Linking the foraging performance of a marine predator to local prey abundance

Abstract

Summary 1.Knowledge of the functional response of predators to prey densities conditions our understanding of food webs. Such links are still poorly understood within the higher trophic levels of marine ecosystems. 2. We present the first field study recording the foraging effort and foraging yield of a seabird (the Great Cormorant, Phalacrocorax carbo ) as well as the abundance and quality of prey within its foraging area. 3. We confirm that Great Cormorants foraging off West-Greenland show the highest foraging performance recorded for a marine predator (between 17 and 41 g fish caught per minute underwater). Former work suggests that such high foraging yield should be based upon the exploitation of extremely profitable prey patches. 4. Contrary to this hypothesis, average prey abundances estimated within the foraging areas of the cormorants were low (0·03-0·09 prey m − 2 , depending on methods), as was the average calorific value of the prey items (4·2 kJ g − 1 ). 5. Our study suggests that Great Cormorants remain highly successful predators even when exploiting modest prey resources. These findings have implications for our understanding of predator-prey relationships, and for the management of Great Cormorant populations.

Fish are not safe from great cormorants in turbid water

“…The question of great cormorant foraging performance in relation to water turbidity therefore remains open, and beyond experiments with captive cormorants in artificial setups (Enstipp et al 2007), field data are scarce (Grémillet et al 2004).…”

Dive depth and plumage air in wettable birds: the extraordinary case of the imperial cormorant

“…They frequent both coastal and inland waters from polar to tropical regions, breeding in colonies numbering from just a few individuals to many thousands of birds (Johnsgard 1993). The obvious success of this group has been attributed to their efficiency as aquatic predators (Gremillet et al 2001(Gremillet et al , 2005a, this being related to their swimming and diving abilities (Gremillet 1997, Gremillet et al 2004) that are, in part, a result of their wettable plumage (Ribak et al 2005, Gremillet et al 2005b, which results in reduced buoyancy. Despite this, cormorants are often cited as diving shallowly, primarily exploiting the first 10 m of the water column in dives that rarely exceed 1 min (for review see Cooper 1986).…”

Foraging under contrasting oceanographic conditions: the gentoo penguin at Kerguelen Archipelago

“…In response to this heterogeneity, many marine birds and mammals can exhibit considerable variation in their foraging behaviour or at-sea distribution (e.g. Croxall et al 1988, Wilson 1995, Boyd 1996, Weimerskirch 1998, Rodary et al 2000, Charassin & Bost 2001, Beauplet et al 2004, Grémillet et al 2004a.…”