Preliminary Investigations of Constructed Oyster Reef Habitat in Lower ...
Preliminary Investigations of Constructed Oyster Reef Habitat in Lower Delaware Bay Jaclyn Taylor and David Bushek Haskin Shellfish Research Laboratory, Rutgers University, Port Norris, NJ 08349
Introduction
Methods
In June 2006, a preliminary small-scale oyster habitat enhancement project began at Rutgers’ Cape Shore Hatchery Facility near Green Creek, NJ in the lower Delaware Bay (Figure 1). The Cape Shore is an extensive high energy intertidal zone with consistently high oyster (Crassostrea virginica) settlement, but high predation, disease and other factors apparently limit the formation and persistence of oyster reefs. To evaluate variations of one reef construction method, experimental reefs of varying height were constructed in the intertidal zone where the high oyster settlement provides the potential for reefs to develop. The shell bag reefs are being monitored for stability, oyster recruitment and growth, sedimentation rates and colonization by benthic and motile species.
Shell Bag Reefs On June 22, 2006, 200 shell bags were deployed to form three 1.5m x 3m reefs varying in height from one to three layers of shell bags (e.g., Figure 2a). The reefs were placed 10 m apart, but were not replicated for this preliminary investigation. New Jersey
Oyster Recruitment and Mortality On October 5, 2006 oyster spat, scars and boxes were counted on all three shell bag reefs using a 10 cm² quadrat. Sixteen replicate counts were taken across the total area of each reef. Sedimentation Sediment levels were measured bimonthly through October using a line level and a meter stick at low tide. A permanent stationary pole was placed in the center of each reef to secure the line level. Measurements were taken in eight directions (N, S, E, W, NE, SE, SW, NW) around each reef from 0 to 2m out in .5m intervals (40 data pts per reef). Change in sediment levels were determined by subtracting the original sediment levels on June 26 from the bimonthly measurements.
Cape Shore Delaware
Delaware Bay
Fauna Sampling After oysters had recruited (Figure 2b), five standard minnow traps (6.35mm wire mesh) were secured to each constructed reef as well as an adjacent control sand site at low tide (20 traps total), then retrieved on the following low tide 12 hr later. Individuals captured in traps were identified to species, enumerated and measured for total length and biomass. Collections were repeated on two daytime tides (Oct 16 and 25) and two nighttime tides (Oct 25 and 26).
Figure 1: Location of study site in Delaware Bay
a)
a)
Average Oyster R ecruitment 6000 Oysters/m2
5000
*
4000 3000
Averag e B im o n th ly S ed im en t C h an g e
2000
0 Reef 1
Reef 2
Reef 3
S he ll Ba g Re e fs
Average Oyster Mortality
b)
b)
Percent Mortality
0.3 0.25 0.2
8 6 4 2 0 -2 -4 -6 -8 -10 June 26 July 12 July 27
A ug 8
A ug 21
S ept 6 S ept 18
Oct 3
0.15
Reef 1
0.1 0.05 0 Reef 1
Reef 2
Reef 3
She ll Ba g Re e fs
Figure 2. a) 2-layer shell bag reef constructed on June 22, 2006. b) 2-layer shell bag reef on Oct. 5, 2006 showing heavy oyster set.
Sediment Change (cm)
1000
Reef 2
Reef 3
Figure 4: The average bimonthly sediment level changes (cm) around the three shell bag reefs. June 26 was the first date sampled, therefore there is no recorded change. Error bars represent variation in sediment levels around the reefs.
Figure 3: a) The average oyster spat abundance per m² for the three shell bag reefs. b) The average percent mortality (oyster scars + boxes) per m² for the three shell bag reefs.
Results Oyster Recruitment and Mortality Spat counts indicated oyster abundances for the 1- and 2-layer shell bag reefs were nearly double that on the 3-layer reef (Figure 3a). ANOVA indicates a significant (p-value 0.001) difference in spat abundance, and a Tukey post hoc comparison revealed the decrease in oyster recruitment for the 3-layer reef was significant. Percent oyster mortality (= [scars+boxes]/[scars+boxes+live]) increased from the 1-layer reef to the 3-layer reef (Figure 3b). However, ANOVA indicated that differences in mortality were not statistically significant (p = 0.08). Sedimentation Relatively large changes in sediment levels occurred initially around the 2- and 3-layer shell bag reefs, but diminished after six weeks (Figure 4). In contrast, sediment levels fluctuated at a relatively constant rate throughout the sampling period around the one layer reef. Changes in mean sediment levels were not consistent in direction or magnitude among reefs indicating that changes were not a result of a larger scale process such as the migration of a sand bar that routinely occurs at this site. Fauna Sampling Nine different species were captured on the reefs whereas only two were captured on the sand control plot (Table 1). Average abundance and biomass of the three most common species and of all species captured during the four sampling periods are shown in Figures 5 and 6. Chi-square tests of the hypothesis of equal distribution of abundances among reefs were significant at 0.001 for Palaemonetes pugio and total abundance, 0.01 for Nassarius obsoletus and 0.05 for Pagurus longicarpus. Fauna biomass showed similar patterns except that the presence of a single 320 g eel skewed the total values (Figure 6; data shown without eel). Average Faunal Biomass
Table 1: Species sampled in minnow traps
30
Average Biomass (g)
Average Abundance
Average Faunal Abundance
Species (Common Name) Sand Reef 1 Reef 2 Reef 3 Anguilla rostrata (eel) 0 0 0 1 Calinectes sapidus(blue crab) 0 0 0 1 Crangon septemspinosa (sand shrimp) 0 2 1 0 Menidia menidia (silverside) 0 0 0 1 Nassarius obsoletus (mud snail) 0 15 11 6 Pagurus longicarpus (hermit crab) 6 18 7 17 Palaemonetes pugio (grass shrimp) 1 37 17 22 Panopeus depressus (mud crab) 0 1 0 1 Rhithropanopeus harrisii (mud crab) 0 1 0 0
25 20 15 10 5 0 Control
Shell bag 1
Shell bag 2
Shell bag 3
20 15 10 5 0 -5 Control
Reefs
All spp.
Palaemonetes pugio
Nassarius obsoletus
Shell bag 1
Shell bag 2
Shell bag 3
Reefs
Pagurus longicarpus
Figure 5: Average faunal abundance per tidal cycle during October 2006. (n=4 tides)
All spp.
Palaemonetes pugio
Nassarius obsoletus
Pagurus longicarpus
Figure 6: Average faunal biomass per tidal cycle during October 2006. (n=4 tides)
Discussion Oysters set and grew well on all three shell bag reefs. As of October 2006, mortality was low and oysters were growing rapidly. Recruitment counts and visual inspections indicated lower levels of early recruitment on the top layer of the three layer reef as well as an increase in spat mortality. We hypothesize that this recruitment and mortality pattern is the result of desiccation from increased exposure, however, differential settlement of larvae may also account for part of the observed patterns. Sediment elevation data indicate that higher vertical relief initially causes an increase in sediment dynamics, but these appear to stabilize over time. Preliminary data also reveal a significant increase in the abundance and biomass of motile species associated with the constructed shell bag reefs compared to the adjacent sand flats . A more comprehensive study is needed to evaluate the impact of intertidal shell bag reefs on sediment dynamics and their ability to reduce shoreline erosion, however, these findings clearly indicate the potential value of intertidal oyster reefs in lower Delaware Bay for increasing species diversity.
Acknowledgements:
We wish to thank F. Fuentes, E. Scarpa and P. Segars for their field assistance. This project was funded by a Rutgers University Research Council grant.
Introduction
Methods
In June 2006, a preliminary small-scale oyster habitat enhancement project began at Rutgers’ Cape Shore Hatchery Facility near Green Creek, NJ in the lower Delaware Bay (Figure 1). The Cape Shore is an extensive high energy intertidal zone with consistently high oyster (Crassostrea virginica) settlement, but high predation, disease and other factors apparently limit the formation and persistence of oyster reefs. To evaluate variations of one reef construction method, experimental reefs of varying height were constructed in the intertidal zone where the high oyster settlement provides the potential for reefs to develop. The shell bag reefs are being monitored for stability, oyster recruitment and growth, sedimentation rates and colonization by benthic and motile species.
Shell Bag Reefs On June 22, 2006, 200 shell bags were deployed to form three 1.5m x 3m reefs varying in height from one to three layers of shell bags (e.g., Figure 2a). The reefs were placed 10 m apart, but were not replicated for this preliminary investigation. New Jersey
Oyster Recruitment and Mortality On October 5, 2006 oyster spat, scars and boxes were counted on all three shell bag reefs using a 10 cm² quadrat. Sixteen replicate counts were taken across the total area of each reef. Sedimentation Sediment levels were measured bimonthly through October using a line level and a meter stick at low tide. A permanent stationary pole was placed in the center of each reef to secure the line level. Measurements were taken in eight directions (N, S, E, W, NE, SE, SW, NW) around each reef from 0 to 2m out in .5m intervals (40 data pts per reef). Change in sediment levels were determined by subtracting the original sediment levels on June 26 from the bimonthly measurements.
Cape Shore Delaware
Delaware Bay
Fauna Sampling After oysters had recruited (Figure 2b), five standard minnow traps (6.35mm wire mesh) were secured to each constructed reef as well as an adjacent control sand site at low tide (20 traps total), then retrieved on the following low tide 12 hr later. Individuals captured in traps were identified to species, enumerated and measured for total length and biomass. Collections were repeated on two daytime tides (Oct 16 and 25) and two nighttime tides (Oct 25 and 26).
Figure 1: Location of study site in Delaware Bay
a)
a)
Average Oyster R ecruitment 6000 Oysters/m2
5000
*
4000 3000
Averag e B im o n th ly S ed im en t C h an g e
2000
0 Reef 1
Reef 2
Reef 3
S he ll Ba g Re e fs
Average Oyster Mortality
b)
b)
Percent Mortality
0.3 0.25 0.2
8 6 4 2 0 -2 -4 -6 -8 -10 June 26 July 12 July 27
A ug 8
A ug 21
S ept 6 S ept 18
Oct 3
0.15
Reef 1
0.1 0.05 0 Reef 1
Reef 2
Reef 3
She ll Ba g Re e fs
Figure 2. a) 2-layer shell bag reef constructed on June 22, 2006. b) 2-layer shell bag reef on Oct. 5, 2006 showing heavy oyster set.
Sediment Change (cm)
1000
Reef 2
Reef 3
Figure 4: The average bimonthly sediment level changes (cm) around the three shell bag reefs. June 26 was the first date sampled, therefore there is no recorded change. Error bars represent variation in sediment levels around the reefs.
Figure 3: a) The average oyster spat abundance per m² for the three shell bag reefs. b) The average percent mortality (oyster scars + boxes) per m² for the three shell bag reefs.
Results Oyster Recruitment and Mortality Spat counts indicated oyster abundances for the 1- and 2-layer shell bag reefs were nearly double that on the 3-layer reef (Figure 3a). ANOVA indicates a significant (p-value 0.001) difference in spat abundance, and a Tukey post hoc comparison revealed the decrease in oyster recruitment for the 3-layer reef was significant. Percent oyster mortality (= [scars+boxes]/[scars+boxes+live]) increased from the 1-layer reef to the 3-layer reef (Figure 3b). However, ANOVA indicated that differences in mortality were not statistically significant (p = 0.08). Sedimentation Relatively large changes in sediment levels occurred initially around the 2- and 3-layer shell bag reefs, but diminished after six weeks (Figure 4). In contrast, sediment levels fluctuated at a relatively constant rate throughout the sampling period around the one layer reef. Changes in mean sediment levels were not consistent in direction or magnitude among reefs indicating that changes were not a result of a larger scale process such as the migration of a sand bar that routinely occurs at this site. Fauna Sampling Nine different species were captured on the reefs whereas only two were captured on the sand control plot (Table 1). Average abundance and biomass of the three most common species and of all species captured during the four sampling periods are shown in Figures 5 and 6. Chi-square tests of the hypothesis of equal distribution of abundances among reefs were significant at 0.001 for Palaemonetes pugio and total abundance, 0.01 for Nassarius obsoletus and 0.05 for Pagurus longicarpus. Fauna biomass showed similar patterns except that the presence of a single 320 g eel skewed the total values (Figure 6; data shown without eel). Average Faunal Biomass
Table 1: Species sampled in minnow traps
30
Average Biomass (g)
Average Abundance
Average Faunal Abundance
Species (Common Name) Sand Reef 1 Reef 2 Reef 3 Anguilla rostrata (eel) 0 0 0 1 Calinectes sapidus(blue crab) 0 0 0 1 Crangon septemspinosa (sand shrimp) 0 2 1 0 Menidia menidia (silverside) 0 0 0 1 Nassarius obsoletus (mud snail) 0 15 11 6 Pagurus longicarpus (hermit crab) 6 18 7 17 Palaemonetes pugio (grass shrimp) 1 37 17 22 Panopeus depressus (mud crab) 0 1 0 1 Rhithropanopeus harrisii (mud crab) 0 1 0 0
25 20 15 10 5 0 Control
Shell bag 1
Shell bag 2
Shell bag 3
20 15 10 5 0 -5 Control
Reefs
All spp.
Palaemonetes pugio
Nassarius obsoletus
Shell bag 1
Shell bag 2
Shell bag 3
Reefs
Pagurus longicarpus
Figure 5: Average faunal abundance per tidal cycle during October 2006. (n=4 tides)
All spp.
Palaemonetes pugio
Nassarius obsoletus
Pagurus longicarpus
Figure 6: Average faunal biomass per tidal cycle during October 2006. (n=4 tides)
Discussion Oysters set and grew well on all three shell bag reefs. As of October 2006, mortality was low and oysters were growing rapidly. Recruitment counts and visual inspections indicated lower levels of early recruitment on the top layer of the three layer reef as well as an increase in spat mortality. We hypothesize that this recruitment and mortality pattern is the result of desiccation from increased exposure, however, differential settlement of larvae may also account for part of the observed patterns. Sediment elevation data indicate that higher vertical relief initially causes an increase in sediment dynamics, but these appear to stabilize over time. Preliminary data also reveal a significant increase in the abundance and biomass of motile species associated with the constructed shell bag reefs compared to the adjacent sand flats . A more comprehensive study is needed to evaluate the impact of intertidal shell bag reefs on sediment dynamics and their ability to reduce shoreline erosion, however, these findings clearly indicate the potential value of intertidal oyster reefs in lower Delaware Bay for increasing species diversity.
Acknowledgements:
We wish to thank F. Fuentes, E. Scarpa and P. Segars for their field assistance. This project was funded by a Rutgers University Research Council grant.
