Pleistocene Bluefish (Pomatomus saltatrix)

The bluefish, a powerful fast-swimming predator, is popular with many saltwater anglers because it fights hard when hooked.  They inhabit tropical and semi-tropical waters year round, but a segment of the American population migrates as far north as Nova Scotia when water temperatures exceed 64 degrees F during summer.  They prey on small fish, such as silversides, menhaden, sand eels, and their own young; as well as squid and sea worms.  The blues prefer shallow coastal waters and especially like to congregate the down current side of shoals between narrow gaps of land. Here, they wait in underwater holes for  small fish swept over submerged sandbars by strong currents. Populations of bluefish have historically and mysteriously fluctuated, and like most other marine species of fish are in danger of being overfished.

There is little fossil evidence of bluefish dating to the Pleistocene.  In the ocean fish remains usually won’t survive the ravages of time.  But a few middens do hold evidence of bluefish.  The Old Oak midden in Florida–a mound consisting of shellfish and vertebrate bones made by generations of the Weeden Island Culture–contained bluefish bones.  This culture existed from 1200-200 years ago. However, bluefish didn’t suddenly pop into existence then.  Without paleontological evidence it’s still possible to learn something interesting about a species’ past by studying their DNA. One genetic study determined western Atlantic bluefish populations off the coast of America have been reproductively isolated from eastern Atlantic and Mediterranean populations for 480,000 years.  And there are 2 populations of bluefish living in the Mediterranean Sea that have been reproductively isolated from each other for over 100,000 years.  The isolation of these 2 Mediterranean populations is associated with barriers that formed during Ice Ages.  These underwater geographic barriers still remain.

The divergence of the western and eastern bluefish populations occurred during Marine Isotope Stage 13a–a warm peak with a cold split.  Deep waters of the mid-Atlantic are the present day barrier that keeps these 2 populations reproductively isolated from each other.  Blues prefer shallow water and are unlikely to traverse deeper waters where baitfish may be scarce over large areas.  The last time a population of bluefish traversed the mid-Atlantic could have been a summer migration that may have gone off course chasing a massive school of menhaden.  It seems likely this occurred during a stadial when the distance between the continents was smaller.  Perhaps a normal summer migration swam into unseasonably cool water that occurred because the Gulf Stream suddenly shut down, and they headed east because they became disoriented.  Bluefish probably originated long before the Pleistocene, before continents were as widely spaced as they are today.

figure 1

Map of the Georgia bight.  The shaded area was above sea level from ~80,000BP-~7,000 BP.  Bluefish prefer shallow coastal water, so their range must have been shifted off the continental shelf during Ice Ages.

Bluefish range map.  DNA studies suggest American and eastern bluefish populations have been isolated from each other for 480,000 years.

Bluefish

John Lawson said bluefish taste like salmon.  I’ve never caught one, nor have I ever seen one in the grocery store.

Image result for School of Bluefish

School of bluefish.

During glacial periods when much of the continental shelf was above sea level, bluefish must have inhabited bays and river mouths.  There were steep drop-offs adjacent to the continental shelf with deeper water than bluefish prefer.  The Gulf Stream shut down during stadials (the coldest periods of Ice Ages), and there would have been no bluefish migration north because the water temperatures were too cold.  Bluefish range was restricted to tropical/semi-tropical waters during stadials.  The northern bluefish migrations resumed during interstadials when the Gulf Stream periodically restarted and began carrying tropically heated water farther north again.

References:

Hersey, John

Blues

Vintage Books 1987

Miralles, Laura; et. al.

“Paleoclimate Shaped Bluefish Structure in the Northern Hemisphere”

Fisheries 2014

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2 Responses to “Pleistocene Bluefish (Pomatomus saltatrix)”

  1. m Says:

    “John Lawson said bluefish taste like salmon.”

    They are an oily fish and unpalatable to most, especially in larger sizes.

    https://www.google.ca/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=bluefish+taste+like

    • markgelbart Says:

      In the book I am reading–Blues by John Hersey–he has a recipe for bluefish at the end of every chapter. To make them palatable, he removes the skin (and along with it most of the oil). He also says they taste much better fresh and should be eaten less than 24 hours after being caught. That would explain why I haven’t ever seen them in a grocery store.

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