Inter-annual variation in the diet, provisioning and growth of Cassin’s auklet at Triangle Island, British Columbia: responses to variation in ocean climate
From Marine Ecology Progress Series, Vol. 229: 221–232, 2002 Published March 20
April Hedd1,*, John L. Ryder1, Laura L. Cowen2, Douglas F. Bertram1, 3
1Centre for Wildlife Ecology, Department of Biological Sciences, and 2Department of Statistics and Actuarial Sciences, Simon Fraser University, 8888 University Drive, Burnaby, British Columbia V5A 1S6, Canada
3Environment Canada, Canadian Wildlife Service, Pacific and Yukon Region, RR1 5421 Robertson Rd, Delta, British Columbia V4K 3N2, Canada
ABSTRACT: We studied parental provisioning and chick growth rates of Cassin’s auklet Ptychoram- phus aleuticus at Triangle Island, British Columbia, Canada, from 1996 to 1999. Auklet reproductive performance and ocean climate conditions during these years were highly variable, and reflected in the diet composition. Chick growth was maximal when the diet was predominated by copepods, in particular Neocalanus cristatus. Provisioning and growth were high in 1999, intermediate in 1997 and poor in 1998. Exceptional was 1996, when growth was low but provisioning rates were high. Pro- visioning and growth were depressed late in 1997 and throughout 1998 when larval rockfish Sebastes spp. (5200 cal g–1) replaced N. cristatus (6236 cal g–1) in the nestling diet. Zooplankton sur- veys indicated that N. cristatus was substantially more abundant in May 1999 than in May 1998, and during 1999 the auklets foraged in areas with the highest concentrations of copepods. Through impacts on prey availability, variation in ocean climate affects the reproductive performance of Cassin’s auklet. Performance tends to be favorable in years when spring is late and cold, and poor when spring is early and warm. Equations for predicting food delivery rates from 24 h mass changes of chicks were highly year-specific, precluding their application in other years or at other sites where Cassin’s auklets breed. Between-year differences were also found in relationships between adult provisioning and chick growth. These were strongly positively related in 1999, positively related in 1996 and 1997, but unrelated in 1998; differences attributed to the magnitude of temporal variation in the nestling diet. Finally, we detected annual differences in parental response to chick needs. In 1999, parents delivered more food to chicks in poor condition and less to those in better condition, responses not observed in 1998. Different responses between years may have reflected variation in the availability of prey.
KEY WORDS: Cassin’s auklet · Ptychoramphus aleuticus · Provisioning · Growth · Diet · Copepods · Neocalanus cristatus · Rockfish · Euphausiid · Ocean climate variation
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