Relationship between Steller sea lion diets and fish distribution in the eastern north Pacific

Abstract

Distributions of fish species were compared with diet information for Steller sea lions (Eumetopias jubatus) to assess the level of correspondence between potential prey availability and sea lion feeding habits. Fish distributions were compiled as part of the Sea Around Us Project at the UBC Fisheries Centre, and were based on published distributions and habitat preferences (e.g., latitude, depth). Sea lion scat samples were collected during the 1990s from seven geographic regions from Oregon to the western and central Aleutian Islands. The frequencies of occurrence of four prevalent species (walleye pollock, Theragra chalcogramma; Pacific herring, Clupea pallasii; Pacific cod, Gadus macrocephalus; and North Pacific hake, Merluccius productus) in the Steller sea lion diet were compared to their distributions in the North Pacific Ocean. The data suggest that Steller sea lion diets broadly reflect the distributions of these major prey species. However, some of the fish species that were regionally predicted to be present in high abundance were not proportionally reflected in the Steller sea lion diet, suggesting that other factors in addition to fish abundance influence their diets. Introduction The Steller sea lion population declined by more than 80% in western Alaska between the mid-1970s and the early 1990s (Trites and Larkin 1996, Loughlin 1998, NAS 2003) while the smaller eastern population increased. Accordingly, the western and eastern populations of Steller sea lions were listed as “endangered” and “threatened,” respectively, un132 Bredesen et al.—Steller Sea Lion Diets and Fish Distributions der the U.S. Endangered Species Act. In an attempt to better understand their role in the ecosystem and the differences between the decreasing western population and the increasing eastern population, considerable research has focused on determining the diet of Steller sea lions (Merrick et al. 1997, Sinclair and Zeppelin 2002). Comprehensive changes have occurred to the biomass and composition of the marine community off the Alaska coast since the oceanic regime shift of 1976. Increases were noted following the regime shift in the abundances of flatfish, gadids, and salmonids (Hare and Francis 1995, Hollowed et al. 2001, Benson and Trites 2002, Wilderbuer et al. 2002). A small mesh survey around Kodiak Island (1953-1997) also noted increases in groundfish such as cod and pollock, as well as a decline in the abundance of forage species such as capelin and shrimp (Anderson and Piatt 1999). Such changes in the abundances and distributions of key prey species composition may be related to the decline of Steller sea lions in western Alaska. One of the leading hypotheses for the decline in western Alaska is nutritional stress caused by a shift in ocean climate that favored the abundance of less nutritious fishes over those that had higher fat content (Alverson 1992, Rosen and Trites 2000, Trites and Donnelly 2003, Trites et al. 2006). Like other pinnipeds, Steller sea lions have often been classified as generalist feeders. However, it is not clear whether Steller sea lions merely eat what is available to them, or whether other factors influence which prey they consume. We therefore sought to assess the level of correspondence between prey distribution and sea lion diets. Methods Predicted fish distribution maps were obtained from the Sea Around Us Project (Fisheries Centre, University of British Columbia, www.seaaroundus.org) and were based on published distributions and habitat preferences (e.g., latitude, depth) (Watson et al. 2004). These maps represent the expected percent of world distribution of individual fish species and are an indication of relative abundance of each species across the North Pacific Ocean. Scat samples were collected during the 1990s (Riemer and Brown 1997, Sinclair and Zeppelin 2002, Trites et al. unpubl. data) from seven geographic regions from Oregon to the Central and Western Aleutian Islands (Oregon [OR], British Columbia [BC], Southeast Alaska [SEA], Gulf of Alaska 1 [GOA1], Gulf of Alaska 2 [GOA2], Eastern Aleutian Islands [EAI], and Western Central Aleutian Islands [WCAI]). Frequently occurring species (walleye pollock, Theragra chalcogramma; Pacific herring, Clupea pallasii; Pacific cod, Gadus macrocephalus; and North Pacific hake, Merluccius productus) in the Steller sea lion diet were compared to their predicted distributions. The importance of each prey species was deter133 Sea Lions of the World mined by the percent frequency of occurrence (%FO) in scat samples from each region (Croxall 1993). Proportionally sized circles, representing the %FO, were plotted for comparison with fish distributions in respective regions. Percentages of both diet and distribution data were arcsine transformed to satisfy the assumptions for statistical analysis. Results and discussion The data suggest that there is a relationship between fish distributions and Steller sea lion diets (Figs. 1 and 2). For example, in the northern part of the sea lion’s range, walleye pollock has a high relative abundance and is an important part of the sea lion diet (Fig. 1A). Although not statistically significant (r = 0.63, P > 0.05), the correlation is positive and consistent with the relationship found for the other prey species. In the southern part of the sea lion range (e.g., Oregon), the predicted relative abundance of walleye pollock is lower and North Pacific hake is predominant both in relative abundance and in the sea lion diet (r = 0.74, P = 0.05) (Figs. 1A, 1D, and 2). A similar pattern can be seen with Pacific herring (r = 0.80, P = 0.03), arrowtooth flounder (r = 0.83, P = 0.02), and, to a lesser extent, Pacific cod (r = 0.28, P > 0.05), where the frequency of occurrence of the prey species in the diet of Steller sea lions is higher in regions that also have high predicted relative fish abundance (Figs. 1B, 1C, and 2). While the relationship between relative fish abundances and Steller sea lion diets seems strong, it is not as straightforward as Figs. 1 and 2 might suggest. For example, in the Gulf of Alaska the biomass of arrowtooth flounder (Turnock et al. 2001) is estimated to be approximately six times the biomass of walleye pollock (Dorn et al. 2001). However, pollock is two to eight times more prominent in the Steller sea lion diet (%FO) than arrowtooth flounder (Sinclair and Zeppelin 2002) (Tables 1 and 2). Atka mackerel is also 35 times more prominent than rockfish in the diet of Steller sea lions in the Aleutian Islands region (Sinclair and Zeppelin 2002) despite the fact that both have a similar biomass estimate (Lowe et al. 2002, Spencer and Ianelli 2002) (Tables 1 and 2). Although not statistically significant, Atka mackerel (r = –0.18, P > 0.05) and Pacific ocean perch/Rockfish (r = –0.29, P > 0.05) show a weak negative correlation between the percent of world distribution and percent frequency of occurrence in Steller sea lion diet (Fig. 3). Many factors likely influence the prey that sea lions choose to eat. These include the presence of spines, and the vertical and horizontal distribution of prey in the water column. Such factors may account for some of the apparent discrepancies between diets and relative prey abundances. For example, arrowtooth flounder may make up a large portion of the relative biomass, but may be harder for sea lions to locate and capture because they tend to be solitary and not school in easily 134 Bredesen et al.—Steller Sea Lion Diets and Fish Distributions LEGEND % of world distribution % Frequency of Occurence in Steller sea lion scat WCAI EAI GOA2 GOA1 SEA