Abundance and prey capture success of Common Terns (Sterna hirundo) and Pied Kingfishers (Ceryle rudis) in relation to water clarity in south-east coastal Ghana

Background Water clarity may negatively influence rate of plunge diving and prey capture success of piscivorous plunge-diving birds, and therefore has implications for their conservation in polluted urban wetlands. We studied the relationship between water clarity and the abundance and prey capture success of Pied Kingfishers (Ceryle rudis) and Common Terns (Sterna hirundo) in two polluted coastal waters of south-east Ghana—the Weija Lake and Densu Delta Ramsar Site.

Methods On each wetland, data on abundance and prey capture success of plunge-divers were collected in four spatio-temporal quadrats of 100 m?×?100 m and analysed with concurrent measurements of water quality parameters using GLM regression with Pearson's correlation coefficients.

Results Mean prey capture success of Pied Kingfishers (34.7?±?13.1%) and Common Terns (35.3?±?11.0%) were similar but the two species responded differently to water clarity. The abundance of Common Terns was significantly higher in less transparent/more turbid water while that of Pied Kingfishers showed no significant relationship with turbidity and transparency. In contrast, the prey capture success of Common Terns was neither related to transparency nor turbidity, as opposed to that of Pied Kingfishers which was significantly higher in more turbid/less transparent waters. Correlations between capture success and bird abundance, as well as capture attempts were insignificant, suggesting that increased fish abundance associated with cloudy water may not necessarily promote higher abundance and capture success of foraging birds. Thus, when foraging in less transparent water, capture success may depend more on predator avoidance by fish prey than lower prey detectability of foraging birds.

Conclusion Within a gradient of 15-51 cm transparency studied, lower water clarity did not constrain prey capture success of Common Terns and Pied Kingfishers. Further studies on the foraging ecology of plunge-divers in coastal Ghana are however required to make firm conclusions on the relationship between water clarity and foraging birds and fish prey abundances, as well as capture success.