A clupeid fishery has existed for many years in the Niger River system. The traditional fishing method is by “Attala” net, a type of lift net, measuring 5 m2 , operated from the side of a canoe and moved slowly through the water with the aid of paddles. The technique requires expensive equipment and is effective when used at night in sheltered places, for harvesting clupeids in the top 1–2 m of water.
After impoundment of Kainji Lake in 1968, a thriving clupeid fishery continued below the dam at Faku and at the northern extremity of the lake near Yelwa. Operations did not extend into the lake due mainly to the fact that here the open water conditions are hazardous to the safe operation of canoes at night and also the fish tend to lie at a greater depth than in the river.
Since that time, notable increases in the clupeid population of the lake have been observed through the regular sampling of this stock by Kainji Lake Research project teams using a lift net in conjunction with light attraction, (Otobo in press).
The efficiency of this method is adversely affected by the extremely turbid condition of the lake during the period of the white flood, the effects of which do not allow the lake to attain a very high degree of clarity at any time.
Two species of clupeidae occur in Kainji Lake: Pellonula afzeliusi standard length 20 – 75 mm; Sierrathrissa leonensis standard length 13 – 34 mm. Because of the small size of the latter, a very small mesh size is necessary for their capture.
The expert arrived at Kainji in September 1973 and in view of the turbid state of the lake, a small mid-water trawl was constructed for use while conditions for this type of gear were most favourable.
Designated No. 1, the net had a designed vertical and horizontal opening of 8 m and mesh sizes ranged from 200 mm at the wings to 12 mm at the cod-end, Fig. 1. Sufficient floats were attached to the headrope to ensure that the net floated at all times and was thus prevented from sinking during the shooting and hauling operation or when towing speed was reduced for any reason.
This was necessary to avoid fouling the many obstructions on the lake bottom. The net was rigged in the manner shown in Pig. 2. The net was used in conjunction with two flat bottomed boats of 9 m length, powered by 33 hp outboard motors. The boats were not well suited to this type of work and proved difficult to control whilst towing the trawl due to the absence of any kind of keel or skeg.
The experiment continued with the construction of a second trawl of the same overall dimensions but mesh sizes were reduced throughout, starting at 87 mm in the wings and terminating in 9 mm at the cod-end. Fig, 3.
Smaller boats with 15 hp outboard engines were used in an attempt to reduce the fuel consumption. These engines each produced a static bollard full of approximately 100 kgs and when used in the fishing operation proved powerful enough to produce some good catches of clupeids, although the number of larger fish caught was noticeably less.
The cod-end mesh size was later reduced to 6 mm as the escape rate through the meshes was still high.
Tables 1 and 2 show the catches obtained during this period of experimental fishing, most of which was carried out in the southern part of the lake near the project headquarters.
The effect on catches of reducing the cod-end mesh size can clearly be seen when the figures are reproduced as an average catch rate per hour.
| Net | 1 | fitted | with | 12 | mm | cod-end | - | 15 | kg | per | hour |
| " | 2 | " | " | 9 | mm | " " | - | 55 | " | " | " |
| " | 2 | " | " | 6 | mm | " " | - | 177 | " | " | " |
The twine sizes of netting used for trawl construction may be increased to produce a stronger and more durable net. Fig. 4.
Echo sounder recordings made between September and February show an almost continuous band of echoes appearing between 3m and 6 m depth. From February to June the water becomes gradually clearer, with the result that the band of echoes widens and occurs at progressively greater depth.
There was some doubt regarding the origin of these echoes and an experiment was conducted to determine whether the source was fish or some other organism in high concentration.
By removing the six inflatable floats and replacing these with 37 rigid plastic gill net floats, equal in buoyancy to the weight of the leaded footrope (0.5 kg), it was possible to operate the net at any required depth by adjustment of the length of towing warp or the size of towing weights.
An area of deep water near the dam site was selected to reduce the risk of damaging the gear on the lake bottom.
Three pulls were made with varying amounts of towing weight or warp length, while the trawl in action was monitored by an echo sounder from above. Both headrope and footrope were clearly recorded, showing the vertical gape of the trawl and its position relative to the surface and the other supposed fish echoes. Figs. 5, 6, 7.
While it is accepted that not all the echoes received are from fish, the results of this experiment indicated that clupeids are found in quantity only within the main band of echoes recorded.
Results also confirm earlier beliefs that the smaller Sierrathrissa leonensis are prevalent in the upper levels while Pellonula afzeliusi prefer the lower levels of this band.
Throughout the programme of experimental and exploratory fishing with the mid-water trawl, there was found to be a correlation between echo sounder trace intensity and fish abundance, with the exception of a period in March and April when good traces were recorded at times but poor catches were made. At this time, quantities of jelly fish were also present in the catch and it is thought that these organisms were responsible in part for the strong echoes received.
A programme of exploratory fishing was initiated in January. Seventeen stations were established around the lake, to be sampled at two weekly intervals. Fourteen of these stations were located at approximately eight kilometre intervals over the old river bed, while the remaining three were situated in shallower areas.
The programme had two main aims:
To locate areas that may support a commercial fishery and
Produce catch data for stock assessment analysis.
In addition to the regular sampling of stations with the trawl, random echo-sounder surveys were carried out in areas where it was impossible to fish due to the presence of submerged trees or where the water was too shallow to operate the net safely.
A standard pull for this programme was set at thirty minutes duration but in some areas it was impossible to tour for this period of time due to concentrations of gill nets or insufficient clear area in which to tow a trawl for 1½ km which was the approximate distance covered in the time. The standard pull was therefore reduced to fifteen minutes, for all stations to minimize the chances of net damage.
During his brief visit to the project, Dr. Bazigos made a rapid assessment of this programme and on his recommendation the number of stations was reduced to eleven. (Fig. 8).
Table 3 gives a summary of catches, presented as a catch per unit effort measured in kilograms per hour.
These figures give an indication of the areas which may support a future fishery. In addition to an adequate abundance of fish, an essential prerequisite for any proposed clupeid trawl fishery is a large area of open water, sufficiently clear of obstructions to allow the net to be operated without risk of damage. The presence of trees severely restricts and largely precludes the use of any type of moving gear in many areas. (Fig. 9).
The main basin of the lake has a large area from which the trees were cleared before impoundment, but the stumps remain and at low water level these are unavoidable. This area also, at times contains large concentrations of gill nets.
The deep southern part of the lake appears to be the most suitable area for the introduction of a clupeid trawl fishery, as here all the requirements are met, including easy access to fuel supplies at New Bussa and to well established marketing facilities for the species at Faku.
Villages such as Oroba and Monai in Kwara State and Anfani in N.W. State are well sited as bases from which to participate in a fishery in this locality. This part of the lake which, in the open water areas most convenient for trawling operations is virtually unexploited by any other type of gear, presents an opportunity to introduce a new fishery without interference with the activities of the present fishing community.
