Sea Otter Survey of Adak Island

Transcription

1 Sea Otter Survey of Adak Island 1998 Summary Report J.A. Estes and M.T. Tinker U.S. Geological Survey- BRD A-316 Earth and Marine Sciences University of California, Santa Cruz Santa Cruz, CA, 9564

2 Sea Otter Skiff Survey of Adak Island: 1998 Sunmmry Report Introduction Jnfmmation on sea otters fi:om the Aleutian archipelago indicates that populations have declined substantially through the 199s. The most detailed record of population change is for Adak Island, where smveys have been done in most years since 1991 (Estes and Tinker, unpublished data). Data from Adak have been collected by skiff smveys, using consistent methodologies and personnel throughout. This report swnmarizes results of the most recent Adak Island sea otter smvey, conducted between 17 July and 25 July, The objectives of the 1998 sutvey were (1) to continue the ongoing data-set for Adak Island, which cuttently represents the most precise and long-tetm source ofinfmmation on sea otter trends in the central-western Aleutians, (2) to determine whether the previously detected decline in sea otter numbers is continuing, and (3) to continue monitoring nulllbers of several other species of coastal marine wildlife whose populations are thought to be linked to sea otter abundance. Methods The SUivey was conducted by runniug a skiff with two or more observers along the coastline of Adak and all adjacent islets. The skiffwas tun as close to shore as possible, generally along the outer margins ofthe surface kelp (Alariafistulosa) canopy between 2 and 5 meters from shore, and recording the nulllber of sea otters, common eiders, and pinnipeds (harbor seals and Steller sea lions) seen. Each otter was tallied as either an independent ("A") or as a female with a dependent pup (''P"). While the probability of sighting otters by this method has not been detetmined, replicated surveys of other areas have shown the technique to provide consistent results. fu addition to the skiff survey of the Island perimeter, a shore-based survey was conducted of Clam Lagoon. Following the methodologies used in past surveys, sea otters were counted from stations around the lagoon by an experienced observer using a 5x Questar telescope and binoculars. Survey Results During the 'I 998 survey of Adak Island, a total of 611 sea otters were counted around the outer coast and within Clam Lagoon, 86 of which were dependent pups (Table 1). Excluding the Clam Lagoon counts provides a total of 55 sea otters, or 434 excluding dependent pups (Table 2). These results show that the sea otter population at Adak continues to decline at a rate of about 25% yt - 1 (Figure 1). The number of sea otters counted in these SUiveys has declined by 48 percent since 1995 (when 98 were counted) and 8 percent since 198 when 2,42 were counted (Figure 2). Because the 198 count was made by Fish and Wildlife staff dming a multispecies sutvey using different methods and observers from our surveys, we suspect that it is biased low. Thus the actual magnitude of the decline probably exceeds 8%. 2

3 .. i Sea Otter Skiff Survey of Adak Island: 1998 Sununary Report Table 1 Summary of sea otter skiff counts made between 1995 and 1998 of Adak Island (outer coast plus Clam Lagoon). Sea otter counts are summarized for independent adults, mother-pup pairs, total number of animals excluding dependent pups, and totalnwnber of animals including dependent pups. Counts are also shown for harbor seals and common eiders. Year #Ind. Adults #Mom-pup pairs Total# Adults Total# incl.dep.pups Harbor Seals Eiders l 1995 I : 16 : clttfr: :: : :: : : :::~~~::::::::::::::c::.: : : :.:::::::nr : : : : ::::: :: ::::. :::.::::. : : :: ::1-~f : : ::.::::.::-:. ::.:. : :.:. ::: :. ::.:~:f~.:: : : :: : : : :: : ::::.. :.j: : :: :. :.::.:.~~r :: :. :.::.::c::~1f J (i998l 43"9 :. 86 :. 525 : 611 : i68 :.. 28 ~i : Table 2 Summary of sea otter skiff counts made at Adak Island between 198 and The 198 data are from a multi-species survey of Adak made by the US Fish and Wildlife Service, while data for are from surveys done by the USGS-BRD sea otter project. Data are for the entire outer coastline of Adak, but do not include Clam Lagoon. Sea otter counts are summarized for independent adults, mother-pup pairs, total number of animals excluding dependent pups, and total number of animals including dependent pups. Counts are also shown for harbor seals and conunon eiders. Year #Ind. Adults #Mom-pup pairs Total# Adults Total# incl.dep.pups Harbor Seals Eiders 198 : )... : l~~i f :~i J J~f ":I: ~i~"~ "E:~ J~t J /A t :~1, ?..!... ~ j 17! 598! 75! 279! 331! 1998 : ~" -r 434 ' r sos r i6s r 2s 4 : 3

4 Sea Oher Skiff Survey of Adak Island: 1998 Summary" Report t ' 8 12 ~... i 1 ~... r u 6 - L. Q).. E 4oo :I z Sea Otter Skiff Surveys, Adak Island Whole Island, Including Clam Lagoon [ -..._ a.. ~ ~ -~:::: :~~~~:p.-. :~~: 1.* --Harbor Seals l - ->E- Eiders I -~-~-- - ~ I... x _ ~ 2 f:: :-:.-.:-:.-:-::::.:-.:: :-_{:. :-:."":" _-:: A ~ ~ Year of Survey Figure 1 Number of sea otters counted during skiff surveys of Adak Island made between 1995 and Numbers include data from shoreline counts of Clam Lagoon. Counts ofharbor seals and common eiders obtained during the surveys are also provided. 25 ~ Sea Otter Skiff Surveys, Adak Island Whole Island, Excluding Clam Lagoon ' -Q) 15 u L. Q).c E :::::J z 2 ~ \ 1. I 5 ~ \ \ \ - -+-Total #Adults Total# incl.dep.pups.. *.. Harbor Seals - >E- Eiders ol/'v - - -~ _. --= ~:.-: :;-: : :-:...., ' ' j-. ~ -: Year of Survey --l ~ Figure 2 Number of sea otters counted during skiff surveys of Adak Island made in 198 and between 1995 and Data are also shown for harbor seals and eider ducks, based on counts made during the sea otter surveys. 4

5 Sea otter Skiff Survey of Adak Island: 1998 Summary Report Focusing just 11 the 1111hem half of Adak Island, iiom the southem e11d ofkagalaska Straits through to the Bay oflslands (inclusively), a more detailed picture ofthe sea otter population declille can be gained by the addition of data fi:om 1991 and 1993 surveys of this area. Sea otter numbers (excluding dependent pups) along the northem perimeter of Adak have decreased fi:om 1,794 in 1991 to 269 in 1998 (Tabie 3). This represents an exponential rate of decrease of25% yr 1 (i.e. r = -.253, where r is the instantaneous rate of population change; Figure 3). Table 3 Surrunary of sea otter skiff counts made between 1991 and 1998 of the outer coastline of northern Adak Island, from the southern end of Kagalaska Straits tluough to the Bay of Islands (inclusively). Sea otter counts are sununarized for independent adults, mother-pup pairs, total number of animals excluding dependent pups, and total number of animals including dependent pups. The pup to adult ratio is shown for each year, and counts are also provided for harbor seals and eider ducks. Year # Ind. Adults #Mom-pup Total# pairs Adults Total# incl.dep.pups pup :adult ratio Harbor Seals Eiders Rate of Population Decrease North Adak 1., a::xj... en 14 4) :s J ).c E a::xj ::I z 4 y = e- 253 x R 2 = Year (1991 = year ) 7 Figure 3 Number of otters counted along the northern perimeter of Adak Island, from Kagalaska Straits to Bay oflslands (inclusively), between 1991 and Year "" is set as 1991, and the exponential regression of sea otter number vs. year is shown as a smoothed line, with the regression equation and R 2 value indicated above. 5

6 Sea Otiet Skiff Survey of Adak Island: 1998 Summary Report While sea otter numbers have c.ontinued to decrease at a constant rate around the outer coast of Adak through the 199s, the number of otters within Clam Lagoon has changed little dming this same period. Between 1995 and 1998, the Clam Lagoon population has remained essentially constant at approximately 1 otters (Table 4). Table 4 Number of sea otters counted during shore-based surveys of Clam Lagoon at Adak Island. Sea otters are counted from stations around the lagoon using a 5x Questar telescope and binoculars. Counts from 1997 and 1998 were made during late July, while the 1995 and 1996 numbers represent mean values from bi-monthly surveys. Year Adults Pups pp:ad ratio Total# Conclusions and Recommendations The 1998 sea otter survey of Adak Island demonstrates that trends and demographic pattems seen in prior years are continuing. While the number of otters counted in coastal waters around Adak declined by about 25 percent dming the past year, the ratio of dependent pups to adults has remained approximately constant, as has the number of otters in Clam Lagoon. These findings are consistent with our earlier conclusions that the sea otter population decline is not due to. reproductive failure and is not occurring in Clam Lagoon. We believe that the weight of available evidence indicates that increased predation, most likely from killer whales, is responsible for the decline. Because the sea otter population at Adak Island is continuing to decline and has reached an alanningly small size, we recommend that monitoring be continued on a regular basis-every year if possible, and no less often than every several years. Surveys and anecdotal observations fi:om elsewhere in the Aleutian Islands indicate that the declines are not restricted.to Adak, although the current information for these other areas is spottier and less detailed than the record fi:om Adak. Thus we also recommend that additional surveys be done to determine the magnitude and geographical scale of sea otter population declines in western Alaska. Acknowledgments We are grateful to Becky Howard and Craig Hill for their assistance with the 1998 surveys, and to the US Fish and Wildlife Service- Alaska Maritime Wildlife Refuge for logistical support. 6