Contemporary spatial distribution pattern, breeding status and habitat suitability criteria for the African skimmer, Rynchops flavirostris Vieillot, 1816 (Aves Laridae) in the Zambezi Valley, Zimbabwe

摘要

Reports indicate a decline in spatial distribution, reproduction and population size of the African Skimmer, Rynchops flavirostris Vieillot, 1816 (Aves Laridae) in the Zambezi Valley, Zimbabwe following construction of Lake Kariba. The objective of the present study was to assess con- temporary distribution and breeding status of R. flavirostris in Lake Kariba and upstream and downstream of the lake in Zimbabwe. The study also developed habitat suitability criteria for Habitat Risk Index (Ri) and established the dominant substrates, soil moisture levels, soil tem- perature and food abundance for R. flavirostris in lentic and lotic environs of the Zambezi Valley, in order to establish the appropriate habitat conditions for the species. A survey method was used to determine R. flavirostris distribution and breeding status within the valley. Soil moisture level was measured by categorising levels into wet, damp, and dry soils and the category with the highest frequency was used to define habitat substrate moisture levels. Dominant substrate was visually estimated basing on proportion of soil particle sizes. Habitat risk was assessed on the basis of potential risk from anthropogenic and natural factors. Seine net fishing was con- ducted to estimate food abundance at sampling sites in Lake Kariba. Significant differences in sample means of measured habitat attributes between sites occupied by R. flavirostris and those unoccupied by the species were tested by two sample t tests with separate variance. One-way ANOVA was used to test for significant differences in habitat conditions among sampling sites. Three-way ANOVA was used to test for significant interactions of habitat conditions on R. flavirostris abundance in lentic and lotic environs. Preference ratios for habitat conditions were used to develop habitat suitability criteria. Results revealed that R. flavirostris was inhabiting and breeding in Lake Kariba and upstream and downstream of the lake. The species tolerance of habitat risk was observed to vary significantly (p0.05, One-way ANOVA) among the three sections of the valley. Within the lake, R. flavirostris mostly preferred sandbars of moderate Habitat Risk Index (0.4R0.7). Contrary, R. flavirostris showed absolute preference to sandbars with low Habitat Risk Index (R0.4) in habitats upstream and downstream of the lake. Signi- ficant differences (p0.05; One-way ANOVA) in R. flavirostris abundance among sites of varying substrates were observed. Habitat Suitability Indices showed that R. flavirostris pref- erentially selected predominantly silty and sandy habitats in both lentic and lotic environs. Signi- ficant difference (p0.05; One-way ANOVA) was also observed on R. flavirostris abundance among sites with different soil moisture content in the valley. R. flavirostris showed low preference for dry habitats in both lentic and lotic environs. No significant difference (p0.05; One-way ANOVA) in temperature was observed on sandbars occupied by R. flavirostris within the lake and upstream and downstream of the lake. R. flavirostris preferentially selected sandbars with food abundances greater than 0.15 fish/m 2 within Lake Kariba. This study conclusively revealed that food availability, dominant substrate, soil moisture level and habitat risk emanating from both natural and anthropogenic factors at sandbars were important determinants of habitat suitability criteria for R. flavirostris in the Zambezi Valley.