Water transparency is an important factor affecting fish availability (underwater visibility) for diving birds. The diet of Great Cormorants Phalacrocorax carbo in relation to water transparency (range 1.4–4.0 m) was studied by pellet analyses at the submontane reservoir Dobczyce, Poland, from June to November. Although water transparency proved to be related to the birds' distribution, in the range of turbidities studied, no relationship was found with either fish species and fish size taken. Of 14 species in the diet, Roach Rutilus rutilus was dominant in all monthly samples (35–91% in 2002, 56–82% in 2004). Numbers of Great Cormorants and water transparency (range 0.4–1.4 m) were studied in the turbid lowland reservoir Goczałkowice during the migration period in autumn (August–November 2011 and 2012). Observations here suggest that the effect of water transparency on food uptake and habitat choice was only apparent below 0.6 m Secchi depth. The number of foraging Great Cormorants was not only affected by Secchi depth, but by a complex of factors (year, month, place, Secchi depth and water depth). We argue that, especially when the number of Great Cormorants is low, only highly detailed measurements of these factors (both spatially and as time series) can show the complex relationship between prey distribution, environmental conditions and the predator's behaviour.