Managing Centrarchid Fisheries in Rivers and Streams

<em>Abstract.</em>—We reexamined a local population of Guadalupe Bass <em>Micropterus treculii </em>after cessation of a stocking program that significantly reduced the number of individuals that were hybridized with Smallmouth Bass <em>M. dolomie</em> u. Analysis of five years of post-stocking fish collection data show that the percentage of hybrids has increased in all four study sections of the upper Guadalupe River since stocking stopped. We found that hybridization had increased from 0.0% to 9.9% in Johnson Creek, 19.8% to 28.8% in North Fork Guadalupe River, 5.6% to 12.0% in South Fork Guadalupe River, and 24.2% to 26.5% in Guadalupe River main stem sections. This represents an average increase of 6.8% for all streams combined (range = 2.2–9.9%) from the last year of stocking. We also examined hybridization levels in individual stream fragments created by instream barriers (<em>n </em>= 76) to assess spatial variability along tributary and main stem corridors. Although hybridization appears to be increasing on a stream-wide basis, finer scale analysis show that hybrid distribution was spatially complex and there was a substantial portion of the study area with little or no hybridization. There were gradients of decreasing hybridization in an upstream direction in each of the tributaries and marked break points upstream of which hybridization was considerably lower (<10%). We also found that hybridization was highly variable among individual stream fragments (range 0%–71.4%) including those immediately adjacent to one another. These findings suggest that instream barriers may restrict genetic mixing throughout the system and contribute to partial isolation of populations within stream fragments. As such, further targeted intervention efforts are likely necessary to further reduce hybridization in fragments that still contain high proportions of hybrids. The patchy distribution that we observed may be a critical component to consider in assessment and restoration efforts; not only for Guadalupe Bass but also in other endemic black basses threatened by hybridization.