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Horseshoe crab movements in the Great Bay estuary

Since 2005 we have been tracking the movements of horseshoe crabs in the Great Bay estuary, NH using a variety of ultrasonic telemetry techniques. We investigate small scale movements using a fixed array system, involving three buoys that communicate with a shore station (Vemco VRAP system). Information about large scale movements over periods of time ranging from days to months is obtained using both manual tracking methods and VR2 "listening stations". To the left are pictures of each of these methods in operation.

We seek to answer the following questions during these investigations:

1. Do horseshoe crabs express the same types of circadian and tidal rhythms observed in the laboratory in the field? If so, do they express different rhythms at different times of year? If not, why not?

2. What conditions in the estuary appear to have the greatest impact on horseshoe crab movements? Tides? Salinity? Thermal gradients?

3. Do horseshoe crabs exhibit a predictable pattern of seasonal movements, moving into the estuary in the spring, and down toward the coast in the summer/fall?

Some Preliminary Data

Small Scale Movements During Mating
We used ultrasonic telemetry to track horseshoe crabs while they were mating near the Jackson Estuarine Laboratory. This enabled us to determine if they were active during each high tide, if they returned to the same beach at each high tide, whether they were more active in the day or night, and what they did between tides. Some sample data is shown on the left. It is a video made from telemetry data that was imported into ArcView and then animated using the Animal Movement Extension.

Mating Surveys
We were curious if more horseshoe crabs mate during daytime high tides than nighttime high tides so we did surveys during both tides on the same day, throughout the mating season. Our data indicates that, while there is considerable variability, about the same number of animals mate during the day as during the night.

Seasonal Movements
The ultrasonic tags we use last for about a year. Therefore, we are able to monitor the seasonal movements of horseshoe crabs. They in the spring they move into shallow water at least a month before they mate. The animals we tracked in 2005-6 then moved up into the estuary to mate before moving back down the estuary in the summer. They then overwintered in fairly deep water, but not more than 3-4 kms from where they mate.