Horseshoe Crab Egg Surveys

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Since 2005, horseshoe crab egg sampling has occurred annually on NJ’s Delaware Bay beaches to assess feeding conditions for shorebirds at this critical stopover site. While much shorebird foraging occurs on beaches at high tide, over the stopover period there are periods when foraging may be less productive, for example during lulls in crab spawning and at lower tide stages.  However, the critical need for these birds to gain weight and arrive at the Artic at the appropriate time and physical condition, drives their need to feed constantly – even during spawning lulls and at lower tides. Recent research by Joanna Burger has shown that many shorebirds will commonly forage in the intertidal flats moving in and out with the tide.  In addition, there has been a trend of shorebird concentration near creek mouths and on ebb shoals adjacent to these creek mouths.  To date, there has been no corresponding surveys of horseshoe crab egg abundance and densities on flats, shoals and creek mouths.

With a vital need to understand the unique and important role alternative habitats serve for horseshoe crabs and shorebirds, our team proposed adding horseshoe egg sampling surveys in alternate feeding and spawning areas (such as creek inlets, flats, and shoals). These data gave us a one-year snapshot comparing egg abundance across these different habitats.  Additionally, this work aided our understanding of the importance of all habitat sections within the bay to horseshoe crabs and shorebirds as well as different patterns in their uses of all habitat types – patterns that were never been examined before.

During the field season our team conducted 5 weeks of sampling shallow egg abundance and 9 weeks of sampling egg cluster abundance on beaches throughout the bay. We added 10 new sample areas at creek mouth shoals between Villas and Gandy’s beach as well as dedicated egg sampling on flats between Villas and Pierces point. On flats, egg sampling was conducted 3 times a week for 6 weeks along random transects perpendicular to the shore on flats. Our efforts focused on alternate feeding and spawning habitats in the bay (flats, shoals, and tidal creeks) to gain understanding of the different uses of these sites and importance for both horseshoe crab spawning, egg development and juvenile success and shorebird food availability and habitat needs.