Effects of the Exxon Valdez Oil Spill on Birds: Comparisons of Pre- and Post-Spill Surveys in Prince William Sound, Alaska ’

Home , Pigeon guillemot, Prince William Sound

The Condor 99:299-3 13 D The Cooper Ornithological Society 1997

EFFECTS OF THE EXXON VALDEZ OIL SPILL ON BIRDS: COMPARISONS OF PRE- AND POST-SPILL SURVEYS IN PRINCE WILLIAM SOUND, ALASKA ’

STEPHENM. MUFZHY AND ROBERT H. DAY ABR, Inc., P.O. Box 80410, Fairbanks, AK 99708, e-mail: [email protected]

JOHN A. WIENS Departmentof Biology, Colorado State University,Fort Collins, CO 80523

KEITH R. PARKER Data Analysis Group, 5100 Chern, Creek Road, Cloverdale, CA 95425

Abstract. We used data from pre- and post-spill surveys to assessthe effects of the Exxon Valdez oil spill on the abundanceand distribution of birds in Prince William Sound, Alaska. We conductedpost-spill surveys during mid-summer (1989-1991) in 10 bays that had been surveyed prior to the spill (1984-1985) and that had experienceddifferent levels of initial oiling from the spill (unoiled to heavily oiled). We evaluated whether there were changesin overall abundanceacross all bays between the pre-spill and post-spill sampling periods, and changesin abundancein unoiled/lightly oiled bays versus moderately/heavily oiled bays that would suggestoiling impacts. Of 12 taxa examined for changesin overall abundance, 7 showed no significant change, 2 (Bald Eagle and Glaucous-winged Gull) increasedin abundance,and 3 (Red-necked Grebe, Pelagic Cormorant, and Pigeon Guille- mot) decreasedin abundanceduring all three post-spill years. Of the 11 taxa examined for differences in use of oiled versus unoiled habitats, 7 showed no significant response, 1 (Black-legged Kittiwake) exhibited a positive responseto oiling, and 3 (Pelagic Cormorant, Black Oystercatcher,and Pigeon Guillemot) exhibited negative responsesto initial oiling. We conclude that the impacts of this oil spill on abundanceand distribution of birds were most evident in 1989, the year of the spill, and were most pronouncedfor Pigeon Guillemots. By 1991, signs of recovery were evident for all taxa that showed initial oiling impacts. Key words: Prince William Sound,Alaska, Exxon Valdez, oiling impacts,oil spill, marine birds, before-after comparisons.

INTRODUCTION this high (e.g., Erikson 1995), there is little When the Exxon Valdez went aground in Prince doubt that many thousandsof birds were killed William Sound (PWS), Alaska, on 24 March as a direct consequence of the spill (Fry 1993, 1989, the ensuing spill of approximately Parrish and Boersma 1995, Wiens 1995), mostly 41,000,OOOL of Alaska North Slope crude oil near large seabird colonies in the Gulf of Alas- became the largest marine oil spill in U. S. his- ka. Perhaps as many as 42,000 birds died in tory. Marine birds, which are among the most PWS (Piatt et al. 1990, Ecological Consulting, conspicuous victims of oil spills at sea, experi- Inc. 1991), with the greatest mortality docu- enced substantial mortality: nearly 30,000 car- mented for sea ducks, cormorants, murres, casses were collected throughout the spill-af- grebes, and murrelets. fected area (PWS, Kenai Peninsula, Kodiak Is- We initiated a researchprogram in June 1989 land archipelago) during the spring and summer to examine the effects of the oil spill on marine of 1989 (Piatt et al. 1990). Simulation models birds. One aspectof this researchevaluated both that used functions relating carcass recovery impacts to and recovery of use of oil-affected rates to total mortality produced a “best esti- habitats by birds in PWS and along the Kenai mate” of total mortality of 375,000-435,000 Peninsula during a 3-year period (1989-1991) birds (Ecological Consulting, Inc., 1991). Al- after the spill (Day et al. 1995, in press; Wiens though not all scientistsagree that mortality was et al. 1996). During that 3-year period, 15 sampling cruises were conducted during different seasons,and analyses of impacts and recovery ’ ’ Received 1 August 1996. Accepted 22 January reported by these studieswere based exclusively 1997. on post-spill data. Another way to gauge the ef-

12991 300 STEPHEN M. MURPHY ET AL. fects of an oil spill on birds is to compare abun- with the post-spill data for that site, thereby al- dances of birds before and after the spill. If mor- lowing a clear assessmentof effects of oiling on tality or displacement from contaminated habi- bird densities. This analysis has been described tats was great, we would expect reduced densi- as a “pre/post pairs” design (Wiens and Parker ties of birds, particularly in the most heavily 1995) and approximates a BACI (before-after/ oiled sites. Here, we compare data collected in control-impact) design (Skalski and McKenzie mid-summer during 3 years following the Exxon 1982, Stewart-Oaten et al. 1986, 1992, Schroeter Vuldez oil spill (1989-1991) with historical data et al. 1993), which was developed for evaluating collected before the oil spill. effects of planned perturbations (e.g., large-scale Studies in PWS conducted in 1972 and 1973 industrial developments). (Dwyer et al. 1976) and during the summers of 1984 and 1985 (Irons et al. 1988) provide the METHODS only systematic pre-spill information on avian abundance and distribution. The data collected STUDY AREA by Dwyer et al. had poor geographic correspon- Prince William Sound (PWS) is a semi-enclosed dence with our study area and were not useful body of salt water located in the northern Gulf for making historical comparisons.The data col- of Alaska (Fig. 1). It is surrounded by the Chu- lected by Irons et al (1988) had complete geo- gach Mountains and is characterized by tide- graphic overlap with our study area and good water glaciers, fjords with steep, forested slopes temporal overlap with surveys we conducted and rocky shorelines, and numerous islands, is- during mid-summer (July-August). We acquired lets, and intertidal rocks. The climate of the re- the original survey data from the U. S. Fish and gion is cool-maritime with frequent and intense Wildlife Service and determined that 320 km of storms; precipitation is highest in fall and winter shoreline in the 10 bays that we were sampling (Isleib and Kessel 1973, Royer et al. 1990). also had been surveyed by Irons et al. (1988). Approximately 16% of the 4,800 km of shore- In this paper, we use mid-summer data from line in PWS was oiled by the Exxon Valdez oil Irons et al. (1988) and from surveys that we con- spill (Neff et al. 1995). Oiling of shorelines was ducted in 1989-1991 to evaluate two hypothe- discontinuous and patchy, and only some of the ses: bays in the general path of the spill were inun- H,l: Densities of birds in the 10 study bays did dated with oil. We selected 10 bays for study not differ between 1984/1985 (pre-spill) and that were typical of western PWS: they were 1989-1991 (post-spill). fjords with rocky shorelines, although they differed in size, shape, exposure to larger water H,2: Changes in densities of birds from pre-spill masses,and other habitat features. Nine of these levels did not differ between oiled and unoiled 10 bays were on or near the Knight Island group bays during 1989-1991. (Fig. 1) and were in the general path of the spill (Galt et al. 1991). The study bays ranged in area The first hypothesis was a simple “before- from 1.5 to 24.3 km*; four were unoiled or light- after” baseline comparison (Wiens and Parker 1995) that evaluated whether there were signif- ly oiled, and six were moderately to heavily icant changes in overall densities of birds in the oiled. Of the 10 bays that we surveyed, 9 were study area. We made no distinction between surveyed by Irons et al. (1988) in 1984 and the sampling sites (bays) that were oiled and those 10th (Galena Bay) was surveyed in 1985; hence, that were unoiled and did not separateoiling ef- the single survey of the 10 study bays is referred fects from natural factors that might have con- to here as 1984/1985. tributed to overall population changes between We used bays as our basic sampling unit be- the two time periods. cause they are discrete areas that can be de- The secondhypothesis addressed a fundamen- scribed quantitatively in terms of initial expo- tally different question, one about impacts sure to oil. Bays were not chosen randomly, but causedby oiling. Among-bay differences in hab- instead were selected to be representative of the itat, natural temporal variation, and interobserv- types of bays found in western PWS and to es- er differences were accounted for by comparing tablish a gradient of oiling conditions (Day et al. Irons et al’s (1988) data from each sampling site 1995). EXXON VALDEZ OIL SPILL AND BIRDS 301

PWS STUDY SITES 1 - Shelter Bay 2 - Hogan Bay 3 - Snug Harbor 4 - Bay of Isles 5 - Drier Bay 6 - Lower Herring Bay 7 - Eshamy Bay 6 - Hening Bay 9 - Northwest Bay IO - Galena Bay

-LEVEL o - None-Light 0 - Moderate-Heavy

FIGURE 1. Locations of study bays in Prince William Sound, Alaska.

DATA COLLECTION ically drove our boats -50 m from shore, al- Our survey proceduresgenerally were the same though the actual distance from the shoreline of- as those used by Irons et al. (1988). In both stud- ten was greater because of the need to avoid ies, nearshore areas were surveyed during mid- intertidal and subtidal rocks. Irons et al. (1988) summer (-20 July to 10 August) by slowly driv- reported driving their boats - 100 m from shore; ing a small boat along the shoreline of a bay and overall, we think that similar survey tracks were identifying and counting all birds seen on the followed by both survey teams. Because both water within 200 m of shore and in the air over teams used experienced observers and traveled this zone. Although including flying birds in slowly during surveys, we do not think differ- transect survey counts may bias density esti- ences in distanceswould affect survey results. mates (Tasker et al. 1981), we did not have a We developed an oiling index to provide a choice as to whether we would include flying continuous measure of the initial exposure of birds becausethey were included in the pre-spill each bay to oil (Day et al. 1995). Data used to data set. We do not, however, consider inclusion calculate the oiling index had been mapped in of flying birds to have been a major source of the field by Shoreline Cleanup Assessment bias in this instance because (1) we were not Teams in 1989, and thus representeda mutually conducting our surveys at sea, where this prob- agreed upon (by government and Exxon scien- lem is most acute, (2) accurate population esti- tists) and consistent system for measuring the mates were not a goal of this researchprogram, amount of oil on the shorelines.Given the fjord- and (3) we accounted for natural variability by like structure of the bays and substantial tidal using replicate surveys. We also carefully ranges that occur in PWS, we believe that these searchedfor and counted birds in the intertidal quantitative measures of oil on the shorelines and supratidal (5 100 m from high-tide line) provided a reasonable indication of the amount zones; these zones appear to have been surveyed of oil to which avian habitats in a bay initially thoroughly by Irons et al. (1988) only for Bald were exposed. Oiling on shorelines was cate- Eagles and Black Oystercatchers(see Appendix gorized at five levels, ranging from “no oil” to 1 for scientific names of bird species). We typ- “heavy oil” (Neff et al. 1995). For each bay, we 302 STEPHEN M. MURPHY ET AL.

NORTHWEST

SNUGHARBOR

HOGAN

HERRING

SHELTER

BAY OF ISLES

ESHAMY I DRIER m

LOWER HERRING UNOILED AND : MODERATELY AND - LIGHTLYOILED , HEAVILY OILED GALENA I I ” I ” I ” ” I ” I ’ ” 0 50 100 150 200 250 300 OILING INDEX FIGURE 2. Oiling index values for the study bays in Prince William Sound, Alaska (modified from Day et al. 1995). Oiling indices reflect the initial oiling exposurefor each bay. Dashed line separatesunoiled and lightly oiled bays fro& moderately and heavily oiledIbay& calculated an oiling index value (see Day et al. sary to make the relationships additive by log- 1995) that could range between 0 (100% no oil) transforming the density estimates before con- and 400 (100% heavy oil). Actual index values ducting the statistical tests (Stewart-Oaten et al. calculated for the bays surveyed in PWS ranged 1986). We added a constant of 0.167 to all den- between 0 and 288.2 (Fig. 2). For analyses, we sity estimates to avoid calculating a log of zero combined the four unoiled and lightly oiled bays (Mosteller and Tukey 1977). (index values < 100) to form a group of “un- For each taxon, we calculated the amount of oiled” bays and the six moderately and heavily change (A) in densities that occurred in each bay oiled bays (index values 2 200) to form a group (i) during each post-spill year (after) relative to of “oiled” bays. the pre-spill baseline (before):

DATA ANALYSIS AND INTERPRETATION A, = In density (after) - In density (before). Although the length of shoreline surveyed in (1) some bays differed slightly between 19840985 These log-transformed changes in densities then and 1989-1991, we standardized all counts of were used to calculate the percent change be- birds from nearshore surveys to linear densities tween pre-spill and post-spill years across all (birds/km of shoreline surveyed) to facilitate bays and the percent difference in densities be- comparisons. If Irons et al. (1988) recorded tween oiled and unoiled bays in each post-spill < cu. 20 individuals of a taxon in the 10 study year relative to the pre-spill baseline. bays, that taxon was omitted from our analyses. The overall change in densities between pre- Consequently, data that we collected on shore- spill and post-spill years across all bays for a birds other than oystercatchers, wading birds, given taxon was calculated by taking a mean of and corvids could not be compared with histor- the changes for all 10 bays: ical data. Irons et al. (1988) visited each bay only once. overall change (& = 2 where IZ = 10. We conducted multiple (3-5) visits to a bay on n ’ each cruise, so we averaged density estimatesto (2) calculate mean densities for each year. We expected the effects of oiling to be multiplicative The percent change across all bays and the per- (i.e., effects would be proportional to population cent difference between oiled and unoiled bays levels in the various bays) and not additive (i.e., then were calculated as: effects would be constant regardless of popula- percent change = (e” - 1) X 100. (3) tion levels in the various bays). Because statistical tests assume additivity, it thus was neces- We used a one-sample t-test (Zar 1984) for EXXON VALLIEZ OIL SPILL AND BIRDS 303

COMPARISON BEFORE-AFTER PIWPOST PAIRS

Bay B I A 1 I-A. I

I I 4 1.-Ai--+ ’ 3 -AC‘ ’

FIGURE 3. Approachesused to examinethe effectsof the Z&on Vuldezoil spill on birdsusing historical data (Irons et al. 1988) and data collected during 3 years after the spill (1989-1991) in 10 bays in Prince William Sound, Alaska. The before-after comparisons(H,,l) evaluated whether the mean differences (D) between pre- spill (B) bird densities and post-spill (A) densities differed from zero. The pre/post pairs comparisons(H,2) evaluated whether the mean difference between pre-spill (B) bird densities and post-spill (A) densities in oiled (0) bays differed from mean differences in unoiled bays (u). each taxon to evaluate whether there was a sig- The combination of high variance and small nificant change in density over all bays (I-&,1: A sample size could reduce power to detect an oil- = 0; Fig. 3). We considered the overall abun- ing effect and increase the likelihood of making dance to have decreased (increased) across all Type II errors (i.e., failing to detect a spill im- bays if the mean change in densities was signif- pact that actually did occur). To counter this icantly lower (higher) than zero. problem, we established a priori that statistical The difference in densities between oiled and significance for all comparisonswould be tested unoiled bays for a given taxon was calculated as with a two-tailed a = 0.20. Thus, we sought to the difference between the mean difference for increase the likelihood of detecting oiling effects oiled (0) bays and the mean difference for un- in these statistical analyses, realizing that by do- oiled (u) bays: ing so we also increased the probability of de- difference in density = &, - A,,, tecting some oiling effects that did not actually where n = 6 and 4, respectively. (4) occur (Type I errors) (Shrader-Frechetteand Mc- Coy 1993, Mapstone 1995, Wiens and Parker The percent difference between oiled and un- 1995). To aid in the interpretation of test results, oiled bays then was calculated as: we also tallied results at a = 0.05 and 0.10. percent difference = (ecn,t- &) X 100. (5) Recovery was evaluated for all taxa that showed initial (1989) negative impacts by ex- We used a two-sample t-test (Zar 1984) for each taxon to evaluate whether changesin mean amining both trends in densities and statisticsin densities differed_ between oiled and unoiled 1990 and 1991. That is, we evaluated the percent bays (H,2: A, - A, = 0; Fig. 3). We conducted change in densities between pre- and post-spill tests only on those taxa that occurred in at least years, the results of statistical comparisons, and two bays in each of the two oiling categories the power analyses to determine if a detected during the post-spill years. We considereda neg- impact had diminished in subsequentyears. ative (positive) oiling impact to have occurred if Finally, we conducted power analyses to as- the mean change in densities between one or sesses the probability of Type II errors and, more of the post-spill years and the pre-spill hence, our ability to detect impacts. Power in- year was significantly lower (higher) in the oiled creaseswith increasing (Y level and sample size bays than in the unoiled bays. and with decreasing variance (Osenberg et al. 304 STEPHEN M. MURPHY ET AL.

TABLE 1. Pre-spill counts, percent changes, results of t-tests, and overall responsesof birds based on com- parisonsof densitiesduring mid-summer before (1984/198.5;Irons et al. 1988) and after (1989-1991) the Exxon Vuldez oil spill in 10 bays in Prince William Sound, Alaska. A negative (positive) percent change indicatesthat densitiesdecreased (increased) in the post-spill year. Percent change was calculatedfrom log-transformeddata. Significance codes for t-tests: * = P 5 0.20, ** = P 5 0.10, *** = P % 0.05.

Percent change Pre-spill OVeIdl Speciedtaxon cO”“t 1989 1990 1991 response Red-necked Grebe 19 - 17.7** -17.7** - 17.4** Decrease Pelagic Cormorant 28 -32.0*** -22.0*** -l&4*** Decrease Harlequin Duck 103 -13.5 -6.4 -11.9 None Common Merganser 101 9.5 -7.6 13.6 None Bald Eagle 81 -3.9 55.8*** 50.9*** Increase Black Oystercatcher 24 -7.3 -9.4 5.3 None Bonaparte’s Gull 131 -2.4 11.6 -4.3 None Mew Gull 825 -8.1 20.8 -33.2 None Glaucous-wingedGull 579 75.9* 248.2*** 197.4*** Increase Black-legged Kittiwake 920 52.4 -4.4 38.8 None Pigeon Guillemot 179 -55.7*** -56.5*** -37.5*** Decrease “Total murrelets” 798 1.7 15.3 69.1** None

1994). It also is a function of the amount of ate changes in overall abundance between pre- change (A) to be detected, becauseit is easier to and post-spill years. During a single survey of detect a large change than a small one. Our in- the 10 study bays in 19840985, Irons et al. terest in conducting power analyses was in eval- (1988) recorded 3,788 individuals of these 12 uating the strength of, and therefore our confi- taxa, with numbers for individual taxa ranging dence in, the statisticalresults. To facilitate com- from 920 for Black-legged Kittiwakes to 19 for paring our tests among different taxa, we con- Red-necked Grebes (Table 1). During our sur- ducted the power analyses using a standardized veys of these same bays, we recorded averages amount of change. Because of the large amount of 3,756, 5,175, and 3,592 individuals of these of oil spilled, there were expectations of dra- 12 taxa in 1989, 1990, and 1991, respectively. matic impacts on many taxa (e.g., Piatt et al. Because we surveyed approximately 11% more 1990, Fry 1993); therefore, the expected effect shoreline than did Irons et al. (1988), we ad- size would be large. Moreover, the degree of justed the abundance totals to reflect that differ- natural variability in marine bird abundance in ence. The adjusted totals indicate that numbers these environments may be great (Zwiefelhofer of birds were lower in 1989 and 1991 (-12 and and Forsell 1989). Therefore, we computed -16%, respectively) and were higher in 1990 power for a hypothetical change of either a (+22%) than in 1984/1985. 100% increase or a 50% decreasein abundance Percent changes in overall abundance in the (i.e., a two-fold increase or decrease). 10 study bays between 1984/1985 and the 3 Power was calculated by the procedures de- post-spill years for individual taxa ranged from scribed in Winer et al. (1991). Sample standard a 57% decrease by Pigeon Guillemots in 1990 deviations were used in all before-after analy- to a 248% increase by Glaucous-winged Gulls ses, and pooled variance estimates were used in in that same year (Table 1). In 1989, 8 of 12 the pre/post pairs analyses. Power was computed (67%) taxa had decreasedin overall abundance with cx = 0.20. Results were classified as high from the pre-spill baseline, whereas4 (33%) had power (2 90% chance of detecting a two-fold increased. In 1990,7 of these 12 taxa (58%) had change), moderate power (60-89% chance), and decreased in overall abundance from the pre- low power (< 60% chance). spill baseline and 5 (42%) had increased. By 1991, six (50%) taxa had decreased and six RESULTS (50%) had increased in abundance from the pre- OVERALL CHANGES IN ABUNDANCE spill baseline. General patterns. Twelve taxa occurred in suf- Before-afier comparisons. In the 198411985 ficient abundance in 198411985 for us to evalu- vs. 1989-1991 comparisons, 6 (50%) of the 12 EXXON VALDEZ OIL SPILL AND BIRDS 305

TABLE 2. Percent differences, results of r-tests, and overall responsesof birds to oiling based on relative changesin densitiesduring mid-summer in oiled (n = 6) and unoiled (n = 4) bays after (1989-1991) the Exxon Vuldezoil spill in Prince William Sound, Alaska. A negative (positive) percent difference indicatesthat densities in oiled bays were lower (higher) than in unoiled bays during a particularpost-spill year. Percent difference was calculatedfrom log-transformeddata. Significance codes for z-tests:* = p 5 0.20, ** = P 5 0.10, *** = P 5 0.05.

Percent difference

Specieskaon 1989 1990 1991 Overall response Red-necked Grebe -7.6 -7.6 -8.4 None Pelagic Cormorant naa -26.7* na Negative Harlequin Duck 13.9 4.6 1.7 None Common Merganser -49.2 36.8 -7.4 None Bald Eagle -3.9 -8.8 27.2 None Black Oystercatcher -27.5* -13.8 -11.5 Negative Mew Gull 21.2 122.8 63.9 None Glaucous-wingedGull 52.8 13.1 193.4* None Black-legged Kittiwake 215.0* 305.7* 132.2 Positive Pigeon Guillemot -51.6** -51.2* -21.8 Negative “Total murrelets” -47.9 -30.1 11.7 None

1 na = no analysis

taxa examined showed no significant changesin oiled bays relative to unoiled bays, whereas 6 abundance during any of the 3 post-spill years (60%) had increased. (Table 1). During all 3 post-spill years, Red- Pre/post pairs comparisons.In 1989, 3 (30%) necked Grebes, Pelagic Cormorants, and Pigeon of 10 taxa examined decreased significantly in Guillemots decreased significantly, whereas oiled versus unoiled bays relative to pre-spill Glaucous-winged Gulls increased significantly. baseline, indicating oiling effects (Table 2). Two additional taxa (Bald Eagle and “total mur- Black Oystercatchersand Pigeon Guillemots ex- relets”) did not differ significantly in 1989 but hibited negative responses to oiling, whereas did in 1990 and/or 1991 (Table 1). We used “to- Black-legged Kittiwakes exhibited a positive re- tal murrelets” for these comparisons because sponse. In 1990, 3 (27%) of 11 taxa examined Irons et al. (1988) recorded44% of the murrelets exhibited significant oiling effects, with Pelagic in our 10 study bays as “unidentified murre- Cormorants and Pigeon Guillemots responding lets.” Our observations during 1989-1991 were negatively and Black-legged Kittiwakes re- of > 99% Marbled Murrelets and < 1% Kitt- sponding positively. In 1991, only 1 (10%) of litz’s Murrelets. Bald Eagles were classified as 10 taxa (Glaucous-winged Gull) exhibited a sig- exhibiting an overall increase becausethey were nificant oiling effect, and it respondedpositively significantly more abundant in both 1990 and (Table 2). Thus, significant positive responsesto 1991. oiling were detected in all 3 post-spill years, whereas negative responseswere found only in OILING EFFECTS 1989 and 1990. Three species responded nega- General patterns. Differences in densities be- tively to oiling in at least 1 year, but only Pigeon tween oiled and unoiled bays relative to the pre- Guillemots showed significant negative re- spill baseline ranged from a 52% decrease by sponsesin more than 1 year. Pigeon Guillemots in 1989 to a 306% increase by Black-legged Kittiwakes in 1990 (Table 2). NATURAL VARIATION AND STATISTICAL POWER In 1989, 6 (60%) of 10 taxa examined had decreased in abundance in oiled bays relative to Natural variation. One of the most challenging unoiled bays, whereas 4 (40%) of 10 taxa had aspectsof using historical data for impact anal- increased (Table 2). In 1990, 6 (55%) of 11 taxa yses is sorting out the effects of a perturbation decreasedin oiled bays relative to unoiled bays, from natural temporal variability that occurs in- whereas 5 (45%) taxa increased. In 1991, 4 dependently within the system (Osenberg et al. (40%) of 10 taxa analyzed had decreased in 1994, Wiens, in press). Natural variability can 306 STEPHEN M. MURPHY ET AL.

TABLE 3. Power analyses for before-after comparisons (H,l) of densities of birds during mid-summer 1984/1985and 1989-1991 and for pre/postpairs comparisons (H,2) of densitiesduring mid-summer in oiled (n = 6) and unoiled(n = 4) baysafter (1989-1991) the ExxonValdez oil spill in Prince William Sound, Alaska. Power was calculatedfor a hypothetical two-fold decreaseor increasein density between years for the before- after comparisonand for a hypothetical two-fold difference in density between oiled and unoiled bays for the pre/post pairs comparisons.Pre-spill (19840985) data are from Irons et al. (1988).

Before-after compansons Prdpost pairs comparisons

Speciealtaxon 1989 1990 1991 1989 1990 1991 Red-neckedGrebe 1.oo 1.00 1.oo 1.00 1.oo 1.oo PelagicCormorant 1.oo 1.00 1.oo 1.oo HarlequinDuck 1.00 0.95 0.85 0% 0.55 0:5”0 CommonMerganser 0.85 0.85 0.95 0.30 0.45 0.55 Bald Eagle 1.oo 1.00 1.oo 0.70 0.85 0.80 Black Oystercatcher 1.oo 1.00 1.oo 0.95 0.90 1.oo Bonanarte’s Gull 1.00 1.oo 1.00 Mew&Gull 0.55 0.70 0.60 0:3”0 o::o 0:30 Glaucous-wingedGull 0.80 0.95 0.90 0.40 0.55 0.45 Black-leaped Kittiwake 0.70 0.60 0.70 0.40 0.35 0.35 Pigeon C%illemot 1.oo 0.95 1.oo 0.75 0.65 0.70 “Total murrelets” 0.40 0.70 0.70 0.30 0.40 0.50

1 na = no analysis.

be measured, however, if there is an adequate significant effects on overall population levels time series of data prior to the perturbation. from environmental perturbations is confounded Unfortunately, such a multi-year data set of by this high variability. These results also indi- bird abundance collected prior to the spill does cate that the abundance of birds in bays can fluc- not exist for PWS. Because of the importance of tuate substantially between years and that an this issue, however, we examined the natural an- evaluation of overall changes in abundance be- nual variability in abundance (densities) of sea- tween years is likely to produce statistically sig- birds around Kodiak Island (located southwest nificant results as frequently as 20% of the time, of PWS in the Gulf of Alaska), using data col- even in the absence of a perturbation. Undoubt- lected during fall and winter in 1979-1983 edly, there are differences between the variabil- (Zwiefelhofer and Forsell 1989). For both fall ity measured during fall and winter on Kodiak and winter data sets, we calculated the percent- Island and that which occurs in PWS during age of annual change in abundance over succes- summer. The Kodiak data are useful nonetheless sive years for 19 taxa for which a complete because they involve the same general suite of 5-year data series was available (primarily from taxa in habitats that are fairly similar to those in Uyak and Uganik bays; the procedure for cal- PWS. culating percent change followed that described Statistical power. Power analyses for the be- in the Methods section for before-after compar- fore-after analyses indicated that our ability to isons in PWS). The degree of annual variability detect significant changes in densities between differed among taxa and between seasons,but 1984/1985 and the 3 post-spill years was mod- annual changes in both seasons combined ex- erate (60-89%) to high (2 90%) for most taxa: ceeded a two-fold increase or decreasein overall mean power for the 36 comparisons was 88% abundance in -21% of the comparisons (n = (Table 3). Overall, power was high for 22 of the 152 for all taxa, seasons,and years). This thresh- 36 comparisons, moderate for 12, and low (< old of a two-fold change in abundance was ex- 60%) only for 2 comparisons (Mew Gulls and ceeded in at least one annual comparison by 9 total murrelets in 19840985 vs. 1989). Mew (47%) of 19 taxa in fall and by 8 (42%) of 19 Gulls showed nonsignificant declines in 2 of 3 taxa in winter. post-spill years, but there was no consistentpat- From these results, we conclude that natural tern from year to year that would suggest an annual variability often is high for marine birds impact. Total murrelets increased significantly in during fall and winter in southcoastal Alaska. 1991 but had nonsignificant increases (with low Consequently, one’s ability to detect statistically power) in 1989 and 1990. EXXON VALDEZ OIL SPILL AND BIRDS 301

TABLE 4. Responses of marine birds to oiling based on pre/post pairs comparisons, power analyses, and general patterns of abundance in 10 bays in Prince William Sound,Alaska, after the ExxonValdez oil spill, mid-summer, 1989-1991.

Response to oiling TZXOll

No significant response and moderate-high power Red-necked Grebe HarlequinDuck Bald Eagle No significantresponse, but low powerand negativetrends” CommonMerganser “total murrelets” No significantresponse, but low powerand positivetrend+ Mew Gull Glaucous-wingedGull Significantnegative response PelagicCormorant Black Oystercatcher PigeonGuillemot Significant positive response Bl‘&k-legged Kittiwake

ANegative trend defined as Z - 10% relative difference between oded and unoiled bays (see Table 2) h Positive trend defined as > 10% relative difference between oiled and unoded bays (see Table 2).

Power analyses for the pre/post pairs compar- oiling, but power was low and relative densities isons indicated that our ability to detect signifi- in oiled bays were > 10% different from those cant changes in densities between oiled and un- in unoiled bays. Densities in oiled bays were oiled bays was low to moderate for most taxa: higher than in the unoiled bays relative to the mean power for 31 comparisonswas 61% (Table pre-spill baseline for Mew Gulls, so this species 3). Power was high for 7 (23%) of 31 compar- was classified as “no significant response but isons, moderate for 7 (23%) comparisons, and low power and positive trends.” Glaucous- low for 17 (55%) comparisons. Power was low winged Gulls also were placed in this response in at least 1 year for 6 (55%) of the 11 taxa category because they had low power in all 3 analyzed. Four of these six taxa showed positive years and nonsignificant positive trends in 1989 responsesto oiling, whereastwo (Common Mer- and 1990. Although a significant positive re- ganser and total murrelets) appearedto have had sponse was detected in 1991, the available evi- at least initial negative responsesto oiling. dence did not justify classifying this species as positively impacted. Black-legged Kittiwakes, CLASSIFICATION OF OILING RESPONSES on the other hand, did show significant positive We classified the responsesof each taxon to oil- responsesto oiling in 1989 and 1990 and were ing based on (in decreasingorder of importance) the only species classified as “significant posi- the results of the pre/post pairs analyses (includ- tive response” (Table 4). ing the temporal pattern of significant results), Densities of Common Mergansers and total the power analyses, and the general patterns of murrelets were lower in oiled than in unoiled abundance in oiled and unoiled bays. Based on bays relative to the pre-spill baseline; thus, these these criteria, we concluded that the taxa we ex- two taxa were classified as “no significant re- amined fell into five fairly distinct responsecat- sponsebut low power and negative trends” (Ta- egories that ranged from no significant response ble 4). Pelagic Cormorants, Black Oystercatch- and moderate-high power to significant negative ers, and Pigeon Guillemots respondednegatively response (Table 4). to oiling during at least 1 of 3 post-spill years For 7 (64%) of 11 taxa, no significant overall and were classified as “significant negative re- effects could be attributed to oiling. For 4 of sponse.” Pelagic Cormorants showed a negative these 7 taxa, however, statistical power was low response only in 1990 but were not present in and there was > 10% relative difference be- enough bays in 1989 and 1991 to conduct anal- tween oiled and unoiled bays (Tables 2 and 4). yses. Consequently, there were insufficient data Thus, only Red-necked Grebes, Harlequin to evaluate whether recovery was occurring. Ducks, and Bald Eagles were categorized as “no Black Oystercatchers and Pigeon Guillemots significant responseand moderate-high power.” were negatively impacted in 1989 (and 1990 for Common Mergansers, Mew Gulls, and total Pigeon Guillemots) but showed no significant mm-reletsalso showed no significant responseto negative response in 1991. We suspect, there- 308 STEPHEN M. MURPHY ET AL. fore, that recovery was underway by that year, sonal context, however. Red-necked Grebes are although the relative difference between oiled present in PWS primarily during winter and are and unoiled bays for both of these species still uncommon visitors during summer (Isleib and was > 10% in 1991 (Table 2). Kessel 1973). Indeed, Irons et al. (1988) saw It is noteworthy that all seven pre/post pairs only 19 Red-necked Grebes in their pre-spill comparisons that were significant had P values surveys of the 10 study bays (Table l), and 12 > 0.075 and 5 0.20 (Table 2). Thus, with the of those grebes occurred in 1 bay. Although we possible exception of the 1989 pre/post pairs recorded only one Red-necked Grebe during our comparison for Pigeon Guillemots (P = 0.075), 1989-1991 mid-summer surveys, we question if we had partitioned (Yin any way or used more the biological significance of this apparent de- restrictive decision-making criteria (e.g., (Y = cline because this species generally is uncom- 0.05), none of the pre/post pairs comparisons mon in PWS during summer and our statistical would have been significant. Our use of a large evaluations of annual changesare based on scant IX, however, representsan emerging trend in im- data. Pelagic Cormorants also are much more pact assessmentsthat considers Type II errors common in PWS during winter (Isleib and Kes- (i.e., failing to detect an impact that did occur) se1 1973), and Irons et al. (1988) counted only to be equally or more problematic than Type I 28 in the 10 study bays during summer errors (i.e., falsely identifying an impact that did 1984/1985. Pigeon Guillemots, on the other not occur) (Eberhardt and Thomas 1991, Shrad- hand, are more common in PWS during summer er-Frechette and McCoy 1993, Mapstone 1995). (Isleib and Kessel 1973), so both the pre-spill and post-spill data sets were adequateto support DISCUSSION the conclusion of significant and persistentover- Oil spills at sea can affect marine birds along all declines in abundance during each of the 3 three primary pathways, namely, by altering post-spill years. Pigeon Guillemots, however, population size and structure, reproduction, and were thought to be declining in PWS prior to habitat occupancy and use (Wiens 1995). The the spill (Oakley and Kuletz 1993). The fact that analyses presented here addressedeffects of the the magnitude of the difference in Pigeon Guil- oil spill on overall bird abundance and distri- lemot abundance decreased in each successive bution, and therefore provide insight into effects post-spill year, however, suggeststhat there was on the first and third of these pathways. The be- an initial decline in overall abundance immedi- fore-after analyses (HJ) indicated that half of ately after the spill but that numbers of birds the 12 taxa that we examined exhibited no sig- were increasing l-2 years later. nificant changes in overall abundance during In the pre/post pairs analysis (H,2), the con- mid-summer between 1984/1985 and 1989- founding effects of natural variation were re- 1991. Fewer taxa showed negative trends in duced because the historical data were used to abundance compared to the pre-spill baseline estimate the differences between oiled and un- during each successive year after the spill (8 oiled bays that would exist in the absenceof the taxa declined in 1989,7 in 1990, and 6 in 1991), perturbation (Stewart-Oaten et al. 1992). Our fi- suggestingthat declines in abundance were most nal classification of overall impacts for the 11 pronounced in the year of the spill and that re- taxa examined indicated that 7 taxa showed no covery was occurring in 1990 and 1991. Over- significant response to oiling, 3 (Pelagic Cor- all, we documented few dramatic declines or in- morant, Black Oystercatcher and Pigeon Guil- creases in the abundance of most bird species lemot) showed significant negative responsesto during the three summers after the Exxon Vuldez oiling, and 1 (Black-legged Kittiwake) had a sig- oil spill. In addition, power analyses indicated nificant positive response to oiling. Statistical that we should have good confidence in these power was a greater consideration in these anal- results. yses than in the before-after comparisons, be- Species that were significantly less abundant cause there were fewer degreesof freedom after during all 3 post-spill years than they were prior partitioning the data into oiled and unoiled cat- to the spill included Red-necked Grebes, Pelagic egories. The consequence of low power would Cormorants, and Pigeon Guillemots. The de- be to increase the likelihood of failing to detect creases in abundances for Red-necked Grebes significant differences (Type II errors). Four of and Pelagic Cormorants must be placed in a sea- the 7 taxa that showed no significant response EXXON VALDEZ OIL SPILL AND BIRDS 309 to oiling had low power or trends that led us to post-spill data set that we used but did not use reconsider those statistical results (Table 3). Of historical data. Instead, Day et al. assessedoiling these questionable taxa, Common Mergansers impacts with two analytical techniquesdescribed and total mm-relets exhibited negative trends in by Wiens and Parker (1995) as “single-time gra- 1989, suggesting that these taxa initially re- dient analysis” and “multiple-time trend-by- sponded negatively to oiling. Thus, an inclusive time analysis.” Although they assessedimpacts accounting of the taxa that were affected nega- and recovery based on six sampling periods tively by the oil spill includes Pelagic Cormo- spacedthroughout the entire year, only analyses rants, Common Mergansers, Black Oystercatch- of nearshoresurvey data from mid-summer sam- ers, Pigeon Guillemots, and total murrelets (Ta- pling periods in 1989-1991 were directly com- ble 4). Most of these impacts were evident only parable to our analyses. During mid-summer, in 1989, however, and signs of recovery were Day et al. recorded negative oiling impacts for evident for all taxa for which we had data in five species: Pelagic Cormorants, Harlequin subsequentyears. Pigeon Guillemots showedthe Ducks, Common Mergansers, Black Oyster- most persistent negative effects, with significant catchers,and Mew Gulls (Table 5). decreasesin oiled bays in 1989 and 1990 and a Thus, in addition to the three significant and nonsignificant but substantial decrease(22%) in two probable taxa that we identified as nega- 1991. Considering that the decreases in 1989 tively impacted during mid-summer, four other and 1990 were significant and > 50%, the 1991 taxa (“loons,” Harlequin Ducks, and “scoters” results suggestthat recovery was beginning. [Klosiewski and Laing 19941; Harlequin Ducks Black-legged Kittiwakes, Mew Gulls, and and Mew Gulls [Day et al. 1995, in press]) were Glaucous-winged Gulls appearedto be attracted identified as negatively impacted by the two oth- to spill cleanup activities. Their apparent posi- er studiesevaluating spill effects on birds (Table tive responseto oiling probably was a secondary 5). When these studies are viewed collectively, consequence of the spill, as some of the most they suggestthat up to 10 (38%) of the 26 taxa heavily oiled bays provided anchorage for the studied were negatively impacted by the spill in hundreds of vessels and their crews that partic- summer. By mid-summer 1991, however, there ipated in the shoreline cleanup. was evidence of recovery from at least one study The results of the pre/post pairs analyses for all of these taxa except “loons” and “sco- (I-&,2) can be compared directly with the results ters.” However, neither of these taxa was abun- of two other analyses that addressedthe same dant in our study bays during summer either be- general question of oiling impacts to birds in fore or after the spill and neither was evaluated PWS. Klosiewski and Laing (1994) conducted in this study or in mid-summer by Day et al. nearshoresurveys in PWS during summer 1989- (Table 5). 1991 in a subset of the areas that had been sur- Based on year-round sampling, Day et al. veyed by Irons et al. (1988) in 1984/1985. Al- (1995) found evidence of initial impacts to 19 though their study had only partial geographic (54%) of the 35 species evaluated in 1989, overlap with our study area, their survey and whereas the percentage of impacted species de- analytical techniques were similar to ours and clined to 40% in 1990 and 10% in 1991. In com- also can be described as pre/post pairs analyses parison, this study showed that 20-40% of the (Wiens and Parker 1995). Based on comparisons specieswere impacted in mid-summer 1989 and with Irons et al. (1988), Klosiewski and Laing that no species showed significant negative im- (1994) found that six taxa (“loons,” Harlequin pacts by 1991, although several species still Duck, “scoters,” Black Oystercatcher, Mew showed some (nonsignificant) evidence of per- Gull, and Arctic Tern) exhibited negative oiling sistent effects. Thus, the overall results of these impacts (Table 5). The number of impacted taxa two studies were quite similar (Table 5). ranged from zero in 1989 to five in 1991; how- Although there was broad concurrence among ever, if the same two-tailed (Y that was used in all three studies in the classification of taxa that this study (cx = 0.20) is applied to their results, were not impacted and in the percentageof taxa those numbers change to one taxon (Black Oys- that were impacted, there was a lack of concur- tercatcher) in 1989 and six taxa (including rence in the classification of negatively impacted murres) in 1991. taxa. This lack of concurrence may be related to Day et al. (1995, in press) also used the same how the three studies classified oiled areas in 310 STEPHEN M. MURPHY ET AL.

TABLE 5. Comparison of the results of three studies that assessedimpacts of the Exxon Vuldez oil spill on marine-oriented birds during mid-summer in Prince William Sound, Alaska, 1989-l 99 1. Negatively impacted taxa that showed evidence of recovery are depicted in bold-faced type.

Historical comparisons

Soeciesltaxon This study USFWS study” Habitat-use study” No Evidence of Impact Red-necked Grebe No impact na’ na “Goldeneyes” na No impact na Bald Eagle No impact No impact No impact Wandering Tattler na na No impact Spotted Sandpiper na na No impact Red-necked Phalarope na No impact No impact “Shorebirds” na No impact na Glaucous-wingedGull No impact?“ No impact No impact “Murres” na No impact na “Total murrelets”e No impact? No impact No impact Homed Puffin na No impact na Belted Kingfisher na na No impact Steller’s Jay na na No impact Black-billed Magpie na na No impact Northwestern Crow na na No impact Common Raven na na No impact Evidence of Impact “Loons” na Negative impact Pelagic Cormorantf Negative impact No impact Eegative impact Harlequin Duck No impact Negative impact Negative impact “Scoters” Negative impact Common Merganser8 f;“, impact? No impact Egative impact Black Oystercatcher Negative impact Negative impact Negative impact Mew Gull No impact? Negative impact Negative impact Black-legged Kittiwake Positive impact No impact Positive impact Arctic Tern na Negative impact No impact Pigeon Guillemot Negative impact No impact Positive impact

d U.S. Rsb and Wddlife Service comparisons with 1984/1985 baseline (Klosiewski and Laing 1994). DDay et al. (1995, in press): mcludes nearshore data from mid-summer (July/August) cruises in 1989-1991 (na = no analysis. * ? = taxon that had nonsignificant differences but low power (see Table 3). *Analyzed as Marbled Murrelets by Day et al. (1995, in press). ‘Analyzed as “cormorants” by Klosiewski and Laing (1994). c Analyzed as “mergansers” by Klosiewski and Laing (1994).

Prince William Sound and how those classifi- shoreline that was in the general path of the spill cations affected data analyses. Day et al. (1995) to be oiled, whether or not field surveys showed used an oiling index based on the amount of oil that it actually was oiled. Although the analyti- that initially contaminated the shorelines of the cal procedures used in this study and by Klo- study bays to describe the relative amount of oil siewski and Laing were similar, differences in in each bay as a quantitative, continuous vari- impact classifications for individual taxa proba- able. This strategy was highly accurate and fa- bly were related to differences between the stud- cilitated the use of regression statistics in hy- ies in the classification of shoreline oiling. The pothesis testing, which were more powerful for distribution of oil on shorelines in the general detecting impacts than were the proceduresused path of the spill was highly variable (to the ex- in both of the other studies. In this study, we tent that some entire bays were unoiled), and a used the same oiling index as Day et al., but then quantitatively based measure such as the oiling categorized each bay as oiled or unoiled (Fig. 2) index we used would seem to provide a more to accommodate the BACI-like analytical de- accurate assay of oiling conditions. Indeed, Ba- sign. Klosiewski and Laing (1994) also catego- ker et al. (1990) considered this level of reso- rized areas as oiled and unoiled for use in a lution to be critical, stating that “consideration BACI-like analysis, but they considered any of the overall, long-term impact of a particular EXXON VALDEZ OIL SPILL AND BIRDS 311 spill must take into account the relative propor- noted that most communities of organisms in tions of oiled and unoiled habitats in the area of temperate waters will recover from massive oil interest.” spills within 2 years, with some effects detect- able up to 10 years after the spill. CONCLUSIONS Our researchindicates that, despite substantial Our evaluation of pre- and post-spill abundances mortality in the aftermath of the spill and sig- of birds in 10 bays in Prince William Sound in- nificant impacts to the distribution of up to 40% dicated that 3 of 11 taxa evaluated had declined of the taxa in PWS during the first summer fol- significantly from numbers recorded 4-5 years lowing the spill, the abundance of birds during before the spill. Although this analysis could not mid-summer generally was not significantly af- factor out natural sourcesof variation, there was fected and redistribution of birds back into no evidence of large-scale, post-spill changesin heavily oiled bays was occurring for nearly all overall abundance of most taxa. Our inability to taxa by the second summer after the spill. The reject the null hypothesisof no large-scale, post- redistribution of birds into the heavily oiled bays spill changes in overall abundance of most taxa indicates that these habitats are at least in the in our 10 study bays (even using a broad statis- process of recovering and that the birds them- tical definition of impact) suggeststhat most bird selves find the habitat suitable for occupancy. populations in PWS during mid-summer were Becausehabitat recovery is a prerequisite for re- not impacted catastrophically. The Pigeon Guil- covery of population or reproductive measures lemot was the one species that was both com- (Morrison 1986, Wiens 1995), the prognosis for mon during summer in PWS and showed per- recovery in these other dimensions appears to sistent declines in overall abundance relative to be good. the pre-spill baseline. Our analysis of oiling impacts indicated that ACKNOWLEDGMENTS the effects of the spill on bird abundancein oiled We thank the U.S. Fish and Wildlife Service, and es- bays during mid-summer were most pronounced pecially D. B. Irons, for granting permission to use in 1989 and that, by 1991, most marine bird spe- data collected from PWS prior to the spill. We also cies in PWS were using oiled bays at levels that thank D. C. Zwiefelhofer, U.S. Fish and Wildlife Ser- vice, for allowing us to use data from Kodiak Island. were indistinguishable from levels of use in un- We gratefully acknowledge the personnel of ABR, oiled bays. Once again, Pigeon Guillemots Inc.,who worked on this project:L. N. Smith, D. A. showed the greatest negative impacts and the Flint, L. C. Bvme. S. G. Sueckman. J. R. Rose. B. K. fewest signs of recovery. Lance, C. B. johnson, A. i. Zusi-Cobb, B. A. Cooper, R. M. Burgess, R. J. Ritchie, A. A. Stickney, T. D. The conclusions from both analyses are Davis, E L. Craighead, T. J. Mabee, M. K. MacDonald, strengthenedby the fact that we sampled many D. C. Anthon, and I. E. Kellogg. This manuscript was of the most heavily oiled bays in the entire spill critically reviewed by B. E. Lawhead, R. J. Ritchie and area, thereby increasing our chancesof detecting B. A. Anderson (ABR, Inc.) and D. B. Irons, S. I? impacts if they were present. These conclusions Klosiewski, and D. C. Zwiefelhofer (U.S. Fish and Wildlife Service). Exxon Company, USA funded this also are supportedby Day et al. (1995, in press), research. The views expressed here are our own and who evaluated effects of the spill on habitat use, do not necessarily reflect those of the reviewers or of and by Wiens et al. (1996), who evaluated ef- Exxon. fects on bird communities. The analyses sup- porting those papers were based upon the same LITERATURE CITED post-spill data used in this paper, although their BAKER,J. M., R. B. CLARK,P. E KINGSTON,AND R. H. analyses did not incorporate comparisons with JENKINS.1990. Natural recovery of cold water pre-spill data. Regardless, both Day et al. and marine environmentsafter an oil spill, p. 173-177. Wiens et al. found that impacts were substantial In Proceedings of the Thirteenth Annual Arctic and Marine Oilspill Program Technical Seminar, and most apparent in 1989, but that recovery 6-8 June 1990, Edmonton, Alberta, Canada. En- was well underway by 1991. Our conclusions vironment Canada, Ottawa, Ontario, Canada. also are in agreement with assessmentsof im- CLARK,R. B. 1982. The long-term effect of oil pol- pacts and recovery by researchers who have lution on marine populations, communities, and ecosystems: some questions. Phil. Trans. Roy. studied other large marine oil spills (Clark 1982, Sot. 297B:185-192. Baker et al. 1990, Mielke 1990, Ritchie and DAY, R. H., S. M. MURPHY,J. A. WIENS,G. C. HAY- O’Sullivan 1994). For example, Clark (1982) WARD,E. J. HARNER,AND L. N. SMITH. 1995. Use 312 STEPHEN M. MURPHY ET AL.

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vironmental impact assessment,p. 55-70. In D. APPENDIX 1. Scientific names of bird speciesmen- Baird, L. Maltby, I? W. Greig-Smith, and I? E. T. tioned in text. Douben [eds.], Ecotoxicology: ecological dimensions. Chapman and Hall, London. Common name Scientificname WIENS,J. A., T 0. GRIST,R. H. DAY, S. M. MURPHY, AND G. D. HAYWARD.1996. Effects of the Exxon Red-necked Grebe Podiceps grisegena Valdez oil spill on marine bird communities in Pelagic Cormorant Phalacrocorax pelagicus Prince William Sound,Alaska. Ecol. Appl. 6:828- Harlequin Duck Histrionicus histrionicus 841. Common Merganser Mergus merganser WIENS,J. A., AND K. R. PARKER. 1995. Analyzing the Bald Eagle Haliaeetus leucocephalus effects of accidental environmental impacts: ap- Black Oystercatcher Haematopus bachmani proaches and assumptions.Ecol. Appl. 5:1069- Wandering Tattler Heteroscelus incanus 1083. Spotted Sandpiper Actitis ma&aria WINER,B. J., D. R. BROWN,AND K. M. MICHELS.1991. Red-necked Phalarope Phalaropus lobatus Statistical principles in experimental design. Mc- Mew Gull Lams canus Graw-Hill, New York. Glaucous-wingedGull Lams glaucescens ZAR, J. H. 1984. Biostatisticalanalysis. 2nd ed. Pren- Black-legged Kittiwake Rissa triductyla tice-Hall, Englewood Cliffs, NJ. Arctic Tern Sterna paradisaea ZWIEFELHOFER,D. C., AND D. J. FORSELL.1989. Marine Pigeon Guillemot Cepphus columba birds and mammals wintering in selectedbays of Marbled Murrelet Brachyramphus marmoratus Kodiak Island, Alaska: a five-year study. Unpubl. Kittlitz’s Murrelet Brachyramphus brevirostris Rep., U.S. Fish Wildl. Serv., Kodiak, AK. Homed Puffin Fratercula comiculata Belted Kingfisher Ceryle alcyon Steller’s Jay Cyanocitta stelleri Black-billed Magpie Pica pica Northwestern Crow Corvus caurinus Common Raven Corvus cot-ax