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Adaptations help tuna hunt

By Staff | Dec 24, 2009

Tis the season for tuna and that platter of fresh sashimi.

It looks like the Hawaii-based longline bigeye quota will not be met before the end of the year, meaning there should be no shortage of fish.

Price will be another thing. Most of the tuna in the markets will come from the long-range Hawaii-based longline fleet offloading in Honolulu. The freshest ahi will be from the local ika-shibi and “weekend warrior” fishermen.

Bigeye tuna are excellent swimmers with a number of remarkable adaptations that make them efficient predators of the ocean. Unlike many other fish, they cannot pump water over their gills, but instead swim with their mouth open, which forces water over their gills. While this is an efficient way of getting water over their gills, it means that if they stop swimming, they will suffocate.

The gills of tuna cover a surface area up to 30 times larger than other fish, giving a large surface over which water can flow, enabling about half the oxygen present in the water to be absorbed.

As well as adaptations to enhance the amount of oxygen reaching the muscles, a unique system to regulate their body temperature enables tuna to maintain their body temperature above that of the ocean. This system is a countercurrent heat exchange system that prevents heat from being lost to the surroundings. This maximizes the efficiency of the muscles and maintains good brain and eye function.

The hearts of bigeye tuna are also much larger than those of other fish — about ten times as large — relative to the size of the body. This gives bigeye tuna an unusual ability to function effectively at the low ambient temperatures encountered while foraging in cold subsurface water. Nonetheless, bigeye tuna must make return trips to warmer surface waters to warm themselves up.

Bigeye tuna are thought to have a life span of up to ten years, with individuals achieving sexual maturity at the age of four. Mature bigeye tuna may spawn every 1-2 days during periods of the full moon, over several months, at least twice a year. They release an incredible 3,000,000 to 6,000,000 eggs each time.

In the Eastern Pacific, spawning has been recorded between 10-degrees North and 10-degrees South, peaking between April and September in the northern hemisphere, and between January and March in the southern hemisphere.