

It is convenient that as the fishable areas of mainland sites become smaller with declining water level, fishing areas of the Foge Island complex become larger, thereby allowing the fishing effort to be sustained throughout the year.

Clearing areas of Foge Island may not be beneficial as these sand banks are subject to a considerable amount of scouring by wave action during the covering and uncovering periods. The movement of sand at these times is therefore likely to expose more roots and other debris at each low water season.

A schematic diagram of the seine net used for experimental fishing is shown in Fig. 12.

A summary of seine net catches landed at Shagunu and various parts of Foge Island is shown below.

A rowing boat with provision for net stowage, suitable for beach seine net operations is shown in Fig, 13.