Physiology behind the great power of skipjacks
The tunas of the world are divided into two major groups: the large or “true” tunas (all with the first name of Thunnus) and the so-called little tunas or “little tunnies.”
The little tunas, as their name suggests, do not grow to a huge size, but what they lack in pounds, they more than make up for in power. Just seeing a school of skipjack (aku) hurtle through the water, like so many mini-torpedoes, is enough to make one realize the incredible power of these “pocket rockets,” and on matched tackle, they seem possessed of strength far beyond their size.
The power-to-weight ratio of little tunas has a lot to do with their physiology (internal make-up). They are amazingly adapted for high-speed life in the open ocean. For starters, the skipjack has proportionately more red muscle (which is designed for long-distance swimming) and, therefore, the ability to “slug it out” over a long period.
The second feature of internal biology of the skipjack that relates to its energy and metabolic rate is that it is the most warm-blooded of any fish that swims. We always tend to think of fish as being cold-blooded, and it is true that the vast majority have no control over their internal temperatures, being at the mercy of ambient water temperatures. However, some of the highly advanced pelagic fishes have evolved a form of heat regulation – notably the billfishes, some sharks (such as makos) and tunas.
All of the tunas have achieved this ability to some extent, but the skipjack stands out, being able to maintain its internal temperature at up to 15 degrees centigrade above surrounding water temperatures. The warm body temperature gives the skipjack advantages over cold-blooded prey, especially when diving to deeper, cooler depths.
The skipjack is easily recognized by its horizontal belly stripes. It is sometimes confused with bonito, but the latter has horizontal stripes over most of its body and also possesses sharp teeth, unlike skipjack. In life, the dark back of the skipjack ripples with seemingly fluorescent, purple hues, while the white underside reflects pink and mauves.
Another color variation that skipjack often undertake is to produce vertical bars when excited or feeding.
The skipjack is one of the most widely distributed of all pelagic fishes. Its range extends at least between the latitudes of 40 degrees North and South in all three major oceans.
The widespread distribution is primarily due to the skipjack’s broad thermal tolerance, from less than 15 degrees to over 30 degrees centigrade. It has long been known that skipjack concentrate at convergences of water masses.
The growth rates of skipjack are not known with certainty. The best “guesstimates,” which are based on tag recapture data and examination of growth bands, are that skipjack live for perhaps only four years, reaching a spawning size by their second year of age.
The maximum size of skipjack is quoted in scientific literature as 108 centimeters fork length (3’6”), which would weigh around 72 pounds. The IGFA all tackle world record for skipjack is 41 pounds, 14 ounces on 30-pound class line. The Hawaii state record is 40 pounds, 8 ounces.
Like many other aspects of its biology, the reproduction of the skipjack is nothing short of amazing. This was the first tuna species proven to not only produce huge numbers of eggs, but to spawn every day (or so) for months on end.
The numbers of eggs spawned each day by a single female skipjack range between about 100,000 and 2,000,000, which considering the sheer biomass of skipjack in the tropical oceans, is a mind-boggling concept. Is it any wonder that this little tuna has been dubbed “the cockroach of the sea?”