This is a summary of our findings so far. Scroll down to learn the details of how we have reached these conclusions.
Horseshoe crab mortality events disrupted the enjoyment of the canals in July 2021 and 2022.
Live crabs are entering the canals, likely seeking a sandy shore to nest on. We believe some of these crabs are then dying inside of the canals.
Dead crabs are not quickly washed out of the canal by the tides, so they decompose in the canal causing a very unpleasant smell.
Water quality conditions are not ideal but are not poor enough to directly cause the crabs to die.
Daytime water temperatures were high but did not reach known limits.
Nighttime dissolved oxygen concentrations reached levels low enough to cause stress, but not death.
Heavy metals, man-made chemicals, and pathogenic bacteria may pose a risk to marine and human health.
Heavy metal mercury levels in the water are well below the EPA's Ambient Water Criteria for saltwater aquatic life (EPA, 1980). Heavy metal mercury levels in the sediment range from .46 parts per billion (ppb) to 91.96 ppb which is still under NOAA's low-range effects of 150 ppb (The United States, Department of the Navy, 2018).
Man-made chemicals like PCBs and DDTs are present in the sediments.
The PCBs found in the sediment range from 0.46 ppb to 1434 ppb. Which significantly exceeds NOAA's median-effects range of 180 ppb (The United States, Department of the Navy, 2018).
The DDTs found in the sediment range from 325 ppb to 3556 ppb. Which is significantly higher than NOAA's Medium-range effects guidelines of 46.1 ppb (The United States, Department of the Navy, 2018)
Thirteen different bacteria were commonly identified in the water while twenty-five different bacteria were commonly identified in the sediment
Water moves very slowly inside the canals.
We are continuing to seek appropriate tools to measure the flow of water and the exchange of materials between the canals and open water.
In the summer of 2022, we conducted several periods of observations of horseshoe crabs in the canal system at 94th St. in Ocean City. We observed many crabs exhibiting behaviors associated with nesting and spawning. Large females with smaller attached males were observed repeatedly attempting to climb up the wooden walls at the ends of the canals. As they reached the water surface, they would fall off the wall, sink to the bottom, and repeat the process (Figure 2). This behavior suggests that the female crabs were seeking sandy or muddy shoreline to lay their eggs at the edge of the water while the males were hanging on in hopes of fertilizing the eggs. We also observed several instances in which the female crab had died, but the male crab continued to hold on to her (Figure 3). We hypothesize that when the crabs fail to find a soft shoreline to spawn on, they continue this habitat-seeking behavior until they become exhausted and die. In summer 2023, we will attempt to address this hypothesis and seek solutions to help the crabs complete their reproductive mission in hopes that they will then exit the canals alive and well.
In addition to our observations, we also conducted a drifter study to understand what happens to floating objects in the residential canals. We deposited four groups of painted oranges in different parts of the canals (Figure 4). We used oranges because they float near the surface, much like dead horseshoe crabs and other debris found in the canals. After one full tidal cycle (one high tide and one low tide), we retrieved the oranges, recording the color and location of each orange retrieved (Figure 5). We found that floating oranges deposited near the ends of the canals moved around in the back portion of the system, but did not exit (Figure 5). Those deposited near the middle moved toward the back, and those deposited near the mouth moved to the middle and got stuck under a dock. We found that this was similar to the way that floating dead horseshoe crabs behaved within the canals (Figure 6).
Figure 1: A pair of horsehoe crabs nesting just above the water line on a beach in the Maryland Coastal Bays. (Photo: M. Sexton)
Figure 2: A pair of horseshoe crabs searching for nesting habitat at the end of a residential canal. (Photo: M. Sexton)
Figure 3: A dead female horseshoe crab in the residential canal. The live male remains attached. (Photo: M. Sexton)
Figure 4: Map of residential canal system with starting locations for drifter study. (Image: R. Thompson)
Figure 5: Final locations of oranges in drifter study; low section numbers are near the mouth of the canal, while high section numbers are near the ends. (Image: R. Thompson)
Figure 6: Dead horseshoe crabs collecting around a structure in the canal. (Photo: R. Thompson)
We deployed a water quality meter (YSI exo2 water quality sonde) near the end of the canal to take water quality measurements every 15 minutes over a 48-hour period. The results showed a predictable pattern of high water temperature during the day and low dissolved oxygen concentrations in the late night/very early morning hours (Figures 7 and 8).
Although horseshoe crabs are known for being tolerant of low oxygen conditions, as little as 5 minutes of exposure to low oxygen conditions seen during the late night hours can cause measurable physiological stress (Allender et al. 2010).
Temperature conditions during the deployment were very warm, but did not reach the known limits. Although the temperature at which horseshoe crabs begin to experience physiological stress is not clearly defined, we know that the adults seek habitats with temperature less than 35o C for nesting, and larvae do not develop above this temperature (Vasquez et al. 2015).
Figure 7: Dissolved oxygen concentration during water quality monitoring period. Concentrations below the red line represent hypoxic conditions. (Image: M. Blackwood)
Figure 8: Water temperature during water quality monitoring period. Horseshoe crabs are known to seek habitat with temperatures below the red line. (Image: M. Blackwood)