Burkett 



Chapter 22 



Food Habits and Prey Ecology 



in Carscadden 1984). At spawning time, capelin appear in 

 schools of considerable size along the shores of gravelly 

 beaches. Spawning occurs in the evening at high tide right at 

 the water's edge. Studies of the beach both during and after 

 spawning indicate that a specific type of ground is selected, 

 the fish tending to avoid both rocky and sandy patches. The 

 eggs are extremely adhesive and immediately become firmly 

 cemented to the gravel (Hart and McHugh 1944). 



Capelin mature at 3 or 4 years of age with faster growing 

 fish maturing earlier (Winters in Carscadden 1984). In the 

 spawning populations, 3- and 4-year-olds usually predominate. 

 Spawning mortality is high, usually greater than 80 percent 

 (Carscadden and Miller in Carscadden 1984). 



Like other pelagic fish species, capelin populations exhibit 

 large variations in abundance of year classes, and natural 

 fluctuations in abundance are often complicated by the 

 presence of fishing mortality. Carscadden (1984) evaluated 

 fluctuations in capelin biomass in the northwest Atlantic and 

 concluded they were the result of natural variation in year- 

 class strength. The causes of the variation were not well 

 understood, but temperature and onshore wind-induced wave 

 action have been correlated with emergence of larval capelin 

 (Frank and Leggett in Carscadden 1984). 



Carscadden (1984) considered the relationship between 

 Atlantic Puffins and capelin as described by Brown and 

 Nettleship (1984) and concluded that a complex of natural 

 environmental and biological factors would probably affect 

 the abundance and behavior of capelin predators, rather than 

 a single one such as abundance of capelin. Brown and 

 Nettleship (1984) concluded that the management of the 

 capelin fishery in the northwest Atlantic should "proceed 

 cautiously" until the relationships between the capelin and 

 its predators were better understood. 



Vader and others (1990) evaluated the relationship 

 between Common Murres, Thick-billed Murres (U. lomvia), 

 and capelin in Norway. A complete collapse of the Barents 

 Sea stock of capelin occurred between 1985 and 1987, and 

 in 1987 fishermen noted a near-complete absence of sand 

 lance. The low sand lance population resulted in a complete 

 breeding failure of Shags (Phalacrocorax aristotelis) in West 

 Finnmark, where Shags are normally totally dependent on 

 sand lance during the breeding season. A sudden drop in 

 breeding Common Murres also occurred in 1987. The authors 

 concluded that the capelin and sand lance food shortage 

 caused the large drop in Common Murres and the reduced 

 breeding of Thick-billed Murres. The authors thought the 

 larger prey spectrum utilized by the Thick-billed Murres 

 allowed that population to fare better than the Common 

 Murres in the face of the food shortage. The causes of the 

 decline in capelins probably included overfishing, 

 uncommonly large year-classes of the predatory cod (Gadus 

 morhua), and a reduction in recruitment due to changes in 

 the physical oceanography of the Barents Sea (Hamre; 

 Ushakov and Ozhigin in Vader and others 1990). 



The importance of capelin in the diet of the murrelet in 

 the Gulf of Alaska (Sanger 1983) indicates the need to 



monitor and manage carefully this resource. Other smelt 

 species may be important in murrelet diet; unidentified 

 osmerids have been documented as murrelet prey over a 

 broad geographic range (table 1). Further research is needed 

 on the importance of smelt in the diet of the murrelet, 

 especially in Washington, Oregon, and California. 



Prey Ecology Summary 



The marine environment, especially in an eastern boundary 

 current system, is not static (Ainley and Boekelheide 1 990: 

 376). In his book on the Ancient Murrelet, Gaston (1992: 74) 

 wrote of a diagram of the food web of Reef Island: "A 

 complete diagram of the food webs of Hecate Strait would 

 probably cover a baseball field at this scale, and would take 

 several lifetimes of research to construct." Sanger's (1983) 

 compilation contains numerous food web diagrams which 

 depict the complex interactions in the marine environment. A 

 food web and a model of the trophic-level interactions 

 influencing murrelets at any site in North America would be 

 complex indeed, but much information on life history of prey 

 species and the murrelet at sea must be gathered. 



From the studies discussed above, some variability in 

 reproductive success of the murrelet can be expected because 

 of the naturally dynamic nature of their prey base and the 

 marine environment. Anthropogenic influences can compound 

 prey fluctuations; thus, marine research and management 

 should be designed to minimize or avoid adverse changes in 

 seabird reproduction and marine trophic-level interactions. 

 Anthropogenic and environmental influences will continue 

 to affect marine ecosystems. Management must therefore 

 entail monitoring and the ability to change course in response 

 to observed effects. Cumulative impacts in localized areas of 

 murrelet abundance should be anticipated and averted. 



Size of Prey Items 



A compilation of prey item size in the diet of adult and 

 subadult murrelets from systematic studies indicates the 

 majority of fish taken ranged from 30.1 to 60.0 mm (table 

 4). The largest combined sample size was for sand lance, 

 and the distribution indicated a heavy reliance on fish up to 

 60.0 mm, although fish greater than 90.0 mm were also 

 taken. Sanger (1987b) calculated a mean value of 45 mm 

 (total length) for sand lance which correlates well with the 

 distribution of prey size revealed in table 4. Smaller size 

 classes (0.1-30.0 mm) of scorpaenids and Cymatogaster 

 aggregate were taken by murrelets; this could be a function 

 of availability or preference. Larval and juvenile fish (0.1- 

 60.0 mm) appear to be the main size classes eaten by adult 

 and subadult murrelets. Larval fish are underrepresented in 

 murrelet diet because they are digested quickly (Carter 1984), 

 therefore, the overall importance of larval fish for murrelets 

 is difficult to assess. 



The size of prey items in the diet of hatching-year and 

 nestling murrelets is markedly different (table 5) though a 

 comparison of fish lengths in tables 4 and 5 reveals adult/ 

 subadult and hatching-year murrelet prey size to be similar. 



240 



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



