Determining food requirements in marine top predators: a comparison of three independent techniques in Great Cormorants,Phalacrocorax carbocarbo

Abstract

Assessment of food requirements is a key feature in the evaluation of the ecological status of the marine megafauna. However, this remains technically difficult because prey intake by marine top predators occurs mainly under water, out of sight. In this paper, we compare three independent methods currently available for use in quantitative dietary studies: (1) time-energy budget; (2) stomach-temperature measurements; and (3) automatic weighing. To this end, concurrent measurements were performed on Great Cormorants (Phalacrocorax carbo carbo) breeding in Normandy. According to the time-energy budget method, breeding males required 690 g of fish while incubating, 1050 g when rearing small chicks, and 1350 g when rearing large chicks; corresponding values for breeding females were 500, 760, and 970 g. These measurements are similar to estimates derived from automatic weighing data, which gave a mean food intake of 540 and 390 g for incubating males and females, 1150 and 830 g for those tending small chicks, and 1410 and 1010 g for those tending large ones, respectively. Stomach-temperature measurements, which can only be performed for birds raising small chicks, were lower (640 g fish in males and 450 g in females) than those obtained using the other two methods. We compare these results with former estimates obtained at the same study site and for other Great Cormorant subspecies and discuss the relative accuracies of the three techniques. Finally, we stress that better assessment of the ecological status of marine top predators requires further technical improvements and additional investigations outside of the reproductive phase.

Postprandial response of gastric pH in leopard sharks (Triakis semifasciata)and its use to study foraging ecology

“…Evans et al, 1988;Gardner et al, 2002). As with leopard sharks, a rise in gastric pH with meal ingestion, followed by a decrease to baseline levels, has been observed for birds (Duke et al, 1975;Peters, 1997a;Gremillet et al, 2000) and humans (James, 1957;Gardner et al, 2002). Humans continuously secrete acid, with an unstimulated (i.e.…”

“…Gastric pH has been measured continuously for snakes (Secor, 2003), birds (Peters 1997a,b;Gremillet et al, 2000), ruminants (Enemark et al, 2003) and humans (e.g. Evans et al, 1988;Gardner et al, 2002).…”

“…Presently the only technique used to directly quantify feeding events and meal size in marine endotherms is in situ measurement of stomach temperature. Stomach temperature is relatively easy to measure, and changes markedly as food enters the stomach (Wilson et al, 1992;Gremillet et al, 2000;Gunn et al, 2001;Klimley et al, 2001). However, changes in stomach temperature can only be observed if the predator has a different body temperature than that of its prey.…”

“…In contrast, for marine endotherms, frequency and timing of feeding events have been made by recording associated changes in stomach temperature using gastric temperature data-loggers or telemetry devices (Wilson et al, 1992;Gremillet et al, 2000;Gunn et al, 2001). Unfortunately, this technique is only effective for quantification of feeding events for marine ectotherms when the body temperature of the ingested prey differs considerably from the predator's own body temperature.…”

Eating the Invaders: The Prevalence of Round Goby (Apollonia melanostomus) in the Diet of Double-Crested Cormorants on the Niagara River

“…Dietary and behavioral information of cormorants migrating through Michigan and for resident non-breeders (subadults and nonbreeding adults) are generally lacking (Gremillet et al 2000). A review by Wires et al (2001) found that population estimates for non-breeders ranged from one to four non-breeders for each breeding pair.…”