Dive Dry with Dr. Bill

#458: When Is a Mackerel Not a Mackerel?

How would you like to go through life knowing that almost everyone wants to eat you? You'd have to be constantly on guard, watching for the slightest sign that the kelp bass hiding in the kelp canopy, that shark cruising past the edge of the kelp forest or that sneaky sea lion bolting towards you from below hoped to swallow you whole! There's even the unseen threat of a pelican or cormorant approaching from the sky and crashing through the water's surface to take a big gulp. You think you have stress in your life? Imagine the poor jack mackerel!

To compound things, humans refer to this fish as the jack mackerel (Trachurus symmetricus) when it isn't even a mackerel at all! It's a member of the jack family, Carangidae, along with species such as the pilot fish that swims alongside sharks and the yellowtail which may munch on its little brother. Another close relative is the Mexican scad (Decapterus scombrinus) that I filmed from inside the cage while waiting for the great white sharks off Guadalupe Island. In the past this species was called the horse mackerel, and it is related to neither of those critters. It has also been called the Spanish mackerel or Spanish jack. So in addition to a stressful life, it must have a mild identity crisis.

Two features set these fish apart from most others in our waters. The lateral line along their side takes a major dip behind the pectoral fin that is very prominent. There is also a dark spot at the rear of their gill cover or operculum. The elongate body has two separate dorsal fins and a strongly forked tail or caudal fin. The color is metallic blue to olive green on the back, silvery on the sides and white on the belly. The green color variant almost looks like a different species. The region near the head and around the eye may be quite dark, and the entire body may darken somewhat as the fish ages. They are reported to reach a maximum size of 32 but are usually less than 22 inches.

This jack is known from SE Alaska to the tip of Baja and into the SW Gulf of California. It has also been reported off Acapulco and the Galapagos. They move into their northern range as the temperatures increase in spring and summer, reaching the Gulf of Alaska by August or September. El Nino years may drive more of them north. Although large schools of juveniles are frequently seen inshore, adults are commonly found 200 miles off the coast and even up to 600 miles. The young-of-year are mostly found in southern and central California inshore waters. Jack mackerel are usually found within 30 feet of the surface, but have been known to go to depths of 600 feet.

The members of the genus Trachurus have an interesting evolutionary history. It is believed they first evolved in the early Miocene (as much as 23 million years ago) in the southern European regions of Russia. The early ancestor later became Trachurus picturatus, a species that currently resides in the eastern Atlantic and Mediterranean. Scientists believe some ventured into the western Atlantic, crossing into the Pacific when the Isthmus of Panama was submerged. When the land mass there emerged, it isolated the Pacific populations and they evolved to become Trachurus murphyi which is found off South America, and our species found here on the west coast of North America.

Like most species of baitfish, the jack mackerel feeds largely on zooplankton (animal plankton), especially small crustaceans like copepods (including their eggs and larvae). They also take fish and fish larvae and the larger ones will eat squid, krill, anchovies and lanternfish (in deeper water). In turn they are munched on by predatory fish including swordfish and white sea bass, birds and marine mammals. They were commonly taken by Native Americans.

Human consumption of jack mackerel is an interesting story. Back when they were known as horse mackerel, people shied away from them preferring other species. Then in the late 1940s the Pacific sardine fishery collapsed... and Cannery Row needed something new to place in the tins. They turned to horse mackerel, landing a mere 10 million pounds in 1946 but 130 million the following year! In 1948 the US Pure Food and Drug Administration allowed the name to be changed from horse to jack mackerel and the fishery took off. The fishery for the southern species off Peru and Chile is one of the largest in the world. Funny how a name means so much... but then how many of you would be buying Patagonian toothfish if it hadn't been renamed Chilean sea bass (which it isn't). Of course no one should be buying that threatened deep sea species these days.

The majority of females (about 70%) are sexually mature by the end of their first year at a length of about nine inches. Spawning occurs when water temperatures reach 57-61 F. Some sources state the spawning season goes from March through October, while others say spawning may occur any month of the year in warmer waters in both inshore and offshore waters. Larger fish are believed to spawn earlier than smaller ones. Jack mackerel are batch spawners and don't put all their eggs in one "basket." Females spawn on average every five days and may do so twenty five times a year. A female may produce 800 to 437,000 eggs which hatch in a mere 2-3 days. The larvae are planktonic and recruit out of the plankton to nearshore waters where giant kelp or eelgrass may be present. The juveniles form very large, dense schools which we frequently see in the dive park and elsewhere along Catalina's coast. If they are very good boys and girls, and don't get munched by the big bad predators, they may live up to 30 years. I guess like those of us who were young in the 60s, they don't trust anyone over 30. Of course we grew out of that!

© 2011 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 29, 10:00 AM weekdays and on Charter Communications Cable channel 33 at 7:30 PM on Tuesdays in the Riverside/Norco area. Please help me climb out of self-imposed poverty... buy my DVD's (see this link). Yes, take Dr. Bill home with you... we'll both be glad you did!

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Close-up of jack mackerel in school and image showing black spot on gill cover (A) and
dip in the lateral line (B); how I like to see jack mackerel, and how some others like to see them.

This document maintained by Dr. Bill Bushing.
Material and images © 2011 Star Thrower Educational Multimedia