Monday, February 17, 2014

Soft Shell Clams in the News, Pt 2

Green Crabs:  

DISCLAIMER:  As a scientist, I'm paid to be a skeptic.  (Some would say I go above and beyond the cause by being a cynic as well, but that's just their opinion . . . .)  I bring this skepticism to my interpretation of green crabs in the news.  Being skeptical doesn't mean I don't think green crabs are an ecological problem, and an economic problem.  It means I think we should base opinion and policy on valid research results.  Not that quality research always translates easily into good management decisions!

Green crabs (Carcinus maenus) are an invasive species thought to have come to New England sometime in the mid 1800's, from a sailing ship.  In the old days, these ships were ballasted for trips with little cargo with cobbles taken from the intertidal.  When they arrived at a port and took on more cargo, they dumped this ballast overboard.  Often, the ballast stayed moist throughout the voyage, so the hitchhikers on the cobbles fared just fine, and made a new home when thrown overboard.  (This is how common periwinkles got here too.)  

Recently, there's been a lot of news about an "exploding" population of green crabs in Maine.  One news report quotes someone as saying "They just boil out of the water" -- and uses these words:  swarms, horror movie, and infestation.  An interesting use of language.  Unfortunately, Maine hasn't invested in science very well in the past (or present), and we have no actual surveys that can inform us about how green crab populations are changing -- just anecdotal evidence.  The state hasn't conducted a survey of green crabs for over 20 years, leaving us reliant on our own poor judgement about how many crabs there are now compared to previous years. 

Far be it for me to be a judge of good journalism, but I have to call out some of our Maine journalists for not doing their homework.  If my students repeated in writing what I've seen in recent articles about green crabs, they'd be pretty unhappy with their grades.  Here's a "fact" I've heard repeatedly:  green crabs can eat 40 soft shell clams a day.  First off, we should all be left scratching our heads on this figure.  How big are these clams they're talking about?  Full size?  Because that simply seems physically impossible.  Babies, just settled from the larvae?  Possible in terms of fitting inside a crab, that's possible, but the likelihood of a crab finding 40 recently settled clams seems poor.  Remember, they have to find their prey, handle it (break the shells back to get to the adductor muscle), then eat it.  To top all this off, nowhere can I find an actual paper that documents this "40 clams a day" figure.  If anyone can find it, please let me know -- I'd be happy to see it.

What do we know about green crab foraging?  This study by Ropes provides some pretty basic information.  They feed from spring to fall. (At least they did in 1968 when the paper was written; with warming water temps, they could be eating year-round.  Since clams settle out of the larvae in late summer/early fall, any extension of the period crabs feed in could make a major impact on clams.)  They are omnivores (I did not know that until today!); in New England eating Spartina (from salt marshes) for example.  They eat a variety of animals; Ropes found their stomach contents contained a mean of ~1.3 clams for large crabs and ~0.2 for small crabs.  Large crabs seem to concentrate on soft-shell clams more than small crabs.  This study suggests large crabs can eat ~21 small clams per day, dependent on how dense the crabs were -- more crabs meant fewer clams eaten per crab (I assume they are too busy interacting with each other).  Apparently Glude (1955) found crabs in a lab ate ~15 clams a day, but I've had trouble finding a copy of that paper. 

I have no doubt crabs are responsible for substantial declines in clam populations.  What's the difference between 40 and 15 clams a day?  Either way crabs have a substantial effect on clam recruitment.  A decline in clam populations was already noted in the middle of the last century (and even then attributed to the invasive green crab), so in fact we are working on a shifting baseline anyways -- what we think of as a normal clam population is probably much smaller than what it actually is (what it would have been 400 year ago).  But I'd like to see more attention paid to basic science (Sea Grant, fund some surveys over time) and better journalism.  Things I think will help clams:

1)  Fund FOARAM -- Federal Ocean Acidification Research and Monitoring.  This act was passed by Congress in 2009 but has never been fully funded.  If we're a society that values science we should fully fund FOARAM, which at less than $20 million/year, is a bargain.  What we don't know will hurt us.
2) Establish a true clam ecology program in Maine, with surveys of clam recruitment, crab density, and harvests.
3) Continue to try some crab exclusion programs.  Or try some experiments in making their habitat more complex (complexity reduces predation).  

Now, Janet's rant is over.  I'm heading down to Kittery next weekend to walk the beach -- I hope to have some terrific info and photos, with no science or whining.  


  1. And here, another article with the 40 clams a day.

    I emailed the journalist this time to see what his source is.

  2. Unfortunately, he says that information comes from "our correspondent in Newburyport". Sigh.