Conserve Wildlife Blog

Ecological Lockdown for Horseshoe Crabs – A Delaware Bay 2020 Update

May 19th, 2020

Guest post by: Dr. Larry Niles

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 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 know 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 have settled down to about 500,000 males 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 their kill or their windfall profits.

We also know that New York fishers are taking crabs from the Delaware Bay population and landing them as a New York harvest. Nearly half 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 above, 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 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 for the crabs.

Read part 2 of this update, “Close to the Finish Line”.

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2 Responses to “Ecological Lockdown for Horseshoe Crabs – A Delaware Bay 2020 Update”

  1. helen holmes says:

    I live on Barnegat Bay and we moved here in 1998 and we are across from Seaside Heights and the Island Beach State Park. At that time we got maybe a dozen to 18 pairs of mating horseshoe crabs. The water here is shallow and the beach small so I have had to watch for the occasional one that would end up on the beach on its back so I could go down and return him or her to the water..

    This area was hit badly by Sandy and it took time for the Bay to get a somewhat superficial cleaning out of larger things on the bottom. The horseshoe crabs kept coming but in smaller numbers. Then a group came in to create and seed an oyster bed around 2014 or so. During that lengthy process the hundreds of blue claw crabs near us disappeared and have never returned. The work was done without serious supervision and not only were the crabs disturbed but people were, too. All the scraps of lumber were just tossed in the water and that made for a new job: I had to go out there after they left and pick up the numerous scraps of lumber that had landed on the horseshoe crab small beach. So far though no oysters. However, some horseshoe crabs – maybe half a dozen pair – do make it up here and seem to be happy but their numbers have not increased. One thing has changed. When we moved here they tended to come in May and since Sandy they now come in mid to late June. So something on their journey has changed but I have no idea why the later arrival date and it doesn’t seem to tie into the temperature of the water and it has been like this for the past 5 years..

    Just thought I would toss it out because of our shallow water it tends to be warm enough for them almost every year – maybe not this year but soon we will know though one person is already swimming out here.

    Now I realize this is a relatively tiny handful of horseshoe crabs but to me anyway they are important and because I have known them for 22 years and I thought I would just pass on what I have noticed.

  2. David Bushek says:

    Interesting observations Helen. As for Delaware Bay the news is much better. The crabs are back in force and nesting, and large flocks of birds are feasting on their eggs! Things are looking up!