Bagrus Docmac Research Articles

Haplochromine Cleaner Fishes: a Taxonomic and Eco-Morphological Description of Two New Species

In addition to the many trophic groups of haplochromine cichlids already known from Lake Victoria, two new ones (i.e. zooplanktophages and cleaner fishes) have been discovered by the Haplochromis Ecology Survey Team which operates in the Mwanza Gulf of the Lake. The present paper deals with the taxonomic description and eco-morphology of the two (new) species of haplochromine cleaner fishes. Cleaning symbiosis has been described mainly for marine fishes. Among cichlids only juvenile Etroplus maculatus were known as (facultative) cleaners 1 . The cleaning behaviour of the Victoria haplochromines is deduced from their stomach contents which revealed that one species feeds mainly and the other partly on carplice and leeches. No other cleaner fishes are known to feed on these kinds of parasites which in Lake Victoria occur externally on smooth-skinned fishes (Protopterus aethiopicus, Bagrus docmac, Synodontis sp. and Clarias sp.). The preference for firmly attached, flat parasites correlates with the scraper-like teeth of the haplochromine cleaners. Virtually all other cleaner fishes have teeth with pronounced, pointed cusps and feed on mobile parasites or (elongate) parasites which are only proximally attached to their hosts.

Analysis of catch data from 1968 to 1976 from nine fish landings in the Kenya waters of Lake Victoria

Fish landing data collected by the Kenya regional fisheries officers from Homa Bay, Kaloka, Karungu, Marenga, Wichlum, Kendu Bay, Usenge, Dunga and Ngegu from 1968 to 1976 were statistically analysed to determine trends in the traditional fisherman's catch in the Kenya waters of Lake Victoria. Over the nine years a significant decline occurred for total catch and catches of Haplochromis spp., Tilapia spp. and Protopterus aethiopicus. Engraulcypris spp. catch increased significantly; while catches of Bagrus docmac and Clarias mossambicus fluctuated and no clearly defined trend emerged, although B. docmac catch seemed to be declining.Overall indications were that as the catch of traditionally preferred fish declined, Engraulicypris spp., B. docmac, C. mossambicus and recently introduced Lates niloticus were becoming more important to the total catch.

Calculating Mortality Rates and Optimum Yields from Length Samples

An equation is derived for yield per recruit of a fishery (or other exploited animal population) as a function of fishing intensity and age of first capture. The equation has the advantage that it does not require explicit estimates of natural mortality or individual growth rate parameters. Linear length growth is assumed until maximum size is reached, and mortality parameters are expressed relative to growth rate. Mortality parameters are estimated from average length samples of separate populations experiencing different fishing efforts in the same fishery. The equation may be used to compare existing fishing efforts and age of first capture with optimal values. Samples of the catfish Bagrus docmac from Lake Victoria (East Africa) are used to illustrate the method. Key words: yield equation, Beverton–Holt, fish lengths, Lake Victoria, Bagrus docmac, fishing effort, recruitment age

The food of Bagrus docmac (Forsk.) (Pisces: Siluriformes) and its relationship with Haplochromis Hilgendorf (Pisces: Cichlidae), in Lake Victoria, East Africa

The diet of the siluriform fish Bagrus docmac (Forsk.) is described in relation to various localities in Lake Victoria, East Africa. Invertebrates which are the principal food of young fish, but generally of little significance in adults, are particularly poorly represented in fishes from Tanzanian waters and most important in those from the Kavirondo Gulf where the ontogenetic changes in diet differ from those found elsewhere in the Lake. B. docmac starts feeding piscivorously at about 10 cm standard length with the transition from a principally invertebrate to a primarily piscivorous diet occurring at 15 cm S.L. In adult B. docmac fish are found in over 95 % of individuals with food. Haplochromis is the all important food type, and Tilapia, the most valuable commercial fish in the lake, is not preyed upon at all. Bathymetric changes with respect to the invertebrate food are noted, and it is shown that piscivorous feeding occurs mainly by day and invertebrate feeding by night. Quantitative estimates of consumption of Haplochromis by B, docmac are computed from the results of a series of 24 h fishing surveys and found to exceed published data from elsewhere. The formula for the computation of a daily ration is given as Log R=2.973 Log L—3.36089 and it is calculated that the mean annual consumption of Haplochromis by B. docmac is approximately 75% of the mean standing stock of the prey fish, as compared with a commercial catch of about 5% of the mean standing stock.

Hosts, distribution and pathology of infections with larvae of Eustrongylides (Dioctophymidae, Nematoda) in fishes from East African lakes†

Eustrongylides sp. larvae were found to be numerous in Bagrus docmac, Clarias mossambicus and Haplochromis spp. from Lake Victoria, rare in Haplochromis spp. and absent from B. docmac and C. lazera from Lake George, East Africa. Larvae encyst in the mesenteries, spleen and in Haplochromis spp. also in the gonads. Unencysted larvae were observed in muscles. Cyst walls consisted of fibroblasts, collagen strands, and were often infiltrated with fat cells. Fatty changes were observed in infected spleens; the muscle tissue, around the unencysted worms, was digested and infiltrated with fibroblasts. Infected gonads, particularly ovaries were deformed and in heavy infections degenerated completely. The biological implication of this infection with larval Eustrongylides sp. on the host populations in Lake Victoria is discussed.

THE FOOD OF NON‐CICHLID FISHES IN THE LAKE VICTORIA BASIN, WITH REMARKS ON THEIR EVOLUTION AND ADAPTATION TO LACUSTRINE CONDITIONS

The food of twenty‐six species of non‐cichlid fishes in the Lake Victoria basin is described. Most species, for which data were sufficient to show this, are facultative feeders, their food being determined by their size, their feeding grounds, and the age of the moon. The size of an organism and its position in the aquatic environment usually determine its importance as prey, and this is considered a reason why so many fishes in the lake are insectivorous. Only three species of non‐cichlid are extensively piscivorous: these feed heavily on Haplochromis, slightly on Engraulicypris, and hardly at all on Tilapia. Several birds and fishes feed on Engraulicypris, but otherwise there are few natural predators of non‐cichlids. Ontogenetic changes in feeding behaviour of Mormyrus kannume and Bagrus docmac support the hypothesis (recently confirmed for B. docmac) that in the lake juveniles of these two species live on rocky shores. The evolutionary stages involved in the colonization of lakes from rivers are discussed, with particular reference to breeding habits and food. The non‐cichlids in Lake Victoria are considered not to have progressed far towards becoming adapted to a lacustrine environment.

Hæmoglobin in Branchiura

HAEMOGLOBIN dissolved in the blood plasma has been recorded in all the major divisions of entomostracan Crustacea except one1. This respiratory blood pigment has been found in all species of Phyllopoda examined, in most Cladocera, in a few Ostracoda, in some harpacticid and parasitic Copepoda and in some Rhizocephala, but not hitherto in the Branchiura. This is a small group of so-called fish lice, which includes the well-known Argulus foliaceus (L.). I have been unable to detect haemoglobin in the pale blood of species of this genus, but Dolops ranarum (Stuhlmann) has red blood and I find that it shows strong absorption bands of oxyhaemoglobin. The animals were fouhd parasitizing the siluroid fish Bagrus docmac (Forskal) in Lake Victoria, and I was able to examine them, thanks to the staff of the East African Fisheries Research Laboratory at Jinja, Uganda.