Let me clarify what I meant by degrading. I didn't necessarily mean shorelines eroding away. I meant the general health of the system. Go back far enough (late 19th century, early 20th century, before the ship channel was dredged) and this is a completely different ecosystem compared to what it was naturally. Turner's Bay, Prien Lake, and Lake Charles all had Cypress in them. They were freshwater swamps. Calcasieu Lake was a thriving estuary that was probably brackish at best, maybe salt at different periods.
Then the calcasieu river was straightened and dredged deeper, allowing more saltwater into the system. Now you've got a saltier system, who's major freshwater input is not nearly as effective anymore.
So by deteriorated or degraded, I mean health, the salinities. The marsh has greatly changed. Back in first half of the century, the marshes were predominately fresh and intermediate on the east side of lake. Think cutgrass, bulrush, sedges, lilies in the deeper areas. Similar to what the northeast part of the Cameron-Creole is now, if anyone is familiar with that area or lacassine for something many are familiar with. Much of the marsh in this part of the state was that way. Add the saltwater super highway that is the ship channel, and salinity, and likely water levels (although probably not significantly), increased. Freshwater plants are poor tolerators of salt, and died off from the stress. This was over time now.
With a dying marsh, you get detritus (dead biotic material). This is what shrimp and crabs feed on. The more you have, the more shrimp and crabs you will produce. Simple enough, right?
So this would most likely explain the better than average shrimp and crab harvests before the wiers. Once the wiers and levees were constructed, it cut off a lot of that ingress and egress out of the marsh, but has saved that marsh from dying off. Sure, the marsh was able to transition from fresh and intermediate communities to brackish and saltmarsh species, but if it hadn't, Big Lake would be Giant Lake. That marsh would be open water.
So my hypothesis would be that the decrease you saw is due to a change in what was happening in that marsh. It could be due to a lot of other factors, something MG has pointed out with his study with oysters and body index.
As far as a saltwater barrier, we may get to see, at least somewhat, what this system was like before 1900.
Some will say salinity is not a big issue. Me being a vegetation guy, I say it is, especially dealing with fresh and intermediate wetlands.
P.S., I know i'm a little long winded, my appologies for that.
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