Wednesday, April 10, 2013

The Bay Out of Balance

The Bay News has this blog post from some legal blog at UC Berserkly

In general, I think he's got a point, but being a lawyer and not a scientist, and one on the left coast, he doesn't seem to have a grasp of the basic facts of Chesapeake Bay

Dear Washington Post: Chesapeake Bay *is* unbalanced

An article in the Washington Post yesterday ran with the headline, “Crabs, supersized by carbon pollution, may upset Chesapeake’s balance.” Not to nitpick, but Chesapeake Bay is unbalanced and has been that way for well over a century.

The article references some interesting research from the University of North Carolina that looks at the effects of ocean acidification on blue crab and oyster populations. Ocean acidification is the result of increased carbon levels in the ocean due primarily to excessive anthropogenic carbon emissions. It turns out that blue crabs grow much larger in high-carbon waters. Oysters, however, struggle to grow in high-carbon waters. One possible outcome: the larger crabs could eat all the weaker oysters in the Bay.
I've been debating whether to blog about this study or not.  The Post article was just awful, a series of horror stories made up from whole clothe about how the carbon inputs from fossil fuels would cause monstrous crabs, which would eat all the poor oysters (the ones that we didn't), who won't grow as well due to the CO2.

The fact is, carbon dioxide in estuarine systems like the Bay is a very complicated story.  Concentrations of CO2 in Bay water can vary, on short time and scales, from many times atmospheric to fractions thereof.  Atmospheric changes in CO2 due to are likely to impose a slight baseline shift in a wildly fluctuating curve.  And crabs live on the bottom, where concentrations of CO2 tend to be higher since benthic respiration is concentrated there.

But before I trash the scientific paper, I owe them duty to really read it and evaluate it.  I haven't done that part of my homework, yet.

Oyster populations in Chesapeake Bay are somewhere around 1% to 2% of what they were before 1850. The Bay has never recovered from that devastating loss of its primary filter mechanism.

Credit: South Carolina Oyster Restoration and Enhancement
Nice graph, it's more effective than mine.  But then, it's not really his either.

The article goes on to claim that the oyster “showed signs of a modest recovery” in 2011. The evidence: both Virginia and Maryland watermen harvested 3x to 5x more oysters in 2012 than in 2005.

I would like to see reporters take a bit more nuanced approach when reporting on “recoveries” of aquatic populations. Sure, an increased catch may be correlated with increasing population size. And there are definitely signs that the Bay’s oyster population has risen slightly in the past few years. But that increased catch could very easily overwhelm any increased population size, leading to a population crash instead of a population boom. There are population studies that attempt to account for these various factors for oysters and other aquatic species. It cannot be that difficult to cite a population study, at least for the highly-studied Chesapeake Bay. So please stop taking the easy way out by conflating catch size with population size.
The point is fair that fishing success does not equal population but the fact is there simply is no significant amount of "fisheries independent" population data for oysters in Chesapeake Bay.  I was part of a very small one years ago that has since been discontinued, and it was notable among all the others that depended on commercial fishing.  So yes, it would  be very difficult to cite such a study, as they simply don't exist currently.  You might be shocked at how much data is missing from the "highly studied" Chesapeake Bay.

Oh, and the report ends by noting that juvenile blue crab populations doubled after Virginia stopped allowing dredging in rivers during the winter, just when female crabs start to reproduce. I know it seems obvious that if you don’t kill crab and crab habitat during reproduction you might end up with more crabs, but it has taken a surprisingly long time for Virginia to acknowledge this simple causal relationship. So congratulations Virginia!
It might seem obvious, but the year to year recruitment of crabs is so variable, that it would come as no shock to a fisheries biologist to have new restrictions followed by an awful crab year.  It takes a lot of years to make good statistics.

And I reject the article’s implication that we may have to catch more crabs to avoid harming the oyster population. To achieve a sustainable balance in Chesapeake Bay requires a much higher blue crab and oyster population, along with about a dozen other hard-to-achieve ecosystem improvements.
Oh, and here, I absolutely agree.  This is the kind of thing the watermen come up with as an excuse for increased crabbing. or increased fishing on striped bass, which they often recommend as a cure for crab woes, since stripers eat crabs. 

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