Bull sharks in rivers……

I wonder if they are in the Hudson, or any other rivers in the tri-state area?

In September 1937, two commercial fishermen in Alton, Illinois, twenty-four miles north of St. Louis, pulled a five-foot, eighty-four-pound bull shark out of the Mississippi River, 1,740 miles from the Gulf of Mexico.

Herbert Cope and Dudge Collins had been finding their bait traps destroyed by something large enough to tear through the mesh. They reinforced the traps with wire, baited them with chicken viscera, and set them again. The next morning they hauled up a dead bull shark, tangled in the wire. The Alton Evening Telegraph ran the story. Nobody could explain what an ocean predator was doing in a river north of St. Louis.

Fifty-eight years later, in 1995, workers at the Rush Island Power Station in Festus, Missouri, found a bull shark jammed against the intake screen of a cooling water canal. Festus is about 900 miles from the Gulf. Two confirmed bull sharks in the upper Mississippi River, separated by six decades, both far enough inland that the ocean is in another climate.

In 2021, Cincinnati Museum Center paleontologist Ryan Shell and Nicholas Gardner of WVU Potomac State College published a study in Marine and Fishery Sciences reviewing hundreds of reported bull shark sightings on the Mississippi. Out of all of them, only the 1937 and 1995 catches held up to scrutiny. The rest were incomplete, misidentified, or unverifiable. Their conclusion was that bull sharks are almost certainly traveling the river far more often than anyone documents, engaging in what Shell and Gardner called cryptic movement: coming and going without being seen or caught.

The bull shark is the only large ocean shark that can survive indefinitely in fresh water. Every other shark species that enters a river either dies within days or turns around. The bull shark rewires its own body chemistry. In saltwater, its blood carries high concentrations of urea and salts to match the osmotic pressure of the ocean. When it enters fresh water, the kidneys increase urine output by as much as twenty times the normal rate to flush out the excess water flooding in through its tissues. The rectal gland, which normally pumps salt out of the body, throttles down to conserve sodium and chloride that would otherwise be lost in the dilute river water. Urea levels in the blood drop by more than fifty percent. The entire system runs in reverse, and the shark swims on as if nothing changed.

This is not a new behavior. Bull sharks have been documented 2,500 miles up the Amazon River in Iquitos, Peru. They live in Lake Nicaragua, where ichthyologist Thomas Thorson proved in the 1960s that the sharks were not a landlocked species, as previously believed, but were swimming upstream through the rapids of the Rio San Juan from the Caribbean. An eight-foot bull shark was caught in the Potomac River in 2010 by a commercial fisherman who had worked those waters for thirty years and never seen one. They turn up in the Brisbane River in Australia regularly enough that swimmers are warned.

The Mississippi has a temperature floor. Bull sharks are warm-water animals and cannot tolerate the river in winter. But from late spring through early fall, the water temperature through the lower and middle Mississippi is well within their range. Shell and Gardner noted that the fossil record shows bull shark teeth in the Mississippi drainage going back to the Miocene, millions of years ago, though not as far north as St. Louis. Whether the 1937 and 1995 animals represent a new northward push or the first time anyone happened to catch a shark that had been swimming past St. Louis for centuries is a question the data cannot answer.

The Mississippi River is 2,340 miles long and drains forty percent of the continental United States. Somewhere in it, at least occasionally, there is a shark.

Source: Shell & Gardner (2021), Marine and Fishery Sciences / Thomerson et al. (1977) / Burr et al. (2004) / Alton Evening Telegraph.