Evolution of the tilapia fishery with specific reference to the Nile tilapia (Oreochromis niloticus Linne)

As a fishery, the immensely large (c. 68,800 km2 ) Lake Victoria is a unique ecosystem which together with a riverine connection to the Lake Kyoga basin share a common endemic "Victorian" fish fauna (Greenwood 1966). Until the 1950s, the single socio economically most important species of fish in these two lakes was the native Oreochromis esculentus Graham (Graham 1929) even though the lake also contained a second native tilapiine, 0reochromis variabilis , and over 300 other fish species (Beauchamp, 1956).

Fishing gear selectivity of Lates niloticus L. (Nile perch), Oreochromis niloticus L. (Nile tilapia) and Rastrineobola argentea Pellegrin (Mukene) in lakes Victoria, Kyoga and Nabugabo

Catch effort data on which fisheries management regulations are sometimes based are not available for most lakes in Uganda. However, failure to regulate fishing gears and methods has been a major cause of collapse of fisheries in the country. Fisheries have been damaged by destructive and non-selective fishing gears and methods such as trawling and beach seining, by use of gill nets of mesh size which crop immature fish and by introduction of mechanised fishing. Selectivity of the gears used to crop Lates niloticus 1. (Nile perch), Oreochromis niloticus 1. (Nile tilapia) and Rastrineobola argentea (Mukene) which are currently the most important commercial species in Uganda were examined in order to recommend the most suitable types, sizes and methods that should be used in exploiting these fisheries . Gill nets of less than 127 mm mainly cropped immature Nile ti1apia and Nile perch. To protect these fisheries, the minimum mesh size of gill nets should be set at 127 mm. Seine nets of 5 mm do catch high proportions of immature Mukene while those of 10 mm catch mainly mature Mukene. When operated inshore, both sizes catch immature Nile perch and Nile ti1apia as by-catch. To protect the Mukene fishery and avoid catching immature byecatch, a minimum mesh size of the Mukene net should have been 10 mm operated as Lampara type net offshore but since most fishennen have been using the 5 mm seine for over five years the minimum size should not be allowed to drop below 5 mm pending further thorough investigations. Beach seining, trawling and are destructive to fisheries and should be prohibited until data that may justify their use is available.

The biology, ecology, population parameters and the fishery of the Nile tilapia, Oreochromis niloticus (L)

Oreochromis niloticus (the Nile tilapia) and three other ti1apine species: Oreochromis leucostictus, Tilapia zi11ii and T. rendallii were introduced into Lakes Victoria, Kyoga and Nabugabo in 1950s and 1960s. The source and foci of the stockings are given by Welcomme (1966) but the origin of the stocked species was Lake Albert. The Nile tilapia was introduced as a management measure to relieve fishing pressure on the endemic tiapiines and, since it grows to a bigger size, to encourage a return to the use of larger mesh gill nets. Ti1apia zillii was introduced to fill a vacant ,niche of macrophytes which could not be utilised' by the other tilapiines. Tilapia rendallii, and possibly T. leucosticutus could been introduced into these lakes accidently as a consquence of one of the species being tried out for aquaculture. The Nile perch and Nile tilapia have since fully established themselves and presently dominate the commercial fisheries of Lakes Victoria and Kyoga. The original fisheries based on the endemic tilapiines O. escu1entus and o. variabilis have collapsed. It is hypothesized that the ecological and limnological changes that are observed in Lakes Victoria and Kyoga are due to a truncation of the original food webs of the two lakes. Under the changed conditions, O. niloticus to be either playing a stabilizing role or fuelling nutrient turnover in the lakes. Other testable hypotheses point to the possible role of predation by the Nile perch, change in regional climate and hydrology in the lake basins.

Fishing gear selectivity in relation to their manufacture and to the management of fisheries of the Nile perch, the Nile tilapia, and Rastrineobola argentea (Mukene) in Lakes Victoria and Kyoga

The fisheries of Lakes Victoria and Kyoga have changed from the native tilapiine species and are now dominated by two introduced species; Nile perch and Nile tilapia, and one native species; Rastrineobola argentea (mukene). Because of the differences in the size of the species, it may be necessary to change the type and sizes of nets used.

Some characteristics of the fish fauna of a dam in the Lake Victoria region of Tanzania including the effects of multispecific stocking with tilapia species

Malya dam had been stocked with several species of Tilapia. However all but the species endemic to the region, T esculentus, made only marginal contributions to the fishery. T. zilli particularly had failed to establish itself, probably because its niche had been previously filled by T. rendalii. Planktivorou Tilapia species predominate in Malya dam but in smaller dams in the region herbivoorous tilapia are relatively more abundant. Six non-cichlid species and one of Haplochromis were found in the dam. Tilapia esculenta the most economically important species, was estimated to grow within 9-10 cm first year and 16-18 cm second year. The largest, specimen caught was 30.5 cm and the smallest mature individual 19 cm. The fish of the dam grew more slowly and matured at a smaller size than those of the same species in Lake Victoria The biological pattern of T. esculenta in Malya dam was similar to that of this species in Lake Victoria.

Yield isopleths of Tilapia esculenta Graham 1928 in Lake Victoria and Tilapia nilotica (Linnaeus) 1757 in Lake Albert

The yield equation given by BEVERTON and HOLT (1957) has several parameters which are difficult to estimate for tropical freshwater fish species. Nevertheless, some simplifying assumptions can be made and the most relevant parameters used to enable the construction of yield isopleths. Tilapia esculenfa has the following parameters: maximum length (L ∞=33.8 c.m. growth rate (K) = 0.32, natural mortality rate (M)=0.17 and the length at maturity (1 m)=22 cm. The optimum yield is obtained by catching the fish at a length of first capture of 26 em and a fishing mortality rate of 0.5. Tilapia nilotica with L ∞=49 cm, 1 m=36 cm, K=0.50 and M= 0.30 gives optimum yield when caught at a length of first capture of 35-36 cm with a fishing mortality rate of 0.5-0.6. The stuned Tilapia nilotica of Lake Albert has L ∞=17 cm, K=2.77,1 m=12 cm and M=3.37. With such a very high natural mortality, maximum yields would be obtained hy using a length of first capture less than 9 cm and a fishing mortality rate exceeding 1.8.

Preliminary observations on the feeding of Tilapia nilotica Linn. in Lake Rudolf

Tilapia nilotica is commercially very important throughout the Ethiopian region including the major rivers in West Africa, the Chad basin, the Nile and its associated lakes. The Tilapia fishery of Lake Rudolf is at present small, but potentially important, particularly on the eastern shores of the lake where fishing intensity is low. Preliminary results from observations on the feeding of Tilapia nilotica in Lake Rudolf are presented. The fish exhibit a regular diurnal feeding rhythm, commencing between 05.00 hours and 08.00 hours and ceasing between 14.00 hours and 18.00 hours. The largest fish appear to feed longer. Quantitative estimates of the daily food intake indicate less material to be ingested than by populations in other lakes. The lysis of algae, intestinal pH and food material are also investigated.

Preliminary observations on cage culture of Tilapia esculenta Graham and Tilapia xillii (Gervais) in Lake Victoria waters, at the Freshwater Fisheries Institute, Nyegezi, Tanzania

Cage culture of Tilapia is not suggested as a substitute for any known techniques in fish culture, but as one of the various techniques of obtaining more fish under controlled conditions. This fact has been very well accepted in various countries. Whererever facilities exist, this line of fish culture should be vigorously explored as a possible avenue in increasing fish production. High density stocking, management under controlled conditions, easy technique of fabricating the cage at relatively low cost, having no demand on land area, absence of prolific and effective breeding and easy availability of fish when a person needs it are a few of the attractions of the technique. The studies indicate that it is desirable to have different meshes for the cages, such as, small meshed cages for rearing fry to fingerlings stages, and larger meshed cages for rearing fingerlings to table sized fishes. II' the meshes are small, the resistance will be more and less water wilt pass through. While feeding with powdered food material, because of brisk activity of feeding fish, a part of the feed appeared wasted. This can be easily overcome if we would resort to feeding fish with cheap pelleted feeds which will no doubt reduce wastage. Precaution has to be taken against damage of the net and thereby loss of fish and against poaching by unauthorised persons. In the present attempt has been demonstrated the possibility of utilizing locally available species of Tilapia for cage culture and obtaining moderately satisfactory growth rates.

A preliminary report on the use of methallibure in tilapia culture

The aim of this research was to retard the onset of maturation and breeding using Methallibure (I.e.I. 33,828), thereby allowing more time for somatic growth. The process of development of the gonads was studied, laboratory experiments were performed upon a breeding stock of Tilapia in an attempt to postpone spawning, and the results of these investigations were tested, on fish in ponds at the Fish Culture Research Station in Israel.

Growth rates of Tilapia esculenta (Graham) and Tilapia zillii (Gervais) under cultivation in ponds at Nyegezi, Tanzania

Aquaculture in Tanzania is still on a subsistence level and most of the ponds are maintained as part time job. The ponds are too small, shallow and over crowded with stunted Tilapia spp. In the present paper the results of experiments conducted in ponds at Nyegezi with T. esculenta and T. zillii are presented. This was part of an overall project of developing techniques of fish cultures with Tilapia under the limited existing conditions at Nyegezi. In a mono - species culture experiement with Tilapia zillii in nine month's time an average size of 172.8 mm/115.0 g was attained. In another experiment with T. zillii and T. esculenta in thirteen month's time, T. zillii attained an average size of 180.2mm/106.6 g and T. esculenta 193.6 mm/118.8 g. In another experiment with intensive feeding schedule an average size of 179.3 mm/126.6 g was attained by T. zillii and 191.0 mm/125.0 g by T. esculenta in four month's time. A locally prepared supplimentary feed with local Brewery Waste and Fish Meal (10:1) was readily accepted by both species of Tilapia. T. zillii voraciously fed on Cabbage leaves, Cauliflower leaves, Chinese cabbage leaves, Cassava leaves and on the common weed Comalina sp. Though all the items mentioned above were readily accepted by T. zillii feeding with Comaltna sp. was the easiest and most convenient because of its availability. In an intensive feeding experiment with vegetable leaves/Comalina sp. and the locally prepared supplimentary feed the fishes attained table size in four months time. Cement cistens of 5 X 3 X 1½ m size could be conveniently used for breeding both species of Tilapia. T. zillii had semi adhesive eggs and they were deposited on the sides of the cement wall. The number of young ones in a brood ranged from 160 to 314 in T. esculenta and 687 to 4,356 in T. zillii.

On the ecology and explorations of the fisheries of an East African rift valley lake, Part 1: On the bionomics and population structure of Tilapia nilotica Linnaeus 1757 in Lake Baringo, Kenya

An ecological survey of the fisheries of Lake Baringo, Kenya was carried out between August, 1972 and August, 1973. The bionomics and population structure of T. nilotica is described. Sampling was done with multifilament gillnets of graded mesh sizes from 51 mm to 178 mm in approximately 12.5 mm increments. The Lake was divided into three sampling and ecologically different zones - the south, central and north zones. The size range of T. nilotica of both sexes caught was between 5 and 27 cm (mode 16 cm) with a mean length of 16.07 cm. For all the collections, males dominated (55.3%) and a higher proportion of males were caught in January, August and November. The smallest mature male and female was 9 and 10 cm respectively. Males grow faster and mature at larger sizes than females. 50% of all males and females mature at 17.4 and 16:4 cm respectively. The periods of intense spawning were between August and October and January to April. The Tilapia were feeding best in central and north zones and the feeding intensity was reduced in January. Two endoparasites Contracaecum sp. and Clinostomum sp. were isolated from the Tilapia. The "condition" of the fish was better in the north than in the other two zones.

Hybridization between Tilapia zillii (Gervais) and Tilapia andersonii (Casteinau) at the Freshwater Fisheries Institute, at Nyegezi, Tanzania

As a part of an overall project on fishculture development techniques in Tanzania, hybridization between Tilapia zillii and Tilapia andersonii was carried out at the Freshwater Fisheries Institute, Nyegezi, Tanzania. T. andersonii, a plankton feeder, is not indigenous to Tanzania but was introduced in 1968 from Zambia for certain specific purpose. T. zillii, a macrovegetation feeder, is present locally and is common. In the present studies T. zillii (245.0 mm/260.0 g) female was hybridized with T. andersonii (288.0 mm/350.0 g) male. Under cement cistern conditions it was only after about four months of acclimatization that hybridization between the two occurred. About 1,637 interspecific hybrid fry were produced in a single brood. Eggs were adhesive and parental care shown by the female, the male being driven away. Growth under cistern conditions was slow, attaining a size of 134.8 mm/44.3 g in 10 months. But this growth rate need not be taken as ideal. In body shape, colouration and other morphometric characters the hybrids had inherited from both parents. The number of gill rakers among the hybrids was eighteen which was intermediate between T. zillii (12) and T. andersonii (27). Among one hundred and seventy two specimens (106.0 mm - 168.0 mm) cut and examined the sex ration was hundred per cent males and all of them were between II and IV stages of maturity. This is the first report of fish hybridization from Tanzania and possibly the first report on hybridization between T. zillii and T. andersonii. The full significanoe of the findings and its role in African fishculture is discussed.

Evaluation of external anchor tags for estimation of growth and survival in tilapia (Oreochromis shiranus chilwae Trewavas)

The effects of tagging with Roy FD-68B T-bar anchor tags on estimates of growth in Tilapia (Oreochromis shiranus chilwae) were investigated in a pond and in a field experiment. In the pond experiment, mean length increments of tagged and marked fish were compared. In the field experiment growth of tagged and "untouched" individual wild fish were compared by measuring scale circuli spacing (Circ.), which is correlated to instantaneous growth rate. Length increments of tagged and untagged/marked fish were not significantly different in either experiment. In the pond experiment, the total mortality rate in the small tagged fish was significantly higher than in the marked fish. The recoveries of tagged fish in the pond experiment increased with fish size. Recoveries exceeded 80% at lengths over 13 cm TL. The ratios of tagged to marked recoveries were 1.02 and 0.74 for large and small fish respectively. The study shows that tagging of Tilapia with Roy anchor tags does not in general alter the growth rates of the fish.