Tuesday, December 7, 2021

Starting Now… The Greatest Water Migration on Earth!



By Bob McConville
Master Naturalist

B6-CBN-5-1-11Undoubtedly, many full time residents of Marco Island and surrounding areas are noticing some changes in the birds we see at this time of year. Spring has sprung and our winter guests have taken flight back north as warm temperatures return. In addition some feathered friends from Central and South America are just arriving as the southern hemisphere begins to cool.

In the waters, many aquatic animals are also traveling north, following the Gulf Stream current as they, too, migrate to chase their sources of food. But this does not happen just in the northern part of the globe. One of the greatest migrations on the planet is beginning right now and the catalyst for this unbelievable phenomenon is caused by what most would consider an insignificant little creature. The culprit is a sardine.

Just about 6 weeks ago we experienced the first day of spring or what is called the vernal equinox whereby both the northern and southern hemispheres receive equal amounts of solar energy. As the earth continues its orbit around the sun, the planet will tilt and our part of the world receives more energy, warming us for the next several months. Conversely, the area south of the equator will have less direct sunlight which cools it down. Not only do the land masses cool but so does the water and that begins this amazing sardine run which puts into motion a feeding frenzy that attracts the largest biomass of predators on record. It is happening right now, in May.

As the waters cool, this seems to trigger the migration of the sardines



northward. In addition to decreasing sea surface temperatures other factors will affect the run, such as wind direction, atmospheric conditions and the strength of the currents.B6-CBN-5-1-13

This takes place in southern Africa, along the eastern coast. As the waters cool the sardines will spawn and move northward along the coast. The numbers will continue to grow until, in some years, it will be in the billions! This gathering of fish is called a “shoal” and this migration is now being called “The Greatest Shoal On Earth”.

Let me explain how large this shoal can be. In some years it has been seen to be 2 miles wide and 12 miles long which encompasses 24 square miles. Marco Island is 4 miles long and 6 miles wide or 24 square miles. Imagine a shoal of fish placed side-by-side and one behind the other all across Marco Island. That’s a lot of sardines, but I’m not done yet. Now stack those fish one on top of the other until they are 30 feet high. That can be the size of a sardine migration along Africa’s coastline!

Now the fun begins. A food source of this magnitude will not go unnoticed by any predator and there are plenty waiting for this annual event. The first indicator that the fish are present is the Cape Gannet, a diving bird that will be seen above the shoal. Once they locate the prey the Gannetts will dive, by the thousands, at speeds reaching 100 miles per hour, to devour their meal.

Cormorants, gulls and terns will also get in on the action. From below the sea



the predators are even more numerous. Several varieties of sharks will join in the frenzy including the Dusky, Blacktip, Spinner, Grey Nurse and, primarily, the Bronze Whaler. Gamefish such as Bluefish, King Mackerel, Tuna and others add life to the party.

But one aquatic species that actually separates many of the sardines into “bait balls” is responsible for much of the work. That would be the cousin of one of the most popular creatures around Marco Island, the dolphin.

During one seasonal run it was estimated that 18,000 dolphins were on the hunt. They would separate the shoal into smaller numbers for better control and ease of capture. The dolphins would corral groups of sardines into bait balls about 30 to 50 feet in diameter and about 30 feet deep. While several dolphins moved under and around the captives to keep them in hand the others would dart in and out of these spheres of food for easy eating. This also made it more simple for sharks, gamefish and birds to attack and feed.

So do all of the sardines experience the “end of days”? No.

As the coastal waters warm the sardines will move into deeper waters to make their way back home where it will all happen again the next year.

This is truly one of the most amazing migrations that takes place on our planet and probably THE most amazing in our waters. Mother Nature has, again, created another wonder for all to learn about and understand.


Bob is the owner of Stepping Stone Ecotours and also a naturalist on board the Dolphin Explorer survey program. He is a member of Florida SEE (Society for Ethical Ecotourism). Bob loves his wife very much!


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