Bass Fishing

The Raptor Effect

Fish Facts
The author with a nice largemouth.
The author with a nice largemouth.

During a recent Missouri fishing tournament at the Lake of the Ozarks, my partner Mike, after having a tough time landing any quality largemouth bass, suggested we turn to flipping jigs and worms under docks. In short order we were rewarded with a number of keeper bass. At this point Mike remarked that the bass were collecting under the docks because the cooler water that’s found there. As a biologist, avid SCUBA diver and retired high school chemistry teacher, the cool water explanation just didn’t make sense. There was no question we were finding bass under the docks but the question for me was why.

Typically when we talk about the location of largemouth bass in summer or fall we speak of two main factors, oxygen content and food source location. However, there is a third factor that I think is a crucial motivating factor defining where a bass will most likely be found that has not been addressed. That third factor is what I call the Raptor effect.

I understand how a bass, during the hot summer months will drop down to lower levels in the lake to rest in oxygen rich cold water. This temperature change has a substantial effect on the amount of oxygen in the water because warm water expands the air it holds and rarifies the oxygen. Warm water holds 5% less oxygen than deeper water that is 20 degrees colder. In addition to the rarefication of oxygen in warmer water, the higher temperature water causes organic matter in the warmer water to undergo decomposition at a higher rate. This decomposition of plant and animal matter also removes oxygen and acts further to make the warm water less hospitable for bass. As a result, bass will move to water that is deeper and cooler.

The truth of the matter is that the water under a dock is nearly the same temperature as the other surface water refuting the idea that cooler water under a dock bring in the bass. I have been diving around docks with a thermometer and confirmed that the water temperature is not cooler under a dock. There is another reason for the bass accumulation under a dock and it isn’t oxygen level.

The second reason given for abundance of big bass under boat docks is the presence of bait fish. When Willie Sutton in 1934 was asked by the FBI why he robbed banks he said, “That is where the money is.”  It goes to reason that the presence of big bass under docks takes place because of the abundance of food. Here again I believe this assumption is unfounded. 

Looking under docks as both a free diver without tanks or SCUBA diving, I have found that bait fish are not prevalent under docks. Yes, brush piles do afford protection for small fishes and structure certainly does hold fish but a floating dock does not hold baitfish any more than open water. As a matter of fact, schools of shad and minnows have a tendency to move away from structures like docks so as to keep the school intact. There is a rarely spoken of reason why largemouth amass under docks and it is not because docks hold baitfish. I believe it is because the dock affords protection but not from what lies below but instead what attacks from the sky - raptors.

For over 150 million years raptors, including eagles, vultures and ospreys have circled over lakes devouring fish that are close to the surface. The acute eyesight of raptors enable them to look into the water and snag fish from the surface as eagles do or in the case of the osprey, dive as far as a three feet underwater. 100 million years ago these raptors had a wingspan of 30 feet and are thought to have dove under water in excess of 10 feet. Fish that were slow to adapt to deeper water have been the target of these raptors. In response to the pressure form aerial assault, bass have evolved to exist away from the raptors by staying at a protected depth and/or hiding beneath a protective structure. 

The tendency for bass to move deeper on sunny bright days or when the water is clear further exemplifies how the bass are wary of assault from the air. The tendency for a bass to move to shallower water at dusk or be found under structure such as a log or piece of flotsam further indicates how bass are cautious of attack from above. 

This brings us to the initial question, why do bass hide under boat docks. It seems that this behavior is an adaptation to prevent the bass from being snatched from above. In order to hide from the aerial attack of a raptor, bass will move depending on the amount of available light, clarity of the water or presence of surface structure. 

Boat docks are a formidable structure and act as a perfect protection from attack above the water. It must be remembered that anglers are also aerial predators. Just as the bass has evolved, we need to adjust our aerial assault in response to the movement of the bass. Keeping in mind the raptor effect and the logic behind the movement of bass we can become better anglers which will helps enable us to find and catch more fish.