Burkett 



Chapter 22 



Food Habits and Prey Ecology 



northwest of Vancouver Island. The study spanned two 

 breeding seasons (1970 and 1971), and 86 adult and subadult 

 Marbled Murrelets were collected between March 25 and 

 August 10 (years combined). The diets were essentially 

 the same for both sexes, and samples from subadults and 

 adults were identical, so the data were pooled for a total 

 sample of 75 individuals. Additionally, six newly fledged 

 murrelets were taken between July 1 and August 4, 1 97 1 , 

 and their food habits were analyzed separately. The 

 percentage of murrelets collected that contained prey ranged 

 from 87 to 100 percent. 



Sand lance (Ammodytes hexapterus) made up 67 percent 

 of the food items in the diet of the adults and subadults. 

 Euphausiids were the next most important food item and 

 contributed 27 percent of the items. Two species of euphausiids 

 were consumed, Euphausia pacifica and Thysanoessa 

 spinifera, with relative importance values of 2 percent and 

 25 percent, respectively. The next most important food item 

 was the viviparous seaperch (Cymatogaster aggregata), with 

 a value of 3 percent. Overall, sand lance, euphausiids, 

 seaperch, scorpaenids, and osmerids made up 98 percent of 

 the murrelet diet. Including the less common food items 

 which occurred in very small amounts, at least nine different 

 types of prey were identified (table 1). 



The six samples of newly fledged young selected different 

 prey than adult/subadult murrelets (table 1). Sand lance still 

 dominated the diet at 65 percent (similar to 67 percent for 

 adult/subadult murrelets), but the seaperch was the next 

 most important prey species, rather than euphausiids, with a 

 value of 35 percent. The euphausiid, T. spinifera, and 

 amphipods made up trace amounts of the remainder of the 

 fledgling diet. 



The difference in adult and juvenile diets can be partially 

 explained by looking at the difference in abundance of prey 

 items taken by the adult/subadult murrelets over the course 

 of a breeding season. The euphausiid, T. spinifera, was 

 found more commonly in the adult/subadult diet during the 

 mid-April to mid-May period and was more important than 

 the sand lance at this time, but euphausiids diminished greatly 

 in the diet after the early part of the breeding season. However, 

 T. spinifera remained important in the diet of adult Ancient 

 Murrelets through mid-July when the study concluded. Sealy 

 attributed this difference in diet to the offshore movement of 

 E. pacifica (affinity for deeper water than T. spinifera) and, 

 to some extent, offshore movement of T. spinifera as the 

 spring progressed and water temperature rose. He also 

 attributed the diet change to reduced abundance of T. spinifera 

 due to loss of females after reproduction. Additionally, he 

 noted that adult Ancient Murrelets feed further offshore than 

 Marbled Murrelets or juvenile Ancient Murrelets, and he 

 believed the food supply of the Ancient Murrelet was spotty 

 and unpredictable. 



Sealy tested for a measurable change in prey avail- 

 ability mid-summer by examining the stomach contents 

 of 13 individuals of seven species, including the Ancient 

 and Marbled Murrelet, from six mixed-species feeding 



assemblages. Between 9 May and 26 June 1971 he conducted 

 plankton hauls where collected birds had been foraging. The 

 results indicated that only Thysanoessa was available and 

 taken by those individuals examined in May, and later samples 

 in June found only Ammodytes available and being consumed. 

 He concluded that fishes such as Cymatogaster and 

 Ammodytes tend to spend the winter and early spring in mid- 

 water offshore, but migrate to the surface and move inshore 

 in late spring, thus possibly becoming available to murrelets 

 at this time. 



Plankton hauls made in 1971 also indicated that the 

 murrelets were more selective in their feeding habits when 

 compared to prey availability (Sealy 1975c). Organisms such 

 as ctenophores, amphipods, and polychaetes were obtained 

 in the plankton hauls, but none of these organisms were 

 found in the food samples analyzed. Zooplankton sampling 

 by Project NorPac (Dodimead 1956) during summer 1955 

 (primarily in August) resulted in a similar difference in prey 

 availability; copepods were by far the most numerous 

 organisms with a total volume of more than 65 percent, 

 while euphausiids composed less than 10 percent of the total 

 volume (LeBrasseur 1956). 



Sealy (1975c) concluded that murrelets seldom feed 

 more than 500 m from shore, usually in water less than 30 m 

 deep. His work demonstrated that euphausiids made up only 

 a small part of the overall diet during the breeding season, 

 but were dominant during the early part of the breeding 

 season. He thought the breeding season was possibly 

 ultimately controlled by the cycles of abundance of fishes 

 near shore, especially the sand lance, which were taken by 

 the murrelet in great quantities in the study area. 



Krasnow and Sanger (1982) 



Krasnow and Sanger (1982) collected murrelets at sea 

 in the vicinity of Kodiak Island in the winter of 1976/ 

 1977. They collected 18 murrelets (all with food) between 

 December 1976 and April 1977 at Chiniak Bay, a large 

 bay on the northeast end of Kodiak Island; a second sample 

 of 19 murrelets (16 with food) was collected from Chiniak 

 in February 1978. Two other sites were sampled during 

 the breeding season of 1978. At Izhut Bay, a small bay 

 north of Chiniak Bay, Krasnow and Sanger collected 34 

 murrelets (25 with food) between April and August 1978 

 and from Northern Sitkalidak Strait, which is located on 

 the southeast end of Kodiak, they collected 26 murrelets 

 (17 with food) between May and August 1978. The 

 percentage of murrelets collected which contained prey 

 ranged from 65 to 100 percent. 



Krasnow and Sanger calculated an Index of Relative 

 Importance (IRI) value for the foods consumed by murrelets 

 according to Pinkas and others (1971). During the 19767 

 1977 winter, fish, primarily of the family osmeridae, were 

 the most important prey, followed by euphausiids of the 

 genus Thysanoessa, and mysids (table 2). A total of 1 1 

 different prey items were identified (table 2), compared to 

 nine from Sealy 's (1975c) breeding season study (table 1). 



224 



USDA Forest Service Gen. Tech. Rep. PSW-152. 1995. 



