Horseshoe Crabs on Beaches Near Active Oyster Aquaculture Farms ...
Horseshoe Crabs on Beaches Near Active Oyster Aquaculture Farms on the New Jersey Delaware Bayshore Patricia Woodruff, David Bushek Haskin Shellfish Laboratory, Rutgers University The beaches of the Delaware Bayshore provide important spawning habitat for the Atlantic horseshoe crab (Limulus polyphemus). Spawning peaks around new and full moons during late April to mid-June. A portion of these beaches are also home to several New Jersey oyster farms. In response to concerns that farms may impede access to nesting beaches, this study compared horseshoe crab (HSC) abundances on beaches with and without farms. This was done through an expansion of the ReTurnThe-Favor (RTF) volunteer program orchestrated by The Wetlands Institute of Stone Harbor, NJ. RTF counts only live flipped or trapped (impinged) crabs. Here, all live horseshoe crabs were counted on the beach.
Site 1
Results Sixteen walks were completed and over over 5,400 crabs counted with highest abundances in late May and Early June
Flipped vs total
Total Number of Horseshoe Crabs Counted on Each Walk
1200
The proportion of flipped crabs was relatively constant across sites averaging 31%. 600
1000
500 No. of flipped crabs
Introduction
Photo credit: David Bosco
800 600
Site 2
400
400 y = 0.31x R² = 0.63
300 200 100
200 0 0 22-Apr
0 29-Apr
6-May
13-May
20-May
27-May
3-Jun
10-Jun
17-Jun
Total number of crabs counted per walk from April 22 to June 22, 2016. Red arrows indicate full moons, grey indicate new moons. Typically, the highest abundance of HSC is seen with the full moon in May. The highest count just after the full moon corresponded to a storm event on May 22nd. This is consistent with RTF information.
Site 2, with the most farming gear, tended to have a higher proportion of crabs on any given date. Distribution of Crabs at Each Site by Date
Site 3
150 yard segments Site 1, no farm Site 2, moderate farm Site 3, Rutgers lab farm
0.7 0.6
Photo of typical layout of rack and bag system used on oyster aquaculture farms.
0.5
Site 4, no farm
0.4
Site 5, no farm
0.3 0.2
Methods
0.1
site 1, no farm
Site 4
site 2, farming
site 3, lab
site 4, prior farm
4-May
6-May
8-May
20-May
21-May
23-May
24-May
30-May
2-Jun
3-Jun
5-Jun
21-Jun
1000 Total no. of crabs
1500
2000
A regression of the number of flipped vs total number of crabs indicated that the number of crabs flipped was related to the number of crabs that were present. Because wind-induced waves flip crabs, the more crabs present during storms, the more that are found flipped. This is consistent with RTF observations and strategy to increase rescue efforts during periods of high wind.
Discussion Farming gear did not reduce the presence of horseshoe crabs on spawning beaches. In fact, the trend was for more crabs to be present on beaches where farming was occurring. This was true even on the two higher quality beach sites (one and two). The primary factor that did appear to have an impact on the presence of crabs was beach condition. Beaches south of the Cape Shore Laboratory, where horseshoe crab spawning was historically abundant, have eroded to peat layers leaving little spawning habitat. Beaches to the north have eroded as well and continue to erode but sufficient sand and slope remain to maintain higher levels of HSC spawning. These conditions are likely the result of recent changes in sea level and do not appear to be related to current or historical farming of oysters in the intertidal zone.
1989
0
The beach was split into five adjacent 150 yard segments, creating five sites. Two of the sites had active farms, three did not. Beach quality decreased from site 1 to 5.
500
2016
site 5, no farm
Impinged crabs were most abundant at the Rutgers Lab where Hurricane Sandy exposed and damaged a gabion wall protecting the dune.
Walks were scheduled around full and new moons in April, May and June to correspond with known periods of peak spawning. Walks were conducted at night three hours after high tide per RTF protocols except that counts included all live, not only flipped and impinged crabs.
140
Proportions flipped (red), impinged (green) or right side up (blue) Note the rapid increase during the 1990s
120
Observers recorded numbers of live flipped, impinged and right-side up crabs
Site 5
No of Crabs
100 80 60 40 20
Photo credit: Jennifer Gius
Right side up HSC
Flipped HSC
0 site 1 north no gearsite 2 moderate gear
site 3 lab
site 4 no gear
site 5 south no gear
Two impinged HSC Total bars represent the average number of horseshoe crabs counted over the time series with one standard error shown. Despite apparent differences among some sites, a single factor ANOVA was not significant (p = 0.06) and a Tukey’s post-hoc multiple comparison found no differences among site pairs.
Acknowledgments: A big thanks to the fearless field crew that helped count crabs at all odd hours of the night: Mike Acquafredda, Sarah Borsetti, Jennifer Gius, Ricky Klie, Horst Lehmann, Lauren O'Neil, Nicole Pray, Jenny Shinn and Mike Whiteside Thanks also to Joe Caracappa, Jessica Mikus and Jenny Shinn for helping with statistical programs And thanks to The Wetlands Institute RTF program for all their great work and assistance
Site 1
Results Sixteen walks were completed and over over 5,400 crabs counted with highest abundances in late May and Early June
Flipped vs total
Total Number of Horseshoe Crabs Counted on Each Walk
1200
The proportion of flipped crabs was relatively constant across sites averaging 31%. 600
1000
500 No. of flipped crabs
Introduction
Photo credit: David Bosco
800 600
Site 2
400
400 y = 0.31x R² = 0.63
300 200 100
200 0 0 22-Apr
0 29-Apr
6-May
13-May
20-May
27-May
3-Jun
10-Jun
17-Jun
Total number of crabs counted per walk from April 22 to June 22, 2016. Red arrows indicate full moons, grey indicate new moons. Typically, the highest abundance of HSC is seen with the full moon in May. The highest count just after the full moon corresponded to a storm event on May 22nd. This is consistent with RTF information.
Site 2, with the most farming gear, tended to have a higher proportion of crabs on any given date. Distribution of Crabs at Each Site by Date
Site 3
150 yard segments Site 1, no farm Site 2, moderate farm Site 3, Rutgers lab farm
0.7 0.6
Photo of typical layout of rack and bag system used on oyster aquaculture farms.
0.5
Site 4, no farm
0.4
Site 5, no farm
0.3 0.2
Methods
0.1
site 1, no farm
Site 4
site 2, farming
site 3, lab
site 4, prior farm
4-May
6-May
8-May
20-May
21-May
23-May
24-May
30-May
2-Jun
3-Jun
5-Jun
21-Jun
1000 Total no. of crabs
1500
2000
A regression of the number of flipped vs total number of crabs indicated that the number of crabs flipped was related to the number of crabs that were present. Because wind-induced waves flip crabs, the more crabs present during storms, the more that are found flipped. This is consistent with RTF observations and strategy to increase rescue efforts during periods of high wind.
Discussion Farming gear did not reduce the presence of horseshoe crabs on spawning beaches. In fact, the trend was for more crabs to be present on beaches where farming was occurring. This was true even on the two higher quality beach sites (one and two). The primary factor that did appear to have an impact on the presence of crabs was beach condition. Beaches south of the Cape Shore Laboratory, where horseshoe crab spawning was historically abundant, have eroded to peat layers leaving little spawning habitat. Beaches to the north have eroded as well and continue to erode but sufficient sand and slope remain to maintain higher levels of HSC spawning. These conditions are likely the result of recent changes in sea level and do not appear to be related to current or historical farming of oysters in the intertidal zone.
1989
0
The beach was split into five adjacent 150 yard segments, creating five sites. Two of the sites had active farms, three did not. Beach quality decreased from site 1 to 5.
500
2016
site 5, no farm
Impinged crabs were most abundant at the Rutgers Lab where Hurricane Sandy exposed and damaged a gabion wall protecting the dune.
Walks were scheduled around full and new moons in April, May and June to correspond with known periods of peak spawning. Walks were conducted at night three hours after high tide per RTF protocols except that counts included all live, not only flipped and impinged crabs.
140
Proportions flipped (red), impinged (green) or right side up (blue) Note the rapid increase during the 1990s
120
Observers recorded numbers of live flipped, impinged and right-side up crabs
Site 5
No of Crabs
100 80 60 40 20
Photo credit: Jennifer Gius
Right side up HSC
Flipped HSC
0 site 1 north no gearsite 2 moderate gear
site 3 lab
site 4 no gear
site 5 south no gear
Two impinged HSC Total bars represent the average number of horseshoe crabs counted over the time series with one standard error shown. Despite apparent differences among some sites, a single factor ANOVA was not significant (p = 0.06) and a Tukey’s post-hoc multiple comparison found no differences among site pairs.
Acknowledgments: A big thanks to the fearless field crew that helped count crabs at all odd hours of the night: Mike Acquafredda, Sarah Borsetti, Jennifer Gius, Ricky Klie, Horst Lehmann, Lauren O'Neil, Nicole Pray, Jenny Shinn and Mike Whiteside Thanks also to Joe Caracappa, Jessica Mikus and Jenny Shinn for helping with statistical programs And thanks to The Wetlands Institute RTF program for all their great work and assistance
