Potential Influences of Whaling on the Status and Trends of Pinniped
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TWENTY-SEVEN Potential Influences of Whaling on the Status and Trends of Pinniped Populations DANIEL P. COSTA, MICHAEL J. WEISE, AND JOHN P. Y. ARNOULD Although this volume focuses on whales and whaling, the are. Pinnipeds are 1 to 2 orders of magnitude smaller in mass depletion of great whales over the last 50 to 150 years per- than whales, which result in greater mass-specific rates of turbed the marine interaction web, thus influencing many food consumption. Thus the pinnipeds have physiological other species and ecosystem processes (Estes, Chapter 1 of and environmental scaling functions that must be consid- this volume; Paine, Chapter 2 of this volume). Such interac- erably different from those of the great whales. For example, tion web effects have been hypothesized for several pinniped although some pinnipeds have remarkable abilities to fast, species. For example, the reduction of great whales in the even the most extreme durations of fasting in pinnipeds fall Southern Ocean may have caused seal and penguin popula- easily within the abilities of large cetaceans. The relatively tions to increase because of reduced competition for their small size of pinnipeds compared with cetaceans results in shared prey, krill (Laws 1977; Ballance et al., Chapter 17 of a much higher mass-specific metabolism and thus a shorter this volume). In addition, pinnipeds share some of the same fasting duration. These differences should constrain pin- predators, especially killer whales, as large whales do. nipeds to operate at smaller spatial and temporal scales Declines in whale populations may thus have caused the than the large cetaceans, thus making pinnipeds more sen- decline of certain pinniped populations because of redirected sitive to variations in prey abundance and distribution. predation by killer whales (Springer et al. 2003; Branch and Smaller size is also linked to a shorter generation time in Williams, Chapter 20 of this volume). These purported indi- pinnipeds, which makes their populations more vulnerable rect effects of whales on pinnipeds are poorly documented to environmental disturbances but also affords them a and controversial. Since most of the arguments are area- or greater potential for population growth. All of these char- species-specific, a global overview of the known patterns acteristics suggest that pinniped populations should be and causes for pinniped population change is topical and more responsive to changes in their environment than the relevant. large whales are. Because of differences in body size and life history, Pinnipeds have a nearly cosmopolitan distribution in pinnipeds are both easier to study, and possibly more the world oceans, although most species occur in temper- sensitive to environmental fluctuations, than most cetaceans ate to polar regions. Abundances range across species from 342 a few hundred to tens of millions of individuals. Estimates and Cape fur seals—all forage in seasonally productive, of abundance or trends in population numbers are the high-latitude ecosystems. most useful indicators of population status. Most popula- tions were severely depleted by commercial harvesting. Phocid Population Trends However, species distributions and population abundances before sealing are often unknown, because sealing ships ARCTIC SPECIES did not keep adequate records. Furthermore, reliable mod- There are six species of ice-breeding phocids in the northern ern abundance estimates are lacking for many species. hemisphere (harp, hooded, bearded, ringed, spotted, and Despite these problems, the history and trends in abun- ribbon seals), many of which annually migrate between sub- dance of the majority of pinnipeds is reasonably well arctic and arctic regions. Because of the difficulty in con- known. ducting surveys in this harsh environment, the abundance of In this chapter we review the current status and trends of many of these species is not well known. pinniped populations worldwide, and, where possible, we Harp and hooded seals are both divided into three stocks summarize the known or suspected reasons for recent (eastern Canada, White Sea, and West Ice), each identified declines. Trends in pinniped populations attributed to natu- with a specific breeding site. Recent modeling efforts indicate ral biological processes are evaluated in terms of reproductive that a harvest of 460,000 young harp seals per year is hold- strategies, physiological limitations, and the resultant sus- ing the eastern Canada stock stable at about 5.2 million ceptibility to disturbance in prey resources and predation individuals (Healey and Stenson 2000). The other harp seal brought about by these factors. stocks are smaller—approximately 1.5 to 2.0 million in the Pinniped Population Trends White Sea and 286,000 on the West Ice. The best current population estimate for hooded seals is 400,000 to 450,000 The present-day abundances of species do not always animals (Stenson et al. 1993). Marked increases in the reflect their pre-exploitation numbers. Some species that number of harp and hooded seals occurred on Sable Island were decimated to near-extinction are now very abundant, in the mid-1990s (Lucas and Daoust 2002). whereas others have either not recovered or have recov- Populations of bearded seals were decimated by early com- ered and subsequently declined. Population abundance in mercial sealing. Russia continued a commercial harvest of pinnipeds ranges over four orders of magnitude across bearded seals, with catches exceeding 10,000 animals yr−1 species from the Mediterranean and Hawaiian monk seals, during the 1950s and 1960s. In the 1970s and 1980s quotas which number in the hundreds of individuals, to the were introduced to limit harvests on declining populations crabeater seal with an estimated abundance of 10 to 15 to a few thousand animals annually (Kovacs 2002). Today, million individuals (Table 27.1). Phocids are generally bearded seals are an important subsistence resource to arctic more abundant than otariids. Fifteen of the 19 phocid peoples, with a few thousand animals taken annually for use species number greater than 100,000 individuals, whereas as human food, dog food, and clothing. Reliable estimates of only 8 of the 17 otariid species number greater than the total population of bearded seals do not exist. Early esti- 100,000 individuals. mates of just the Bering-Chukchi Sea population ranged from Pinnipeds range throughout the world oceans. Although 250,000 to 300,000. Discrepancies in recent survey efforts in the preponderance of species occurs in the northern hemi- 1999 and 2000 have precluded an updated estimate, but the sphere (Figures 27.1 and 27.2), the southern hemisphere abundance may be much greater than previously described contains far more individuals. The abundance of crabeater (Waring et al. 2002). and Antarctic fur seals alone exceeds the combined abun- Currently, five distinct subspecies of ringed seals are dance of all northern hemisphere species. The lesser num- recognized. Population estimates for most of these are ber of species in the southern hemisphere may reflect a outdated, and there are many uncertainties in the estima- northern hemisphere center of origin for otariids and pho- tion and sampling methods. Nonetheless, Bychkov (in cids (Costa 1993; Demere 1994; Demere et al. 2003). The Miyazaki 2002) estimated that there were 2.5 million in the larger numbers of individuals in the southern hemisphere Arctic Ocean and 800,000 to 1 million in the Sea of likely result from highly productive Antarctic and sub- Okhotsk in 1971. The Baltic ringed seal population Antarctic waters coupled with an abundance of predator- decreased from 190,000 to 220,000 animals at the begin- free islands. The relative scarcity of human settlements ning of the twentieth century to approximately 5,000 dur- (which invariably lead to habitat loss, direct and indirect ing the 1970s. In the mid-1960s, the remaining seals were pinniped/fisheries interactions, and hunting pressure) may afflicted by sterility, likely caused by organochlorides also contribute to the larger sizes of southern hemisphere (Harding and Harkonen 1999; Reijnders and Aguilar 2002), pinnipeds. The relative abundance of phocids is likely due which inhibited natural population growth during the sub- to their generally inhabiting the highly productive polar sequent 25-year period. Ringed seals are hunted in many and subpolar regions (Bowen 1997). Similarly, the three regions (Miyazaki 2002). Thus, the decrease in seal numbers most abundant otariid species—the northern, Antarctic, was a consequence of excessive hunting in combination WHALING EFFECTS ON PINNIPED POPULATIONS 343 TABLE 27.1 Pinniped Population Numbers and Trends Worldwide Common Name Species Population Size Trend Northern Hemisphere Eared Seals Otariidae Guadalupe fur seal (GFS) Arctocephalus townsendi 7,000 Increasing California sea lion (CSL) Zalophus californianus 237,000–244,000 Increasing Northern fur seal (NFS) Callorhinus ursinus 1,400,000 Decreasing Steller sea lion (SSL) Eumatopias jubatus <75,000 Decreasinga Galápagos sea lion (GSL) Zalophus wollebaeki 5,000 Fluctuating Galápagos fur seal (GAFS) Arctocephalus galapagoensis 12,000 Fluctuating Japanese sea lion Zalophus japonicus Extinct Extinct Walruses Odobenidae Pacific walrus Odobenus rosmarus divergens 200,000 Decreasing Atlantic walrus Odobenus rosmarus rosmarus >14,000 Unknown Earless Seals Phocidae Hooded seal (HOS) Cystophora cristata >400,000 Increasing Gray seal (GS) Halichoerus grypus Unknown Increasing