*
Inter-Individual Variation and Effect of Wave
Height on Foraging upon Mussels by the
California Sea Ötter, Enhydra lutris
Steven W. Randle
5706 E. Bryce Ave
Orange, CA 92667
srandle Oleland.stanford.edu
Advisor: Jim Watanabe
Hopkins Marine Station
Stanford University
June 7, 1996
Permission is granted to Stanford University to use the citation and abstract of this paper,
Randle: Sea Otters and Mussels
ABSTRACT
Foraging on intertidal mussels by the California sea otter, Enhydra lutris,
was examined to determine its effects on the mussel beds around Point
Cabrillo, Pacific Grove, CA. Frequency of success for foraging on mussels and
foraging on other types of prey were observed, as well as the mean number of
mussels removed and eaten by otters. The average duration of individual
feeding bouts and the average number of dives per minute were measured in
order to discover the mean number of mussels removed a day. These data
were analyzed comparing individual otter eating habits. Frequency of success
for mussel (94.53%) and non-mussel foraging (69.05%) and were significantly
different (p««0.001). There was also significant inter-individual variance in
mean number of mussels removed (p«40.001) and mean number of mussels
eaten (pæ20.001) among different otters. Foraging on mussels in relation to
increasing wave heights showed a significant decrease in number of mussels
removed (p20.005) and mussels eaten (p40.001) as wave height increased.
Success ratio also decreased as wave height increased (p20.01). In addition,
the frequency of foraging on mussel beds increased with tidal height and was
more prevalent in the early evening compared to the other times of the day.
Because otters have such a high metabolic rate, they must eat up to 25%
of their body weight daily. Using previously published data (Costa, 1978),
mean number of mussels per dive, and average number of dives per minute, a
sea otter would have to forage for mussels for approximately 294 minutes per
day to obtain sufficient kilocalories.
Randle: Sea Otters and Mussels
INTRODUCTION
Sea otters are the smallest marine mammal and were once thought to be
extinct in California, due to exhaustive hunting. The population now stands at
approximately 2000 individuals (USFWS/CDFG 1990 census, M. Staedler,
pers. comm.) and ranges south from San Luis Obispo to the northern boundary
of Ano Nuevo Point (Riedman and Estes, 1988). The original distribution
stretched from central Baja to northern Japan (Riedman and Estes, 1988). The
removal from and subsequent reintroduction of sea otters in central California
has had a large effect on the coastal communities they inhabit.
Sea otters live close to shore of areas composed of rocky substrate,
rarely swimming past of the outer limit of the kelp canopy. Occasionally otters
live in sandy bottom areas or are observed more than one kilometer from the
shore (Riedman and Estes, 1988b, 1990). However, rocky substrate supports a
higher density of otters than a soft bottom community does: 5 otters/km2 versus
0.8 otters/km? (California Department of Fish and Game, 1976). Sea otters
spend much of their time in the large kelp forests of Macrocystis pyrifera, either
resting or foraging.
Unlike other marine mammals, sea otters lack blubber. Instead they
have a thick, dense fur, that must be constantly groomed to keep a thin layer of
air trapped inside it for insulation. To reduce the amount of heat lost to the cold
waters off central California, otters maintain a high level of heat production
(Costa and Kooyman, 1982). Because of this high metabolic rate, an average
Randle: Sea Otters and Mussels
sized female sea otter (20 kg) requires about 6000 kilocalories per day. This
typically corresponds to 20-25% of its weight, daily (Costa 1978, Östfeld, 1982).
For this reason, otters spend approximately 25% of their time foraging, with the
rest of their time divided between resting (-60%) and grooming (-15%).
There are three peak times of foraging activity per day. One peak occurs
in the early morning, the second at late afternoon, and the third peak happens
around midnight. (Shimek and Monk, 1977, Loughlin, 1977). The extent of the
foraging activity throughout the night is currently under debate, since there is
such difficulty in making nighttime observations.
The California sea otter's diet consists of practically any macro¬
invertebrate that can be found near the coast. Ötters feed on species ranging
from turban snails, clams, sea urchins, and mussels to octopuses, abalone, and
kelp crabs, even eating the occasional fish or sea bird (Riedman and Estes,
1990, 1988a).
The object of this study was to find out to what extent otters feed on the
mussels at Point Cabrillo, Pacific Grove, California. At Point Pedras Blancas,
San Luis Obispo County, California, otters were observed removing mussels in
clumps, creating gaps in the mussel beds. Measurements of the increase in
size of these gaps and the number of new gaps created have been used to
determine the frequency of occurrence of sea otter foraging (VanBlaricom,
1988).
The questions addressed by this study were how the behavior of foraging
on mussels varied among individuals and how increasing wave height affected
Randle: Sea Otters and Mussels
this behavior. It has been established that sea otters commonly forage on
mussels (Mackey, 1982, VanBlaricom 1988), and the type of otters foraging on
mussels are usually females, mothers with pups, and recently independent
juveniles. The variation in diet of different otters has clearly been demonstrated
(Lyons, 1991), and individuals exhibit sustained preferences for particular prey
(Riedman and Estes, 1990). The variations among individuals foraging on a
single prey species, in this case mussels, has not been examined, so this was
chosen as an area to be explored. Since sea otter predation and wave force
are the two main factors acting on mussel mortality, the effects of waves on sea
otter foraging was chosen as a second factor to investigate.
Randle: Sea Otters and Mussels
MATERIALS AND METHODS
This study was carried out at Point Cabrillo (also known as Mussel Point).
at Hopkins Marine Station, Pacific Grove, California, from April 22 to May 30,
1996. Observations were made between 6:00 a.m. and 8:00 p.m. The rocky
intertidal community on the exposed side of the point is dominated by the
California mussel, Mytilus californianus, and sea otters forage there regularly,
There were two main sites on the point where the otters were observed (Fig. 1).
Observations were made from points along the shore with 20x binoculars or a
Questar spotting scope. While watching otters forage on the mussels beds
frequency of success, proportion of dives in which an otter returned to the
surface with prey, number of mussels removed, number of mussels eaten.
duration of dive, duration of foraging bout, number of dives per minute, and
number of otters foraging throughout the day were all recorded. When possible,
the identity of the otter was also noted.
The number of mussels removed is probably underestimated. Ötters
commonly remove mussels in clumps and then place them on their chests while
eating. Only those mussels which were too small or too difficult to crack open
and thus actively discarded or those mussels accidentally dropped could be
counted. Any mussels that fell off the otters chest (whether intentional or not)
while it rolled over to clean off debris or wash of the mussels went uncounted.
When making observations of otters foraging subtidally on food other than
mussels, only the frequency of success was measured consistently,
Randle: Sea Otters and Mussels
Identifications of the prey item and individual otter were sometimes difficult,
depending on the distance from shore.
Tidal height estimates were provided by TideMaster» - Zephyr Services,
Pittsburgh, PA. Wave height was obtained from the National Data Buoy Center
Buoy Number 46042, located in the northern Monterey Bay (36.8° N, 122.4° W).
The wave height was recorded for each hour, and then classified into one of
four categories: calm (25.0 ft), mild (5.0-6.9 ft), medium (7.0-8.9 ft), and rough
(29.0 ft).
Marking of individual mussels with fingernail polish was attempted, but
proved unsuccessful as the majority of mussels that the otters were foraging
were below MLLW and did not have sufficient time to dry. The polish stayed on
only about 70% of the mussels, so is was impossible to tell if the mussel had
been removed by an otter, wave force, or if the paint had just worn off.
In order to determine the average caloric value of a mussel, samples
were collected and their length and weight were measured. The wet weight
was then converted to a dry weight, considering that a mussels is approximately
83% water (Peak, 1983). The ratio between the lengths and dry weights
correlated to those found by Salkeld (1995). This dry weigh was then converted
to kilocalories per mussel using the constant 4.66 kcal/g (Costa, 1978).
The following comparisons were made using chi-squared tests: the
frequency of success for mussel versus non-mussel foraging, the frequency of
success for mussel foraging between different otters, and the ratio of mussels
removed and mussels eaten by individual otters. Also, the frequency of success
Randle: Sea Otters and Mussels
across the tour wave height categories was analyzed with a chi squared test.
Methods of statistical analysis were obtained from Sokal and Rohlf (1981).
Single factor analysis of variance (ANÖVA) was used to test for
differences in mean number of mussels removed and mean number of mussels
eaten for different individuals and at different wave height categories, as well as
inter-individual mean dive time. Duration of dive was separated into nine time
categories and the mean number of mussels removed per dive depending on
dive length was compared. Two factor ANÖVA was used to analyze the effects
of time of day and wave height on the mean number of otters foraging on
mussels and the effects of tidal height and wave height on mean number of
otters foraging on mussels. The time of day was grouped into four different
categories: early morning (0600-0959), late morning (1000-1259), afternoon
(1300-1659), and early evening (1700-2059). The number of otters was log¬
transformed flog(x+1)] before analysis.
The test for tidal height was carried out in a similar fashion, with four
groups: low low (g1.49 ft), high low (1.5-2.49 ft), low high (2.5-3.49 ft), and high
high (23.5 ft). The same type of test was run for tidal height as was for time of
day. The data were log-transformed and tested for independent effect and
interaction effect.
Randle: Sea Otters and Mussels
RESULTS
Behavior of sea otter foraging on mussels remained highly consistent
among the different individuals around Point Cabrillo. Most often, the otter dove
very close to shore in shallow water for about 20 seconds and removed a clump
of mussels. Then the otter ate the mussels one at a time, leaving the remaining
mussels on its chest or under flaps of skin under the axilla of each forelimb
while eating. While otters that were not feeding on mussels were frequently
observed using tools to crack open their prey, tools were very rarely used by
otters eating mussels. Generally, the otter split the mussel with its molars, pried
it open and scraped off the meat with its incisors. This characteristic behavior,
along with the bright orange color of the inside of the mussel, facilitated
identification of otters foraging on mussels. Occasionally otters climbed up a
vertical rock face to reach mussels, using the swell to lift them higher, and the
force of gravity to help remove the mussels. Although otters were sometimes
observed foraging on horizontal rock faces, the majority of feeding was done on
submerged vertical faces. The highest spot from which otters removed mussels
was approximately 3.5 feet above MLLW. Ötters were seen hauled out on
areas with mussels, and twice observed to bite at the mussels, but they were
never seen actually removing or eating these mussels. No noticeable gaps in
the mussel beds caused by otter foraging were noticed at Point Cabrillo.
However, gaps would be difficult to see since the majority of foraging is done on
beds that are usually submerged.
Randle: Sea Otters and Mussels
After removing the mussels, the otters float on their backs and consume
prey. The otters foraging off Point Cabrillo usually stayed close (-3 m) to the
mussel beds. Even on relatively rough days, with offshore wave heights of 6 to
7 feet, the otters made no obvious effort to distance themselves from the shore.
After eating a mussel, an otter commonly rolled over face down in the water to
either remove shell debris from its body, wash off a mussel, or possibly drink
water.
There were 14 individually identifiable sea otters and numerous
unidentified sea otters that fed on mussels at Point Cabrillo, and three other
identitied otters that were never seen eating mussels. Of these 14, only one
was a male, and it dove for mussels only on one occasion. However, this male
and one other male were regularly seen stealing mussels from females.
Usually, the male swam quickly to the area, stole the mussels from two or three
dives, and then the female quit diving and started to groom herself, so the male
would leave. Shortly after, the female usually resumed foraging again.
Sometimes the female and the male left the area together. The reactions from
the females varied from a violent scuffle to willing relinquishment of the
mussels.
Even though there were 14 otters seen foraging on mussels, there was
only one individual that was observed almost every day. Approximately half of
the observations include this otter, since she was foraging on mussels for the
majority of her diet. There was another otter that was observed frequently for
the first two and a half weeks, then not seen again until the final few days of the
Randle: Sea Otters and Mussels
study. There were three mother-pup pairs in the area around Point Cabrillo.
and two pairs were seen foraging on the mussels beds in the area. Seven of
the 13 females otters that foraged on mussels had nose scars, and two of these
were the mothers with pups; the remaining six otters were assumed to be
juveniles or younger adults. Using Garshelis' (1983) criteria for estimation of
age, only two of these otters were older than six years. Most of the others were
probably under four years of age.
Frequency of success for otters foraging on mussels was significantly
higher than the combined frequency of success of foraging on all other types of
prey (Chi-squared, p««0.001, Table 1). The frequency of success for individual
otters was also examined and was proved to differ significantly (Table 1). The
frequency of success for otters foraging on mussels also varied significantly with
wave, with lowest frequency of success when wave height was highest (Table
1). Next, the ratio of mussels eaten to the number of mussels removed was
examined at varying wave heights, and this ratio decreased significantly as
wave height increased (Table 1). There was no significant difference in the
ratio of the number of mussels removed to the number of mussels eaten among
different individuals. The pooled ratio of mussels eaten to mussels remoyed
was 0.88, and figure 2 shows that ratio of the mean number of mussels removed
and mean number of mussels eaten for seven otters remains fairly consistent.
There were a significant inter-individual differences of both the mean
number of mussels removed and the mean number of mussels eaten (ANOVA,
p«0.001, Table 2). The mean number of mussels removed for the population
10
Randle: Sea Otters and Mussels
was (-S.E.) 3.540.1 mussels per dive, with a maximum of 13 mussels, and
several occurrences of a single mussel being removed. The mean number of
mussels eaten per dive was 3.040.1 mussels per dive, ranging from zero to
twelve mussels. There were also significant differences in the mean duration of
mussel dives for different otters (ANÖVA, p««0.001, Table 3). The mean
duration of dives for the population was 18.440.5 seconds, with a shortest dive
of 6 seconds, and the longest was 47 seconds. Also, the mean duration of dive
decreased significantly with increasing wave height (ANÖVA, p-0.014, Table
4). Only dives made at calm and medium wave heights were analyzed, since
there were very few dives timed at mild wave heights, and none at the roughest
conditions.
Otters made, on average, 0.95 dives per minute while foraging on
mussels. The mean number of mussels removed increased significantly with
duration of dive (ANÖVA, p«0.001, Table 4 ), reaching a peak of 4.9 mussels
removed for dives lasting 30 to 34 seconds. The mean duration of foraging
bouts for the population was 75.744.8 minutes per bout, and it did not vary
significantly among individuals (ANÖVA, p=0.995, Table 5). Increasing wave
height did not effect duration of foraging bout (ANOVA, p-0.680, Table 5).
However, the mean duration of foraging bouts increased slightly as wave height
increased. The mean number of mussels removed and mussels eaten both
decreased significantly (ANÖVA, p40.001, Table 6) as wave height increased.
The average number of otters foraging on mussels throughout the day
(Fig. 3a) increased slightly in the morning and remained constant until 3:00
Randle: Sea Otters and Mussels
p.m., when it sharply dropped, then increased to a peak at 6:00 p.m., then
decreased again. Foraging activity was significantly greater in early evening
compared to both early and late morning. Foraging activity also decreased
significantly as wave heights increased (Fig. 3b, Table 7) Foraging activity at
medium and rough wave conditions remained low throughout the day, and the
peaks of activity that the overall pattern of foraging show are not present for
foraging on mussels (Fig. 3b).
The number of otters foraging increased significantly with increased tidal
height (Fig 4, Table 7). The average number of otters foraging on mussels was
significantly lower on rough days versus all other categories.
Increased wave height did more than influence the individual foraging
behavior, it also reduced the mean number of total otters foraging per day in the
study site (Fig. 5, Table 8). While otters were observed foraging on mussels at
practically all wave heights, at wave heights above 9 ft, this behavior was short
lived and infrequent. The mean number of otters foraging on mussels per day
drops from 7.8 to 1.25, resulting a large effect of the total number of mussels
removed per day depending on the wave conditions. The numbers of otters
foraging on mussels decreased at all times of day and at all tidal heights as
wave heights increased.
The average wet weight of mussels approximately the same size that
otters forage on was measured to be 8.8 grams. This was converted to a dry
weight of 1.5 g/ mussel. Multiplying by 4.66 kcal/g (Costa, 1978) gives 6.99
kcallmussel.
Randle: Sea Otters and Mussels
DISCUSSION
Foraging activity on all types of prey peaks at 7:00 a.m. and at 5:00 p.m.,
separated by a sharp decrease in activity at noon (Shimek and Monk, 1977).
Only one peak at 6:00 p.m. was observed for foraging on mussels. The number
of otters foraging on mussels increased slightly from 6:00 am to 3:00 p.m., with
no noticeable times of increased or decreased activity. Garshelis (1983)
observed substantially higher levels of activity from 8:00 am to 4:00 p.m. in an
area high in both consumption of mussels by otters and in number of mother
and pup pairs. He hypothesized that the majority of the otters foraging on
mussels at off peak times are doing so to avoid competition from other more
dominant otters. In addition, food stealing was most frequently observed in the
early evening.
The lack of a peak of foraging activity in the early morning could be
artifact, due to seasonal variation in tidal height. Since this study was only
carried out in the spring, there were no substantially high tides in the morning.
Kovnat (1982) did not observe a single occurrence of intertidal foraging in the
early morning (0700 to 0930), and attributed this occurrence to intertidal
foraging being of secondary importance to foraging in the subtidal. In order to
test for this effect, further studies would have to be done at other times of the
year, when there are early morning high tides.
In contrast to this study, Kovnat (1982) concluded that large waves have
littie effect on intertidal foraging. As wave heights increased, almost all
Randle: Sea Otters and Mussels
quantifiable aspects of otters' foraging behavior were significantly reduced.
Mean number of mussels removed and mean number eaten decreased, as did
duration of dive. It could be due to sheer wave force, so the otters have to
devote more effort to avoid being smashed into a rock, leaving less effort for
removing mussels. Wave force probably plays a large part in the reduced
frequency of success with increased wave height.
Although VanBlaricom (1988) reported California sea otters swimming
10-20 meters away from the shore after each dive, the otters foraging off Point
Cabrillo did not exhibit this behavior. The otters stayed relatively close to the
shore even at larger wave heights. Also, the ratio of mussels eaten to mussels
removed decreased with increased wave height, and this is most likely due to
an increase in the amount of dropped mussels due to larger waves.
One aspect that did not decrease with increasing wave height was the
mean duration of the foraging bout, which increased slightly with wave height.
Even though this trend was not significant, it makes sense that the otters would
have to forage for a longer period of time in order to eat the same amount of
mussels, since mean number of mussels eaten per dive decreases with
increased wave height.
Using Costa's (1978) numbers for caloric content, considering that an
average female otter weighing 20 kg requires 6000 kilocalories per day, and
using the empirically determined average weight of a mussel, an otter would
have to eat approximately 860 mussels per day if its diet consisted strictly of
mussels. At 0.95 dives per minute and 3.04 mussels eaten per dive, this results
Randle: Sea Otters and Mussels
in a period of foraging of about 290 minutes per day. This is about 20% of the
day spent foraging, which corresponds to previously determined otter activity
budgets. If an otter fed on only red abalone (Haliotis rufescens), then it would
only need to spend 93 minutes a day foraging, and if it fed strictly on kelp crab
(Pugettia producta), then it would take 268 minutes per day (Östfeld, 1982),
However, these two species are not nearly as abundant as Mytilus
californianus. A diet consisting only of mussels would not be energetically
unteasible, but individual otters have prey preferences, and do not feed on any
single species of prey exclusively (Riedman and Estes, 1990, Lyons, 1991).
While watching foraging on mussels and other prey, otters seemed
determined to find one type of prey at a time. The otters at the mussel beds
persisted to eat mussels, even though there were usually several Pisaster in the
same area, they were never observed to eat them. Ötters were commonly
observed to return to the surface with the same species of prey on continuous
subtidal dives.
Randle: Sea Otters and Mussels
ACKNOWLEDGMENTS
First and foremost I would like to thank my advisor, Jim Watanabe, who
went above and beyond the call of duty in helping me with this project. I would
also like to thank Teri Nicholson and Emily Carrington in helping me conceive
the idea for this project, and to Teri for walking me through the first few weeks of
observations. Thanks to Nancy Eufemia for making sure none of us fell into the
cracks and making all of our presentations look professional. Special thanks to
Mike Kenner and Dawn Breese of UCSC for loaning a Questar spotting scope
to me. Thanks to Michelle Staedler of the Monterey Bay Aquarium for taking
time out to talk to me and set me on the right track. Thanks to School House
Rock for teaching me how to count, since these skills were definitely put to the
test this quarter. Also, thanks to Bryan Sun for helping with measurements on
the mussel beds. And finally, another large outpouring of gratitude to Jim
Watanabe, for spending countless hours helping me with statistical analysis,
numerous revisions of this paper, and critiquing my presentation at midnight the
night before it was given.
Randle: Sea Otters and Mussels
LITERATURE CITED
California Department of Fish and Game. 1976. A proposal for sea otter protection and
research, and request for retum of management to the State of California, January
1976. 2 Vols. Unpubl. rep.
Costa, D. P 1978. The ecological energetics, water, and electrolyte balance on the
California sea otter, Enhydra lutris. Ph.D. dissertation, University of Califomia,
Santa Cruz. 75 pp.
Costa, D. P., and G. L. Kooyman. 1982. Oxygen consumption. thermoregulation, and
the effect of fur oiling and washing on the sea otter, Enhydra lutris. Can. J. Zool.
60:2761-2767.
Garshelis, D. L. 1983. Ecology of sea otters in Prince William Sound, Alaska. Ph.D.
thesis, University of Minnesota, Minn. 321 pp.
Kovnat, G. D. 1982. Intertidal foraging by the southern sea otter, Enhydra lutris.
Master's thesis, Stanford University, Stanford, Calif. 60 pp.
Loughlin, T. R. 1977. Activity patterns, habitat partitioning, and grooming behavior of
the sea otter, Enhydra lutris, in California. Ph.D. thesis, University of California,
Los Angeles. 110 pp.
Lyons, K. J. 1991. Variations in feeding behavior of female sea otters, Enhydra lutris,
between individuals and with reproductive condition. Ph.D. dissertation,
University of California, Santa Cruz. 176 pp.
Mackey, F 1982. Underwater observations of sea otter foraging behavior. Center for
Coastal Marine Studies, University of California, Santa Cruz. 38 pp.
Östfeld, R. S. 1982. Foraging strategies and prey switching in the California sea otter.
Oecologia 53:170-178.
Peak, T. 1983. Foraging patterns and prey selection in Leptastrias hexactis.
(Unpublished MS on file at Hopkins Marine Station Library).
Riedman, M. L., and J. A. Estes. 1988a. Predation on seabirds by sea otters. Can. J.
Zool. 66:1396-1402.
Riedman, M. L., and J. A. Estes. 1988b. A review of the history, distribution and
foraging ecology of sea otters. Pages 4-21 in G. R. VanBlaricom and J. A. Estes,
eds. The community ecology of sea otters. Springer-Verlag, Berlin, West
Germany.
Riedman, M. L., and J. A. Estes. 1990. The sea otter (Enhydra lutris): behavior, ecology,
and natural history. U.S. Fish and Wildl. Serv., Biol. Rep. 90(14). 126 pp.
Salkeld, P. N. 1995. Aspects of reproduction associated with the use of a segmented
regression to describe the relationship between body weight and sheil length of
Mytilus edulis. Mar. Ecol. Prog. Ser. 124:117-128.
Shimek, S. J., and A. Monk. 1977. Daily activity of sea otter off the Monterey Peninsula,
California. J. Wildl. Manage. 41::277-283.
Sokal, R. R., and F. J. Rohlf. 1981. Biometry. 2nd ed. W. H. Freeman and Company,
San Francisco. 859 pp.
Randle: Sea Otters and Mussels
VanBlaricom, G. R. 1988. Effects of foraging by sea otters on mussel-dominated
intertidal communities. Pages 48-91 in G. R. VanBlaricom and J. A. Estes, eds.
The community ecology of sea otters. Springer-Verlag, Berlin, West Germany.
VanBlaricom, G. R., and J. A. Estes, eds. 1988. The community ecology of sea otters,
Springer-Verlag, Berlin, West Germany.
Randle: Sea Ötters and Mussels
Table 1:
Frequency of Success: Mussel versus Non-mussel Foraging
Chi-Squared
Type of Foraging Mussel Non-mussel Y of Groups
Successful
898
145
145
Unsuccessful
117
950
210
L of Dives
1160
Freq. of Success
0.69
0.95
Gadj=94.6889
df=1
p0.001
Frequency of Success: Individuals
Chi-Squared
Individual
Otter 2 Otter 3
Otter 4
Otter 5 Otter 11 Otter 13 Y of Individ.
Successful
713
469
54
Unsuccessful
21
L of Dives
490
2
— 150
Feq. of Success
0.94
0.99
0.96 0.92 0.78 0.97
Gadj=15.612
df=5
p0.01
Frequency of Success: Wave Height
Chi-Squared
Wave Height
Calm Mild
Medium
Rough
Z of Wave Heights
Successful
310
328
243
898
Unsuccessful
10
349
320
25
L of Dives
950
Freq. of Success
0.97
0.94
0.95
0.94
0.77
Gadj=11.7814
df=3
p0.01
Ratio of Mussels Eaten to Mussels Removed: Wave Height
Chi-Squared
Wave Height
Calm Mild
Medium
Z of Wave Heights
469
253
Mussels Eaten
354
1076
46
Mussels not Eaten
140
515 416
289
1220
Total Removed
Percent Eaten
0.91
0.85
0.88
0.88
Gadj=55.66
df=2
PO.001
Table 2:
Mussels Removed: Individuals
Anova: Single Factor
SUMMARY
Individual
Average
.25
Otter 4
187
Otter 3
5.11
Otter
4.14
4.17
Otter
Otter
1.83
Otter
3.53
Otter 5
2.80
ANOVA
Source of Variation
Between Groups
133.68
Within Groups
277
1099.09
Total
283
1232.77
Mussels Eaten: Individuals
Anova: Single Factor
SUMMARY
Average
Individual
n
2.87
187
Otter 4
Otter
4.67
21
3.33
Otter
3.83
Otter
1.50
Otter
12
Otter
3.07
2.50
Otter
10
ANOVA
Source of Variation
Between Groups
117.78
Within Groups
944.42 277
Total
1062.21 283
SE
0.129
0.550
0.618
0.562
0.345
0.559
0.490
P-value
22.28 5.6153 50.001
3.97
SE.
0.12
0.525
0.465
0.601
0.289
0.511
0.477
P-value
NS
E
19.63 5.7576 50.001
3.41
20
Randle: Sea Ötters and Mussels
Table 3:
Duration of Dive: Mean Mussels Removed
Anova: Single Factor
SUMMAR
Duration (s)
Average
(6-9)
1.86
(10-14)
3.6:
(15-19
3.68
(20-24)
3.92
(25-29)
(30-34
4.88
(35-39)
3.43

(40-44
3.00
(45-49)
3.00
ANOVA
Source of Variation
S
Between Groups
118.36
Within Groups
541.56 201
Total
659.92 209
S.E.
0.159
0.175
0.222
0.340
0.399
0.972
0.429
2.000
0.000

P-value
N
14.80 5.4913 50.001
2.69
Randle: Sea Ötters and Mussels
Table 4:
Duration of Dive: Wave Height
Anova: Single Factor
SUMMARY
Groups
Count
133
Calm
Medium
ANOVA
Source of Variation
Between Groups
352.41
Within Groups
12072.59
Total
12425.00
Duration of Dive: Individuals
Anova: Single Factor
SUMMARY
Individual
n
Otter 4
145
Otter 2
Otter
Otter
Otter
Otter
ANOVA
Source of Variation
Between Groups
3595.37
Within Groups
8764.13
12359.50
Total
Average
18.98
16.31
dt
209
210
Average (s)
15.56
25.24
21.54
19.00
26.64
26.88
df
194
199
SE
0.725
0.690
NB
352.41
57.76
S.E.
0.515
1.810
1.674
2.000
2.394
3.833
NB
719.07
45.18
22
Randle: Sea Ötters and Mussels
P-value
6.101 0.014
P-value
15.917 50.001
Table 5:
Duration of Foraging Bout: Wave Height
Anova: Single Factor
SUMMARY
Wave Height
Average (min)
Calm
70.37
Mild
77.35
Medium
80.60
ANOVA
8
Source of Variation
946.68
Between Groups
48
Within Groups
58433.90
59380.59
Total
50
Duration of Foraging Bout: Individuals
Anova: Single Factor
SUMMARY
Individual
Average (min)
Otter 4
79.67
Otter
74.50
77.33
Otter
93.00
Otter
Otter
81.67
77.50
Otter
ANOVA
Source of Variation
S
dt
Between Groups
551.11
48900.67 34
Within Groups
49451.78 39
Total
SE
9.091
7.914
7.908
NB
473.34
1217.37
SE
8.513
14.953
9.333
1.000
13.642
16.500
165
110.22
1438.25
23
Randle: Sea Ötters and Mussels
P-value
0.3888 0.6800
—
P-va
0.0766 0.9954
Table 6:
Mussels Removed: Wave Height
Anova: Single Factor
SUMMARY
Wave Height
Average
S.E.
144
Calm
3.58
0.159
Mild
107
3.89
0.243
100
Medium
2.89
0.167
Rough
1.00
ANOVA
Source of Variation
NB
P-value
Between Groups
26.06
78.18
6.2413 0.0004
Within Groups
1465.60
351
4.18
354
Total
1543.79
Mussels Eaten: Wave Height
Anova: Single Factor
SUMMARY
wave Height
Average
S.E
Calm
144
3.26
0.152
107
3.31
Mild
0.218
100
Medium
2.53
0.157
Rough
0.000
1.00
ANOVA
Source of Variation
F P-value
M
df
Between Groups
57.14
19.05 5.3261 0.0013
Within Groups
1255.23 351 3.58
Total
1312.37 354
24
Randle: Sea Ötters and Mussels
Table 7
Time of Day vs Wave Height
Anova: Two Factor
df
Source of Variation
F P-value
NS
Time of Day
4.341
1.447 5.252
0.002
4.785
1.595 5.790
Wave Height
0.001
3.470
Time x Wave
0.386 1.400 0.201
23.418
Error
85
0.276
Total
36.014
100
Tukey Hs
Multiple Comparisons
Waves
Calm
Mild Medium
Mild O.360 ns
O.495
Mediun
O.135 ns
Rough 0.731 0.371 ns 0.236 ns
Time
E
LM
AE
LMO.166 ns
AF O.335 ns 0.168 ns
0.433 0.264 ns
E O.559
Tidal Height vs Wave Height
Anova: Two Factor
Source of Variation
P-value
NS
14.196
Tidal Height
4.732 20.186 0.000
Wave Height
6.228
2.076 8.856 0.000
3.25
0.361 1.542 0.392
Tide x Wave
94
Error
22.036
0.234
45.713
Total
109
Tukey H:
Multiple Comparisons
Waves Calm
Mild Medium
MildlO.170 ns
Mediun
0.398 ns 0.228 ns
Rough 0.759 0.590 0.361
H 14
Times LL
0.398 ns
LO.722 .324 ns
H1.103 0.705 0.381 *
25
Randle: Sea Ötters and Mussels
Table 8:
Mean Number of Ötters Foraging: Wave Height
Anova: Single Factor
SUMMARY
Group.
Average
S.E
Count
Calm
7.80
1.158
Mild
4.18
0.605
Medium
3.00
0.307
Rough
1.25
0.455
ANOVA
dt
Source of Variation
NE
46.79
Between Groups
140.37
Within Groups
115.94
3.62
32
256.31 35
Total
Randle: Sea Ötters and Mussels
P-value
12.9146 50.001
26
Randle: Sea Otters and Mussels
FIGURE LEGENDS:
Figure 1: A map of Point Cabrillo. The two main sites of otter foraging are
indicated.
Figure 2: A comparison of the mean number of mussels removed and mean
number of mussels eaten per dive among 7 otters. Error bars are
standard error.
Figure 3a: Mean number of otter foraging on mussels taken at hour intervals
and plotted against time of day. Error bars are standard error.
Figure 3b: Mean number of otters foraging on mussels throughout the day,
divided up into wave height category. Number of otters taken at each
wave height at hour intervals. Error is standard error.
Figure 4a: Mean number of Ötters foraging on mussels per day plotted against
tidal height. Error is standard error.
Figure 4b: Mean number of otters foraging on mussels per day plotted against
tidal height, and then divided up by wave height categories. Error is
standard error.
Figure 5: Mean number of otters mussel foraging per day at each wave height
category. Error is standard error.
Randle: Sea Otters and Mussels
Figure 1
N
kelp
Locations of Ötter Foraging
kelp






V



28
S
A








9

o
Hopkins
Marine
e.

de. v. 8bre.
Ttst.
Dewoy
Ave.
Ear
3r6gt.
28
Figure 2:
Randle: Sea Otters and Mussels
□ Mussels Removed per Dive
Mussels Eaten per Dive
Randle: Sea Otters and Mussels
Figure 3a:
Average Number of Otters Foraging
on Mussels Throughout the Day





.
0.


600 800 1000 1200 1400 1600 1800 2000
Time of Day
Eigure 3b.
Calm
Mild



3.5 5.
3.5
— —
3
3

—
.
32

2.5

4
2
2
2
15   — — — /4
1.5
1
E
o5E
att
N
L
600 800 1000 1200 1400 1600 1800 2000
600 800 1000 1200 1400 1600 1800 2000
Time of Day
Time of Day
Medium
Rough
+
4
aa-
aaaa-

5 3.5
——
a


3
3 E.

2.5
2.5

—
2

2

1.5

15

1

t

o5kI f
0.5E
R
EN
600 800 1000 1200 1400 1600 1800 2000
600 800 1000 1200 1400 1600 1800 2000
Time of Day
Time of Day
Randle: Sea Otters and Mussels
Figure 4a:
The Number of Otters Foraging on
Mussels Increases with Tidal Height



3

2.

....
f


1.5




4
...
0.5

Tidal Height
Flgure 4a:
Calm
Mild

5    .
5 .






...
Vt
/


2

O



O
o
0
Tidal Height
Tidal Helght
Medium
Rough


5
sa

..
4
4


3



2 L




L
oed
o
Tidal Height
Tidal Helght
Randle: Sea Otters and Mussels
Figure 5:
The Number of Otters Foraging on Mussels
Decreases with Increasing Wave Height

—
............



55
.

EE
calm mild medium spicy
Wave Height