Water that is too salty may have higher incidents of mortalities brought about by predation by large numbers of oyster drills (snails) which can quickly wipe out an oyster reef. Oysters in those same waters may have their shells compromised by boring clams and sponges (which also tend to avoid fresher waters) and by the oyster diseases MSX and Dermo.

While the oysters themselves can survive a very wide range of salinities, they thrive best where there is the correct mixing of fresh and salt waters. This is true throughout the world. Estuarine areas are their preferred habitat.

The Conowingo Hydroelectric Dam was finished on the Lower Susquehanna River in 1929, and was touted as being able to help nearshore oyster reefs better withstand freshets (minor flooding - sometimes lasting several weeks). It did indeed help do that, and many of those nearshore reefs prospered. However, being closer to the shore, they are also closer to pollution where harvesting is usually not allowed.

The most serious problems were to occur during periods of prolonged drought, where only the nearshore reefs would receive the necessary amount of the freshwater so vital for their survival. Oysters on the more distant reefs would succumb to predators and disease. The oyster fishery of the Chesapeake would fluctuate with the cycles of wet years and dry years. During wet years, reefs further offshore tend to thrive, during dry years, they die off.

This problem gradually became more and more severe as additional reservoirs were built on the numerous watersheds that supply the Chesapeake Bay with the freshwater that oysters need. Fishermen or watermen worked the reefs that remained to earn a living for their families. Some of these remaining reefs were most likely over harvested by the large number of fishermen the bay had faithfully supported in the past.

During the late 1990's, some progress was made in oyster recovery, I feel largely as a result of some of the smaller dams being removed in an effort to restore the anadromous striped bass or rockfish, which migrate upstream to spawn. (This and other species were also victims of damming the rivers.) A prolonged drought, set back these restoration efforts.

Granted, other reasons for the demise of oysters share the blame. For example, TBT anti-fouling paints for boats that were common in the 80's had very lethal effects on shellfish. These paints were reported to be especially effective on the larval stages of oysters. They worked too well so they were banned or heavily regulated. But oysters must have freshwater in order to survive. They don't need much. The damming of rivers many miles from the reefs is sure to have a cost. The Chesapeake Bay is just one area paying that cost. Think about that the next time you take a shower.  

Mollusk-eating has never been the most obvious way to satisfy human beings' irrational craving for animal flesh.  At least with cephalopods (octopuses, squids, cuttlefish), one or two, or not many more, might be satisfactory for one diner's course.  But with bivalves (clams, oysters), gluttony is the way to go: the more, the merrier.  And perhaps there is a prestige quotient too: the more, the cooler!

Chickens deserve our true friendship! So do fish! So do other sentient beings! Let us learn to be kind.

During more natural times, bivalves were plentiful and humans lived in ecologically friendly low numbers.  In this case, humans did absolutely no ecological harm by eating bivalves, which is one of the few truly natural meat sources for humans.  I realize that you also advocate for animal rights, but from an ecological perspective, there's no harm from human consumption of bivalves per se.  The problems are unnatural human behaviors that have lowered bivalve populations and human overpopulation.

Nevertheless, it should never be forgotten how "invertebrate" is a lazy crazy cowardly vertebratocentric term, one of the worst words in all the vocabularies of all the sciences.  So, by way of recompense, one rather leans in the favor of the clams.

Meanwhile, notice in the fund-raising pageant that remarkably scruffy photo of none other than Fuzz-Face DR's Generalissimo himself, Chip Giller, who looks as though Sarah had dared him to swallow a jellyfish, and he did.

Chickens deserve our true friendship! So do fish! So do other sentient beings! Let us learn to be kind.