Burkett 



Chapter 22 



Food Habits and Prey Ecology 



Table 2 Comparison of winter diet of Marbled Murrelets in Chiniak Bay, 

 Alaska, between December 1976-April 1977, and February 1978 



a Data from Krasnow and Sanger (1982) 



b Values are Index of Relative Importance values calculated after Pinkas 

 and others (1971). 



In contrast to the results of Sealy (1975c), no Ammodytes 

 were present, but, similar to Sealy's (1975c) study, 

 Thysanoessa was an important prey item. 



The results from the February 1978 collections were 

 extremely different from the 1976/1977 winter data. Mysids 

 dominated the prey items with a cumulative IRI value of 

 11,892 (table 2). Osteichthyes were second, followed by 

 gammarids and capelin (Mallotus villosus). A total of 13 

 different prey items were identified (table 2). Once again, no 

 Ammodytes were noted, and even Thysanoessa was reduced 

 to an IRI value of 4. Sealy's (1975c) breeding period study 

 did not detect mysids and gammarids, but these prey items 

 appear to be more important in the winter diet of murrelets, 

 at least in the Gulf of Alaska (Sanger 1987b, Sanger and 

 Jones 1982). The lack of Thysanoessa consumption in 

 February 1978 by the murrelets is particularly interesting in 

 light of Sealy's (1975c) work. Krasnow and Sanger (1982) 

 reported that murrelets fed primarily in shallow water but 

 obtained their prey throughout the water column. Sanger 

 (1987b) noted that the ability of murrelets to forage at least 

 part of the time near the bottom assures a broader trophic 

 spectrum than a food supply originating with phytoplankton 

 productivity in the water column alone. 



The reduction of capelin in the winter diet of murrelets 

 between the study periods may be due to the dynamic nature 

 of capelin populations. Because capelin live only 3 or 4 

 years and most spawn only once, poor recruitment of a 

 given year class can lead to cycles of abundance and near 

 absence [Warner and Dick in Krasnow and Sanger (1982)]. 

 Fisheries data indicated that the distribution of capelin was 

 different in the 2 years, with most fish being caught in deep 

 troughs in 1 978 [Rogers and others in Krasnow and Sanger 

 (1982)]. Additionally, fewer capelin and more sand lance 

 were fed to Black-legged Kittiwake (Rissa tridactyla) chicks 

 in Northern Sitkalidak Strait during 1978 than in 1977. 

 Productivity of kittiwakes declined from 0.74 young fledged 

 per nest attempt to 0.17, suggesting that the availability of 

 food was depressed below some "critical level" [Baird and 

 Hatch in Krasnow and Sanger (1982)]. Productivity of 

 kittiwakes in Chiniak Bay also decreased, from 1.23 young 

 fledged per nest attempt in 1977 to 0.77 in 1978 [Nysewander 

 and Barbour in Krasnow and Sanger (1982)]. Food samples 

 were not collected at the breeding colonies of kittiwakes in 

 Chiniak Bay in 1978, and thus the assumption that fewer 

 capelin were brought to chicks than during the previous 

 years could not be substantiated. 



If euphausiids were scarce or, for some reason, 

 unavailable to murrelets in early 1978 in Chiniak Bay, then 

 the ability of the murrelet to feed so heavily on detritivores 

 such as mysids and gammarids likely demonstrates prey- 

 switching capability. This adaptive and opportunistic behavior 

 illustrates the result of natural selection pressure due to 

 dynamic prey populations. Alternatively, two factors, small 

 sample size and a difference in the collection period (5 

 months compared to 1 month), could be complicating the 

 results. However, given the information on kittiwake 

 reproduction and capelin being found in deeper waters cited 

 above, it would appear that changes in the marine food web 

 in Chiniak Bay between years and prey-switching behavior 

 by the murrelet are more plausible explanations. 



The results of Krasnow and Sanger' s (1982) study of 

 breeding-season diet at Izhut Bay and Northern Sitkalidak 

 Strait in 1978 pointed to the importance of local differences 

 in the relative availability of major prey species within the 

 same year. The diets from the two different study areas 

 included a high proportion of unidentified Osteichthyes (table 

 3), with ten different prey items identified in the summer 

 diet, comparing with 9 from Sealy (1975c). Euphausiids 

 were more common in the murrelet diet at northern Sitkalidak 

 Strait. For the murrelets and most other seabird species in 

 the Kodiak area, distinct seasonal trends were apparent from 

 spring through late summer 1978. Marbled Murrelets, Tufted 

 Puffins (Fratercula cirrhata), Sooty Shearwaters (Puffinus 

 griseus), and Black-legged Kittiwakes exploited a similar 

 suite of prey. Sand lance and euphausiids were taken during 

 spring, capelin during early summer, and sand lance during 

 late summer. The authors attributed this chronology to the 

 probable seasonal occurrence and distribution of prey as did 

 Sealy (1975c) and Carter (1984) in their study areas. 



228 



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



