Conserve Wildlife Blog

Horseshoe crabs expanded breeding into neap tides

July 7th, 2018

by Larry Niles (Part 1 of 3)

The horseshoe crabs extended their breeding period into the neap tide phase after the cold weather of mid-May decreased water temperature during the spring tides.  The crabs roughly require a water temperature of about 59 degrees F before breeding begins in earnest.  Crabs still breed at a lower temperature, but many more will breed above the temperature threshold.

At the same time, crabs also look for spring tides, the higher high tides that come with full and new moons, because they can breed in sandy places unavailable at lower tides. This year the water cooled during the new moon spring tide and warmed in the neap.  Good spawning during the neap tides of the last week was welcome good news.  This May good spawning conditions will raise average egg densities about 50% higher than last year.

We found eggs lying on the beach in many beaches including Reeds Beach at the Jetty.  This is better news then it first appears.  Last year shorebirds desperately fed on the eggs at the jetty and beach corner at North Reeds to the delight of many visitors.  Little did they know, the birds couldn’t find eggs elsewhere at similar densities and would have avoided the place if they could. The good news this year is they have.

Higher egg densities on the New Jersey’s Delaware Bayshore gave shorebirds a welcome boost.  Most gained weight rapidly.  The weights of red knots and ruddy turnstones show the difference.  Knots rapidly reached the 180-gram threshold considered necessary for birds to reach the Arctic breeding areas in good conditions, and Ruddy Turnstones are in similarly good condition.

Dr. Larry Niles has led efforts to protect red knots and horseshoe crabs for over 30 years.

A view from the Reeds Jetty looking south. Horseshoe crab eggs piled up after a few days of southerly winds but unlike last year shorebirds did not pile into eat them. It a good sign. The jetty exposes the birds to intense disturbance and they would rather feed in more secure places – and they did because they could.

Horseshoe Crab breed on the sod bank of Egg Island and shorebird feed on the eggs as they are being laid. The breeding attempts must fail because eggs cannot incubate in the anoxic environment of marsh mud but its a good sign the crabs are breeding in odd places.

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