**Preassessment Data Report #5:**

**Persistence rates of bird carcasses on beaches of Unalaska Island,
Alaska, following the wreck of the M/V *Selendang Ayu***

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height="5.004861111111111in"}

G. V. Byrd^1^

J. H. Reynolds^2^

*^1^U. S. Fish and Wildlife Service, Alaska Maritime National Wildlife
Refuge, 95 Sterling Highway, Suite 1, Homer, AK 99603*

*^2^U. S. Fish and Wildlife Service, Division of Natural Resources,
MS-221, 1011 E. Tudor Road, Anchorage, AK 99503*

This document should be referenced as:

Byrd, G.V., and J.H. Reynolds. 2006b. Persistence rates of bird
carcasses on beaches of Unalaska Island, Alaska, following the wreck of
the M/V Selendang Ayu. U.S. Fish and Wildlife Service, Alaska Maritime
National Wildlife Refuge, Homer, AK.

cover photo:

A salmon stream in Skan Bay oiled following the wreck of the M/V
Selendang Ayu, Unalaska Island (on beach segment SKN14; 17 December
2004, Art Sowls, Alaska Maritime National Wildlife Refuge).

**ABSTRACT**

Bird carcass surveys were conducted as part of the pre-assessment phase
of response to the M/V *Selendang Ayu* oil spill off the coast of
Unalaska Island. Deposited carcasses were not expected to persist on a
beach for more than a few days due to scavenging and tidal rewash.
Carcass persistence rate was estimated approximately 1.5 months
following the original spill event by experimentally depositing unoiled
carcasses in the field and monitoring their state (intact / scavenged)
and persistence (present / absent) through repeated visits. Daily
scavenging and persistence rates were estimated using standard
mark-recapture models. The probability that an intact unoiled carcass
remained unscavenged for 24 hours was 0.0%. The probability that an
intact unoiled carcass persisted overnight (though scavenged) was 14.6%.
The probability that a scavenged carcass persisted overnight was 79%.
These persistence rates and the relatively low detection probabilities
(Byrd and Reynolds 2006a) suggest that only a very small proportion of
all deposited carcasses were found and recorded during beach searches on
Unalaska Island.

**INTRODUCTION**

On 8 December 2004, the M/V *Selendang Ayu* ran aground and broke in
half in rough seas off Unalaska Island, Alaska (5338\'N, 167 07\'W). An
estimated 354,218 gallons of oil (339,538 gallons of bunker oil \[IFO
380\] and 14,680 gallons of marine diesel and miscellaneous oils) were
discharged. Numerous bird carcasses were recovered during beach searches
in the months following the spill as part of the typical pre-assessment
phase of response to an oil spill, which includes collecting data from
which to estimate the total number of animals affected by the event
(Ford *et al.*, 1987; Page *et al*., 1990; Piatt *et al.*, 1990; Burger,
1992a, Flint *et al.* 1999). Recovered carcasses represent only a
fraction of the total number deposited on beaches (Flint *et al.* 1999).
Recovered carcasses represent only a fraction of the total number
deposited on beaches (Flint *et al.* 1999), including both mortality
related to the event and "natural background mortality."

Carcasses found by observers on a specific beach at any given time after
a mortality event are a result of three processes: (1) the rate at which
carcasses are deposited on beaches, (2) their persistence once on the
beach, and (3) the probability of observers detecting them when
surveying the beach (Flint *et al.* 1999). These components vary
depending on site and environmental characteristics present in each
specific mortality event. For example, carcass deposition rate can vary
with factors such as beach type (Bodkin and Jameson, 1991, Flint *et
al.* 1999), persistence rates can vary with factors such as beach type,
time since deposition, weather, tidal activity, carcass size and
scavenger activity (Bodkin and Jameson, 1991; Burger, 1992b, 1993; Van
Pelt and Piatt, 1995, Ford et al. 1996, Fowler and Flint 1997, Ford in
press), and detection probabilities can vary with factors such as beach
type and weather conditions like presence or absence of snow (Fowler and
Flint 1997).

Persistence rate is defined as the probability that a carcass will
remain on the study area for a given period of time. A combination of
factors at Unalaska Island suggests that persistence rates for carcasses
on beaches are likely to be very low in most instances. For instance,
many beaches are exposed to direct wave action, exacerbated by storm
surges, which might reduce persistence rates by rewashing carcasses.
Additionally, resident populations of scavenger populations occur
commonly in much of the affected area, including red foxes (*Vulpes
vulpes*), common ravens (*Corvus corax*), bald eagles (*Haliaeetus
leucocephalus)*, and glaucous-winged gulls (*Larus glaucescens*), and
some of these scavengers removed carcasses quickly.

Persistence rates can be estimated using mark-recapture analyses
techniques (Pollock *et al.* 1990). In this application, the usual
survival rate estimate is, functionally, a persistence rate.

This report summarizes carcass persistence rate estimates for beach
surveys on Unalaska Island, Alaska in January 2005. Results are
discussed in the context of scavenger satiation. Detection probability
estimates are reported in a companion document (Byrd and Reynolds
2006a).

**Methods**

*Study Design*

Following a reconnaissance, all beaches in the oil spill core area were
classified to one of four types: *exposed* -- high impact beaches
normally exposed to waves, often with steep angles indicating frequent
impacts; *catchment* -- beaches with areas where large amounts of debris
had accumulated; *protected* -- beaches not normally exposed to waves
from the open sea; *unavailable* -- segments that were either too short
(\< 100m) or not safely accessible by the Tiglax-based crew. The spill
area was comprised of 18% exposed beaches, 6% catchment, 31% protected,
and 45% non-accessible coast (cliffs or other unwalkable segments of
coastline). Beach segments were defined based on physical shore zone
character following Owens and Sergy (2000).

Persistence studies require conducting repeated searches on the study
beach segments. Previous studies suggested a revisitation frequency of
every other day (Van Pelt and Piatt 1995, Fowler and Flint 1997). The
persistence study beaches were thus selected from that subset of (i) the
stratified random sample of exposed, catchment, and protected beaches
selected for the Tiglax-based beached animal surveys (Byrd et al. 2005)
that (ii) supported regular repeated access from the ship. This design
constraint eliminated consideration of any exposed beach segments due to
(i) the difficulty of regularly accessing such open areas from a ship
and (ii) the expected high rate of re-wash. Four beaches were
purposively selected for the carcass persistence rate study, three
protected (CNB 10, NGW 2, PTN10) and one catchment (CNB 19) (Figure 1).

Longstanding U.S. Fish and Wildlife Service practice in Alaska prevented
leaving any oiled carcasses on beaches because of a desire to remove
sources for secondary oiling of scavengers as quickly possible,
therefore carcass persistence was estimated using previously frozen
carcasses of marine birds of species present in the spill area. The 112
carcasses available for the study included auklets, murres, puffins,
cormorants, eiders, kittiwakes and gulls (Table 1). The size class
distribution of deployed carcasses was 37 % large, 19 % medium and 44%
small. Each carcass was marked by placing a numbered, metal tag through
the gap between the radius and ulna bones of each wing.

A total of 112 carcasses were distributed across the four beaches in
samples of 22, 26, 32, and 32 carcasses, respectively, on beaches CNB
19, NGW 2, PTN10, and CNB10. Carcasses were deposited along the beach in
a fashion similar to that of natural carcasses as observed earlier in
the response. Each carcass' position was recorded relative to the mean
low tide line (above, below) to assess the effect of tide (i.e., rewash)
on carcass persistence. Fifty percent of carcasses were deposited at the
mean low water line with the remainder deposited at the higher high
water line.

On the three protected beaches a small numbered board of plain plywood
(approximately 10 cm x 18 cm) was placed directly under each carcass to
assess tidal rewash. If a board remained but the carcass was missing,
the carcass was assumed lost due to scavenging rather than tidal action.
When both the carcass and the board were missing, loss was assumed due
to tidal action.

The study was first implemented at beach CNB 19 and rechecked 48 hours
later, at which time all carcasses had been scavenged. All other
implementations of the study were therefore rechecked at least every 24
hours (sometimes within 8-12 hr). During each revisit observers searched
until they were certain they had detected all deployed carcasses that
remained on the beach, recorded the presence of carcasses and numbered
boards as well the condition of the carcass with regards to scavenging.
Thus each visit to each carcass placement location resulted in one of
four carcass outcomes:

-   intact (with board)

-   scavenged in place (with board),

-   removed by scavengers (carcass missing but board present)

-   re-washed (carcass and board both missing)

Further scavenging of an already scavenged carcass was not recorded.

*Analysis of Persistence Rates*

There were three rates of interest: (i) persistence rate of an intact
carcass (intact → intact), (ii) persistence rate of scavenged carcass
(scavenged → scavenged), and (iii) transition rate from intact to
scavenged, but not removed, carcass (intact → scavenged). The time scale
of interest was daily rates rather than hourly or weekly rates. When a
study site had received multiple visits within a day, observations from
visits that were as close to 24 hours apart as possible were used for
the daily rate estimation; observations made within less than 24 hours
of each other were used to estimate diurnal scavenging probabilities.

Interval censored survival models were used to account for variation in
the length of time between observations (Johnson 1979). Let *s* be
constant daily persistence rate and *t* the number of days between
carcass visits (here *t* = 1 or 2). Let *n~t~* be the total number of
carcass-events of revisit interval *t*, i.e., the sum across all but the
last revisit of the number of carcasses remaining deployed at each
revisit. Let *r~t~* be the total number the n~t~ carcass-events where
the carcasses persisted to the next revisit. For example, if there were
three daily visits with 100 carcasses deployed at the first day, 50
remaining the second day, and 5 remaining the last day, n~t~=100+50=150
carcass-events, of which r~t~=50+5=55 persisted to the next interval.

The likelihood for the sample persistence data is given by Equation 1,
assuming that all carcasses were detected if present on every search
('known fate' analysis). The resulting maximum likelihood estimator
(Equation 2) was solved via an iterative procedure using Excel
(Microsoft, Inc., Redmond, WA.). Standard error estimates were obtained
by substituting the estimated daily persistence rate *s* and the other
data into Equation 3 and taking the reciprocal. Confidence intervals
were estimated by assuming an asymptotic normal sampling distribution
for the maximum likelihood estimates.

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*Diurnal Probabilities of loss, Probability of Re-wash*

The proportion of carcasses lost during the day versus the night was
estimated using observations from beaches surveyed multiple times within
a 24 hour period. The proportion of carcasses lost due to tidal re-wash
was estimated using observations from the three beaches with numbered
boards. Both proportions were estimated following the usual binomial
model (Zar 1996). Confidence intervals were calculated following Agresti
and Coull (1998) using the function binconf in the Hmisc package
(Harrell 2004) for the statistical freeware package R (R Development
core team 2005).

*Assessment of the assumption regarding detection probability*

Two deployed carcasses, both LEAU, were not found on later persistence
searches but then located much later on regular beach surveys. In both
cases, the carcasses were moved by scavengers during the first evening
following initial deployment as the associated board was present the
next morning but the carcass was not detected. In both cases, two
(three, respectively) further persistence searches were conducted on the
relevant beaches and the scavenged carcass was not found. No carcasses
were detected on the subsequent persistence searches, thus these
searches ceased. Carcass NGW2-2783/2792 was encountered eleven days
after the cessation during a regular beach survey; all that remained was
the wing tags and some primaries and contour feathers. Carcass
CNB10-2152/2151 was encountered three days after cessation during a
regular beach survey. The detection rate for numbered boards placed with
carcasses was 1, implying that searchers were effectively covering the
beaches.

There were insufficient data to directly estimate a detection rate using
maximum likelihood approaches and a standard Cormack-Jolly-Seber Model
(Pollock et al. 1990). As an alternative, persistence rates for
scavenged carcasses were calculated under three different scenarios
regarding the fate of the two relocated carcasses and persistence
searcher detection rates: (i) the carcasses were assumed to have
remained present on the study beach the full time and a true detection
rate of 1.0 for study carcasses was attained by treating the two
carcasses in question as if they had actually been detected on all
intervening persistence searches (Table 2a); (ii) the carcasses were
assumed to have been initially removed from the study area by scavengers
then returned to the study area later, and thus were not available for
detection during the intervening searches (i.e., temporary immigration)
(Table 2b); and (iii) the detection probability was estimated using an
ad hoc approach that mimicked the logic of the detection probability
estimate from a C-J-S model. That is, detection probability is defined
as the proportion of carcasses marked before time i and were known to be
present after time i, that were observed at time i. Given that the
estimated persistence rate is functionally the product of the
persistence rate and the detection rate, we divided the estimate of
persistence by this detection probability to predict true persistence
rates.

**Results**

***Intact Carcasses***

Of the 112 carcasses, eleven (scenario 1) / ten (scenario 2) persisted
to the second survey by which time they had all been scavenged (Tables
2a, b). The daily persistence rate for an intact carcass remaining in an
intact state was thus 0.0 (Table 3). The estimated daily persistence
rate for an intact carcass remaining as a scavenged carcass was 0.146
with a standard error of 0.034 (Table 3).

***Scavenged Carcasses***

The eleven (ten) scavenged carcasses remaining to the second visit
served as the initial sample for estimating persistence of scavenged
carcasses. There were five recheck interval lengths (Table 2). The
estimated daily persistence rate for a scavenged carcass remaining on a
beach was 0.791 with standard error of 0.066 under scenario 1; 0.622
with a standard error of 0.113 under scenario 2 (Table 3). Under
scenario 3, the estimated detection probability was 0.714 (5 of 7
carcasses detected) with a binomial standard error of 0.171. Dividing
the estimated persistence rate from scenario 1 by this estimated
detection probability yields an adjusted estimated persistence rate \>1.
The approximate lower 95% CI for this estimate is 0.63.

Thus the probability that an intact carcass remains after the first day
is 0.146, at which point it is a scavenged carcass, and the probability
it continues to persist on day t is 0.146 (0.791)^t-1^ (Figure 2). For
example, the probability that a carcass remained present ten days after
deposition is estimated to be 0.146 (0.791)^9^ = 0.018.

***Diurnal Scavenging Probabilities***

Fifty eight carcasses were placed on beaches in the morning, rechecked
in the evening of the same day, and then checked the following morning
(protected beaches NGW2, CNB10, and PTN10). On the evening search of the
placement day 1 carcass was missing and 5 were scavenged. On the
following morning search only 3 remained, each scavenged.

Thus probability of daylight removal or scavenging was 0.103 (6/58, 95%
confidence interval of (0.048, 0.208)). The probability of being removed
during the night given that the carcass was intact in the evening was
0.947 (54/57, 95% confidence interval of (0.856, 0.982)). Of all
carcasses ultimately lost over a 24 hour period, the probability that
they were removed at night was 98.2% (54/55, 95% confidence interval of
(0.904, 0.999)). Some daylight scavenging occurred with probability
0.086 (5/58, 95% confidence interval of (0.037, 0.186)), though all of
these carcasses were subsequently removed by scavengers overnight.

***Rewash Probability***

Marked boards were placed under 90 carcasses, of which 3 showed evidence
of flotation and removal by tidal action within the first 24 hours
following placement (0.033, 95% confidence interval of (0.011, 0.093)).
Only one tagged carcass was subsequently found on a beach other than
where it was marked, demonstrating that scavengers could remove
carcasses from the study beaches to areas not generally searched. Thus
it is feasible, though improbable, that some of the 3 carcasses
attributed to rewash could have been removed by scavengers and their
boards subsequently removed by tide. The rewash probability estimate
could thus feasibly be biased slightly upwards.

**DISCUSSION**

Intact carcasses had very low rates of persistence on the Unalaska
Island study beaches during the period of our study, approximately 48
days after the M/V Selendang Ayu oil spill. All intact carcasses were
either removed or scavenged within 24 hours. For carcasses that remained
on the beach after scavenging, the estimated carcass persistence
probability increased substantially (Table 3). Similar almost
exponential declines in carcass persistence (Figure 2) have been
observed in auklets on Naked Island, Prince William Sound, Alaska,
following the Exxon M/V Valdez oil spill (Ford et al. 1996) and in
common murres in Resurrection Bay, Alaska, following a natural die-off
event (Van Pelt and Piatt 1995).

The minimal evidence of tidal re-wash and the removal of carcasses
almost exclusively during night strongly suggest that most carcasses
were removed by nocturnal scavengers such as red foxes. While degree of
scavenging was not recorded on searches, we suspect that the carcasses
with the highest persistence were heavily scavenged carcasses, probably
by birds, with little remaining food value.

The rates of persistence observed here, while comparable to those found
in Prince William sound and northern Washington (Ford et al. 1996), were
considerably lower than those from some previous studies (see Van Pelt
and Piatt 1995 for review, Fowler and Flint 1997). They are also
considerably lower than estimates of bird carcass persistence in
terrestrial habitats (Tobin and Dolbeer 1990, Linz et al. 1991, Kostecke
et al. 2001, Rivera-Milan et al. 2004).

The very low rates of persistence imply that scavengers were not
satiated 48 days after the initial spill, contrary to the suggestion of
Fowler and Flint (1997) following a spill in the Pribilof Islands.
Carcass persistence rates have been hypothesized to vary with carcass
density as carcasses at higher densities would cause scavengers to focus
on specific areas, thus resulting in lower rates of persistence for
those areas (Linz et al. 1991), though we know of no data that
demonstrates this effect on ocean beaches. By the time of the study the
deposition rate of carcasses from the M/V Selendang Ayu was apparently
insufficient to satiate or overwhelm scavengers.

The very low rewash rate estimate likely underestimates the actual
rewash rate averaged across all relevant beaches during the event.
Firstly, rewash rate was only estimated on protected beaches, which are
the type expected to have the lowest rewash rates. Secondly, rewash
rates during and immediately following the spill event were likely much
higher due to the larger waves associated with the much higher wind
velocities during and immediately after the spill compared to during the
persistence study (V. Byrd, unpubl. data).

As required by current policy, all of the persistence study carcasses
were completely un-oiled. It is possible that degree of carcass oiling
could affect the probability of scavenging and hence persistence, in
which case oiled carcasses may persist at higher rates than reported
here. However, neither Ford et al. (1996) nor Wiese (2002) found
evidence for this effect in either Alaska subsequent to the Exxon Valdez
or in Newfoundland. Since most carcasses found in the beach surveys were
heavily scavenged and consisted of only a few fragments, such as a wing,
any difference in probability of scavenging between oiled and unoiled
carcasses is at most slight.

Although relevant data were collected, too few intact carcasses
persisted to support estimating variation in persistence rates across
beaches, by carcass size, or relative to tide level on the beach.
Previous studies demonstrated variation in persistence rates across
habitat types (Kostecke et al. 2001), beach types (Fowler and Flint
1997), carcass ages (VanPelt and Piatt 1995), carcass size (Ford in
press) and carcass condition (Fowler and Flint 1997). Kostecke et al.
(2001) concluded that carcass persistence rates did not differ by
species, implying that scavengers were neither satiated nor selective.
We conclude that scavenger density on protected and catchment beaches in
the affected area (31% and 5% of the beach segments, respectively) is
uniformly high resulting in consistently low persistence rates.

**Assumptions of estimation models**

While many of the standard assumptions of mark-recapture estimation are
irrelevant when applied to carcass persistence, some do apply. First,
the models used here assume no tag loss. Three untagged scavenged murre
carcasses were found during beach revisits. As the wings of these
carcasses were damaged or missing, it is probable that they were tagged
carcasses from which scavengers had removed the tags. If so, failure to
include these carcasses as recaptures would slightly underestimate
persistence rates. As a conservative approach, all three carcasses were
assumed to have been originally tagged and utilized accordingly in these
analyses.

The persistence model used is a "known fates" model which assumes
perfect carcass detection probabilities at every revisit -- that is, all
carcasses present on the beach during a survey were detected. All
numbered boards placed under carcasses were relocated implying that
detection probability for intact carcasses was 1.0. However, of the 11
scavenged carcasses used in the study, two were moved by scavengers and
not encountered during multiple persistence searches but then
encountered on subsequent regular beach carcass surveys. Less than
perfect detection would cause underestimation of the persistence rate.
Given that (i) this was an experimental study where carcasses were
purposively placed on the beach, hence the initial number and locations
of carcasses were known by the searchers, (ii) the crews were instructed
to search at each visit until they were confident that all carcasses
present were located, and (iii) all numbered boards placed under
carcasses were relocated, the relocations suggest two possible
scenarios. *Scenario A*: the two scavenged carcasses were present and
not detected on the two or three intervening persistence searches yet
relocated on subsequent regular searches. *Scenario B*: they were moved
off of the study beach, like all other carcasses, and thus not available
for detection (at which point they are no longer considered part of the
persistence study, by definition) and later were returned to the study
beach.

Scenario A assumes that even though the persistence study crew(s)
explicitly searched until they were fully confident that no deployed
study carcasses remained anywhere on the beach segment, these two
carcasses were both missed in at least two persistence searches yet
relocated during subsequent regular beach searches which have an
associated detection probability of roughly 40% (Byrd and Reynolds
2006a).

Scenario B assumes that carcasses initially removed from the study beach
by scavengers could later be returned to the beach (by scavengers,
winds, or other means). Within the first night of deployment,
approximately 86% of all intact carcasses were removed from the study
beaches; ultimately over 99% of all deployed carcasses were removed from
the study beaches (Table 2). Other scavenged carcasses were found that
had been moved from their initial deployment location to either other
locations within the study segment, off the study segment and onto other
nearby segments, or off the study segment and onto the nearby grass
(persistence study field notes). Thus scavengers can move carcasses and
not all movements involve removal from beaches. Unfortunately the exact
location of subsequent re-encounter of the two relocated carcasses were
not recorded, i.e., whether near the grass, and thus there is little
contextual information for determining the likelihood of these carcasses
being re-deposited back onto the study beach.

Given that the estimated persistence rate is functionally the product of
the true persistence rate and the true detection probability, the
appropriate estimated detection probability cannot be less than the
estimated persistence rate as this would require true persistence rates
in excess of 1.0. Therefore, the logical bounds for the point estimate
of the detection probability are from 0.791 to 1.0. All estimates of
detection probability from other studies fall below this level making it
clear that detection rates during this controlled experiment exceeded
those for other searches. Unfortunately, the small sample size resulted
in an *ad hoc* estimate of detection probability with such high variance
that its was uninformative. We have no additional data that can be used
to estimate a detection probability appropriate for these data.
Therefore, we assumed that detection probability was 1.0 but we
acknowledge this may result in underestimation of the persistence rate
by an unknown degree.

**Conclusions and Recommendations**

The combination of extremely low persistence rates and relatively low
detection probabilities (Byrd and Reynolds 2006a) suggests only a very
small proportion of total carcasses deposited on the Unalaska beaches by
the event were actually found during beach searches.

**ACKNOWLEDGEMENTS**

Paul Flint made essential contributions to this report. Tremendous
thanks to the following individuals for the their assistance in
implementing this study and in reviewing the report: the crew of the R/V
Tiglax - Kevin Bell, Dan McNulty, Dennis Haunschild, John Faris, Dan
Erickson, Joe Isenhour, Bob Ward, Eric Nelson; Jeff Williams, Leslie
Slater, Becky Howard, Bill Schaff, Greg McClelland, Ty Wyatt, Janis
Krukof, Kent Sundseth, Jeff Lewis, [C]{.underline}ris Dippel, Andy
Aderman, Don Dragoo, Arthur Kettle, Greg Thomson, Steve Kendall, Gary
Wheeler, Pat Walsh, Fred Broerman, Ken Gates, Kristine Sowl, Thomas
Siekaniec, Clay McDermott, Delia Person, Michael Winfree, Suzann
Speckman, Merban Cebrian, Jennie Wetzel, Ingrid Harrald, R. Glenn Ford,
and those individuals and agencies who provided frozen carcasses at
short notice to allow this study to occur.

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Table 1. Species and size of carcasses used for estimating persistence
rates. 'Beach Strata' refers to the type of beaches where the carcasses
were deposited for the study.

  ------------ ----------------------- ------------- ---------------------
  Species ID   Common Name             Carcass Size  Beach Strata

  PECO         pelagic cormorant       Large         Protected

  COEI         common eider            Large         Protected

  KIEI         king eider              Large         Protected

  GWGU         glaucous-winged gull    Large         Protected

  COMU         common murre            Large         Protected, Catchment

  TBMU         thick-billed murre      Large         Protected, Catchment

  Murre        unidentified murre      Large         Protected

  TUPU         tufted puffin           Large         Protected, Catchment

  HOPU         horned puffin           Medium        Protected, Catchment

  BLKI         black-legged kittiwake  Medium        Protected

  RLKI         red-legged kittiwake    Medium        Protected

  LEAU         least auklet            Small         Protected, Catchment

  WHAU         whiskered auklet        Small         Protected

  CRAU         crested auklet          Small         Protected, Catchment

  MAMU         marbled murrelet        Small         Protected
  ------------ ----------------------- ------------- ---------------------

**Table 2a.** Carcass persistence summary data, by beach (row), number
of days between visits (columns) and initial carcass state (left -
intact; right - scavenged), assuming the two relocated carcasses were
present and detected the whole time. For example, 22 intact carcasses
were deposited on beach CNB 19 and revisited two days later, at which
time only four scavenged carcasses remained (Intact t~2~ = 4 = Scavenged
t~0~). Those were revisited one day later (Scavenged t~1~), at which
time three remained, then revisited after another seven days (Scavenged
t~8~), at which time no carcasses remained. Of the 112 intact carcasses
originally deposited, 11 remained at the first revisit, all scavenged..

  -------------- -------- ------ ------ ----------- ------ ------ ------ ------ ------ ------ -------
  **Initial      Intact                 Scavenged                                             
  State**                                                                                     

  **Beach,       t~o~     t~1~   t~2~   t~o~        t~1~   t~2~   t~3~   t~4~   t~5~   t~8~   t~12~
  Type**                                                                                      

  CNB 19, C      22              4      4           3                                  0      

  NGW 2, P       26       2             2           2                                         1

  CNB10, P       32       4             4           1                    1      0             

  PTN 10, P      32       1             1           1             1      1                    
  -------------- -------- ------ ------ ----------- ------ ------ ------ ------ ------ ------ -------

**Table 2b.** Carcass persistence summary data, by beach (row), number
of days between visits (columns) and initial carcass state (left -
intact; right - scavenged), assuming the two relocated carcasses were
removed by scavengers and thus unavailable for detection until being
returned to the beach.

  -------------- -------- ------ ------ ----------- ------ ------ ------ ------ ------ ------ -------
  **Initial      Intact                 Scavenged                                             
  State**                                                                                     

  **Beach,       t~o~     t~1~   t~2~   t~o~        t~1~   t~2~   t~3~   t~4~   t~5~   t~8~   t~12~
  Type**                                                                                      

  CNB 19, C      22              4      4           3                                  0      

  NGW 2, P       26       1             1           1                                         0

  CNB10, P       32       3             3           0                                         

  PTN 10, P      32       1             1           1             1      1                    
  -------------- -------- ------ ------ ----------- ------ ------ ------ ------ ------ ------ -------

Table 3. Daily persistence rate estimates, aka transition probabilities,
with 95% confidence intervals. The estimates for the transitions Intact
→ Intact and Intact → Rewashed are the usual binomial estimates; their
intervals follow Agresti and Coull (1998). The estimates for the
transitions Intact → Scavenged and Scavenged → Scavenged are from
interval censored survival model (Equation 1); their intervals assume
asymptotic normality of the estimates.

  -------------- ------------ ---------------- ---------------- -----------
                 State                                          
                 Tomorrow                                       

                 Persisting                    Disappearing     

  State Today    Intact       Scavenged        Rewashed         Removed

  Intact         0.0\         0.146\           0.033\           0.821^a^
                 (0,0.041)    (0.079, 0.213)   (0.011, 0.093)   

  Scavenged                   0.79\            0.0              0.21
                              (0.662, 0.920)                    
  -------------- ------------ ---------------- ---------------- -----------

a\. Daily probability of Intact → Removed = 1 -- 0.0 - 0.146 -- 0.033;
similarly Scavenged → Removed = 1 -- 0.79.

Figures

1\. (a) Area of western Unalaska Island, Alaska, surrounding spill zone;
(b -- d) detail maps identifying beach segment labels. The current study
was conducted on beach segments CNB 10, CNB 19, NGW 2, and PTN10 within
Makushin Bay (see 1 d).

2\. Persistence probability for initially intact carcasses on beaches at
Unalaska Island, Alaska in January 2005: Prob(persist t days after
deposition)=0.1460.791^(t-1)^. After the initial day of deposition, all
carcasses persist only as scavenged carcasses. Dashed lines mark upper
and lower 95% confidence intervals.

Figure 1 a

![](media/image5.jpeg){width="7.729861111111111in"
height="6.2972222222222225in"}

Figure 1 b

![](media/image5.jpeg){width="6.502083333333333in"
height="7.392361111111111in"}

Figure 1 c

![](media/image5.jpeg){width="6.502083333333333in"
height="5.202083333333333in"}

Figure 1 d

![](media/image5.jpeg){width="6.502083333333333in"
height="6.223611111111111in"}

Figure 2.

![](media/image6.wmf)