Horng & Hayhurst: Haul-Out Effects of the Pacific Harbor Seal on the Rocky Midtidal Community
Abstract
This study, conducted at the Hopkins Marine Refuge in Pacific Grove, CA, examined haul¬
out effects of the Pacific harbor seal (Phoca vitulina richardsi) on the rocky intertidal community.
The study employed a nested sampling design, consisting of three replicate plots each within three
haul-out intensity sites, low, medium and high. Plots were sampled using 20 X 20 cm quadrats
haphazardly placed within a vertical range of 1.5 to 3 feet above MLLW.
Percent cover analysis of sessile species revealed no significant differences among haul-out
intensities. Mastocarpus papillatus cover, however, noticeably decreased as haul-out intensity
increased. Conversely, Mazzaella affinis cover increased as haul-out intensity increased. Average
algal biomass and frond length measurements also failed to demonstrate significant differences
among haul-out intensities. However, trends of increasing biomass and decreasing frond length
with increasing haul-out intensity were observed.
Due to the high level of heterogeneity in the rocky midtidal, only large differences among
haul-out sites could be detected by our experimental design. A lack of significant differences
among haul-out sites does not rule out the possibility of actual impact by harbor seals. Observed
trends indicate that haul-out of harbor seals may have a moderate impact on the rocky midtidal
community.
Horng & Hayhurst: Haul-Out Effects of the Pacisic Harbor Seal on the Rocky Midtidal Community
Introduction
The current world population of harbor seals is estimated to be 500,000 individuals
(Reeves et al. 1992) (Fig. 1). With males weighing up to 140 kilograms and measuring 1.9 meters
in length, and females averaging 80 kg (Reeves et al. 1992), the potential for impact on haul-out
sites by harbor seals is substantial. The harbor seal (Phoca vitulina) belongs to the taxonomic
family Phocidae. Unlike other mammals in the order Pinnipedia, such as the California sea lion,
the phocids are characterized by (among other things) hindflippers which cannot be rotated forward
for movement on land, mammae with two teats, and lack of external ear flaps (Riedman, 1990).
Phocids such as the harbor seal move on land by hunching the body and wriggling to the side
(Reeves et al. 1992). Compared with other pinnipeds which walk on their flippers, the phocids
contact with haul-out substrate is greater in area and involves more dragging action. This further
increases the mechanical stress placed on the sessile communities at haul-out sites.
Behavioral as well as physical factors contribute to the possibility of such disturbances.
Harbor seals spend forty-four to fifty percent of their time hauled out (Newby, 1973; Riedman,
1990; Sullivan, 1982). Moreover, harbor seals exhibit a high level of fidelity to haul-out sites over
periods of years (Reeves et al. 1992). They are also most likely to haul out in sites where other
seals are already present (Ösborn, 1985; Sullivan, 1982), which concentrates their potential
impact.
Few previous studies have addressed the impact of harbor seal disturbance on intertidal
habitats where seals haul out frequently. Boal (1980) reported that seal haul-out in the rocky
intertidal significantly alters algae and microscopic fauna composition. She attributed these
findings to both mechanical and chemical factors.
The goal of this study was to assess the effect, if any, of the Pacific harbor seal (Phoca
vitulina richardsi) on the rocky midtidal community of Hopkins Marine Refuge. The first Pacific
harbor seals observed to haul out regularly at Hopkins Marine Refuge arrived in the early 1970s
(Boal, 1980). Since that time, the local harbor seal population has increased substantially, from
Horng & Hayhurst: Haul-Out Effects of the Pacific Harbor Seal on the Rocky Midtidal Community
approximately 30 seals in 1979 to 100 seals in 1990. In this study, up to 172 hauled out seals
have been observed at one time.
Materials and Methods
The effect of harbor seal haul-out was determined by investigating the distributional and
physical differences in sessile species at sites of high, medium, and low seal use. A fully nested
sampling design was used to distinguish variations within and among sites. The study was carried
out in the Hopkins Marine Refuge, located at Hopkins Marine Station in Pacific Grove, California
(Fig. 2). Field research was conducted from April 24 to May 24 1996.
Determination of Seal Haul-out Intensities
Study plots were determined by identifying locations of low, medium, and high seal haul¬
out intensities. Observations were made from the shore adjacent to the study area using hand-held
8 X 42 binoculars. The number of seals at 17 identifiable plots of similar size was sampled at
hourly intervals from 8 a.m. to 5 p.m. for a total of four days. Tide heights, weather conditions,
and possible disturbances were also recorded.
The number of seals hauled out in each plot was averaged over the 10 time intervals (8 a.m.
to 5 p.m.). The data were then averaged over the four days of observation to give an overall
average of seal intensity per plot at any given time. Due to their similarity in intertidal height, the
plots were assumed to have comparable percentage exposure time. Plots that averaged 0 to 1 seals
hauled out were categorized as low intensity, 7 to 10 seals hauled out as medium intensity, and 14
to 16 seals hauled out as high intensity (Fig 3). In total, three low, three medium, and three high
intensity plots were identified (Fig. 4, 5, and 6).
Determination of Community Composition within Sites
The study employed a fully nested sampling design of three replicate plots within each of
the three seal haul-out intensity categories. Five replicate quadrats were sampled within each plot.
The following categories of measurements were sampled at comparable vertical heights in each
plot: percent cover of all sessile species, algal frond length and algal biomass. Based on vertical
Horng & Hayhurst: Haul-Out Effects of the Pacific Harbor Seal on the Rocky Midtidal Community
zonation patterns in the rocky intertidal as well as observations of seal haul-out patterns, a height
range of 1.5 - 3.0 feet above MLLW was selected for sampling. This range was identified in the
field with the aid of a surveyor's rod and transit and a topographical map of the refuge, "Survey of
Rocky Intertidal Zone, Hopkins Marine Station of Stanford University, Pacific Grove, California
(1980).“ Field work was conducted during low tide and seal disturbances were minimized.
Percent cover of each plot was determined by using a random point contact method within
five randomly placed quadrats. X,Y-coordinates on the grid of the quadrat were chosen from a
table of random digits (Sokal and Rohlf, 1995a). Thirty-five points were used in each quadrat.
This number of points was determined by conducting a sample quadrat and recording species hits
at every point on the grid. From this information, a graph was constructed of the number of points
sampled versus cumulative number of species hit (Fig. 7). The resulting curve leveled off near the
point where x equals 35. This method allowed the determination of an optimal number of points
tested to maximize species representation and minimize time spent per quadrat sampled.
Quadrats measuring 20 X 20 cm were placed haphazardly in the sampling area. In order to
ensure access by seals as well as consistency of the sampled community, only quadrats which
landed on relatively horizontal surfaces were scored. Quadrats that landed in crevices were moved
to the nearest horizontal surface within the allowed vertical range. Both primary and secondary
(cover) species were recorded separately at each point tested in the quadrat. The general location
and characteristics of each quadrat were noted as well. Species were identified in the field using
Dawson and Foster (1982). Additional aid in identification was provided by Jim Watanabe.
Algal frond lengths were measured at five random points in each quadrat. At each selected
point, a ruler was used to determine the fully extended frond length in millimeters. Hits on crustose
algae, such as Hildenbrandia spp. and the Petrocelis phase of Mastocarpus papillatus , were
recorded as 0.0 millimeters.
Finally, algal biomass in each plot was measured with scrapings from two randomly placed
10.5 X 10.5 cm quadrats. Samples were removed with a small paint scraper. As with algal frond
length, biomass measurements did not take into account crustose algae which were difficult to
Horng & Hayhurst: Haul-Out Effects of the Pacisic Harbor Seal on the Rocky Midtidal Community
scrape from rock. The scrapings were placed in labeled plastic bags in the field and transported to
the lab, where they were separated from debris and spread out to dry. Cleaned samples were
placed in a drying oven set at 60 degrees Celsius and weighed repeatedly until their weights no
longer changed. At this time, their dry weights in grams were measured on a mass balance and
recorded.
Data Analysis
Percent cover data for the six most abundant species of algae and numerical abundance data
for Tegula funebralis were analyzed using a two-factor nested analysis of variance (ANOVA).
Percent cover data were subjected to arcsine transformation (sin-1 V p, where p = proportion)
while numerical abundance data were subjected to log (x+1) transformation (Sokal and Rohlf,
1995b). Cochran's Test was used to determine homogeneity of variances among plots (Sokal and
Rohlf, 1995b). Pairwise plot comparisons using Tukey's HSD test were performed to detect
significant differences between plots. The percentages of total variance due to differences among
quadrats, among plots, and among intensity categories were also calculated (Underwood, 1981).
Two-factor ANOVAs were also performed on average algal biomass and average algal frond length
data for each haul-out intensity.
Diversity of the different plots was calculated using the Shannon index:
H'=-Epi ln pi
The quantity pi represents the proportion of individuals found in the ith species (ni/ N, where Nis
the total number of individuals) (Magurran, 1988). The average diversity of the plots was used to
estimate the diversity of the three haul-out intensity categories.
Results
Thirteen species of algae representing nine orders, one species of sea grass (Phyllospadix
scouleri), and thirteen species of invertebrates representing eight orders were encountered and
identified in this study. Eight species of algae and P. scouleri accounted for more than 5% of the
total cover in at least one of the nine plots. Five of the invertebrate species were encountered more
than five times in at least one of the nine plots (Table 1).
Horng & Hayhurst: Haul-Out Effects of the Pacific Harbor Seal on the Rocky Midtidal Community
No significant differences in percent cover for all six algae species analyzed were detected
among haul-out intensities (Fig. 9). Similarly, no significant differences among haul-out
intensities were detected for T. funebralis abundance. Large variations in algae percent cover and
T. funebralis abundance were found among quadrats within plots and among plots within haul-out
intensities (Table II). Cochran's test revealed homogeneity among M. papillatus, M. affinis, and
M. papillatus (Petrocelis phase), but heterogeneity among Hildenbrandia spp., E. muricata, and C.
canaliculatus (Table I).
Two species of algae, Mastocarpus papillatus and Mazzaella affinis, demonstrated definite
trends in percent cover among haul-out intensities (Fig. 10). M. papillatus cover decreased
substantially as seal haul-out intensity increased. In contrast, M. affinis cover increased as haul¬
out intensity increased. Although ANÖVA revealed no significant differences among haul-out
intensities (p = 0.087 for M. papillatus and p = 0.069 for M. affinis) substantial proportions of
total variance were due to haul-out intensity (28.7% for M. papillatus and 25.3% for M. affinis)
(Table II). Calculation of the sources of variance revealed that 28.7% of the total variance for M.
papillatus and 25.3% of the total variance for M. affinis resulted from differences in haul-out
intensities (Table II). A Tukey's HSD test of M. papillatus cover showed that plots categorized
within the same haul-out intensity were not significantly different from each other. However,
certain high haul-out intensity plots were significantly different from certain low haul-out intensity
plots (12:6, 13:6, and 3:7) (Table III).
A two-factor nested ANOVA of algal biomass among the three haul-out intensity categories
revealed no significant differences among intensities. However, a trend of increasing biomass
with increasing haul-out intensity was observed (Fig. 10). Average algal frond length was also not
significantly different among the three intensities. A possible trend of decreasing frond length with
increasing haul-out intensity was noted (Fig. 11).
Estimated diversities of the three haul-out intensities were very similar. No trends were
observed due to high variability between plots of the same intensity (Fig. 12).
Horng & Hayhurst: Haul-Out Effects of the Pacific Harbor Seal on the Rocky Midtidal Community
Discussion
Although no significant differences in percent cover among seal haul-out intensities were
detected for the six most abundant species of algae encountered, definite trends in Mastocarpus
papillatus and Mazzaella affinis were observed. The trend for M. papillatus cover decrease with
haul-out intensity increase is supported by a study of human trampling on marine rocky shore
communities (Brosnan and Crumrine, 1994). The study noted that M. papillatus cover was lower
(from 9 to 1%) in trampled plots than in control plots. Human trampling and seal haul-out could
create similar mechanical stresses on the community by crushing and dislodging the algae. In fact,
seal haul-out most likely creates greater physical stress on the community, as the seals contact a
larger area and spend more time on the rocks.
An explanation for the M. affinis trend observed in this study may lie in the results of a
study by Danek (1979) on M. affinis distribution, which reported a greater abundance of this
species at intermediate wave exposed sites. There were no major qualitative differences in wave
exposure between any plots in our study (Ullmann, 1991). Based on Danek's findings, it seems
possible that M. affinis abundance is enhanced by the physical stress of wave action.
Analogously, the physical stress caused by seal-haul-out may also enhance M. affinis abundance.
There were no statistical differences for algae biomass among seal haul-out intensities,
despite distinct trend of increasing algae biomass with increasing haul-out intensity. Algae frond
length, on the other hand, appeared to decrease as seal haul-out intensity increased. This
difference was also not statistically significant. As mentioned previously, the harbor seals move
on land in a hunching, lurching manner (Randall et al. 1992; Riedman, 1990). The mechanical
abrasion of seals moving on a rock would tend to tear algae with long fronds and encourage the
growth of bushier, more turf-like algae. Turf-like algae have shorter frond length, but greater
biomass. Supporting evidence for this explanation comes from Brosnan and Crumrine (1994)
who found significant declines in foliose algae and increases in relative abundance of turf-like algae
in trampled areas.
Horng & Hayhurst: Haul-Out Effects of the Pacific Harbor Seal on the Rocky Midtidal Community
The results of this study indicate that the rocky midtidal community is highly
heterogeneous. A large percentage of the total variance occurs among quadrats within replicate
plots. The variance at this spatial scale is greater than 50% for all of the algae species analyzed. In
light of this finding, the results presented in a similar study (Boal, 1980) must be re-evaluated. In
Boal (1980), the experimental design consisted of a paired comparison of control and experimental
sites, rather than a nested sampling design. A paired comparison confounds harbor seal haul-out
effects with spatial heterogeneity. It is therefore impossible to accurately attribute the observed
differences between sites specifically to seal haul-out. Furthermore, the vertical range sampled by
Boal (1980) was zero to two meters above MLLW. In a range of this magnitude, the variation
between quadrats would be even greater than was encountered in this study. Yet, without a nested
sampling design, this variation was not accurately assessed,
The results of Boal (1980) were somewhat different from those presented in this report.
She found significantly greater cover of Chondracanthus canaliculatus at haul-out sites, which she
attributed to seal haul-out effects. Our study also indicated a trend of greater C. canaliculatus cover
as haul-out intensity increased (Fig. 8). However, the variations among plots and among quadrats
were so great (94.98% of the total variance) that this trend could not be attributed to seal haul-out
(Table III).
Boal (1980) also found significantly fewer Tegula funebralis on haul-out rocks. She
attributed this difference as well to seal haul-out effects. Analysis of T. funebralis counts from our
study indicated a similar trend with fewer T. funebralis at high haul-out intensity plots. Again, the
high variance among quadrats and among plots (94.33% of total variance) cast doubt on the
assignment of any significant effects to seal haul-out (Table III). Because T. funebralis often
aggregate in crevices at low tide (Morris, R. et al. 1980; Ricketts et al. 1985), seal haul-out would
not be expected to have a significant impact on their abundance. Wara and Wright (1964) indicate
that T. funebralis population density decreases as total algal cover increases. In light of these
findings, perhaps the decrease in T. funebralis with seal haul-out, as noted in both this study and
Horng & Hayhurst: Haul-Out Effects of the Pacific Harbor Seal on the Rocky Midtidal Community
Boal (1980), can be attributed to the species' non-independent relationship with an increase in
algae cover.
The results of our study indicate that Pacific harbor seals may have some impact on the
rocky midtidal community. The lack of significant differences in percent cover, diversity, algae
biomass, and algae frond length among seal haul-out intensities indicates that this impact is not
severe. However, because the limited sample size only allowed detection of the largest differences
between sites, this study cannot conclude that seal haul-out has no impact.
Further data are necessary in order to accurately assess the impact of the Pacific harbor seal
on the rocky midtidal community. In past years, the population of harbor seals has dramatically
increased at Hopkins Marine Refuge and in the surrounding area. If this trend continues, the
possible effects of seal haul-out will become an increasingly relevant topic. Future studies that
examine the effect of seal haul-out in the rocky intertidal should employ an experimental design
which accounts for the high degree of variance at all spatial scales in this environment. Spatial
heterogeneity needs to be considered when examining the effect of any factor on a community as
diverse as the rocky midtidal.
Horng & Hayhurst: Haul-Out Effects of the Pacific Harbor Seal on the Rocky Midtidal Community
Acknowledgments
Sincère thanks to our advisor, Jim Watanabe, for his guidance and unending support from
start to finish. Without his help, this project would still be a question, and nothing more. Many
thanks to Terry Nicholson for her expert advice on anything related to seals. Finally, an apology
to the seals for intruding onto their home.
Horng & Hayhurst: Haul-Out Effects of the Pacific Harbor Seal on the Rocky Midtidal Community
Literature Cited
Boal, J. (1980). Pacific harbor seal (Phoca vitulina richardsi). Haul out impact on the rocky
midtidal zone. Mar. Ecol. Prog. Ser. 2, 265-269.
Brosnan, D. and Crumrine, L. (1994). Effects of human trampling on marine rocky shore
7, 79-97.
communities. J. Exp. Mar. Biol. Ecol. 1
Danek, S. (1979). Rhodoglossum affine (Rhodophyta) intertidal distribution and physiological
ecology. (Unpublished manuscript on file at Hopkins Marine Station Library.)
Dawson, E. and Foster, M. (1982). Seashore Plants of California. University of California Press,
Berkeley, CA.
Magurran, A. (1988). Ecological Diversity and Its Measurement. Princeton University Press,
Princeton, NJ.
Morris, R., Abbot, D. and Haderlie, E. (1980). Intertidal Invertebrates of California. Stanford:
Stanford University Press.
Newby, T. (1973). Observations on the breeding behavior of the harbor seal in the state of
Washington. J. Mamm. 54, 540-543.
Osborn, L. (1985). Population dynamics, behavior and effect of disturbance on haulout patterns of
the harbor seal, Phoca vitulina richardsi. (M.S. Thesis for University of California at Santa
Cruz.)
Reeves, R., Stewart, B. and Leatherwood, S. (1992). The Sierra Club Handbook of Seals and
Sirenians. San Francisco: Sierra Club Books.
Riedman, M. (1990). The Pinnipeds. Berkeley, Los Angeles, Oxford: University of California
Press.
Ricketts, E., Calvin, J. and Hedgpeth, J. (1985). Between Pacific Tides, Sth ed. Stanford:
Stanford University Press.
Sokal, R. and Rohlf, F. (1995a). Statistical Tables, 3rd ed. New York: W.H. Freeman & Co.
Sokal, R. and Rohlf, F. (1995b). Biometry, 3rd ed. New York: W.H. Freeman & Co.
Sullivan, R. (1982) Agonistic behavior and dominance relationships in the harbor seal, Phoca
vitulina. J. Mamm. 63, 554-569.
Ullmann, J. (1991). The influence of substrate characteristics on haulout behavior of harbor seals,
Phoca vitulina. (Unpublished manuscript on file at Hopkins Marine Station Library.)
Underwood, A. (1981). Techniques of analysis of variance in experimental marine biology and
ecology. Oceanogr. Mar. Biol. Annu. Rev. 19, 513-605.
Wara, W. and Wright, B. (1964). The distribution and movement of Tegula funebralis in the
intertidal region of Monterey Bay, CA. Veliger 6, 30-37.
Horng & Hayhurst: Haul-Out Effects of the Pacific Harbor Seal on the Rocky Midtidal Community
Table I
ALGAE
INVERTEBRATES
DIVISION CHLOROPHYTA
PHYLUM ANNELIDA
Order Acrosiphoniales
Order Sabellida
Cladophora columbiana
Serpulorbis spp.
DIVISION RHODOPHYTA
PHYLUM ARTHROPODA
Order Bangiales
Order Thoracica
Porphyra perforata
Balanus glandula
Order Ceramiales
Tetraclita rubescens
Osmundia (Laurencia) spp.
Chthamalus spp.
Order Corallinales
PHYLUM CNIDARIA
Coralline spp.
Order Cryptonemiales
Order Actiniaria
Hildenbrandia spp.
Anthopleura elegantissima
Order Endocladiaceae
PHYLUM MOLLUSCA
Endocladia muricata
Order Gelidiales
Order Archaeogastropoda
Gelidium pusillum
Fisurella spp.
Pterocladia calogloissoides
Tegula funebralis
Order Gigartinales
Order Mesogastropoda
Mastocarpus papillatus
Littorina scutulata
Mastocarpus papillatus (Petrocelis phase)
Order Mytiloida
Mastocarpus jardinii
Mytilus californianus
Mazzaella affinis
Order Neogastropoda
Mazzaella flaccida
Ocenebra circumtexta
Nucella emarginata
Order Plocamiales
Gastroclonium subarticulatum
Achinthina punctulata
Order Neoloricata
(also Phyllospadix scouleri)
Chiton spp.
For algae, indicates cover greater than 5% in at least one of the plots.
For invertebrates, indicates encountering more than 5 times in at least one of the plots.
12
Horng & Hayhurst: Haul-Out Effects of the Pacific Harbor Seal on the Rocky Midtidal Community
Table II
2 Factor Nested Analysis of Variance (ANOVA)
arcsine transformation = sin IV p, where p = proportion (Sokal & Rohlf, 1995)
Mastocarpus papillatus
Cochran's Test = 0.244 homogeneous
Transformation: arcsine
% of Total
F-Ratio
Degrees of
Mean
P Value
Source
Freedom
Square
Variance
Intensity
3.772
0.087
28.71
0.662
Plot(Intensity
3.099
0.175
0.015
20.87
50.41
Error
0.057
Mazzaella affinis
Cochran's Test = 0.270 homogeneous
Transformation: arcsine
F-Ratio
Source
Degrees of
Mean
% of Total
P Value
Freedom
Square
Variance
Intensity
4.323
0.069
0.701
25.32
Plot(Intensity
0.162
0.136
9.86
1.756
Error
0.092
64.82
Chondracanthus canaliculatus
Cochran's Test = 0.438 heterogeneous
Transformation: arcsine
F-Ratio
% of Total
Degrees of
Source
P Value
Mean
Freedom
Square
Variance
Intensity
0.358
0.339
1.303
5.02
Plot (Intensity
0.275
4.400
38.68
0.002
Error
56.30
0.062
Endocladia muricata
Cochran's Test = 0.598 heterogeneous
Transformation: arcsine
F-Ratio
Source
Degrees of
P Value
% of Total
Mean
Freedom
Square
Variance
Intensity
1.874
0.257
0.233
12.46
Plot(Intensity
31.46
0.137
3.856
0.005
Error
56.07
0.036
Hildenbrandia spp.
Cochran's Test = 0.471 heterogeneous
Transformation: arcsine
F-Ratio
P Value
Source
Degrees of
Mean
% of Total
Freedom
Square
Variance
Intensity
0.300
0.075
0.751
Plot(Intensity
49.05
0.250
5.777
0.000
Error
36
0.043
50.95
Horng & Hayhurst: Haul-Out Effects of the Pacific Harbor Seal on the Rocky Midtidal Community
Mastocarpus papillatus (Petrocelis phase)
Cochran's Test = 0.261 heterogeneous
Transformation: arcsine
% of Total
F-Ratio
Source
Degrees of
P Value
Mean
Freedom
Variance
Square
Intensity
1.026
0.414
0.33
0.167
Plot(Intensity
2.709
25.47
0.163
0.028
Error
0.060
74.20
Tegula funebralis
Cochran's Test = 0.445 heterogeneous
Transformation: lo
(X+1)
F-Ratio
Source
Degrees of
% of Total
PValue
Mean
Freedom
Square
Variance
Intensity
3.596
1.438
0.309
5.67
24.98
Plot(Intensity)
2.501
2.800
0.024
0.89
69.35
Error
Horng & Hayhurst: Haul-Out Effects of the Pacific Harbor Seal on the Rocky Midtidal Community
Table II
P-Values for Pairwise Comparison Matrix of Tukey's HSD Test
Entries represent the probability that there is no difference between the plots.
Probability less than 0.05, indicating significant difference between the plots.
For algae, comparison of percent cover.
For T. funebralis, comparison of numerical abundance,
Mastocarpus papillatus
High Intensity
Low Intensity
Medium Intensity
Plots
14
High
13
1.000
Intensity
0.998
0.987
0.197
0.308
0.041
Low
0.956
0.013'
0.024
0.002
Intensity
0.914
0.973
0.523
0.916
0.262
0.187
0.294
1.000
0.039
0.906
0.961
Medium
0.355
0.506
0.091
0.839
1.000
0.985
1.000
Intensity
0.999
1.000
1.000
0.089
0.731
0.083 0.181
0.004'
Hildenbrandia spp.
High Intensity
Low Intensity
Medium Intensity
Plots
13
14
High
1.000
Intensity
0.764
0.735
0.998
0.996
0.990
Low
1.000
0.419
1.000
0.926
Intensity
0.535
0.019*
0.016
0.111
0.004
0.836
0.810
1.000
0.997
0.449
0.503
Medium
0.988
0.983
0.998
1.000
0.847
0.170
1.000
Intensity
0.735
0.765
0.042
0.287
0.960
0.057 0.197
0.000
Mazzaella affinis
No significant differences were found between any of the plots.
Endocladia muricata
High Intensity
Low Intensity
Medium Intensity
Plots
High
1.000
Intensity
1.000
1.000
0.002
0.002
0.002
Low
1.000
1.000
1.000
0.002'
Intensity
0.664
0.664
0.664
0.664
0.181
0.877
0.877
0.877
0.079
0.877
1.000
Medium
1.000
1.000
1.000
1.000 0.664
0.877
0.002*
Intensity
1.000
1.000
1.000
0.004“ 1.000 0.828
0.962 1.000
Horng & Hayhurst: Haul-Out Effects of the Pacific Harbor Seal on the Rocky Midtidal Community
Figure Legend
1. Shaded areas represent the worldwide distribution of harbor seals (Phoca vitulina) (Riedman,
1990).
2. Topographical map of Hopkins Marine Refuge. Arrows indicate locations of the sampled plots.
3. Graph of the average number of seals at any given time during the day for each plot. Intensity
refers to seal haul-out intensity. Large differences among intensities support the categorization of
plots as low, medium, and high in seal haul-out use.
4. Close-up topographical map of Hopkins Marine Refuge, Hewatt's Dome and vicinity,
Numbers and arrows indicate the location of each plot. 12 and 13 are high intensity plots and 14 is
à medium intensity plot. Plot 14 extends to figure 5.
5. Close-up topographical map of Hopkins Marine Refuge, Bird Rock and vicinity. Numbers and
arrows indicate the location of each plot. 6 and 7 are low intensity plots and 9 is a medium
intensity plot. Parts of plot 14, a medium intensity plot, extends over from figure 4. Plot 7
includes both sides of Pete's Rock.
6. Close-up topographical map of Hopkins Marine Refuge, Seal Island and vicinity. Numbers and
arrows indicate the location of each plot. 5 is a low intensity plot, 4 a medium intensity plot, and 5
a high intensity plot.
7. Plot of the cumulative number of species encountered as the number of points in the quadrat
increased. The vertical line marks the number of points sampled for each quadrat in this study
(35).
Horng & Hayhurst: Haul-Out Effects of the Pacific Harbor Seal on the Rocky Midtidal Community
8. Average percent cover was graphed for the six most abundant species of algae encountered.
Error bars for each column graph standard error. Intensity refers to seal haul-out intensity. No
clear trends in general algae cover can be seen among seal haul-out intensities.
9. Average percent cover of Mastocarpus papillatus and Mazzaella affinis was graphed for each
plot. Error bars represent the standard error of that plot. Low, medium, and high refers to seal
haul-out intensities. The graph indicates trends of decreasing M. papillatus cover and increasing
M. affinis cover with greater seal haul-out intensities.
10. Average dry algae biomass (grams / meter2) was graphed for each plot. The triangle symbols
indicate the mean of the three plots of that intensity. Intensity refers to seal haul-out intensity. A
general trend of increasing algae biomass with increasing seal haul-out intensity is indicated by the
graph.
11. Average extended algal frond length (millimeters) was graphed for each plot. The triangle
symbols indicate the mean of the three plots for that intensity. Error bars indicate the standard
error of that plot. Intensity refers to seal haul-out intensity. A possible trend of decreasing algae
frond length with increasing seal haul-out intensity is indicated by the graph.
12. The Shannon index of diversity (H') was graphed for each plot. Symbols represent the average
diversity of those three plots. Average diversity is used to estimate the diversity for that intensity.
Intensity refers to seal haul-out intensity. The graph indicates similar diversities for each seal haul¬
out intensity.
18
Horng & Hayhurst: Haul-Out Effects of the Pacific Harbor Seal on the Rocky Midtidal Community
Figure 1

un.
80
30--
45

Greenan

Norway

es
celand

USs
g
Great
Sntain

Canada

treland—
Pacisic Ocean
Umdesae.

Atlantic
en
Ocean
0.

Distribution of the harbor seal (5 subspecies), Monterey Bay Aquarium
Horng & Hayh
0
scts of th ac ic l 5or Seul o e cky Iiduidal Community
Horng & Hayhurst: Haul-Out Effects of the Pacific Harbor Seal on the Rocky Midtidal Community
Figure 3
15.
Low Intensity
Medium Intensity
High Intensity

7 6 9 14 4 13 3
Plot
21
—:——

—

Tigure
Horng & Hayhurst; Haul-Out Effects of te-Pacific Harbor Seal on the Rocky Midtidal Community

—--.---
— .„ ——

95
9.

—i

—.—-

.. -

Hewatt's Dome
55

T
13

a
—

+ 61

—- ——.

—

1—
—
14
S
G
--:—

. . .
. 90 6.
—-—7
Horng & Hayhurst: Haul-Out Effeets of thePacsic Harbor Seal on the Rocky Midtidal Comminity
Bird Rock

—.—
Pete's Rock

8
-

2
Horng & Hayhurst: Haul-Out Effects, of the Pagific Harbor Seal on the Rocky Midtidal Community
Figure
—
Seal Island
—----
—- - —.-—

1
1
6
59

—-..- - -—

l
2

—
.

—

p
0

R
—
24
—
Horng & Hayhurst: Haul-Out Effects of the Pacific Harbor Seal on the Rocky Midtidal Community
Figure 7
15-

u
10 20 30 40 50 60 70 80 90
Number of Points
Horng & Hayhurst: Haul-Out Effects of the Pacific Harbor Seal on the Rocky Midtidal Community
Figure 8
60-
E Low Intensity
Medium Intensity
50-
High Intensity
20
10-

26
Horng & Hayhurst: Haul-Out Effects of the Pacisic Harbor Seal on the Rocky Midtidal Community
Figure 9
Mastocarpus papillatus
Mazzaella affinis
60-
40

Plot
Horng & Hay hurs: Haul-Out Eliecis et de Pacific Harbo. Seal on the Rocky Midtidal Community
Figure 10
1000-
750-
Low Intensity
Medium Intensity
500-
High Intensity
Mean
A
250-

14 4 13 3 12
Plot
Horng & Hayhurst: Haul-Out Effects of the Pacific Harbor Seal on the Rocky Midtidal Community
Figure 11
40-
30-
E Low Intensity
Medium Intensity
20-
High Intensity
Mean
A
10

olig

14 4 13 3 12
Plot
orng & Hayhurst: Haul-Out Ellects ol die Pacific Harbor Seal on the Rocky Nlidtidal Cer
Figure 12
2.5-
Low Intensity
E
Medium Intensity
E
High Intensity
E
1.5.
Low Intensity Average
A
Medium Intensity Average
0.5-

High Intensity Average

5
9 14 4 13 3 12
Plot
munity