The Horseshoe Crabs of Delaware Bay
An aerial survey of shorebirds along the Delaware Bay has been performed for the last 11 years. The survey covers 80km of beaches in New Jersey and 80 km in Delaware. Survey flights are made once per week for 6 weeks, from May 4th through June 12th. The surveys are performed 3-4 hours after high tide when the shorebirds are feeding on the beaches. The plane is flown at a height of 25m, 30m from the shore, at a speed of 110km/hr. As the plane passes the beaches, the shorebirds are flushed which allows them to be counted and identified.
The survey graphs shown below indicate the peak single day count recorded for each survey year. This gives a minimum estimate of the total number of birds using the bay. Since birds arrive and depart throughout the 6 week survey period there is no easy way to determine the total number of birds using the bay.
The single day peak counts of several species have shown declines over the years of the survey. Sanderlings and Semipalmated Sandpipers have both shown significant declines (Clark, Niles & Burger, 1992). What influence, if any, the horseshoe crab numbers have had on this decline is difficult to isolate given the complexity of the shorebirds' migratory routes.
Aerial surveys performed this year did reveal that the distribution of shorebirds around the bay was shifted from previous years. Red Knots are usually evenly distributed around the bay. This year 91% of the Red Knots on the day of the peak count were concentrated along one stretch of beach on the Delaware side, the only place where horseshoe crabs were spawning in large numbers. In previous years Sanderlings were most commonly found on the southern part of the New Jersey shore. This year they were evenly distributed around the bay. Both these changes appear to be directly related to the distribution of spawning horseshoe crabs this year and the lack of horseshoe crabs on the New Jersey shore.
Sampling of the weights of Red Knots this year seemed to indicate that
they were able to find adequate food, even if it was isolated to
a small section of the bay. There are obvious concerns raised
by this though. What would have happened if the crabs had not spawned
on this one stretch of Delaware's shore also? In that case, many shorebirds
probably would not have been able to find the food they needed along the bay.
There has been a statistically significant decline in the total peak counts of shorebirds since 1986. The Red Knot peak count was the lowest ever in 1996 but recovered some this year and was at the 11 year average. However, the lowest percentage of Red Knots on the New Jersey shore was recorded this year.
The data for Sanderlings and Semipalmated Sandpipers show statistically significant declines over the time period. No trend was detected statistically for Red Knots and Ruddy Turnstones. Further explanation and interpretation of the following graphs can be found in the original report.
