Case Studies in Fisheries Conservation and Management: Applied Critical Thinking and Problem Solving
Case 1: A Tale of Two Oceans: The Demise of Bluefin Tuna
The bluefin tuna saw the pod of false killer whales slash through the tuna school again, injuring several fish. The school pressed on at high speed through the chilly northern Atlantic, leaving both the victims and attacking predators behind. This tuna was built for speed. Its cylindrical body and sickle-like tail are shaped for maximum hydrodynamic efficiency. Even its dorsal fin can retract into a groove on the side of the body, thereby reducing high-speed drag as it pursues prey.
Bluefin tuna have been clocked at over 55 mph (90 km/hr). In fact, the English word ‘tuna’ is derived from the Greek verb ‘thuno,’ or ‘to rush,’ in reference to their hurried lifestyle. All tuna species are constantly on the go, needing rapid forward motion to force enough water over their gills to satisfy their body’s tremendous oxygen demand. To satisfy this demand, tunas have larger gill-surface areas than most fish in order to extract as much oxygen as possible from the surrounding seawater. Additionally, the blood of tunas contains the oxygen-binding compound myoglobin, as well as the hemoglobin found in most animals. Myoglobin can carry even more oxygen than hemoglobin, thus providing additional oxygen to the tuna’s heavily used swimming muscles. Finally, the muscle tissue of tunas contains more blood vessels than the muscle of other fishes, making the meat of tuna much darker in color than most other species found in fish markets worldwide.
Heat is also an important consideration for these marine sprinters. Unlike other fishes, which are generally considered to be ‘cold-blooded,’ many tuna species actually generate heat in their muscles through constant exertion and they display metabolic rates that are almost as high as those of mammals. Tuna have an elaborate countercurrent blood-flow system (the ‘rete mirabile’) that allows them to capture this heat and recycle it to swimming muscles, thereby increasing the muscle’s power output even further.