Foods of Northern Fulmars Associated with High-Seas Drift Nets in the Transitional Region of the North Pacific

Abstract

We examined digestive tract contents and nitrogen stable isotope ratios in breast muscles of northern fulmars (Fulmarus glacialis) salvaged from squid and large-mesh drift nets in the transitional North Pacific. Lantern fishes (Myctophidae) were the principal prey item found in the digestive tracts. Pieces of unidentified fishes (probably Pacific pomfret Brama japonica) and shredded squid tissue (probably neon flying squid Ommastrephes bartrami) indicate scavenging at fishing operations. Although soft-bodied prey such as Velella were not found in the digestive tracts, 815N values suggest that fulmars may feed heavily on such low trophiclevel animals. Northern fulmars (Fulmarus glacialis) have been reported to feed on varying amounts of squids, fishes, zooplankton, carrion, and offal, probably varying with what is most available in various locations and seasons (Hunt and others 1981; Furness and Todd 1984; Hills and Fiscus 1988; Hatch 1993). Most of these studies have emphasized the importance of cephalopods, although several suggest that small fishes may be most important to the overall diet. We have found only two references to lantern fishes in the diets of northern fulmars from other areas (Schneider and others 1986; Hatch 1993). In the Gulf of Alaska, Sanger (1987) assigned northern fulmars to the highest trophic level among the 19 marine bird species studied and suggested that these birds were third-order carnivores. No studies of feeding by northern fulmars have been conducted on their wintering grounds in the central North Pacific, however. As part of our efforts to assess the impact of North Pacific drift net fisheries on marine birds (Gould and Hobbs 1993; Johnson and others 1993), we examined the digestive tracts and stable nitrogen isotopes in the breast muscles of a small sample of non-breeding northern fulmars salvaged from drift nets set in the transitional region of the central North Pacific. STUDY AREA AND METHODS Specimens were salvaged from large-mesh and squid drift nets set by ships from Japan, Taiwan, and the Republic of Korea (Ito and others 1993). In 1991, 29 northern fulmar specimens with complete digestive tracts were obtained in the area bounded by 40° to 44°N and 171°E to 172°W, and between 9 September and 18 November; however, all but 2 of the specimens were taken in October. The sample included 17 females and 12 males, and ages varied from newly fledged juveniles to breeding-age adults, as judged by bursa sizes (length x width) that ranged from 0 to 582 mm2 (x = 294.3, SD = 155.7). Only 2 of the specimens lacked bursas, so most of our specimens had not yet achieved breeding status (Siegel-Causey 1990). In addition, 9 birds were salvaged in 1990 (37° to 46°N, 153°E to 152°W; June to November) and 3 birds with stomachs but without gizzards were examined from 1991. Data from these 12 specimens are included as anecdotal information in this report.