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MARINE MAMMAL SCIENCE, 23(4): 766–802 (October 2007)  C 2006 by the Society for Marine Mammalogy No claim to original US government works DOI: 10.1111/j.1748-7692.2006.00093.x KILLER WHALES AND MARINE MAMMAL TRENDS IN THE NORTH PACIFIC—A RE-EXAMINATION OF EVIDENCE FOR SEQUENTIAL MEGAFAUNA COLLAPSE AND THE PREY-SWITCHING HYPOTHESIS PAUL R. WADE VLADIMIR N. BURKANOV MARILYN E. DAHLHEIM NANCY A. FRIDAY LOWELL W. FRITZ THOMAS R. LOUGHLIN SALLY A. MIZROCH MARCIA M. MUTO DALE W. RICE National Marine Mammal Laboratory, NOAA Fisheries, 7600 Sand Point Way NE, Seattle, Washington 98115, U.S.A. E-mail: paul.wade@noaa.gov LANCE G. BARRETT-LENNARD Vancouver Aquarium Marine Science Center, Vancouver, British Columbia V6B 3X8, Canada NANCY A. BLACK Monterey Bay Cetacean Project, P.O. Box 52001, Pacific Grove, California 93950, U.S.A. ALEXANDER M. BURDIN Alaska Sealife Center, 301 Railway Avenue, P.O. Box 1329, Seward, Alaska 99664, U.S.A. JOHN CALAMBOKIDIS Cascadia Research Collective, 218 1/2 W Fourth Avenue, Olympia, Washington 98501, U.S.A. SAL CERCHIO Molecular Systematics Lab, American Museum of Natural History, Central Park West at 79th Street, New York, New York 10024, U.S.A. 766 WADE ET AL.: KILLER WHALES AND MARINE MAMMAL TRENDS JOHN K. B. FORD Pacific Biological Station, Fisheries and Oceans Canada, 3190 Hammond Bay Road, Nanaimo, British Columbia V9T 6N7, Canada JEFF K. JACOBSEN Department of Biological Sciences, Humboldt State University, Arcata, California 95521, U.S.A. CRAIG O. MATKIN North Gulf Oceanic Society, 3430 Main Street, Suite B1, Homer, Alaska 99603, U.S.A. DENA R. MATKIN North Gulf Oceanic Society, Gustavus, Alaska 99826, U.S.A. AMEE V. MEHTA Boston University Marine Program, Woods Hole, Massachusetts 02543, U.S.A. ROBERT J. SMALL Alaska Department of Fish and Game, 1255 West 8th Street, P.O. Box 25526, Juneau, Alaska 99802, U.S.A. JANICE M. STRALEY University of Alaska Southeast, Sitka, Alaska 99835, U.S.A. SHANNON M. MCCLUSKEY Washington Cooperative Fish and Wildlife Research Unit, School of Aquatic and Fishery Sciences, University of Washington, Seattle, Washington 98195, U.S.A. GLENN R. VANBLARICOM Washington Cooperative Fish and Wildlife Research Unit, School of Aquatic and Fishery Sciences, University of Washington, Seattle, Washington 98195, U.S.A. and Biological Discipline, U.S. Geological Survey, Seattle, Washington 98195, U.S.A. PHILLIP J. CLAPHAM National Marine Mammal Laboratory NOAA Fisheries, 7600 Sand Point Way NE, Seattle, Washington 98115, U.S.A. 767 768 MARINE MAMMAL SCIENCE, VOL. 23, NO. 4, 2007 ABSTRACT Springer et al. (2003) contend that sequential declines occurred in North Pacific populations of harbor and fur seals, Steller sea lions, and sea otters. They hypothesize that these were due to increased predation by killer whales, when industrial whaling’s removal of large whales as a supposed primary food source precipitated a prey switch. Using a regional approach, we reexamined whale catch data, killer whale predation observations, and the current biomass and trends of potential prey, and found little support for the prey-switching hypothesis. Large whale biomass in the Bering Sea did not decline as much as suggested by Springer et al., and much of the reduction occurred 50–100 yr ago, well before the declines of pinnipeds and sea otters began; thus, the need to switch prey starting in the 1970s is doubtful. With the sole exception that the sea otter decline followed the decline of pinnipeds, the reported declines were not in fact sequential. Given this, it is unlikely that a sequential megafaunal collapse from whales to sea otters occurred. The spatial and temporal patterns of pinniped and sea otter population trends are more complex than Springer et al. suggest, and are often inconsistent with their hypothesis. Populations remained stable or increased in many areas, despite extensive historical whaling and high killer whale abundance. Furthermore, observed killer whale predation has largely involved pinnipeds and small cetaceans; there is little evidence that large whales were ever a major prey item in high latitudes. Small cetaceans (ignored by Springer et al.) were likely abundant throughout the period. Overall, we suggest that the Springer et al. hypothesis represents a misleading and simplistic view of events and trophic relationships within this complex marine ecosystem. Key words: North Pacific, killer whale, Steller sea lion, sea otter, harbor seal, fur seal, ecosystem, predation, whaling, population dynamics. Springer et al. (2003) have contended that North Pacific populations of pinnipeds and sea otters (Enhydra lutris) in the Bering Sea, Aleutian Islands, and Gulf of Alaska declined in a marked sequence, and hypothesize that the declines were due to increased predation by so-called “transient-type” (mammal-eating) killer whales (Orcinus orca). In brief, the hypothesis posits that the primary prey of killer whales originally constituted the large whales, that is, baleen whales and sperm whales (Physeter macrocephalus), but that the depletion of this prey source by whaling in the Aleutian Islands and Gulf of Alaska by ∼1970 reduced whale biomass to the point where killer whales were forced to switch to other marine mammal prey. The hypothesis posits that the biomass of this alternative prey was lower than preexploitation whale populations, and the killer whale predation thus caused the sequential decline of harbor seals (Phoca vitulina richardii), Steller sea lions (Eumetopias jubatus), northern fur seals (Callorhinus ursinus), and sea otters. The “prey-switching” hypothesis regarding declines of pinnipeds and sea otters in the North Pacific is superficially attractive. As presented by Springer et al., the picture of whale catch statistics and pinniped and sea otter trend data would indeed suggest that these events happened sequentially: As they describe the sequence of events, whale catches declined in the late 1960s, harbor seals and fur seals declined in the 1970s, Steller sea lions declined in the 1980s, and sea otters declined in the 1990s. As we demonstrate below, however, this assertion of sequential decline throughout the North Pacific is based upon a selective and simplistic analysis of trend data. A more detailed examination of these data, and consideration of information from other WADE ET AL.: KILLER WHALES AND MARINE MAMMAL TRENDS 769 North Pacific populations of these same species, results in a picture that is not nearly as simple or as elegant as that suggested by the Springer et al. hypothesis. In this critique, we systematically review the data and assumptions underlying the Springer et al. paper and suggest that the large whale depletion hypothesis is an unlikely explanation for the observed declines. In brief, the primary assumption regarding the importance of large whales as prey items for killer whales prior to commercial whaling is poorly supported, and the assertion that populations of pinnipeds and sea otters declined in sequence throughout the North Pacific is not born out by the evidence. Alternative analyses presented here reveal—contrary to the suggestion of the prey-switching hypothesis—that, in some regions of the North Pacific, pinniped populations have grown substantially since 1970, despite a history of commercial whaling in those regions and the presence there of killer whales that prey upon pinnipeds. There are essentially four different components to the Springer et al. hypothesis, which we evaluate separately: (1) Was there a large population of mammal-eating killer whales that preyed primarily on large whales? (2) If so, did the removal of large whales by commercial whaling leave this population with insufficient prey biomass, such that the killer whales needed to find an alternative food source? (3) Did pinnipeds and sea otters decline in a sequential manner following commercial whaling? (4) Could killer whale predation be responsible, partly or wholly, for any of the observed declines in pinnipeds and sea otters, independent of the whale depletion hypothesis? In this review, we focused primarily on the eastern North Pacific (waters east of 180◦ W), in waters north of 30◦ N. We reexamined whale catch data, trends in abundance and biomass of potential marine mammal prey of killer whales, and observations of killer whale prey in three regions of the North Pacific: the Bering Sea and Aleutian Islands, the Gulf of Alaska, and the Western Coast of North America (from Southeast Alaska to California) (Fig. 1). Each of these regions is large and contains many independent populations of the killer whale prey species in question. A regional approach allows a more detailed review of the trend and abundance data. Springer et al. combined trend data from the Bering Sea and Aleutian Islands region with the Gulf of Alaska region. We examine those regions separately and also examine a third region not considered by Springer et al., the Western Coast of North America. Marine mammal trend data are available for a large number of species in these areas (by contrast, less information is available for the western North Pacific). We also examine pinniped and sea otter trend data from the Commander (Commadorski) Islands in Russian waters of the northwestern Pacific. The Commander Islands are of particular interest because trend data are available for all four species considered by Springer et al. The islands are adjacent to the area in the Aleutians where sea otter and Steller sea lion declines have occurred, and are essentially part of the region on which Springer et al. based their hypothesis. In particular, we carefully examined data from the four species that are the focus of the Springer et al. paper: harbor seals, northern fur seals, Steller sea lions, and sea otters. These four species all have distributions that range across the North Pacific at higher latitudes. Some of the information summarized and synthesized here is either unpublished or from sources not widely available, such as government documents. Therefore, much of this information may not have been available to Springer et al., or the wider scientific community, and so we have attempted to provide sufficient detail for the reader to examine the hypothesis in light of the available data. A comparative regional approach is used to examine whether substantial takes of large whales occurred in all three areas and, if so, whether prey switching and declines 770 MARINE MAMMAL SCIENCE, VOL. 23, NO. 4, 2007 Figure 1. Map of North Pacific with labels of place names mentioned in the text. BSAI is the Bering Sea and Aleutian Islands, GOA is the Gulf of Alaska, and WCNA is the Western Coast of North America. The vertical thick lines mark the boundaries between these areas as defined for this study. of pinnipeds and sea otters occurred there, as would be predicted by the Springer et al. hypothesis. A regional approach also allows us to examine in more detail whether declines in potential killer whale prey occurred and whether or not those declines were sequential. METHODS REGIONAL APPROACH As noted above, our approach focuses on three main regions: the Bering Sea and Aleutian Islands, the Gulf of Alaska, and the Western Coast of North America (from Southeast Alaska to California). Three types of killer whales have been identified in the northeast Pacific, labeled as “resident,” “transient,” and “offshore” ecotypes (Bigg et al. 1990, Ford et al. 2000) based on aspects of morphology, ecology, genetics, and behavior (Ford and Fisher 1982; Baird and Stacey 1988; Baird et al. 1992; Hoelzel et al. 1998, 2002; BarrettLennard 2000). Only one type, transients, is known to prey on marine mammals (Ford et al. 1998; Saulitis et al. 2000). Until recently in Alaska, transient killer whales had been studied intensively only in Southeast Alaska and in the Gulf of Alaska (from Prince William Sound, through the Kenai Fjords). In the Gulf of Alaska, Matkin et al. (1999) described two communities of transients. Neither of the WADE ET AL.: KILLER WHALES AND MARINE MAMMAL TRENDS 771 two Gulf of Alaska communities commonly associate with transient killer whales that range from California to Southeast Alaska (the “West Coast community”), although there is at least one case of Gulf of Alaska transient whales associating with West Coast transients in Southeast Alaska (D. Matkin et al., in press). The West Coast transient community ranges from Southeast Alaska to California (Goley and Straley 1994). All three communities have significant differences in mtDNA and nuclear (microsatellite) DNA (Barrett-Lennard 2000) and are considered discrete populations. Over 300 whales have been identified in the West Coast population (Black et al. 1997; Dahlheim et al. 1997; Ford and Ellis 1999). A substantial number (>75) of transient killer whales have been identified in the two Gulf of Alaska populations (Dahlheim 1997; Matkin et al. 1999). Recent studies in western Alaska have confirmed that at least several hundred transient killer whales are also found along the Alaska Peninsula and Aleutian Islands. 1, 2 In summary, there are substantial populations of mammal-eating killer whales in all three regions that we consider here. 3 We summarize information on commercial whale catches as well as on abundance, trend, and biomass (both historically removed and presently available) for all relevant species of marine mammals in the three regions. Although complete data are not available for many species, considerable information exists. We also examine what is known about predation by mammal-eating killer whales in the North Pacific. Most available data on abundance of marine mammal species are from coastal or nearshore areas. Conveniently, the distribution of harbor seals, Steller sea lions, northern fur seals, and sea otters is largely confined to this part of the ocean. The exception is that, for some age classes, fur seals have a broad pelagic distribution during some periods of the year. Most of the abundance data come from surveys conducted during the period April to October, as do the majority of the whale catch data. The majority of northern fur seals are on or near rookeries in summer and are therefore within the area we examine, the one main exception being that some (perhaps most) juveniles remain at sea for several years. Similarly, we have also restricted the whale catch data to broad regions adjacent to land and excluded catches from pelagic regions far from land. Fortuitously, then, most marine mammal abundance data are from the areas where Steller sea lions, harbor seals, northern fur seals, and sea otters also represent potential killer whale prey. In these areas, it would be conceivable for killer whales to turn to pinniped and/or sea otter prey if other species became unavailable. In contrast, it seems less likely that killer whales from pelagic habitats in the North Pacific, if deprived of a preferred prey source in such areas, would relocate to coastal waters to prey on pinnipeds and sea otters. However, given the known long-range movements of some killer whales, this latter idea cannot be entirely ruled out. Western Coast of North America (Southeast Alaska to California, WCNA) Each of the four killer whale prey species of interest has a breeding population in the WCNA region. Harbor seals are widely distributed from California to Southeast 1 Unpublished data provided by National Marine Mammal Laboratory, Alaska Fisheries Science Center, NOAA Fisheries, 7600 Sand Point Way NE, Seattle, WA 98115, May 2006. 2 Unpublished data provided by North Gulf Oceanic Society, 3430 Main Street, Suite B1, Homer, AK 99603, May 2006. 3 In this review, the term “killer whale” refers to the mammal-eating transient ecotype, unless otherwise noted. 772 MARINE MAMMAL SCIENCE, VOL. 23, NO. 4, 2007 Alaska. Steller sea lions also occur over this range; however, rookeries south of British Columbia are few and relatively small. Northern fur seals have a small breeding population on San Miguel Island in southern California, and sea otters occur as discrete populations in California, Washington, British Columbia, and Southeast Alaska. Other pinniped species, including California sea lions and elephant seals, are also common in portions of this region. Gulf of Alaska (GOA) Only three of the four species of interest have breeding populations in the Gulf of Alaska: Steller sea lions, harbor seals, and sea otters. Northern fur seals in the GOA occur primarily in offshore pelagic waters in lower densities than in the Bering Sea and Aleutian Islands, and they do not haul out in appreciable numbers. Bering Sea and Aleutian Islands (BSAI) In the eastern North Pacific, none of the four species of interest are found in large numbers north of the southeastern Bering Sea. Large concentrations of harbor seals are found in southern Bristol Bay, and several large Steller sea lion rookeries are found in the eastern Aleutian Islands. The main northern fur seal rookeries in the eastern North Pacific are found on the Pribilof Islands. Sea otters are found throughout the Alaska Peninsula and in southern Bristol Bay. None of these species are found in appreciable numbers north of St. Matthew Island and the Yukon Delta. Therefore, the focus here was on species found in the southeastern Bering Sea from April to October. For example, data from bowhead whales (Balaena mysticetus) were not used because bowheads spend the summer in the Beaufort Sea and cannot be considered a likely alternative prey source in summer for killer whales that prey on Steller sea lions. Additional pinniped species in the southeastern Bering Sea include Pacific walrus (Odobenus rosmarus divergens) and spotted seals (Phoca largha). All of the species of interest are also found in the Aleutian Islands. Northern fur seals are found at sea in this area in summer and have established a small but growing rookery on Bogoslov Island in the eastern Bering Sea. Commander Islands The Commander Islands consist of two main islands, Bering and Medney. Substantial colonies of northern fur seals exist on both islands, together with harbor seals, Steller sea lions, and sea otters. Though separated from the Near Islands (the “nearest” of the Aleutian Islands to Russia) by approximately 200 nm, the Commander Islands can be considered part of the same archipelago and are similar in that they are surrounded by a narrow shelf, with a habitat that is primarily oceanic. WHALE CATCH DATA Whaling History in the Eastern North Pacific The first commercial whaling in the North Pacific occurred in the early 1800s with the pursuit of sperm whales (Webb 1988). Most of this occurred in lower latitudes, WADE ET AL.: KILLER WHALES AND MARINE MAMMAL TRENDS 773 near the equator, in Panama Bay, and off Baja California (Mexico). During this early period, the whalers operating furthest north were American vessels that killed sperm whales on the Japan Grounds in the 1820s. Commercial whaling in high latitudes in the North Pacific did not begin until a few whaling ships pushed north in 1835 and discovered right whales 4 in a region from approximately the Queen Charlotte Islands in the east to Kodiak Island and beyond to the west. Shortly after the discovery of these “Northwest” or “Kodiak” whaling grounds, a major right whale fishery developed, and right whales were depleted in this region (and elsewhere in the North Pacific) between 1840 and 1865. Whaling ships subsequently pushed even farther north and began taking bowhead whales in large numbers in 1849. Commercial whaling of gray whales (Eschrichtius robustus) began in about 1845, and by 1874 the gray whale fishery had ended because so few whales were left. It was not until the advent of major technological developments in the late 19th and early 20th centuries (faster steam-powered ships, exploding harpoons, and the use of compressors to prevent dead whales from sinking) that systematic whaling began on the faster rorquals, notably fin (Balaenoptera physalus) and blue whales (B. musculus). Humpback whales (Megaptera novaeangliae) were initially depleted in the North Pacific in areas such as Washington and British Columbia, from about 1905 to 1915 (Webb 1988). Catches in California declined substantially in the 1920s, indicating depletion of the population (Clapham et al. 1997), and humpbacks were depleted along the entire western coast of North America by 1930 (Tønnessen and Johnson 1982). Similarly, catches of blue whales from coastal whaling stations along the west coast of North America declined rapidly in the 1910s, and catches in the 1920s were sustained only at a single whaling station (Akutan) in the Aleutian Islands (Tønnessen and Johnson 1982). In British Columbia, catches of humpback and blue whales also declined dramatically in the early period of modern whaling (1905–1943) (Gregr et al. 2000). In the mid-1930s, both Russian and Japanese factory ships entered the Bering Sea and worked along the eastern North Pacific rim, catching mainly fin, humpback, and sperm whales. However, the majority of catches of these species occurred after World War II. After World War II, Japanese and Soviet whaling ships resumed widespread operations in the North Pacific, including in the Aleutian Islands, Bering Sea, Gulf of Alaska, and along the edge of the continental shelf of Canada and the United States. This period included substantial illegal whaling and falsification of data by the USSR, which depleted populations of several species, notably North Pacific right whales (Doroshenko 2000a). As noted above, humpback and blue whales were thought to be initially depleted in the early 1900s, and the resumption of whaling on these species after World War II was believed to have severely depleted them in the North Pacific by the 1960s (Webb 1988). Consequently, the International Whaling Commission (IWC) banned the taking of blue and humpback whales in 1966. Fin whale catches off British Columbia had also declined during the early 1900s but rebounded during the second whaling era (1948–1967) and then, with sei whale (Balaenoptera borealis) catches, declined again in the mid 1960s (Tønnessen and Johnson 1982; Gregr et al. 2000). Catches of sperm whales in this region during the post-war period showed no trend (Gregr et al. 2000). Fin whales continued to be hunted legally in the North Pacific until 1976, and catches of sei and sperm whales were still allowed until 1981, although this was not binding on some countries that filed an objection to a worldwide moratorium. A 4 Referred to as Balaena glacialis by Rice (1998) and as Eubalaena japonica by Rosenbaum et al. (2000). 774 MARINE MAMMAL SCIENCE, VOL. 23, NO. 4, 2007 moratorium on catches of minke whales (Balaenoptera acutorostrata) came into effect in 1986. Whaling Catch Summary Whaling catch data were obtained from the IWC’s catch database and summarized within each of the three regions defined above. Summaries for the BSAI and GOA regions were restricted to catches north of 50◦ N latitude. Although catches occurred south of this latitude, we selected this boundary to restrict the catches to regions in the vicinity of the pinniped and sea otter species of interest, as explained above. The BSAI region for catches was defined as the area from 180◦ W to 163◦ W longitude. The border between the BSAI and GOA regions was set at 163◦ W longitude because this has been selected as a breakpoint for Steller sea lion trend data between the Gulf of Alaska and the Aleutian Islands and Bering Sea, and it provides a reasonable division between the “Kodiak” (“Northwest”) whaling ground and catches that occurred south of the Aleutian Islands. The border between the GOA region and the WCNA region was set at 130◦ W longitude, as this corresponds well to the population division between Southeast Alaska and the Gulf of Alaska that occurs in many marine mammal species, and it also appropriately includes catches that occurred on the “Vancouver” whaling ground from catcher boats associated with British Columbia shore-stations into the WCNA region. The southern boundary of the WCNA region was set at 20◦ N latitude. Catch data were converted to biomass by multiplying the number of takes by an average biomass per individual (see below). As noted above, it is known that substantial falsification of whaling data occurred in the North Pacific, primarily by the Soviet Union in the 1960s and 1970s. In particular, where true records are available for comparison, catches of sperm whales by the Soviet Union in the North Pacific from 1948 to 1973 were 1.8 times greater than reported to the IWC (Brownell et al. 2000). Catches of humpback whales by the Soviet Union in the North Pacific from 1961 to 1971 were 1.6 times greater than reported (Doroshenko 2000b). Catches of fin whales by the Soviet Union in the North Pacific during this time period were somewhat overreported, presumably to cover up illegal takes of protected species. Although an ongoing revision of the North Pacific catch history to correct this falsification remains incomplete, these “correction factors” will be considered when interpreting the results of the official (incorrect) IWC catch database used here. KILLER WHALE PREDATION OBSERVATIONS Killer whale predation observations were reviewed using published literature and unpublished field data. For all three regions, the number of observations of marine mammal prey taken by killer whales was tallied by species and taxonomic groups. Numbers tallied included all observations of prey seen killed by killer whales, observations of a carcass being eaten by killer whales, or prey remains identified in the stomach contents of killer whales. Prey observations were summarized by regional areas in the North Pacific and were restricted to observations since 1950. Observations that could be classified only as an attack, with no confirmation of a kill, were not included. Each confirmed predation event was counted as a single event, regardless of how many individuals were killed during the event. Predation events reported as occurring on separate days were counted as separate events. Stomach contents were tallied as the minimum number of individuals consumed. Primary reviews consulted WADE ET AL.: KILLER WHALES AND MARINE MAMMAL TRENDS 775 included Rice (1968); Lowry et al. (1987); Jefferson et al. (1991); Matkin and Saulitis (1994); Barrrett-Lennard et al. (1995); Ford et al. (1998); Hatfield et al. (1998); Saulitis et al. (2000); Heise et al. (2003); and Ford et al. (2005). Substantial unpublished data were used from Southeast Alaska (J. Straley and D. Matkin, unpublished data), California (N. Black, unpublished data), and Alaska.1 Details of the sources of the prey data are given in Appendix A. The great majority of the records are from observations, not from stomach contents. There are potential biases in such data, primarily regarding the probability that a given type of predation event will be observed. For example, more killer whale observations occur in protected, inland waters than in open exposed seas, and this may influence the probability of reporting for predation on certain species. Therefore, these data should not be viewed as representing exact percentages of prey preferences for mammal-eating killer whales in the North Pacific, but rather they provide a qualitative sense of what kind of prey are known to be taken. Similarly, stomach content analyses are subject to bias, notably that the hard parts of baleen whales (bones and baleen) are not consumed and therefore are unlikely to persist in stomachs; this is not the case for teeth or bones of odontocete prey. MARINE MAMMAL ABUNDANCE AND TREND DATA Although the primary focus of our analysis was the four species discussed by Springer et al. (2003), we also examined abundance and trend data for other taxa that are available as potential prey for killer whales in the North Pacific. Abundance estimates for each species were compiled and converted to biomass for each region, including all large whale species and all other species of marine mammals that have been observed to be killer whale prey in that region. If more than one stock (population) occurred in a region, the abundance for the species was summed across all stocks within the region. If a population occurred in more than one region, the abundance of the population was split between the regions. Trend data for pinnipeds and sea otters were compiled for the Commander Islands. Compiled tables and details of the sources of the abundance and trend data for each species are given in Appendix B. BIOMASS CALCULATIONS Whale catch data and marine mammal abundance data were converted to biomass by multiplying catch or abundance by an average biomass of individuals of that species (Mizroch and Rice 2006). Calculations of average biomass for large whales were based upon actual length data in the Bureau of International Whaling Statistics/IWC catch database. Weight was estimated for each whale caught based upon a method developed by Mizroch (1983) using length/weight curves developed for the various species concerned, using parameters estimated by Lockyer (1976) for most of the large whales. For Baird’s beaked whales, Mizroch and Rice (2006) estimated parameters directly from length/weight tables published in Balcomb (1989). For each species, the method involved using a single value for body mass at a specific length (e.g., any 20-m fin whale was assigned a value of 48 tons as derived from the length/body mass curves). Although the true body mass of individual whales would undoubtedly have varied from these averaged values, depending on body condition and reproductive state, we consider the figures sufficiently precise for the purposes of this review. Biomass calculations for other marine mammal species were derived from various sources in the literature. The mean body mass of each species used in the biomass calculations is listed in Table 1 of Appendix C. 776 MARINE MAMMAL SCIENCE, VOL. 23, NO. 4, 2007 In order to calculate current available biomass and trends in biomass, the total estimated abundance for each species in each region was multiplied by the average biomass per individual. This provided, for each species, an estimate of the biomass (in million kg) available as prey for killer whales in each of the three geographic areas (see Tables 2–4 in Appendix C). The percentage of the total available biomass represented in various taxonomic groups was also calculated for each of the three geographic areas. Finally, trends in biomass for some species were similarly calculated where abundance estimates through time were multiplied by the average biomass per individual. RESULTS WHALE CATCHES Western Coast of North America In the WCNA region, total whale biomass (summed across all species) taken in commercial catches increased during the 1950s, peaked in the 1960s, and declined during the 1970s, with the last substantial catches occurring in 1978 (Fig. 2). Total biomass taken remained relatively high through 1974. From 1950 to 1978, the greatest biomass taken was that of sperm whales (44%), followed by fin whales (34%), sei whales (11%), humpback whales (7%), and blue whales (4%). Biomass reported taken was dominated by fin whales in the late 1950s and early 1960s. From 1965 to 1978, the biomass of the reported catch was dominated by sperm whales. However, given the under reporting of sperm whale catches and over reporting of fin whale catches by the Soviet Union, the biomass of sperm whales removed in the 1950s and early 1960s might have been on the same order as that of fin whales. Sei whale catches peaked from 1962 to 1970. Humpback and blue whales were caught mainly from the late 1940s to the mid-1960s. Gulf of Alaska Catches in the GOA region after World War II did not resume until 1960. Total whale biomass taken in commercial catches increased rapidly to a peak in 1965–1966 and then declined to a relatively low level by 1968 (Fig. 2). Takes continued at low levels in the early 1970s and had essentially ceased by 1975. From 1960 to 1975 the greatest biomass reported taken was that of fin whales (42%), followed by sperm whales (33%), sei whales (14%), blue whales (6%), and humpback whales (5%). Most of the fin whale biomass was taken between 1961 and 1966; after 1966, the greatest biomass reported taken was that of sperm whales. However, given the under reporting of sperm whale catches and over reporting of fin whale catches by the Soviet Union, it may be that the greatest biomass removed over this entire time period in this region was that of sperm whales. Takes of sei, blue, and humpback whales mainly occurred from 1960 to 1966. Bering Sea and Aleutian Islands Substantial commercial whale catches in the BSAI region began in 1954, remained at relatively high levels until 1966, and had essentially ceased by 1973 (Fig. 2). The peak of biomass removal occurred in 1964–1965. From 1954 to 1973, the greatest WADE ET AL.: KILLER WHALES AND MARINE MAMMAL TRENDS 777 Figure 2. Biomass of whale catches from the International Whaling Commission Catch database in the three regions of interest from 1950 to 2002. The five species with the highest catches are plotted individually. Total also includes catches of additional species (e.g., gray whales). See text for details on conversion of catches to biomass. biomass taken was that of sperm whales (50%), followed by fin whales (39%), sei whales (5%), blue whales (4%), and humpback whales (2%). The greatest biomass taken in the 1950s was that of fin whales, and by 1959 the greatest biomass taken was that of sperm whales, which continued to be the case until 1968. However, given 778 MARINE MAMMAL SCIENCE, VOL. 23, NO. 4, 2007 the under reporting of sperm whale catches and over reporting of fin whale catches that occurred by the Soviet Union, it may be that the greatest biomass removed over this entire time period in this region was that of sperm whales. Humpback and blue whales were caught primarily from the mid-1950s to the mid-1960s. Sei whales were mainly taken in the mid-1960s. KILLER WHALE PREY OBSERVATIONS Western Coast of North America In the WCNA region 73% of all observed predation events (n = 466) were of pinnipeds, including (in order of occurrence) harbor seals, California sea lions (Zalophus californianus), Steller sea lions, and northern elephant seals (Mirounga angustirostris) (Table 1, Appendix B). The next largest category was small odontocetes, with 17% of the observations, including harbor porpoise (Phocoena phocoena), Dall’s porpoise (Phocoenoides dalli), Pacific white-sided dolphins (Lagenorhynchus obliquidens), and longbeaked common dolphins (Delphinus capensis). Large baleen whales represented 8% of the observations, with all of these observations of predation on gray whales. Minke whales represented 1% of the total. Little predation (