The Effect of Artificial Shoreline on Habitat Quality and Mortality of Blue Crabs,
Callinectes sapidus.
PROJECT REPORT
September 2005
Paul Jivoff
Department of Biology
Rider University
2083 Lawrenceville Rd
Lawrenceville, NJ 08648
TEL: 609-895-5421
FAX: 609-895-5782
e-mail: pjivoff@rider.edu
OBJECTIVES
The overall objective of this research was to examine the effect of bulkheading on marine and
estuarine habitats, the diversity of species and the relative abundance of individuals, particularly
the blue crab, Callinectes sapidus. In addition, the relative mortality rates of juvenile blue crabs
in front of artificial versus natural shorelines were compared using tethering techniques.
Specifically, in areas adjacent to natural versus artificial shoreline, I compared:
1.
2.
3.
4.
physical attributes (depth profile, sediment characteristics) of available habitats,
species diversity and abundance of individuals of all species captured,
abundance and population characteristics (age, size, sex, molt stage) of blue crabs,
relative mortality rate of juvenile blue crabs.
RESULTS
Physical Attributes of Shorelines
As expected, the water characteristics among the shorelines were similar (Figure 1).
Over the entire sampling period (May-August combined), water temperature (F2,36=1.14,
P=0.332), salinity (F2,36=0.73, P=0.489), and dissolved oxygen (F2,33=1.69, P=0.200) did not
differ among the shoreline types. However, as predicted, water depth was significantly different
among the shoreline types (F2,18=44.8, P<0.0001) (Figure 1) with bulkheads being significantly
1
deeper than beach (Tukey HSD, P<0.0001) and marsh (Tukey HSD, P<0.0001) habitats. Depth
profiles indicate that bulkheads lack water depths of less than 1 meter; the water depths that
dominate at both beaches and marshes (Figure 2). Marsh shorelines are often characterized by a
sharp drop-off at the marsh edge whereas beaches exhibit a more gradual slope. As a result
beach shorelines were significantly shallower than marsh shorelines at 1 meter (Tukey HSD,
P=0.015) and 2 meters (Tukey HSD, P=0.036) from shore. Sediment composition also differed
among the shoreline types in four out of five grain size categories. Bulkheads contained more
gravel-sized particles than either beach (Tukey HSD, P=0.040) or marsh habitats (Tukey HSD,
P=0.018) (Figure 3). Bulkheads also contained more granule-sized particles than marsh habitats
(Tukey HSD, P=0.002) (Figure 3). Both the beach (Tukey HSD, P<0.0001) and bulkheads
(Tukey HSD, P=0.0001) had greater amounts of medium sand grains than the marsh habitats
(Figure 3). The marsh habitats had greater amounts of silt than either the beach (Tukey HSD,
P<0.0001) or the bulkheads (Tukey HSD, P<0.0001) (Figure 3). By comparing the physical
characteristics of bulkhead habitats to those of directly adjacent natural shorelines it is possible
to assess how bulkheads have potentially altered the habitat. For example, one of the bulkheads
(“Blue Roof Bulkhead”) was directly adjacent to a marsh habitat (“Blue Roof Marsh”) similar to
what the bulkhead presumably replaced. This comparison indicates a considerable loss of silt
concomitant with large gains especially in medium sand but also in granule-sized and gravelsized particles (Figure 4).
Species Diversity and Abundance of Individuals
The total number of species captured (fish and decapods) at the beach (28 species) and
marsh (26 species) was greater than at the bulkhead (18 species) (see Table 1). However, the
Shannon-Weiner index, which takes into account both species richness and species evenness, of
2
bulkheads (1.51) exceeded that of the beach (0.83) and marsh (0.95) habitats. Thus, the beach
and marsh habitats had reduced species evenness because fewer key species constituted the
majority of individuals captured. For example, the beach had 5 species and the marsh had 3
species making up at least 1% of the total number of individuals captured whereas the bulkhead
had 9 species that did so.
The sampling effort varied among the shorelines because the gear and the amount of
shoreline sampled differed. Therefore, the absolute abundance data were converted to catch per
unit area in order to make equivalent comparisons among the shoreline types. For each sampling
day and site, the abundance of each species was divided by the area sampled by the gear (gear
width x length of area sampled). The length of the sampling area was measured at each site.
The width sampled by each gear was 6.6m for seines and 5.3m for trawls.
Overall, the total abundance of organisms captured varied among the shorelines
(F2,405=31.1, P<0.0001) with both the beach (Tukey HSD, P<0.0001) and the marsh (Tukey
HSD, P<0.0001) habitats hosting a greater abundance of organisms (Figure 5). The total number
of fish (F2,298=17.7, P<0.0001) and decapods (F2,94=18.6, P<0.0001) varied among the shoreline
types. The abundance of both fish and decapods exceeded that of the bulkheads (Tukey HSD,
P<0.0001 for each comparison) (Figure 5). Fifteen species that were captured at all three
shorelines, 6 of those species including the blue crab, differed significantly in abundance among
the shorelines (Table 1). All 6 species were more abundant at either one or both of the natural
shorelines as compared to the bulkheads (Table 1).
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Abundance and population characteristics of blue crabs
A total of 72 blue crabs were captured during the sampling period. While blue crabs
were more abundant at both the beach (Tukey HSD, P<0.0001) and marsh (Tukey HSD,
P<0.0001) as compared to the bulkhead, no other population characteristics differed for blue
crabs among the shorelines including average size (F2,69=1.52, P=0.22), age distribution
(2=1.27, df=2, P=0.53), sex ratio (2=0.74, df=2, P=0.69), or incidence of molting (2=1.62,
df=2, P=0.44).
Relative mortality rate of juvenile blue crabs
Juvenile blue crabs tethered at the beach and marsh had greater mortality rates than at the
bulkhead, however the differences were not significant (F2,9=2.28, P=0.16) (Figure 6). This
pattern of mortality does not concur with my initial expectation that mortality would be higher at
the bulkheads due to the lack of shallow water which acts as a refuge and thus the relative
abundance of large blue crabs which previous work has shown are often the main predator on
tethered juvenile blue crabs. Instead, the mortality rates may simply reflect the relative
abundance differences among the habitats; there were simply more blue crabs at both the beach
and the marsh than at the bulkheads to act as predators on the tethered crabs.
Summary and Conclusion
The results of this research suggest that the habitat in front of bulkheading is of lesser
quality than that provided by beaches and marshes, principally by a lack of shallow water and
potentially by altered sediment characteristics. Bulkheads have reduced species richness and
extremely low secondary production as compared to natural shorelines. The juveniles of several
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commercially and/or recreationally important species (e.g., blue crabs, spot, kingfish, bluefish)
preferred natural shorelines and avoided bulkheading. Blue crabs were more abundant at both
beach and marsh habitats as compared to bulkheads however no population characteristics or
juvenile mortality rates differed significantly among the shorelines.
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Table 1. Number of fish and decapods captured at each shoreline (May-August). Species
captured at all three shorelines are underlined, those that differ significantly (P<0.05) in catch per
unit area among the shorelines are in bold.
Species
Fish
Anchoa mitchilli
Anguilla rostrata
Clupeid sp.
Cynoscion regalis
Cyprinodon variegatus
Fundulus heteroclitus
Fundulus majalis
Gasterosteus aculeatus
Gobiosoma bosc
Leiostomus xanthurus
Menidia menidia
Menticirrhus saxatilis
Opsanus tau
Lagodon rhomboides
Pleuronectes americanus
Pomatomus saltatrix
Scophthalmus aquosus
Selene vomer
Sphoeroides maculatus
Sphyraena borealis
Strongylura marina
Syngnathus fuscus
Tautoga onitis
Tautogolabrus adspersus
Total
Decapods
Callinectes sapidus
Cancer irroratus
Carcinus maenas
Crangon septemspinosa
Eurypanopeus dentatus
Palaemonetes pugio
Palaemonetes sp.
Libinia emarginata
Ocellatus occidentalus
Total
Beach
62
0
4
4
3
278
240
2
5
3
13509
6
1
4
12
10
1
0
2
17
5
62
2
0
14232
Beach
34
0
7
536
12
2934
0
1
10
3534
Shoreline
Bulkhead Marsh Total
0
79
141
0
4
4
0
2
6
10
40
54
0
0
3
0
340
618
0
65
305
2
65
69
0
7
12
0
3
6
467
5425 19401
0
1
7
2
5
8
0
1
5
1
1
14
0
13
23
1
0
2
0
1
1
23
1
26
0
0
17
0
28
33
64
59
185
11
5
18
0
1
1
581
6146 20959
Bulkhead Marsh Total
5
33
72
2
0
2
4
6
17
1
5
542
72
70
154
225
8959 12118
11
0
11
12
0
13
3
0
13
335
9073 12942
6
30
28
26
24
22
Mean Values
20
18
Beach
Marsh
Bulkhead
16
14
12
10
8
6
4
2
0
Temperature (oC)
Salinity (ppt)
Dissolved Oxygen (mg/l)
Physical Variables
Depth (m)
Figure 1. Average (+1 SE) values of physical variables for each shoreline (May-August combined).
7
0
1
2
3
4
5
6
7
Distance from Shore (m)
8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
0
0.2
0.4
0.6
Depth (m)
0.8
1
Beach
Marsh
Bulkhead
1.2
1.4
1.6
1.8
2
2.2
Figure 2. Average (+1 SE) depths every meter from 0-22 m from shore at each shoreline.
8
1.3
1.5
12.9
Beach
22.5
61.9
1.1
0.5
15.8
gravel
granule
Marsh
20.9
61.8
3.8 3.6
4.1
15.4
Bulkhead
73.1
Figure 3. Percent composition of 5 size categories of sediments at each shoreline.
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medium
very fine
silt
1.4
0.6
3.7
1.8
Blue Roof
Marsh
92.5
gravel
granule
6.4
7.0
medium
7.6
6.2
very fine
silt
Blue Roof
Bulkhead
72.9
Figure 4. Percent composition of 5 size categories of sediments at one bulkhead and an adjacent marsh site.
10
3.5
Catch per Unit Area
3.0
2.5
2.0
Beach
Marsh
Bulkhead
1.5
1.0
0.5
0
decapod
fish
Organism Category
Figure 5. Average (+1 SE) total abundance and abundance of each major organism category
at each shoreline (May-August combined).
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total
50
45
40
Percent Mortality
35
30
25
20
15
10
5
0
beach
marsh
Shoreline
Figure 6. Average (+1 SE) mortality of tethered blue crabs at each shoreline (May-August combined).
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bulkhead