Vegetation-open water interface and the predator-prey interaction between largemouth bass and bluegills.

Abstract

A field experiment was designed to test effects of manipulating vegetation-open water interface (or edge) on success of largemouth bass (Micropterus salmoides) preying on bluegills (Lepomis macrochirus). Four enclosures were placed in a natural lake, with different levels of structural complexity in each: (1) all vegetation removed (enclosure NV), (2) vegetation left untouched (CV), (3) one strip of vegetation 6-feet wide and 15-feet long removed from the middle creating 30 feet of edge (1S), and (4) two strips 3-feet wide by 15-feet long removed at 3-foot intervals creating 60 feet of edge (2S). Three sizes of largemouth bass (8, 11, and 14 inches, total length) and two size groups of bluegills (1.0-2.9 and 3.0-4.9 inches, total length) were used. Predation rate averaged over all sizes of largemouth bass was 4.4 times greater in the 2S enclosure compared to the 1S. A mathematical model was developed to predict numbers of bluegills encountered by largemouth bass in two enclosure types which contained edge (1S and 2S). Capture probabilities were assumed constant and independent of amount of edge. Results of model runs suggested that edge effects would double numbers of bluegills encountered in the 2S enclosure compared to the 1S, under certain conditions of light intensity, assumed bluegill behavior, and largemouth bass position in open water between vegetated plots. Spatial distribution of bluegills in both enclosures were identical in these simulations. However, bluegill behavior may be affected by width of open water between refuges and perceived density of largemouth bass in the immediate locale. Model results supported this possibility since predicted numbers encountered were sensitive to subtle changes in assumed behaviors of bluegills, and were more consistent with experimental observations. These results demonstrate that reactions to predation risk on a diel scale may cause behavioral responses by bluegills that are difficult to measure, but which significantly alter their ability to avoid detection. Appropriate configurations of edge for a largemouth bass-bluegill community should include knowledge of: (1) biomass and size structure of a largemouth bass population; (2) desired abundance of bluegills; (3) amount of edge needed per largemouth bass as a function of largemouth bass size and number; (4) critical size of refuges; and (5) effects of open water widths between refuges on largemouth bass and bluegill behavior.