Seabird foraging tactics and water clarity are plunge divers really in the clear?

A steep turbidity gradient across 3 shelf water masses and the Gulf Stream on the upper conhnental slope off the coast of the southeastern USA formed the basis for testing relationships between predator foragng tactics and relative detectability of prey.Censuses of seabirds were used to test a hypothesis originally proposed by Ainley (1977): plunge diving (plummeting from the air to subsequently capture prey underwater using the momentum of the fall) is more prevalent in clear as opposed to turbid surface waters.Twelve species of seabirds regularly used plunge bving to obtain prey in the study area: the tropicbirds Phaethon lepturus and P. aethereus, the boobies and gannet Sula dactylatra, S. leucogaster, and S. bassanus, respecbvely, and the terns Sterna maxima, S. sandvicensis, S. hirundo, S. paradisaea, S. forsteri, and S. antillarurn.Only P. lepturus was significantly more common in clear water.Five plunge-diving species were significantly more common in turbid waters.Neither the total number nor the proportion of plunge-diving bird species within the assemblage increased over more transparent waters.Significant decreases in the total number of plunge-diving individuals, and in the proportions of indviduals within the assemblage using plunge diving, occurred over waters with increasing transparency.Because the latter findings are directly counter to predictions arising from Ainley's hypothesis, we conclude that water clarity has yet to be clearly implicated as an influence on the allocation of foraging tactics in aerial seabirds.