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Ecological Lockdown for Horseshoe Crabs – A Delaware Bay 2020 Update.

By Larry Niles

May 18, 2020

When asked to describe the ecological conditions of any one year of our 23 years of work on Delaware Bay, Humphrey Sitters, one of the first biologists to understand the value of Delaware Bay to shorebirds would respond “every year is unprecedented.”  And so it seemed until this year.

Most years begin like all the others but then end with the birds or crabs surprising us.

At first, we fought for the birds and crabs, trying to understand how to help them as fishers fought to catch and sell every crab they could catch.  Then agencies reined in the harvests and crab numbers stabilized.  Things got better.  More people volunteered to take on new roles like stewarding and rescuing crabs in danger.  More shorebirds stayed for longer periods in the bay as groups like American Littoral Society and Conserve Wildlife Foundation of New jersey rebuilt eroded beaches. In the last two years, the birds enjoyed steady improvement as each new year provided a better understanding of their needs.

Now, after a week of very few spawning crabs and virtually no red knots, we must see this year as truly different than all the others. And not because of the deadly COVID-19 virus.  Cool weather, tropical storms and God knows what else seems to put the entire stopover in a sort of ecological lockdown.  We don’t know what is next for birds and crabs.

We know horseshoe crab numbers have not improved, but they have not declined.  Crabs killed by fishers has settled down to about 500,000 male crabs/year.  This must be tempered with our poor understanding of how many crabs die at the hands of the international companies bleeding horseshoe crabs, it could be less than 200,000 or double that number.  The profitable industry provides no information to the public about how many crabs they kill.

We also know that New York commercial bait fishers are taking crabs from the Delaware Bay population and landing them as a New York harvest.  Nearly half of the landed crabs have Delaware Bay genetic signatures.  Also, scallop and clam boats, as well as boats dragging the bottom for flounder and other bottom fish, kill some unknown number of crabs, as bycatch, but even the worst case wouldn’t leave the beaches empty of crabs, as they are now.

The cause of the lack of spawning is more likely cold temperatures.  In a more typical year, the bay’s water temperature would gradually warm through May, usually reaching the threshold for spawning at approximately 59 degrees by early May.  In some years, short periods of cold weather cool bay waters and choke off a nascent spawn.  But in all years, breeding resumes after the May sun has its inevitable way.

This happened in 2017. A cooling in mid-May shut down the spawn, leaving beaches devoid of eggs and shorebirds pursuing them throughout the bay.  In that year, the percentage of birds making good weight declined to dangerous lows, but an end-of-season blossom of eggs provided relief.

This year portends worse. The bay water temperature barely reaches 59 degrees at the Cape May buoy, and the coming week promises more of the same.  As seen in the NOAA graph below, the temperature bumps up to the warmth that crabs need to spawn vigorously, but then the incoming tide of the much colder ocean water, drags the temperature back down.  The unseasonably cool air temperatures will keep it that way.

On the beaches, few crabs spawn.  Almost none appeared until May 16th and then only on isolated stretches that were hard to reckon.  For example, crabs spawned in the middle section of Reeds beach but nowhere else.  Yesterday the crabs came ashore to breed in more areas but oddly restricted.  Crabs spawned heavily on the south end of Reeds Beach but not North Cooks Beach, separated by a tidal creek and only 20 feet away.  This morning we found a slight improvement but far short of a healthy spawn of eggs.

We hope for better this week, but Tropical Storm Arthur, a rare event for May, churns away off the southeast US coast. It’s generating an easterly flow of wind into the Delaware Bay area, causing two problems for birds and crabs.   It will very likely stop shorebirds flying north, delaying the migration.  It will also lock in cool temperatures until at least this coming weekend.  Easterly or northeasterly winds slow the outgoing tides and assist incoming tide bringing in cold ocean water that will chill the bay.  It will also decrease air temperature, further cooling bay water.

At this moment, there are about 7,000 knots known to be in the area, most on the Atlantic coast feeding on mussel spat and whatever else birds can find.  About 200 were seen on the bay this morning, a big improvement from yesterday but far less than the 12,000 seen last year at this time.  But we hope the minor spawn will continue and gradually build egg densities to attract more Red Knots and other shorebirds to the bay.

But the future is far from certain. We hope for warmer weather by the end of the weekend, we can only hope it is not too late.