A variety of factors affect natural feeding in predator fish. Some common factors are food availability, turbidity (water clarity), cover (plants and wood, etc.), temperature, weather, water quality, and other stressors. Specifically, we examine smallmouth bass feeding with a bit on other species.
Keep in mind that depending on species, fish rely on several sensory organs to find and locate objects in the water. Sight is a major method of finding prey, but smell (chemical detection) and sound (pressure and noise) all play a role. In addition, different species rely on one or more of these systems to varying degrees. Also, some species are generalists (eat what is available depending on foraging efficiency—think yellow perch and bluegill), while others are specialists (actively selecting prey to maximize growth—bass and walleye).
One study titled Effects of Turbidity and Cover on Prey Selectivity of Adult Smallmouth Bass by Mark W. Carter, Daniel E. Shoup, John M. Dettmers, and David H. Wahl in Transactions of the American Fisheries Society 139:353-361.2010 examined turbidity (water clarity) and cover as two important variables which influence prey choice by smallmouth bass.
In this study, individual adult smallmouth bass were given a choice of five northern crawfish, five golden shiners, or five round gobies. Smallmouth bass chose round goby in clear water and golden shiners in turbid water in trials without cover. With cover present, smallmouth bass increasingly picked golden shiners, notably as water clarity decreased. Unexpectedly, crawfish (Northern) were negatively or neutrally selected in all trials with both turbidity and cover. Turbidity had a more significant effect than cover on prey consumption rate, which went down as turbidity increased.
These results suggest that turbidity and cover, two important feeding variables, influence prey choice by smallmouth bass. The reduced number of prey consumed per hour with increasing turbidity indicates that smallmouth bass easily capture prey in clear water but have more difficulty as water becomes even slightly more turbid. The authors point out that a possible explanation for these selection patterns is that turbidity reduces the amount of light available in the water column and these reductions increase rapidly with depth.
Because golden shiners are pelagic schooling fish, they inhabited an area higher in the water column where more light was available relative to areas inhabited by the other two prey types. The study notes that round goby and crawfish use cover to avoid predation, whereas golden shiners do not. Thus, it is possible that the presence of cover provided refuge only for the two benthic species, causing predator selection for golden shiners at low and high turbidities. In previous studies, round goby and crawfish each experienced reduced predation when cover was present.
The negative and neutral selection of northern crawfish across all test combinations noted by the authors was unexpected. Multiple studies demonstrate that crawfish dominate the diet of smallmouth bass. Seasonal shifts and ontogenetic shifts between crawfish and other prey have been described, but crawfish are generally recognized as preferred prey for adult smallmouth bass.
Another recent study of smallmouth bass prey selectivity in clear water found that these predators preferred round goby to emerald shiners, and crawfish were consumed the least. In the referenced recent study, optimal-sized prey accounted for energetic differences among round goby, emerald shiners, and crawfish. The low energetic gain achieved from consuming crawfish contrasts with the near-unanimous finding of abundant crawfish in reported diets of smallmouth bass. Similarly, round goby, which are lower in energy relative to other fishes, were preferred by young smallmouth bass when the two species occurred in nearshore habitats of Lake Erie, highlighting the importance of prey availability in determining which prey are consumed. These multiple study results suggest that smallmouth bass do not prefer crawfish more than other prey types, but crawfish are likely the most common prey species available in the habitats frequented by smallmouth bass.
Largemouth bass have shown more difficulty feeding in turbid water. Generally, tolerance for turbid water is higher in Largemouth bass than in smallmouth bass, and Largemouth bass are more likely to successfully forage in turbid water than smallmouth bass. The reduced consumption by smallmouth bass with increased turbidity is similar for walleyes, several other fish-eating species, and many invertebrate and plankton-eating species.
As a generalist predator, Yellow perch selected prey passively based on encounter rates and foraging efficiency compared to the walleye, a specialist fish-eater that actively selects fish prey to maximize growth. Similar turbidity-induced changes in diet have been observed for fish-eating Largemouth bass and skipjack. Turbidity may also cause changes in food choice by plant eaters, given the well-documented ability of turbidity to alter their reaction distances, rates of prey encounter, and predation rates.
Many studies note that insufficient food is the most significant cause of poor fish conditions and population imbalances.
Harvesting is a necessary management tool, along with understanding the food web. One part of population management is understanding the factors that cause poor foraging.
Reprinted with permission from Pond Boss Magazine
