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Marine reserves can help in maintaining biodiversity and potentially be useful as a fishery management tool by removing human-mediated impacts. Intertidal, soft-sediment habitats can often support robust recreational and commercial shellfish harvests, especially for clams; however, there is limited research on the effects of reserves in these habitats. In San Juan County, Washington, several reserves prohibit recreational clam digging. We examined the effects of these reserves on infaunal community composition through comparison with non-reserve beaches during a 6-week period. Clam abundance, overall species richness and total polychaete family richness were greater on reserve beaches compared to non-reserve beaches. Additionally, an experiment within a reserve demonstrated negative impacts of digging on non-target infauna. These effects probably resulted from local disruption and disturbance of the sediment habitat and not from increased post-digging predation, which was controlled. Intertidal reserves could play an important role in sustaining local and potentially regional biodiversity.
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Marine Biology (2006) 149: 1489–1497
DOI 10.1007/s00227-006-0289-1
RESEARCH ARTICLE
Jennifer GriYths · Megan N. Dethier · Amanda Newsom
James E. Byers · John J. Meyer · Fernanda Oyarzun
Hunter Lenihan
Invertebrate community responses to recreational clam digging
Received: 27 April 2005 / Accepted: 10 January 2006 / Published online: 5 May 2006
© Springer-Verlag 2006
Abstract Marine reserves can help in maintaining bio-
diversity and potentially be useful as a Wshery manage-
ment tool by removing human-mediated impacts.
Intertidal, soft-sediment habitats can often support
robust recreational and commercial shellWsh harvests,
especially for clams; however, there is limited research on
the eVects of reserves in these habitats. In San Juan
County, Washington, several reserves prohibit recrea-
tional clam digging. We examined the eVects of these
reserves on infaunal community composition through
comparison with non-reserve beaches during a 6-week
period. Clam abundance, overall species richness and
total polychaete family richness were greater on reserve
beaches compared to non-reserve beaches. Additionally,
an experiment within a reserve demonstrated negative
impacts of digging on non-target infauna. These eVects
probably resulted from local disruption and disturbance
of the sediment habitat and not from increased post-dig-
ging predation, which was controlled. Intertidal reserves
could play an important role in sustaining local and
potentially regional biodiversity.
Introduction
Marine reserves have the potential to help preserve bio-
diversity, boost and sustain overexploited Wsheries, and
serve as benchmarks for undisturbed ecosystems (Pauly
et al. 2002; Thrush and Dayton 2002; Gell and Roberts
2003). Empirical research on subtidal reserves strongly
support the prediction that protected areas enhance bio-
diversity as well as the abundance and size structure of
some Wshery target species (Babcock et al. 1999; Halpern
and Warner 2002). Less attention is paid to intertidal
areas, popular not only for non-extractive recreational
use but also for recreational and commercial shellWsh
harvest (e.g. clams and oysters).
Limited research on intertidal harvesting eVects has
focused primarily on target clam species and abiotic
responses, but little on non-target species and non-
mechanical (i.e. human powered) harvesting practices.
The implications of reserves for target species were dem-
onstrated by Byers (2005) for the primary harvest spe-
cies, the non-native clam Venerupis phillipinarum, which
had higher abundances within reserves. Abiotic
responses to digging disturbance include sediment shifts
to coarser surface deposits that are deWcient in organic
and bioaggregated particles (Anderson and Meyer 1986).
Dernie et al. (2003) also found that the sediment compo-
sition and hydrodynamic regime of an area can alter the
eVects of harvesting disturbance; disturbed plots in mud-
dier sites took longer to inWll than in sandier sites. Their
study did not explore the eVects of any speciWc harvest-
ing practice, but showed that digging disturbances do
aVect overall community composition, with both the
number of individuals and the species decreasing imme-
diately after harvest. Lenihan and Micheli (2000) showed
that hand raking of oysters and hand digging for clams
caused relatively high rates of mortality and reduced
abundance of non-target shellWsh on intertidal oyster
reef habitat. Bait-worm digging also negatively aVected
the non-target clam Mya arenaria through shell damage
and increased exposure to predation (Brown and Wilson
1997); even low-intensity commercial digging for clams
and bait worms aVects benthic community structure in a
short period of time by reducing species richness and
abundance. However, this study was conducted on a
heavily dug beach, and pre-experiment baselines were
Communicated by P.W. Sammarco, Chauvin
J. GriYths (&) · M. N. Dethier · A. Newsom · F. Oyarzun
Biology Department and Friday Harbor Laboratories,
University of Washington, Friday Harbor, WA 98250, USA
E-mail: jeng@u.washington.edu
J. E. Byers · J. J. Meyer
Department of Zoology, University of New Hampshire,
Durham, NH 03824, USA
H. Lenihan
Donald Bren School of Environmental Studies
and Management, University of California,
Santa Barbara, CA 93106, USA
1490
not non-disturbance baselines. Most recently, Skilleter
et al. (2005) showed in a three-tiered study that the recre-
ational and commercial harvesting of callianassid shrimp
increases polychaete spatial patchiness and causes
declines in abundance of polychaetes, soldier crabs, and
amphipods.
Human impacts on soft sediment intertidal areas,
such as exacerbated erosion, nutrient input, beach nour-
ishment, trampling, and harvest, have led to the estab-
lishment of marine reserves to protect some coastal
habitats. The San Juan Islands in Washington State,
USA (Fig. 1) contain several such reserves, closed to
shellWsh harvesting within the last 2 decades to maintain
biodiversity and pristine sites for ecological research.
Because productivity on these beaches is high and local
water quality is good, many surrounding sites outside of
the reserves are subject to heavy recreational harvest for
a variety of clam species. We examined the eVects of
reserves on community composition via a broad survey
of reserve and non-reserve beaches. In addition, we initi-
ated an experiment on one reserve beach to quantify the
impact of recreational clam digging on non-target infau-
nal organisms, and the aspects of disturbance that
impact the community most severely.
Fig. 1 Survey and experimental
sites, San Juan Islands, Wash-
ington, USA (48°32.8N,
123°0.6W). All sites were sur-
veyed for clams and other infau-
na except those denoted by
asterisks which were only sur-
veyed for clams. Argyle Argyle
Creek, University of Washing-
ton Reserve, San Juan Island;
Bell* Bell Harbor, San Juan
Island, ECC English Camp
National Park Closed, San Juan
Island, ECO English Camp
National Park Open, San Juan
Island, ES* Eastsound, Orcas
Island, MB* Mud Bay, Lopez
Island, Reid Harbor Reid Har-
bor, Stuart Island, Shaw Shaw
University of Washington Re-
serve, Shaw Island, SS Spencer
Spit State Park, Lopez Island
1491
Methods
Survey methods
We conducted a quantitative survey of all macroscopic
invertebrates at three reserve and three non-reserve sites
in the San Juan Islands (Fig. 1). The three reserves
included Argyle Creek and Shaw Reserve, both owned
by the University of Washington and established as
reserves in 1990, and a section of English Camp (previ-
ously British Camp, Byers 2005) National Park that has
been closed to shellWsh harvesting since »1977. The three
non-reserves were English Camp Open on San Juan
Island, Spencer Spit State Park on Lopez Island, and
Reid Harbor on Stuart Island. These non-reserve sites
experience intense recreational shellWsh harvesting pres-
sure. Clam abundances but not other invertebrates were
sampled at three additional non-reserve sites (Bell, East-
sound, and Mud Bay, Fig. 1). Sites were chosen to corre-
spond with previous clam surveys by Byers (2005) in
2000. Byers (2005) conWrmed that general physical vari-
ables, such as primary productivity (i.e. chlorophyll),
salinity, and temperature were similar across sites. Sites
were on beaches with mixed mud–sand–pebble sediment,
limited wave exposure, and a minimum combined den-
sity of Protothaca staminea (the native littleneck clam)
and Venerupis philippinarum (the Japanese littleneck) of
16 individuals/m2. This latter biological criterion directly
ensured that the selected reserve and non-reserve sites
were suitable habitat for the primary targets of clam-
mers.
Survey sampling was completed during October and
early November 2003. At each site, we dug test holes to
establish the intertidal vertical distribution of Prototh-
aca, which shares the same vertical range as Venerupis.
Two transects running parallel to the water were then
established between these vertical limits, at approxi-
mately 0.5 and 1.0 m above mean lower low water
(MLLW). Tidal heights were measured using a hand
level and meter stick and compared to NOAA tide pre-
dictions for Friday Harbor, San Juan Island.
In each horizontal transect, clams were sampled in
Wve or six 0.125 m2 cores dug to 17 cm (beyond maxi-
mum clam burial depth, Byers 2005) and sieved in the
Weld through 5 mm mesh. Other species were sampled
with Wve or six 10 cm diameter cores to a depth of 15 cm
and sieved on 4- and 1-mm nested sieves. All samples
were taken 3–6 m apart; distances varied due to diVer-
ences in beach length and permitting constraints at
English Camp National Park. Non-clam infaunal organ-
isms were brought back to the lab, preserved, and identi-
Wed to species if possible, using KozloV (1987), Banse
and Hobson (1974), Hobson and Banse (1981), and
Blake et al. (1996).
Because infaunal communities are known to be sensi-
tive to sediment type, one core was taken up to 10 cm
depth at each tidal height for sediment analysis using 2-
cm diameter, 50-ml plastic centrifuge tubes. Samples
were stored in a cold room at 12°C until analysis and
protocol was guided by Folk (1974) and Puget Sound
Estuary Program (1986). Samples were wet-sieved
through a standard set of sieves (4, 2, 1, 0.5, 0.25, 0.125,
and 0.063 mm) with fresh water. Fine sediments were
collected in a bucket below the sieves and concentrated
by air-vacuum Wltering through pre-weighed 6-m Wlter
paper. Filters with Wne sediments were air-dried, and
their weight calculated. Larger grains retained on the
sieves were dried at 80°C for a minimum of 5 h and
weighed.
Experimental methods
We quantiWed the impacts of digging on non-clam
infauna at the English Camp reserve site (ECC, Fig. 1).
At 0.5 m above MLLW we established 16 treatment
plots in blocks of four (i.e. 4 treatments/block; Fig. 2).
Each plot measured 1£0.6 m and was separated from the
next by 1 m. A digging treatment simulated the distur-
bance created by recreational clammers; a no-digging
treatment simulated a reserve where clamming is prohib-
ited. Clammers are typically knowledgeable about the
vertical range of target clams and dig holes in horizontal
swaths across the beach within the clams’ range. Our dig-
ging was done in half the plots; a 30£30 cm hole was dug
to a depth of 20 cm, and the sediment from the hole
(“Wll”) was deposited to one side of the plot. Holes were
not reWlled, following common clamming practices.
Holes were initially dug during week 0 and the same
points were re-dug during weeks 2 and 4. InWlling rates
of holes will vary with wave activity and sediment type.
We predicted our holes would reWll and redug the holes
to simulate the return of clammers to a dug and reWlled
beach. The no-digging controls were left undisturbed.
To test the role of predators in both digging and non-
digging treatments, half of the plots were protected under
cages made of 1/2 in. hardware cloth (Fig. 2) preventing
access by any marine or terrestrial macropredators (e.g.
crabs, raccoons). Cages were held in place at four corners
by 3/8 in. rebar stakes, and the sides were buried 2-cm
Fig. 2 Experimental design at
English Camp. Two factors with
controls were each replicated
four times on the beach in a
horizontal line. Each plot is
1.0£0.6£0.01 m
Digging – Cage Digging –
No Cage Control – Cage Control –
No Cage
Hole Fill Hole Fill
1492
into the sediment allowing easy migration of infaunal
organisms. The height above the sediment was approxi-
mately 8-cm. Thus there were four treatments: (1) dig-
ging; (2) digging covered with a cage; (3) no-digging; and
(4) no-digging covered with a cage. Treatments were
blocked (one replicate of each of the four treatments per
block) in space to account for any systematic spatial vari-
ation. Caged treatments alternated spatially with non-
caged treatments to confer similar water circulation. A
coin toss was used to assign digging and no-digging treat-
ments within blocks to cage and no-cage treatments.
All experimental treatments were sampled for non-
clam infauna at 1 and 5 weeks, using the same infaunal
coring methods as described above. No-digging treat-
ments were sampled with one core taken anywhere
within the treatment plot. Digging treatments were sam-
pled in the hole and also the Wll. Hole samples were taken
in the sediment inWll not below the depth of the original
hole. Samples were sieved and preserved as described
above. Three sediment cores per treatment type (one core
in three of four plots per treatment) were also taken dur-
ing week 5 only for grain size analysis.
Additional sediment cores were taken during week 5
for the analysis of particulate organic carbon. One core
was taken per disturbance type and frozen until analysis.
Samples were thawed and wet-weighed on pre-weighed
weighboats, then dried at 80°C for a minimum of 24 h
and weighed again. They were then placed in a muZe
furnace at 500°C for 4 h and re-weighed (Byers 2002).
Loss on ignition was used as a proxy for organic content
by comparing the portion of the sediment weight lost
between the dry and combusted samples.
Statistical analysis
Statistical tests were performed with JMP 5.0.1 and ini-
tially tested for normality and homogeneity. Normality
was determined by goodness of Wt under the Shapiro–
Wilk W test. Data were considered homogeneous when
they were not signiWcant (P value >0.05) under both the
Brown–Forsythe and Bartlett tests.
Clam abundances were compared separately between
reserves and non-reserves with one-way Kruskal–Wallis
tests. The non-parametric, one-way test was used
because reserve and non-reserve sample sizes and vari-
ances were unequal, and because higher abundances for
both species inside reserves than on harvested beaches
were predicted (Byers 2005). A one-way Wilcoxon post
hoc test was used to compare abundances among all
sites. Non-clam data were analyzed for species richness,
polychaete family richness, and biodiversity using a
three-way, nested ANOVA design: reserve, site (reserve),
and tidal height (reserve, site). A nested parametric anal-
ysis could not be used for epifaunal species richness or
the Shannon Biodiversity Index due to lack of normality.
The data were separated by high and low tidal heights
and a Wilcoxon non-parametric test performed on each
tidal height separately and combined. Multivariate anal-
yses of the whole community (epifauna and infauna)
were performed using PRIMER software (Clarke and
Gorley 2001). The data matrix of abundances was
square-root transformed, and ordinations performed
using non-metric multidimensional scaling (MDS). Anal-
yses of similarity (ANOSIM) tested the signiWcance of
hypothesized diVerences in communities among treat-
ments (reserve vs. non-reserve) and tidal heights, and the
SIMPER routine analyzed the species most important in
separating treatments. Correlations between community
similarities and physical variables (grain sizes) were
tested using the BEST procedure.
The percent gravel (retained on 4 and 2 mm sieves),
sand (1, 0.5, 0.25, 0.125, and 0.063 mm sieves), and Wnes
were calculated for all sediment samples. With two-way,
crossed ANOVAs we tested the eVect of reserve status
and tidal height on grain size distribution between
gravel, sand, and Wnes, with each grain size analyzed sep-
arately. Additional two-way, crossed ANOVAs tested
for the eVect of individual sites and tidal height on grain
size distribution. Linear regressions were used to check
for correlations between sediment size distributions and
species richness and abundance.
Experimental data from English Camp were analyzed
using a two-way, crossed MANOVA for species richness,
with “Digging” and “Cages” as the Wxed factors. A
repeated measures response was used to test for diVer-
ences between sampled times. Separate analyses were
done for hole samples and Wll samples. We also tested for
the eVect of the treatments on total polychaete abun-
dance with a one-way MANOVA with only digging as a
factor. Linear regressions were used to check for correla-
tions between species richness and either percent Wnes or
percent organic material at this Wner spatial scale. Com-
munity-level analyses of infaunal abundances were done
with PRIMER, as described above.
Results
Survey
The most common clam species in the sampled zones was
the native littleneck, Protothaca staminea (38% of all indi-
viduals). The introduced Japanese littleneck, Venerupis
philippinarum, a favored species for human consumption,
made up 13% of sampled individuals. Other species found
included mud clams (Macoma spp.), butter clams (Saxi-
domus sp.), the non-native softshell clam (Mya arenaria),
and the non-native purple varnish clam (Nuttalia obscu-
rata). Both littleneck species, which are largely targeted
by recreational harvesters, were more abundant inside
reserves than in areas where clam digging is allowed (Fig. 3,
Table 1a). The post hoc test revealed that for each species,
one site drove the diVerence between reserve and non-
reserve sites: English Camp Closed for Protothaca, and
Shaw Reserve for Venerupis. Venerupis was signiWcantly
less abundant than Protothaca at all sites. Site had a sig-
niWcant eVect on the densities of both species.
1493
The infauna at all surveyed sites consisted mostly of
polychaetes, but also included nematodes and phoronids
(Phoronopsis harmeri) and the epifauna included amphi-
pods, Hemigrapsus spp. (shore crabs), and limpets. There
was a signiWcant, positive eVect of reserve status on over-
all species richness (Fig. 4, Table 1a). Post hoc compari-
sons using the Tukey HSD procedure indicate that this
eVect was driven by the Shaw Reserve site, which diVered
signiWcantly from all other sites (Table 1a). The low tran-
sects also had signiWcantly higher species richness than
the high transects (Fig. 4) in all sites. When infauna and
epifauna were analyzed separately, reserves showed a
signiWcantly higher non-clam infaunal species richness
(Fig. 4, Table 1a) and epifaunal richness (Fig. 4,
Table 1a) than non-reserves.
Multivariate analyses showed diVerences in overall
community composition (clams and other infauna and
epifauna). Communities diVered between reserves and
non-reserves (ANOSIM test, P=0.013); reserves had
greater abundances of most clam species and a variety of
polychaetes. Non-reserves had more Macoma nasuta (a
non-harvested clam species), limpets, and Nereis poly-
chaetes. Communities also diVered among tidal heights
(P=0.002).
Sediment composition varied greatly among sites
(Table 2). For example, English Camp Closed had a very
large proportion of Wnes compared to Argyle Creek,
which had a high proportion of gravel. Tidal height did
not consistently aVect grain size distributions for gravel,
sand or Wnes. While there was a diVerence in the propor-
tion of gravel and sand between reserve and non-reserve
sites, percent Wnes were similar. Abundances of Prototh-
aca (r2=0.810) but not Venerupis (r2<0.01) were posi-
tively correlated with percent Wnes (Table 1). Non-clam
species richness was not correlated with percent Wnes
(linear regression, r2 =0.0033, P=0.858). Infaunal com-
munities overall showed only weak correlations with
grain sizes, with the strongest correlation (=0.29) with
Wnes alone.
Fig. 3 Protothaca and Venerupis clam densities at reserve (n=3)
and non-reserve (n=6) sites. Densities for each were averaged for
both high and low tidal elevations. Bars represent §1 SE
0
1
2
3
4
5
6
7
8
9
10
Venerupis Protothaca
S
p
ecies
Reserve
Non-Reserve
Density (#/0.125m
2
)
Table 1 Resu
l
ts.
(
a
)
Survey resu
l
ts
f
or c
l
am a
b
un
d
ance, non-c
l
am
species richness, and sediment composition and eVects on clam
abundance. (b) Experimental MANOVA results for species richness
and polychaete abundance
df 2P value
(a) Survey results
Clam abundance
(one-way Kruskal–Wallis)
Protothaca 1 5.507 0.019
Venerupis 1 7.339 0.007
Venerupis Abundance
compared to Protothaca 1 30.675 <0.0001
df 2P value
Site eVect on density
Protothaca 8 30.481 0.0002
Venerupis 8 25.746 0.0012
df Sum of
squares F ratio Prob >F
Non-clam
species richness
(three-way,
nested ANOVA)
Reserve status 1 56.023 22.7306 <0.0001
Site [reserve status] 4 73.769 7.4826 <0.0001
Tidal height
[reserve status, site] 6 55.083 3.7249 0.0039
Post hoc Tukey test Least square mean Standard error
Shaw 6.10 0.49645
ECC 3.70 0.49645
Argyle 2.40 0.49645
SS 2.60 0.49645
ECO 2.08 0.45320
Reid 1.8 0.49654
df Sum of
squares F ratio Prob > F
Infauna species richness (three-way, nested ANOVA)
Reserve status 1 10.01195 7.4016 0.0089
Site [reserve status] 4 30.52500 5.6416 0.0008
Tide height [reserve status, site] 6 20.38333 2.5115 0.0334
SZProb > Z
Epifauna species richness (Wilcoxon non-parametric)
1,147.5 3.10 0.0019
Gravel df Sum of squares F ratio Prob > F
Sediment composition (two-way, crossed ANOVA)
Reserve status 1 0.41275 14.2259 0.0023
Tidal height 1 0.03980 1.3684 0.2631
Reserve status£
tidal height 1 0.09461 3.2529 0.0945
Sand
Reserve status 1 0.64760 21.8217 0.0004
Tidal height 1 0.00248 0.0836 0.7770
Reserve status£
tidal height 1 0.01189 0.4006 0.5377
Fines
Reserve status 1 0.00086 0.0019 0.9662
Tidal height 1 0.04090 0.0886 0.7707
Reserve status£
tidal height 1 0.00908 0.0197 0.8906
1494
Experimental results
Experimental digging had a clear negative impact on the
infauna in the English Camp reserve. “Hole” samples
had signiWcantly reduced species richness compared with
control samples at both 1 week and 5 weeks (Fig. 5,
Table 1b). The caging treatment had no signiWcant
impact on species richness and there was no diVerence
over time among samples. There was also no diVerence
between the “Wll” samples and the controls due to dig-
ging (Fig. 5, Table 1b), caging, or time sampled.
Digging had a signiWcant impact on the total abun-
dance of polychaetes, which comprised a majority of the
infaunal individuals. Polychaete abundance was greatly
reduced in holes, and there was a change in eVect over
time (Table 1b). At week 1, abundances were reduced to
such low levels that no further reduction could be
observed in week 5. There was also a signiWcant interac-
tion between digging and time, indicating that these two
factors are not independent of each other in their eVect
on polychaetes. Again, there was no diVerence in the Wll
areas versus the controls due to digging (Fig. 6, Table 1b)
or time. Multivariate analyses of the whole infaunal
community further illustrate these trends (Fig. 7). Each
point represents the community in one sample, and
closer points have more similar communities (both spe-
cies and abundances). Control and Fill samples are
similar, with no clear trend among the diVerent weeks.
The Hole samples, on the other hand, are all outside the
‘cloud’ of Control and Fill points, and they show very
high variability both within and among times. ANOSIM
analyses showed strong diVerences among treatments
(P=0.001), but pairwise tests showed no diVerences
between Fill and Control communities. The Fill and
Control cores contained more of almost all families of
polychates (capitellids, orbiniids, lumbrinerids, and spio-
nids) than the Holes, while the Holes contained more of
only two taxa, the opheliid Armandia brevis, and the dor-
villeid Protodorvillea gracilis (represented by very few
individuals). Overall changes through time in the treat-
ments were gradual, with communities diVering between
weeks 0 and 5 but not at intermediate times.
Both the sediment composition and the organic
content were variable among samples, showing no clear
pattern (Table 3). There was no correlation between
Table 1
(
Cont
d
.
)
Interactions between all factors in the species richness two-way,
crossed MANOVA were not signiWcant and not included in the table
df Sum of squares F ratio P value
Correlation with percent Wnes (ANOVA)
Protothaca 1 67.58465 29.855 0.001
Venerupis 1 0.026355 0.007 0.94
df Value Exact FProb > F
(b) Experimental results
Species richness (two-way, crossed MANOVA)
Holes
Digging 1 1.18518 14.2222 0.0027
Caging 1 0.29629 3.5556 0.0838
Time 1 0.13298 1.5957 0.2305
Fill
Digging 1 0.01342 0.1611 0.6952
Caging 1 0.27181 3.2617 0.0960
Time 1 0.00365 0.0438 0.8377
df Value Exact FProb > F
Polychaete abundance (one-way MANOVA)
Hole
Digging 1 1.02819 14.3947 0.0020
Time 1 7.54067 105.5694 <0.0001
Digging£time 1 0.45954 6.4335 0.0237
Fill
Digging 1 0.02301 0/3223 0.5792
Time 1 0.07891 1.1047 0.3110
Digging£time 1 0.34973 4.8962 0.0440
Table 2 Proportion of gravel, sand, and Wnes per tidal height by sur-
vey site
Survey site abbreviations are the same as in Fig. 1 and reserve site
names are in bold. Tidal height deWnitions are as follows:
Low=approximately 0.5 m above MLLW; High=approximately
1.0 m above MLLW. N =1 for all sites. There was no sample for M
B Low
Site Tidal height Gravel Sand Fines
Argyle Low 0.944 0.045 0.011
Argyle High 0.619 0.380 0.002
ECC Low 0.531 0.361 0.107
ECC High 0.173 0.165 0.662
Shaw Low 0.546 0.412 0.042
Shaw High 0.456 0.516 0.028
ECO Low 0.199 0.566 0.235
ECO High 0.261 0.640 0.099
SS Low 0.294 0.681 0.025
SS High 0.290 0.704 0.006
Reid Low 0.094 0.864 0.042
Reid High 0.376 0.578 0.046
Bell Low 0.363 0.589 0.049
Bell High 0.299 0.605 0.096
MB High 0.245 0.727 0.028
ES Low 0.003 0.986 0.012
ES High 0.001 0.990 0.009
Fig. 4 Average non-clam species richness: reserves vs. non-reserve
sites. Species density per core pooling all reserve sites and non-re-
serve sites by tidal height. Bars as in Fig. 3
0
1
2
3
4
5
6
High All
Species
Low All
Species
High
Infauna
Low
Infauna
High
Epifauna
Low
Epifauna
Tidal Hei
g
ht and Spec ies M easured
Species Richness (#/ 7.85
-3
m
2
)
Non-Reserve Reserve
1495
percent Wnes and percent organic material (r2=0.0003,
P=0.158); however, sample sizes were extremely small.
Discussion
Prohibiting recreational clam digging in reserves has a
signiWcant eVect on soft sediment community composi-
tion of protected beaches. Reserve eVects are illustrated
by increased diversity, abundance of harvested clam spe-
cies (Fig. 3), non-clam species richness (including both
epifauna and infauna, Fig. 4), and overall community
composition inside reserves. Our survey characterized
these beaches only during one 6-week time period, but
because the sampling was done at a time of year with lit-
tle digging (see below), it may be conservative in its esti-
mation of an overall reserve eVect.
Physical factors such as wave energy and temperature
vary among sites and can aVect community composition
(Dethier and Schoch 2005), but using carefully selected
sites minimized the inXuence of factors other than clam
digging. There was no diVerence in sediment conditions
among sites that corresponded with reserve status. Addi-
tionally, while other studies show species richness (espe-
cially for polychaetes) correlating positively with the
percentage of Wnes in the beach sediment (Nichols 1970),
we did not Wnd this relationship at any of our sites.
The lack of correlation between species richness and
physical factors underscores that human harvesters
likely drive the observed diVerences among sites. Clam
digging is a popular recreational activity on beaches in
the San Juan Islands, and harvest weights generally fall
between 1,000 and 60,000 lbs each year at sites through-
out Washington State (WDFW 2000). In 2000, our sur-
vey sites had between 400 and 600 user trips (WDFW
2000). In the summer, accessible clam-rich zones of these
popular beaches often appear to be entirely disturbed,
suggesting that turnover of sediment may be rapid. This
magnitude of harvest has the potential to decrease not
only clam abundance, but also cause habitat changes
that directly reduce the richness and abundance of other
species within the aVected area.
Our experimental results from disturbance experi-
ments at the English Camp reserve indicate that digging
and not reWlling clamming holes reduces preferred habi-
tat but does not cause direct mortality of infauna. The
experimental clamming holes showed a signiWcant nega-
tive eVect of disturbance on species richness (Fig. 5), and
Fig. 5 Average non-clam species richness per treatment, English
Camp Closed. Species density per core by treatment sampled. The
unmanipulated control treatment was sampled only at time zero
(n=5) and all experimental treatments were sampled in weeks 1 and
5 (n=8). Bars as in Fig. 3
0
1
2
3
4
5
6
7
Control Control
Cage
Hole Fill Hole Cage Fill Cage
Treatment
Species Richnes s (#/ 7.85
-3
m
2
)
0 weeks
1 week 5 weeks
Fig. 6 Average abundance of polychaetes per treatment over time,
shown as the density per core. Bars as in Fig. 3
0
2
4
6
8
10
12
14
16
18
20
Control Hole Fill
Treatment
Abundance (#/ 7.85
-3
m
2
)
Week 0 Week 1 Week 5
Fig. 7 Non-metric multidimensional scaling plot of community
similarities (Bray–Curtis) for all cores in the digging experiment.
Closer points indicate greater community similarity. Numbers by
each point indicate the time of the sample (0, 1, or 5 weeks)
MDS of Experimental Sites
Treatment
2D Stress: 0.17
Control
Fill
Hole
1
111
1
1
11
5
5
5
5
5
5
5
5
1
1
1
15
51
1
1
1
5
5
1
1
5
1
55
5
5
5
5
5
1
1
1
1
055
105
0
0
0
5
Table 3 Average proportion and standard error of gravel, sand,
Wnes, and organic material per treatment
N=3 for all gravel, sand, and Wnes samples except Wll cage where
n=2. For organic samples n=1
Gravel Sand Fines Organics
Control 0.478§0.052 0.303§0.019 0.218§0.045 0.031
Control Cage 0.209§0.069 0.368§0.043 0.422§0.107 0.041
Hole 0.590§0.037 0.230§0.020 0.181§0.050 0.021
Hole Cage 0.604§0.036 0.252§0.011 0.144§0.032 0.026
Fill 0.619§0.022 0.276§0.015 0.105§0.009 0.032
Fill Cage 0.543§0.039 0.267§0.042 0.190§0.002 0.019
1496
an overall change in the community (Fig. 7), while the Wll
areas were not signiWcantly diVerent from the controls.
This suggests that most non-target infauna transferred
by clam diggers from a hole to a Wll area survive the pro-
cess and do not migrate elsewhere. The majority of fami-
lies found in the Wll areas are classiWed by Fauchald and
Jumars (1979) as discreetly motile or sessile, including
Spionidae, Cirratulidae, Nereidae, Lumbrineridae, and
Terebellidae. Discreetly motile organisms may move to
forage for food, but are functionally sessile in between
feeding bouts. Worms classiWed as motile, primarily Cap-
itellidae and Orbiniidae, were also found in Wll areas but
apparently did not re-colonize the holes, corresponding
to observations by Dernie et al. (2003) that opportunistic
species did not immediately enter disturbed areas.
Organism transfer therefore increased the patchiness of
distributions in the sediment. This consequence of har-
vesting was also shown by Skilleter et al. (2005) for poly-
chaetes and amphipods in areas of callianassid shrimp
harvest.
Potential long-term consequences of sediment trans-
fer, however, include decreased water circulation to the
sediment underneath “Wll” piles and therefore reduced
oxygen and nutrients. Post-digging mortality may be
higher in the summer due to the exposure of disturbed
organisms during daytime low tides, when higher tem-
peratures also increase thermal stress (Woodin 1974).
Additionally, infauna underneath the Wll pile may be
adversely aVected as Wll can increase their burial depth in
the sediment. This may ultimately decrease their ability
to get food and oxygen from the water column (e.g. for
clams) and increase mortality.
Numerous families, for example Capitellids, were
absent or in low abundance in the holes. These worms
are frequently colonizers of disturbed areas (Fauchald
1977; KozloV 1990), including the genus dominating
these samples, Mediomastus. However, digging treat-
ments completely altered the dug area; changes in sedi-
ment grain size, organic matter, and oxygen content
within holes may have made this habitat unsuitable for
these polychaetes, at least over the 5 weeks of this study.
Holes collected standing water and partly Wlled with
muddy, Wne sediments. Fill areas and controls, by com-
parison, had much larger grain sizes and a large amount
of shell hash, which are characteristic of these beaches.
The only species to occur in relatively high abundance
in holes was Armandia brevis, an extremely mobile worm
common in mixed sediments in the region. They
appeared in the samples only from the Wfth week, and
were not found in either the controls or the Wlls. There-
fore, in the short term, holes are suitable habitat for only
a single species on a normally rich and productive beach.
Turning over sediment exposes invertebrates to pre-
dators. Previous work by Ferns et al. (2000) found that
intertidal invertebrate harvesting frequently provides
feeding opportunities for birds. Birds were unlikely to
have aVected our experiment because low tides during
this period occurred after dark, and thus disturbed ani-
mals were exposed when few seabirds were foraging.
However, it is likely that during daytime low tides that
occur during peak clamming season (spring and sum-
mer), bird predation could be a signiWcant cause of
infaunal mortality.
Although crab and Wsh predation on clams can be
substantial in the San Juans (e.g., Byers 2005; Meyer and
Byers 2005), the lack of a cage eVect suggests that crab
and Wsh predation had little eVect on infaunal worms in
the disturbed areas. Woodin (1974) found that Cancer
magister is an important predator on polychaetes in the
low intertidal zone in this region. Crab surveys at English
Camp Closed concurrent with our study found very low
abundances of Cancer gracilis, and no other cancer crabs
(A. Glaub, unpublished), and Byers (unpublished data)
found crabs to be more abundant in warmer months.
During periods of intense clam digging, predatory crabs
and birds are likely to be more abundant and could have
a greater eVect on infauna.
Our experimental results are consistent with those of
Brown and Wilson (1997), who showed that commer-
cial clam and bait-worm digging had a negative impact
on several infaunal species after three months of dig-
ging treatments. Their experiment was conducted on a
heavily dug mudXat, potentially only populated by
organisms already adjusted to a high level of distur-
bance. Skilleter et al.’s (2005) much more intensive har-
vesting experiment also demonstrated negative eVects
on polychaete abundances and increased patchiness.
Despite a less-intensive digging treatment in our experi-
ment, there were still measurable eVects on community
composition, perhaps because of slow hole Wlling at
English Camp. The holes had only partially reWlled at
the time of the second treatment (2 weeks) and not at
all by the third treatment (4 weeks), despite periods of
stormy weather. Thus, redigging non-Wlled holes
resulted in deeper holes over time (intensifying the
treatment) although species response did not increase
after week 1. Fill areas continued to show control levels
of richness and abundance. Limited clam digging could
increase habitat heterogeneity and create a more
patchy distribution of polychaetes without decreasing
overall system diversity. However, high- intensity clam
digging due to high yearly visitation rates could lead to
more severe community impacts as entire beaches tend
toward hole environments.
Clam digging has a substantial inXuence on the abun-
dance of both target and non-target species in soft sedi-
ment communities. Digging disturbance increases
community patchiness and has the potential to decrease
community richness, as seen in the overall comparison of
reserve versus non-reserve beaches. Year round monitor-
ing of clamming beaches and reserve beaches would pro-
vide further insights on digging disturbance. Rotating
closures, especially among intensely dug beaches that are
already recreationally clammed may stimulate recovery
of the whole community. Continuing research on the
impacts of harvesting disturbance in soft sediment inter-
tidal communities is essential, along with the application
of that research to management plans.
1497
Acknowledgements Assistance in the Weld of Amy Glaub, Court-
ney Lyons, Jessica Harm, Abby Lunstrum, Marcos Toran, and Jen-
ny Selgrath was greatly appreciated. Craig Staude helped with the
sediment analysis. Thank you to all the MPA apprentices and the
Friday Harbor Labs community for encouragement and support,
and to the National Park Service for access to the English Camp
Historic Site and English Camp National Park. A grant from the
Mary Gates Foundation made this research possible. The experi-
ments performed in this research complied with all current laws of
the United States.
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