Posts Tagged ‘oxbow lakes’

Floodplain Fish

July 31, 2017

River systems host a hidden world of tiny invertebrates.  Some are microscopic, while others, though visible to the naked eye, remain unseen unless a curious fisherman cuts open the stomach of his catch.  A fish’s stomach might contain small crustaceans including water fleas (Cladocera), seed shrimp (Ostracada), amphipods, copepods and/or isopods.  These minute shrimp-like creatures form the basis of a food chain that supports fish populations.

Image result for Ostracod

Seed shrimp (Ostracods) along with other small crustaceans are an important part of the food chain in aquatic habitats.

In southeastern North America rivers overflow their banks between November and March because cooler temperatures reduce evapotranspiration and dormant riverside vegetation takes in less water.  The flood stage is especially wide in the flat coastal plain region where a sheet of water 2-3 feet deep can cover hundreds of square miles alongside major rivers, though modern dams, ditches, and canals have reduced the former extent of these flooded wetlands.  This flooded land offers more territory for fish to forage and reproduce.  The diet of many fish species changes from the aquatic crustaceans mentioned above to prey that normally lives some distance from the river.  1 study found fish occupying floodplains ate a species of isopod that lives in small pools of water, terrestrial species of crayfish, beetle larva, and caterpillars.  These terrestrial species were not normally found in fish’s stomachs until the flood stage.  Some species of fish even breed over floodplains that become dry land during summer.  The blueback herring (Alosa aetivalis) spawns in flooded hardwood swamps, unlike its relatives the American shad and hickory shad that spawn in the main channel and tributaries of a river.  Blueback herring eggs adhere to twigs on the forest floor.

 

Blueback herring spawn over flooded land.

White bass (Morone chrysops) also spawn on floodplains during high water.  This species is probably the “white fish” mentioned by John Lawson in his book A New Voyage to the Carolinas.  A few years ago, I wrote a blog article identifying the fish Lawson wrote about in his early natural history book.  (See: https://markgelbart.wordpress.com/2014/08/31/identifying-the-species-of-fish-described-by-john-lawson-in-1710-part-2/   )I was able to figure the identity of most of them despite the archaic names and vague descriptions, but his “white fish” stumped me.  Zach Matthews, editor of The Itinerant Angler, suggested to me that Lawson was referring to the white bass.  Lawson’s description that it was found in “freshets” or floodwaters is good evidence he was discussing the white bass.

Image result for white bass

White bass also spawn over floodplains.  This is probably the “white fish” John Lawson discussed in his book A New Voyage to the Carolinas.

There have been plenty of genetic studies of the white bass and its cousin the striped bass because the 2 closely related species are hybridized for sports fishermen.  But I can’t find any genetic studies that explore the evolutionary origin of this genus.  It seems likely white bass diverged from the same ancestor as the striped bass, and this common ancestor was probably an anadromous fish, like the latter species. The initial ancestral population of white bass began spawning on floodplains and became landlocked and unable to return to the ocean when something temporarily blocked access to the ocean.   This explains how the 2 species diverged from each other.  White bass evolved the ability to survive entirely in freshwater habitats and were able to colonize aquatic environments much further inland than striped bass.  White bass collect fat reserves and can endure cold winters.  They became well adapted to the colder temperatures of Pleistocene Ice Ages.  Geneticists could probably use a molecular clock to determine when this divergence occurred, and they may be able to tie the timing to some climatic event.

Fish use floodplains to migrate to new habitats and maintain genetic vigor between populations.  During flood stages many fish from the Okefenokee Swamp swim through flooded habitat to the Suwannee River.  Warmouth, flier, bowfin, pickerel, bullheads, and lake chubsuckers have all been recorded traveling through 2-3 feet of water to the river.  Floods can also connect breeding fish from oxbow lakes with fish from the main branch of the river.  Shiners, bream, catfish, darters, mosquito fish, and starhead minnows often travel through flood waters between oxbow lakes and rivers.  Eels also use these corridors but they don’t breed in freshwater.  Many fish get trapped in oxbow lakes and sloughs after floodwaters recede.  However, oxbow lakes provide better habitat for fish than rivers, often holding 12 X more fish per acre though species diversity is identical.  The most common fish in Altamaha River oxbow lakes are gizzard shad, spotted sucker fish, and channel catfish.

During Ice Ages rivers in the southeast didn’t flood as much as they do today.  The fish best adapted for braided river patterns were most common.  Cut-off channels within river beds probably held concentrated populations of catfish and killifish.  Anadromous fish such as shad and striped bass spawned in areas that have since been inundated by rising sea levels.  Following the end of the Ice Age, there was a supermeandering phase of rivers when flooding was more extreme than it is today.  This caused a resurgence of floodplain fish species.

Reference:

Clark, J.R.; and J. B. Forado

Wetlands of Bottomland Forests

Proceedings of Bottomland Hardwood Forest Wetlands in Southeastern United States 1980

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Pleistocene Paddlefish (Polyodon spathula)

March 8, 2016

A large population of 7 foot long, 150 pound paddlefishes lived in the primeval waters of the Mississippi River Drainage System  from well before the Pleistocene until the 19th century when humans began overexploiting this species.  An even larger population of smaller individuals also swam these waters for eons.  Millions upon millions of paddlefishes existed for tens of thousands of generations, yet, as far as I can determine from the scientific literature, not a single fossil specimen of Pleistocene Age has ever been found.  The paddlefish is a primitive species with a body structure supported by cartilage rather than bone.  There is nothing on their body that is hard and durable, therefore, evidence of their past existence is very unlikely to survive the ravages of time.  Sharks are also primitive fishes supported by cartilage, but at least they have hard teeth that do resist decomposition.  There isn’t even much evidence of paddlefish in the archaeological record, though Indians certainly utilized this species.  The remains of a paddlefish were excavated from a Native-American midden located in Wisconsin.

Scientists do know the paddlefish is an ancient species, possibly originating before the dinosaurs.  William Bemis, a paleontologist, described a Cretaceous Age fossil of a paddlefish found in Montana as “remarkably like (the 2) living species of polyodon.”  The only other extant species of paddlefish in the world occurs in Chinese rivers, and its scientific name is Polyodon gladius.  The 2 living species probably diverged during the Miocene between 25 million-5 million years BP, when climatic changes led to an environmental barrier that divided the American population from the Asian gene pool.  Genetic evidence suggests the American paddlefish has been uniform for a long time.  They travel great distances throughout the Mississippi River Drainage System and interbreed freely and do not live in isolated populations.  One tagged specimen caught in Moon Lake, Mississippi was captured later 870 miles away in Illinois.

American Paddlefish (Polyodon spathula).

The American paddlefish filter-feeding. No Pleistocene aged specimens of this species have ever been found because they are made of cartilage.  The adults have no teeth.

Paddlefish range map.  Paddlefish are now extinct on the periphery of their northeastern range and their populations are in decline elsewhere.

Paddlefish live in large rivers, braided channels, and oxbow lakes.  All of these habitats existed during the Pleistocene.  Braided channels more commonly formed during cold arid phases of Ice Ages.  The lower water table resulted in channels cut off and choked with sandbars.  Warmer wetter climate phases caused an increase in the formation of oxbow lakes as overflowing waters meandered more.  Paddlefish thrived in both habitats wherever there was an abundance of zooplankton.  Paddlefish use their unusual paddle-like structures to locate the tiny crustaceans and insects upon which they filter feed.  Their diet of mini-crustaceans probably explains why their flesh reportedly tastes like lobster.  They are still referred to as “poor man’s lobster,” even though this endangered fish is now rarer in fish markets than actual lobster.

Moon Lake, Mississippi still supports a commercial fishery for paddlefish.  They were so abundant during the early 20th century here that 100 could be caught in a single purse-seine haul.  The catch is much reduced today.

Moon Lake, Mississippi, an oxbow adjacent to the Mississippi River, still has a viable population of endangered paddlefish.

Oxbow lake formation fascinates me.  An oxbow lake is the remains of a meander that gets cut off from the main flow of the river, following a period of high water when the river surges over land to connect the shortest distance between 2 points.  These natural formations provided the only lake habitats over much of the south until man began building reservoirs.  Eventually, sediment builds in oxbow lakes until they evolve into marshes and then dry land.  Old oxbows that dry out are known as meander scars.

References:

Hoover, Jeffrey; et. al.

“Age and Reproductive Condition of an Unusually Large Bighead Carp from the Lower Mississippi River Basin”

Southeastern Naturalist 14 (4) 2015

Theler, James

“Animal Remains from Native American Archaeological Sites in Western Wisconsin”

Transactions of the Wisconsin Academy of Science, Art, and Letters 2010