In 2018 and 2019 we conducted several studies evaluating the impact of aquaculture on horseshoe crabs, horseshoe crab egg availability, and shorebird feeding patterns.
Evaluation of the interaction of horseshoe crabs with intertidal aquaculture structures using side-scan sonar
Using side-scan sonar, we measured horseshoe crab abundance/density in areas that are inshore and offshore of control and aquaculture gear areas to help determine whether crabs primarily move through gear or around it. A secondary goal is to measure the rate of movement – e.g. on a rising tide do we see horseshoe crabs arriving inshore of gear at the same time as they arriving in control areas? And the same goes for a falling tide on the offshore side of gear.
The study area is defined as the shoreline between Highs beach and Green Creek depending on the current distribution of fixed gear. This comprises approximately 1 mile of shoreline that is a mix of aquaculture and open flats. From our ongoing work with horseshoe crabs, we have a good understanding of the relative habitat quality of beaches and their use for spawning.
During 2018 we conducted 14 surveys between April 29 and June 12 in a survey area that stretches from Green Creek to the south and Highs Beach to the north. Working with Stockton University, we established that horseshoe crabs could be effectively detected and counted using consumer-grade side-scan sonar instruments. We confirmed that calmer wave conditions promote a greater degree of crab spawning and also allow for better-quality side-scan imagery.
Example sonar images showing A) a pair of horseshoe crabs in amplexus; B) Oyster aquaculture racks and bags along with scattered horseshoe crab and ray feeding depressions; C) on left, extensive feeding depressions and on right, large numbers of horseshoe crabs along the shore during spawning at high tide; D) feeding depressions and ATV tracks from aquaculture activities.
We used of a modified oyster dredge to validate sonar surveys of horseshoe crab abundance. The dredge was deployed for two-minute tows in regions where horseshoe crabs were detected on sonar and in regions where they were apparently absent. These tows confirmed that the signals observed on sonar were horseshoe crabs.
Our preliminary analysis indicates that horseshoe crab abundance on intertidal flats in 2018 varied considerably over the course of the season, with the highest numbers observed during the early part of the peak spawning period. Weather and water temperature conditions may affect these patterns and more seasons of surveys will be needed to understand these patterns. 2018 was a particularly warm water and calm season compared to the previous two years.
Evaluation of relative abundance of horseshoe crabs moving through fixed oyster aquaculture gear on intertidal flats
It is unclear how horseshoe crabs respond to the presence of intertidal oyster aquaculture gear on intertidal flats on the lower Delaware Bayshore. The crabs must move through and/or around the gear as they move inshore to access spawning beaches.
In 2018 we designed and built nine custom traps that were situated among aquaculture gear and in open areas adjacent to gear. Traps were run for 43 days between May 17 and July 29. We captured horseshoe crabs in traps that were both inside gear and in adjacent open areas. The number of traps deployed within aquaculture gear was small because were limited to a single farm. This small sample size prevented us from making definitive conclusions from trapping about the effect of aquaculture on crab activity without additional seasons of work. But it was a successful pilot year that allowed us to develop and effective trapping design and protocol.
In 2019 we expanded the scale and scope of this activity to cover the majority of oyster aquaculture farms. This provided robust data and clear results on patterns of activity across space, time and mostly importantly, it illustrated clear differences between areas with and without oyster aquaculture gear. We deployed a total of 16 traps that were split evenly between control areas with no aquaculture and aquaculture areas. Traps were deployed for 34 days, totaling 560 total trap checks. There were significantly fewer horseshoe crabs caught in all traps that were set within aquaculture gear, indicating a clear effect on aquaculture on horseshoe crab use of the area.
Evaluation of shorebird use of intertidal flats - comparing shorebird use around commercial aquaculture and key shorebird foraging beaches without aquaculture
To determine the prevalence of horseshoe crab eggs available for shorebird feeding on intertidal flats, we collected 1,550 core samples from intertidal flats between Pierce’s Point and Villas NJ in 2018. We carried out supplemental sampling in 2019 (372 samples). We also examined pilot sampling done in 2017 (582 samples). We found the horseshoe crab egg abundance is generally lower than the availability on beaches, with decreasing abundance with increasing distance from the beach. Also, availability of eggs on flats appears to be related to weather conditions, with more eggs available after periods of high westerly winds.
In summary, availability of horseshoe crab eggs on intertidal flats is patchy and ephemeral with the following general patterns:
- egg abundance is higher in areas closer to shore
- egg abundance increases after high wind/wave events
- egg abundance is higher in the vicinity of creek mouths
- egg abundance may be higher horseshoe crab and ray feeding depressions
In 2018’s field season to study shorebird use of intertidal flats our team conducted two studies. The first study collected data on shorebird foraging on different beaches at low tide - comparing shorebird use around the same commercial aquaculture with reference sites; and by gathering data at key shorebird foraging beaches without aquaculture. These data were studied and published in the Environmental Research Journal (see attached publication). Red knot and other shorebird presences were studied at different differences (<100m, 101-200m, and 201-300m) to assess shorebird foraging activity from mean high tide line and gain insight on their use of the intertidal flats. During the surveys red knots were present during 67% of the surveys, with our data showing that shorebirds generally foraged at the water’s edge and moved out with the tideline, making use of the flats as they were exposed at low tide.
The second study included deploying 100 nanotags on red knots throughout the month of May in 2018 and 2019. Nanotags are small radio-transmitters that send data on their location to MOTUS towers, which are deployed in known coastal migration routes. At present, there are 7 towers in Delaware Bay, 7 on the nearby Atlantic Coast, and more than 100 on their northward migration route.
Over two field seasons our team collected these data from and plotted them by site to assess use of our towers in Delaware Bay (Figure 6). With both years showing a high usage of our towers in the northern section of Delaware Bay beaches (North of Villas) which coincides with egg availability findings as well as our ground counts and aerial surveys for each year.
Overall, our data shows the importance of these sites to shorebirds throughout their migratory period in May.