Ask a Blue Crab: If You Want to Thrive, First You Need to Survive

Ask a Blue Crab: If You Want to Thrive, First You Need to Survive

Dr. James J. S. Johnson

bluecrab-virginiainstitutemarinescience-photo

When a man hath taken a new wife, he shall not go out to war, neither shall he be charged with any business; but he shall be free at home one year, and shall cheer up his wife whom he hath taken.   (Deuteronomy 24:5)

Chesapeake Bay blue crabs, like young men, do well to avoid combat –at least until they have had a decent opportunity to procreate the next generation.  To thrive, first you must survive. And, for populations, that means surviving long enough to become parents.

That’s true with every lifeform God made, from people to pinnipeds, from cranes to crabs.

newlyweds-embracing-chuckespinoza-photo

Consider, for example, how this works with human populations – as the Biblical quotation above (Deuteronomy 24:5) illustrates.

To protect the population success of the Hebrew people, after they escaped slavery in Egypt, God provided a population-protective law – a newly married man was not to be assigned combat duty until he had one year at home, to spend a lot of time with his wife. From a population growth perspective, this is only “common” sense, because reproducing members of a population need to survive long enough to procreate – otherwise how can the local population thrive?  (Actually the reality seems to be fairly uncommon in many nations, nowadays, where human population growth is negative.)

BlueCrab-in-net.NOAA-photo.jpg

And yet the same idea applies to local populations of blue crabs —  a multi-million dollar industry in the Chesapeake Bay – which is the largest bay in the world.

As with humans, for a blue crab population to thrive, surviving (long enough to reproduce) is the first and most basic requirement. If enough young adults don’t live long enough to procreate the next generation , don’t expect population stability – much less population growth!  And the yo-yo-like roller-coaster populations of Chesapeake Bay blue crabs illustrate this fact every year.

A good prognosis for Callinectes sapidus [i.e., Chesapeake Bay blue crab] The comeback for the [Chesapeake] Bay’s most valuable fishery [i.e., Chesapeake Bay blue crabs] is exactly what the experts predicted.  The 2016 wintertime survey estimated there were 553 million blue crabs in the Bay, a 35 percent increase from last year’s [i.e., AD2015’s] tally and the highest population in four years [!@].  The number of spawning-age females was estimated to be 194 million — below the recommended target of 215 million, but double what it had been the year before [i.e., AD2015].

The comeback for the Bay’s most valuable fishery [i.e., blue crabs!] is yet another sign that harvest restrictions placed on both states [i.e., Maryland and Virginia] in 2008, while painful for the crab industry, may be helping the fisheries to bounce back. Among the cuts crabbers absorbed:  limits on the number of bushels of female crabs they could take in a day, seasonal closures in Maryland of the fishery for female crabs and a complete closure of the Virginia winter dredge fishery, which overwhelmingly focused on female crabs.

[Quoting from Rona Kobell, “Crabbers, Scientist Seeing More, Larger Blue Crabs This Spring”, Chesapeake Bay Journal, 26(5):11 (July-August 2016).]

bluecrabs-in-bushel-calvertbeacon-photo

It is noteworthy – in light of the Mosaic avian wildlife protection law in Deuteronomy 22:6-7 (which prohibits the predatory taking of nesting female birds found in the wild)– that protective conservation of females is the key to reproductive success of blue crabs – and thus also to the Chesapeake crustacean’s population stability and population growth.

While scientists hoped that crabs would recover quickly and watermen wanted the restrictions loosened, the story wasn’t quite that linear. The crab population rebounded, more than doubling from 2008 to 2010, but then it slumped again.  The juvenile crabs dropped dramatically in 2013, from 581 million to 111 million [!].  Scientist didn’t know the cause [from the slump], but some watermen suspected many were gobbled up by an influx of red drum [fish, which is a predator of blue crabs – especially red drum juveniles that hunt for crabs in the Chesapeake Bay’s shallows] into the Bay.  Others thought many crabs expired in oxygen-poor water [perhaps due to extra-warm water, which holds less dissolved oxygen than does cooler water].  Whatever the reason, the 2013 results were indicative of a yo-yo population that’s tough to manage.

Crabs are notoriously unpredictable, cannibalistic creatures that typically live less than three years. They reproduce prolifically, but their survival depends on a number of external factors, beginning with favorable winds to blow the larvae back into the Bay after their eggs hatch.  They also need thick underwater grasses for habitat; until recently, sediment from rainstorms and algae blooms from excess nitrogen and phosphorus [e.g., phosphate waste] have blocked the light [that otherwise shines through the surface water] that [submergent] aquatic vegetation needs to grow.

But this year [i.e., AD2016], researchers at the Virginia Institute of Marine Science and Maryland Department of Natural Resources reported the greatest extent of Bay grasses [in shallow waters] they’d seen in three decades. Canby [i.e., C. J. Canby, a crabber who fishes 500+ crab-pots near Annapolis] and others who frequent the Bay say the water is clearer than they’ve seen in a long time, and data bear that out, at least in some areas of the Chesapeake.  . . .

Canby’s traps were fuller than any June he recalled. The skittering crabs were a beautiful blue, and so large that his customers were asking him to sort them into several sizes instead of just mixed bushels.  . . .   “They’re getting bigger,” Whalen [i.e., Bruce Whalen, manager of Cantler’s Riverside Inn, a crab house near Annapolis] said of this year [i.e., AD2016]’s crabs.  . . .

Better oyster harvests in the last several years have brought more watermen back into the [Chesapeake Bay’s oyster] fishery. Some conservationists worry the same thing will happen eventually with crabbing should the [blue crab] population bounce back, but Canby doesn’t think so.  It’s hard, unpredictable work, and it’s expensive, he said, what with the price of [crab] pots, bait and manpower increasing.  And sometimes, the wind blows blustery and the chop doesn’t stop and the equipment breaks down and the person you would call to fix it is … you.  “You gotta love this,” Canby said, “because there’s a lot of better ways to make money.”

[Quoting from Rona Kobell, “Crabbers, Scientist Seeing More, Larger Blue Crabs This Spring”, Chesapeake Bay Journal, 26(5):11-13 (July-August 2016).]

© Jay Fleming

In other words, vulnerable crabs need shallow-water vegetation to hide in, so they are less of a visual “target” for predators, such as Red Drum fish. [ See also, regarding Red Drum [Sciaenops ocellatus] fish juveniles, Karl Blankenship, “The Mystery of the Missing Blue Crabs”, in Chesapeake Bay Journal, 23(8), page unknown  (posted 11-6-AD2013), at http://www.bayjournal.com/article/the_mystery_of_the_missing_blue_crabs .]

So, there you have it – something as simple as having lots of clear, sunshine-filled shallow water, in the shallows of the Chesapeake Bay, makes all the difference for blue crab population successes.

bluecrab-underwater-grass-harrisseafood-photo

If the crabs have sufficient hiding-places, where they can evade potential predators, they can survive long enough to procreate – in the millions each spawning season.

Déjà vu!  —  surviving (long enough to successfully reproduce) is unsurprisingly critical, for thriving as a local population.  That’s why wildlife protection statutes often allude to “critical habitats” (i.e., the habitats where reproduction of the next generation either succeeds or fails), not just any habitat (such as a migratory stopover, or a winter home).

God is the inventor of habitat factors that provide protection from potential predators — as well as the provider of defensive survival traits (ecologically employed by prey populations, “fitting” them to avoid their predators).

Accordingly, Biblical creationists can appreciate this simple reality:  as the all-wise Creator of the uncountable complexities of every living creature — from people and pinnipeds, to cranes and crabs  —  God deserves credit for SURVIVAL OF THE FITTED.

PHOTO CREDITS:

Newlyweds embracing:  Chuck Espinoza (copyrighted photo used per Fair Use statute)

Blue Crab on Carolina blue background:  Virginia Institute of Marine Science

Blue Crab in net: NOAA

Blue Crabs in bushel:  Calvert Beacon  https://www.calvertbeacon.com/chesapeake-bay-blue-crabs-beginning/

Blue Crab in underwater grass:  Jay Fleming (copyrighted photo used per Fair Use statute)

Blue Crab underwater, near grass:  Harris Seafood (photo used per Fair Use statute)


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