The feeding ecology, ontogeny and larval feeding in Labeo Victorianus Boulenger 1901 (Pisces: Cyprinidae)

Abstract

Labeo victorianus was one of the most popular commercial fish of Lake Victoria highly cherished by all communities within the lake basin. The fish was over fished and by the early 1960s it had virtually disappeared from the catches of the lake. There has been growing interest by both biodiversity conservators and fish consumers to restore the population of Labeo victorianus. It was identified in the Lake Victoria Environmental Management Project (LVEMP) as one of the species for restoration. Several investigations were carried out in the current study to provide the scientific knowledge that will facilitate the domestication of Labeo victorianus as a way of restoring its declined population.

Ripe L. victorianus broodstocks were collected from the spawning grounds and induced under culture conditions using fresh extracts of the pituitary gland of the mirror carp (Cyprinus carpio) following a method used by Hecht et al. (1997) for catfish. The ontogenetic developmental of its digestive system were noted to provide the knowledge required for designing its diets and larval feeding program. This information would facilitate commercial production for Labeo victorianus seed fro aquaculture.

An experimental investigation of the most suitable larval diets for L. victorianus was also carried out. Several diet types ranging from zooplankton, phytoplankton and dry diet were tested as starter diets for L. victorianus larvae. Suitability of the diets was compared on the basis of survival and growth.

In another part of the study, the structure of the digestive system of Labeo victorianus was studied with the aim of determining the functional adaptability to the feeding ecology. By comparing the results with adaptations of similar structures of fish species of known feeding ecology, inference was made on the feeding ecology of Labeo victorianus. Results of this work were expected to provide clues on the feeding habits of this fish.

The last investigation analyzed the composition of the gut content of Labeo victorianus to provide a better understanding of the diets and feeding habits of this fish. The analyses were carried out on gut contents collected throughout an annual cycle to see if there was seasonality in the feeding habits. The gut contents were isolated by food type and organisms identified. They were quantified using both the point and the frequency of occurrence methods to determine their importance in the diets.

Labeo victorianus larvae hatch out with undeveloped digestive system and unable to start feeding. They rely on the yolk sac for at least 3 days before they area able to start feeding, although the yolk sac lasts up to 5 days. Therefore, feeding of the larvae can not start until 3 days post hatch. By Time 2 (T2), the mass of larvae was significantly higher in fish fed on either cladocerans or a combination of cladocerans and dry feed (Scheffe post-hoc tests, P < 0.05) (Fig. 2.9). Fish fed cladocerans exhibited the second highest mass, but it was not significantly greater (p > 0.5) than the dry feed or rotifer treatments. Survival was lower (P < 0.5) for L. victorianus larvae supplied with micro-algae than those supplied with either zooplankton alone or a combination of zooplankton and microalgae soon after the onset of exogenous feeding (at day 20). The fish fed on zooplankton exhibited a significantly higher mass than fish fed on micro-algae and the combination (Scheffe post hoc tests, P < 0.05). By this time survivorship in the dry feed treatment (70±6.0% of the original number) was lower than in the other three food treatments (posthoc tests, P < 0.05), but there was no difference in survivorship among the cladoceran, rotifer, and cladoceran/dry feed combination (88.7±9.0%, 86±9.2% and 91±7.8% of their original numbers, respectively) (post-hoc tests, P > 0.05). It was concluded that cladocerans and a combination of dry diet were the most suitable starter diets for L. victorianus larvae.

The R. Sio and R. Kagera populations of L. victorianus did not differ significantly (p > 0.001) in all the morphotypes investigated, meaning that these two populations are very similar in their feeding adaptation. However, in the morphotypes that determine the normal position in the water column, L. victorianus was less (P < 0.5) adapted for benthic habitat than (C. gariepinus and B. docmak) but not as pelagic (p < 0.5) as the Nile tilapia. It was therefore concluded that L. victorianus was adapted for substrate habitats in relatively shallow waters.

In its feeding ecology, all samples of L. victorianus collected at night (1900 hrs) and dawn (0500hrs) had empty guts. This indicates that this fish does not feed at night. This fish feeds on a diverse range of diet types ranging from detritus, plant parts, zooplankton, diatoms, and insect parts. By both the point and the frequency of occurrence methods the most dominant five items that were found in the guts of the R. Sio population were detritus, fine sand particles, cladocerans, Baciliophyta (diatoms), and Cyanophyta, in that order, and they contributed 84.6% of the gut content. The most dominant five items found in the guts of the R. Kagera population were detritus, fine sand, plant tissue, Chlorophyta, and Baciliophyta, in that order, and they contributed 90.5% of the gut content. In both of these cases there was no evidence of the nutritional value of sand, and it was concluded that it was probably ingested by accident during feeding on rocks. The most dominant diet types, that could be categorized as epiphytes, tend to grow on surfaces of objects (substrates). This finding suggests that L. victorianus could be grown on periphyton, such as the ‘acadja’ system.

The implications of the findings of the current study for commercial aquaculture are discussed.