Leo A.J. Nagelkerke

Resource partitioning by Lake Tana barbs predicted from fish morphometrics and prey characteristics

We develop a food-fish model (FFM), whichquantitatively relates properties of aquaticfood types (size, shape, escape velocity,habitat, mechanical properties and chemicalquality) to feeding structures of cyprinidfish. The model is based on functionalmorphology and experiments on search, capture,selection, and internal processing of food byfish. The FFM shows which food properties aremost critical in feeding and how fish canoptimise coping with them. Relative food sizeimposes the highest demands, followed by preyvelocity, food habitat and mechanicalproperties. These overrule taxonomic affinitiesof food types. Highly demanding food types(large, fast prey, suspensions of plankton,benthic prey and mechanically tough items)impose incompatible morphological requirementson fish. We apply the FFM to the endemic Barbus species flock of Lake Tana (Ethiopia),since the structural diversity of its 14species reflects recent adaptations to trophicniches. We predict their potentials inutilising different food types by quantitativecomparisons of 35 parameters, measured for eachspecies, with the values for each foodspecialist derived from the FFM. These dietpredictions are tested against gut contentsfrom 4,711 fish, sampled over seasons andhabitats. Gut contents and predictions show agood overall fit. The value of the model isshown by its resolution in predicting resourcepartitioning among the barbs. For the 14 barbsa trophic hierarchy with six major trophicgroups is reconstructed which closely matchesthe predictions. Trophic specialists (> 65%by volume of a single food type) are alsostructurally specialised, whereas less extremeanatomical structures characterise trophicgeneralists, allowing them to switch betweenfeeding modes. Trophic generalists are bestdefined by behavioral flexibility, sincefeeding modes integrate both fish and foodcharacters. The FFM is of practical use inevaluating the role of morphological diversityin an ecosystem and enables the analysis oftrophic interactions in fish communities and ofthe cascading effects by environmental change.Such an approach can be instrumental in thedevelopment of management strategies forfisheries and in conservation of biodiversity.

Temporal and spatial distribution of microcrustacean zooplankton in relation to turbidity and other environmental factors in a large tropical lake (L. Tana, Ethiopia)

The spatial and seasonal distribution of microcrustacean zooplankton of Lake Tana (Ethiopia) was monthly studied for 2 years. Concurrently, various environmental parameters were measured and related to zooplankton distribution. Canonical Correspondence Analysis (CCA) was used to estimate the influence of abiotic factors and chlorophyll a content in structuring the zooplankton assemblage. Among the environmental factors, zooplankton abundance correlated most strongly with turbidity. Turbidity was negatively correlated with species abundance, especially for Daphnia spp. and to the least extent for Diaphanosoma spp. Analysis of variance (ANOVA) was used to determine spatial (littoral, sublittoral and pelagic zone) and temporal (four seasons) variation in zooplankton abundance. We observed significant temporal differences in zooplankton abundance, with highest densities during dry season (November–April). Only cladocerans showed significant differences in habitat use (highest densities in the sublittoral zone). %

Evolution of MHC class II &; chain-encoding genes in the Lake Tana barbel species flock (Barbus intermedius complex)

Abstract Major histocompatibility complex (MHC) class II protein polymorphism is maintained in allelic lineages which evolve in a trans-specific manner, passing from one species to descendant species. Selection pressure on peptide binding residues should be greatest during speciation, when organisms move into new environments and their MHC molecules encounter new pathogens. The isolation of MHC genes from teleost fishes, the most diverse group of vertebrates, has created possibilities for testing this hypothesis. The large barbels of Lake Tana have undergone an adaptive radiation within the last 5 million years, producing 14 morphotypes which inhabit different ecological niches within the lake. We studied the variability in class II beta chain-encoding genes of four of these morphotypes using polymerase chain reaction amplification and DNA sequencing. The sequences obtained were orthologous to four of the known class II genes from the common carp, from which barbels diverged approximately 32 million years ago. When subjected to phylogenetic analysis, the 48 sequences clustered into groups which represent allelic lineages. A comparison of nonsynonymous and synonymous substitutions between the peptide binding region codons and non-peptide binding region codons of these sequences revealed that they are under strong selective pressure.

In Lake Tana, a unique fish fauna needs protection.

tion of biodiversity usually focus on terrestrial habitats, especially on rain forest ecosystems (Myers 1979, Simberloff 1984, Wilson 1989). Because of this bias, the value of aquatic communities, less accessible for direct observations, is often not fully appreciated. The purpose of this article is to draw attention to a unique freshwater systemLake Tana-and especially to its fishes. Thus we want to urge the international scientific community to increase knowledge on the origin and state of the current biodiver-

The ‘small barbs’ Barbus humilis and B. trispilopleura of Lake Tana (Ethiopia): Are they Ecotypes of the Same Species?

Four species of ‘small barbs’ (Barbus, subgenus Enteromius Cope, 1869) are known from Lake Tana, isolated in the Ethiopian highlands: B. humilis, B. trispilopleura, B. pleurogramma (all Boulenger, 1902) and B. tanapelagius de Graaf, 2000. However, only three species appear valid from cluster analysis using 32 morphometric characters and taking specimens from different locations in the southern Gulf of Lake Tana during August–October 1999. B. humilis and B. trispilopleura significantly differ from B. tanapelagius and B. pleurogramma in up to 36 characters. However, B. humilis and B. trispilopleura cannot be distinguished from each other by morphometric analysis or by gut contents. Specimens from clear, shallow rocky areas with vegetation have a darker back, will be more susceptible to birds, have significantly higher infection by cestodes, smaller size at first reproduction, lower fecundity, and correspond most to the B. trispilopleura phenotype. Specimens in turbid deeper water without vegetation are most similar to the B. humilis phenotype. We conclude that both species actually are extremes (ecotypes) of a continuum, belonging to a single biological species. The observed variation may well be induced by habitat-dependent predation pressure by birds. The high frequency (57%) of spot numbers intermediate between Boulenger’s number for B. trispilopleura (3) and for B. humilis (0) demonstrates the continuum best. Pigment spots and colour change in response to aquarium conditions and are in this case no valid taxonomic characters. Both characters may reduce the risk of predation. It is concluded that B. trispilopleura is a synonym of B. humilis. For future research we recommend to use the most appropriate name, B. humilis, for both types.

Underwater Towed Video: A Novel Method to Estimate Densities of Queen Conch (Lobatus gigas; Strombidae) Across Its Depth Range

ABSTRACT Queen conch (Lobatus gigas) populations living deeper than 20 m are rarely studied, because of the limitations of conventional survey methods using divers [i.e., belt transect (BT), towed-diver]. Acrucial management goal for conch populations is to maintain adult densities at adequate levels to ensure reproduction, which is highly density dependent. Therefore, accurate estimates of adult conch densities, both in shallow and deep areas, are essential. The rapid technical progress of video systems has made it possible to develop new cost-effective ecological sampling tools, which can be used to survey areas previously hardly accessible. A lightweight towed video array was used, which was able to survey adult conch throughout the species entire depth range (ca. 0–60 m depth), in a safe and efficient manner. The towed video method (TVM) was compared with a conventional BT method using scuba divers, in its ability to identify adult live and dead conch. A series of intercalibration transects was conducted in a high-complexity (HC) and in a low-complexity (LC) habitat by having the towed video followed by a diver conducting a concurrent standard BT, covering the exact same surface area as the towed video. In both the HC and LC habitat, adult live queen conch had similar counts with both methods. Adult dead conch were not mistaken for live conch but were significantly underestimated with the towed video compared with the BT. The results validate the use of TVM as a reliable sampling tool to estimate densities of live adult conch in both HC and LC habitats throughout the species depth range.