Turbidity negatively affects fisheries. It reduces light penetration and reduces the production of algae that is the foundation of the food web. Eventually the fine silt particles that make water turbid settle out smothering organisms that live on the bottom, turning hard bottom areas into soft muck, and eventually filling in reservoirs.
As an angler, I have always been interested in the effects of turbidity on catching bass. I’ll confess my biases because maybe you share some of them. Clear water opens up choices in lure selection, muddy water reduces lure options. I feel more confident about getting bites in clear water because bass, like anglers, are visual predators. I really feel like I’m wasting my time fishing deep structure in muddy water; if I can only see inches into the water, how is the bass going to find my offering 15 or 20 feet deep? Yet, I repeatedly read about top-tier tournament anglers seeking “off-color” water for a better bass bite.
A recent study by fishery researchers Eric Dibble and Thad Huenemann at Mississippi State University cuts through some of the murkiness. Largemouth bass were offered fathead minnows or golden shiners as prey in large aquaria with turbidity levels ranging from zero turbidity (treated tap water) to very muddy — less than 2 inches of visibility (how far you could see into the water). The bass were 6 to 9 inches long, and the prey fish were small enough to be readily eaten. The bass were not fed for three days before the experiment, and feeding trials lasted 20 minutes. The prey fish were attached to a 3-inch piece of light monofilament to restrict their movement. This is hardly a natural condition, but Dibble and Huenemann specifically wanted to assess the effect of turbidity on detection of prey and exclude any effects of predator avoidance by the prey.
The results were simple: prey capture decreased as turbidity increased. Bass in the clearest water captured prey in 100% of the trials. Percentage of bass capturing prey decreased with turbidity, and only 15% of the bass in the muddiest conditions captured prey. Time to capture prey also decreased with increasing turbidity. Bass in clear water averaged only 200 seconds to capture prey, but the few bass that captured prey in the muddiest water averaged about 1100 seconds per capture.
Clearly, turbidity affects food intake, which in turn affects bass growth. The reduction in feeding with increasing turbidity probably would have been even greater if the prey were free to avoid the bass.
Do these findings apply to larger bass? Probably — the feeding behavior of 9-inch bass is the same as the feeding behavior of larger bass, although the bigger bass have the luxury of eating a wider size range of prey.
Biologists have documented slower growth of bass and other fishes presumed to be sight feeders in turbid water. Yet, some reservoirs that are consistently turbid support healthy bass populations, and anglers catch fish. As might be expected from the lower production of algae, which in turn reduces the abundance and production of all links in the food chain, these turbid reservoirs tend to support fewer pounds of bass per acre.
Largemouth bass have other sensory systems that aid prey detection and capture in low-light conditions when vision may be ineffective. The senses of taste and smell may contribute to prey detection, but the lateral line system that is able to detect changes in water flow is probably the main back-up system for sight feeders like bass. Research on other fish (pike) has demonstrated that the lateral line system plays a key role in prey capture, even in clear water.
Although bass can use other sensory systems to find prey, the message for anglers is clear: find the clearest water available.
Clear water increases the abundance of bass and benefits their food consumption and growth. Bass evolved for millions of years in dynamic environments, but these systems probably had clear water. Many streams and reservoirs in Mississippi, as well as in other states, are consistently turbid. The explanation is that the turbidity results from watersheds with highly erodible soils. This is true, but the soils only erode when natural ground cover is removed. Accounts from early settlers of Mississippi report crystal clear water in rivers and streams, even those in the Mississippi Delta.
Muddy waters are typical of agricultural areas, and numerous analyses throughout the U.S. have found declines in fish diversity and production in lakes and streams with agricultural watersheds. However, new studies are also proving that turbidity can be reduced where improved land management practices have been implemented. Turbid water may be common, but it is not the natural condition, and it is not the condition we want for our fisheries. Turbid water is a condition that can be repaired.