Evaluation of Fish and Invertebrate Assemblages Associated with Torpedograss (Panicum repens) in Lake Conroe, Montgomery County, Texas
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Torpedograss (Panicum repens) is a non-indigenous perennial species of rhizomatous graminaceous grass that currently persists along the majority of the vegetated shoreline of Lake Conroe, Montgomery County, Texas. For this study, invertebrate and fish assemblages associated with varying densities of torpedograss were studied seasonally from fall 2015 through summer 2016. Fish assemblages were sampled through the use of exhaustive electrofishing within blocknetted areas containing torpedograss. Invertebrates were sampled using a drop sampler, which was used to collect standardized samples of invertebrates from the water column, torpedograss vegetation, and benthos. Diet composition of piscivorous and insectivorous fishes were also observed. All fish species collected that comprised at least 2% of the total number of fishes, with exception to largemouth bass, were low dissolved oxygen tolerant species. Kolmogorov-Smirnov tests for length frequency for bluegill (Lepomis macrochirus), largemouth bass (Microperus salmoides), and western mosquitofish (Gambusia affinis) showed no significant difference in length frequency distributions across torpedograss densities; however, significant differences were observed for largemouth bass and western mosquitofish based on seasonality. With regards to invertebrates, a canonical correspondence analysis showed that torpedograss density and weight had the strongest positive correlation (pseudo-F = 25.9, p-value = 0.002) to taxa composition and densities, representing 43.6% of explained variation. Season and location of taxa within the water column each had a significant positive correlation to the number and diversity of invertebrate taxa and represented 15.8% and 42.4% of explained variation respectively (pseudo-F = 13.0, p-value = 0.002), (pseudo-F = 49.0, p-value = 0.002). The most common invertebrate taxa found within fish diets were also the most common taxa found within torpedograss patches, and chironomid larvae consistently had the highest frequency of occurrence and prey-specific abundance in bluegill, largemouth bass, and golden topminnows (Fundulus chrysotus). With regards to fishes, a redundancy analysis showed that torpedograss density and weight had the strongest positive correlation (pseudo-F = 5.1, p-value = 0.002) to fish taxa composition and densities, representing 61.8% of explained variation. Season combined with torpedograss density had a significant positive correlation to the number and diversity of invertebrate taxa and represented 28.4% of explained variation respectively (pseudo-F = 1.7, p-value = 0.09). Stable isotope analysis supports the use of torpedograss as a foraging location, as the δ^13C (‰) values for the largemouth bass, bluegill, and western mosquitofish were all similar, and correlated to the δ^13C (‰) value of the invertebrates and periphyton tested. The δ^13C (‰) value of torpedograss is relatively close to that of the western mosquitofish, suggesting that the western mosquitofish at some point in its life could be consuming torpedograss itself. These findings provide insight into the fish and invertebrate communities that are utilizing torpedograss as habitat and a potential foraging location. As such, management, rather than eradication, of torpedograss may be preferable when making fisheries management decisions.
Mynatt, Christopher Matthew (2016). Evaluation of Fish and Invertebrate Assemblages Associated with Torpedograss (Panicum repens) in Lake Conroe, Montgomery County, Texas. Master's thesis, Texas A & M University. Available electronically from