Once this has been triggered, he says, the body releases large quantities of the hormones adrenaline and noradrenaline. These hormones then trigger secondary responses—increased heart rate and blood flow to the working muscles and a decrease in flow to nonessential areas like the gut. This improve a fish’s ability to flee or fight.

“That shot of adrenaline increases blood flow to the gills,” Tufts continues, “so all the respiratory structures of the gills, the secondary lamellae, become involved in gas exchange. At rest, only about half of these respiratory structures receive blood. The increase in adrenaline levels also stimulates the release of extra red blood cells from the spleen into the circulation. This, too, improves the transport of oxygen.”

Thus, many of the most important physiological adjustments a fish makes are intended to ensure that it receives an adequate supply of oxygen when it’s hooked. Important metabolic changes also take place in the fish’s muscles. “To make long runs and jumps on the end of your line,” Tufts reports, “fish rely on energy reserves in their white muscle tissue, primarily phosphocreatine, adenosine triphosphate, and glycogen. These compounds can be mobilized quickly, even in the absence of oxygen. We call them anaerobic energy stores for this reason. Once these reserves are depleted, however, the fish no longer can jump or struggle, and it appears worn out.

“Consumption of anaerobic energy reserves also increases the levels of some important metabolic end products in the fish’s muscles, such as lactic acid. The hydrogen ions that result from the production of lactic acid can have important negative consequences in the fish.”

After a fish is released, it has several ways of dealing with the build-up of harmful by-products resulting from the vigorous exercise. Research shows that it takes anywhere from a few hours to a full day for a fish to reduce the levels of lactate and protons and to restore muscle energy reserves.

In extreme cases, some bass may not be able to recover from the stress of being caught. They typically die several hours into the recovery phase. This is the early phase of “delayed mortality” anglers often hear about but rarely witness unless they visit harbors routinely used for tournaments. When Tufts observed it during his research, he said the fish often appeared normal when released.

“Fortunately,” Tufts notes, “this delayed mortality syndrome is probably more prevalent in experimental studies, where fish are also experiencing other stresses, such as surgery and confinement for observation, than it is on the lake.”

Are Some Fish Tougher Than Others?

Researchers in this field have concluded that the same physiological processes occur in all fish species, but specifics vary among species. Different fish have varying amounts of muscle energy reserves that are depleted during angling. Levels of muscle energy reserves typically match the lifestyle of a species. Steelhead and salmon, for example, whose lifestyle requires them to leap tall waterfalls and power through rapids during long migrations, have higher levels of muscle energy reserves than more sedentary species like bass, walleyes, and crappies.

“These differences provide a physiological basis for what anglers observe—some fish fight harder than others. Other factors are involved, including the shape and size of the fish, but species differences in muscle energy reserves largely explain why some fish fight harder and longer than others.”

Other factors affect the way a fish responds physiologically to being hooked, fought, landed, and released. “Many studies have shown that the time it takes to land a fish can have important consequences to a fish’s health and survival,” Tufts says.