The incidence of predation by largemouth bass Micropterus salmoides on fish in Lake Conroe, Texas, was examined over 7 years at two disparate levels of habitat complexity. When areal coverage of submersed vegetation ranged from 39 to 44% of the reservoir's 8,100 hectares, largemouth bass 100 mm and smaller in total length consumed fish infrequently; most did not consume fish until they reached lengths of 140 mm and more, Following the elimination of all submersed vegetation by grass carp Ctenopharyngodon idella, fish were consumed by most largemouth bass 60 mm or longer. The initiation of piscivory at smaller sizes resulted in significantly faster first‐year growth for all largemouth bass year‐classes produced after vegetation removal. Although shifts in the structure of the forage fish community occurred, ample fish prey existed for largemouth bass before and after vegetation removal. The onset of piscivory remained similar for largemouth bass collected along the dam riprap, where vegetation was absent throughout the study. These observations support the hypothesis that habitat complexity, as mediated by vegetation abundance, was the principal factor regulating piscivory by largemouth bass in the littoral zone of Lake Conroe.
Over 3,600 hectares of submersed aquatic vegetation in Lake Conroe, Texas, were eliminated 1 year after 270,000 grass carp Ctenopharyngodon idella were stocked in 1981-1982. Seventeen fish species were commonly collected in cove rotenone samples and the biomasses of eight species declined (P < 0.10) after vegetation removal. The most notable declines were observed for several small, phytophilic Lepomis spp., for bluegill Lepomis macrochirus. and for crappie Pomoxis spp. Biomass of largemouth bass Micropterus salmoides did not decline (P = 0.12) but the density of age-1 and older fish did decline (P = 0.02). Biomass and density of two cyprinid species and channel catfish Ictalurus punctatus increased. Although biomass of longear sunfish Lepomis megalotis did not increase (P = 0.11), mean size declined and density increased an order of magnitude (P = 0.02). Density of threadfin shad Dorosoma petenense increased nearly fivefold after vegetation removal, coincident with a decline in mean size; however, variability was high and the difference could not be declared significant. Biomass of gizzard shad D. cepedianum fluctuated due to inconsistent year-class production that was not directly related to vegetation coverage. Seining revealed that populations of three cyprinodontid species, bantam sunfish Lepomis symmetricus. and brook silversides Labidesthes sicculus collapsed following vegetation removal, whereas catches of inland silversides Menidia beryllina and threadfin shad increased significantly. Gillnetting revealed that large year-classes were produced by yellow bass Mor one chrysops and white bass M. mississippiensis following vegetation removal. Although abundance of white crappie Pomoxis annularis declined in offshore regions sampled by gill nets, catches of black crappie P. nigromaculatus were similar before and after vegetation removal. No change in abundance or structure over this 7-year study could be detected for at least 10 populations. The original largemouth bass-crappie-hybrid striped bass (Mor one chrysops x M. saxatilis) fishery was replaced by a channel catfish-white bass-hybrid striped bass-largemouth bass-black crappie fishery after vegetation removal. The observed response of many species to vegetation removal could be predicted given published information, but mechanisms governing the dynamics of several important species were unclear.
We counted annul! on otoliths, scales, anal fin rays, and anal fin spines of striped bass Morone saxatilis to determine precision of age estimates from several readers and relative accuracy of the estimates from the different structures. Our principal objective was to determine if estimates from scales, spines, and rays, which can be removed without harming the fish, were similar to those from otoliths. Among-reader variation was similar for spines, scales, and rays, and lowest for otoliths. Variation increased with fish total length (TL) for otolilhs and scales. Age estimates from scales, spines, and rays were usually within 1 year of the otolith age estimate for striped bass shorter than 900 mm TL. However, for striped bass longer than 900 mm TL, estimates from spines and scales were lower than estimates from otoliths by averages of 1.6 and 3.0 years, respectively. Scales, spines, and rays can provide relatively accurate and precise age estimates for striped bass up to about 900 mm TL (age 10 from our samples). For longer fish, the choice of a structure for age determinations should depend on whether the improved accuracy and precision expected from otoliths is worth killing the fish.
We assessed the importance of initial predation as a source of mortality of stocked fingerling Florida largemouth bass Micropterus salmoides floridanus (30–46 mm total length). Florida largemouth bass were tagged with coded wire tags and stocked (3,000 fish/site) at six sites in Elm Creek Embayment of O. H. Ivie Reservoir, Texas—the location of an earlier study in which the immediate mortality of stocked fish was found to be highly variable. Stocked fish were eaten by at least five species of predators that varied widely in size. Ninety‐three percent of the stocked fish recovered from predator stomachs were consumed by juvenile and adult largemouth bass M. salmoides and smallmouth bass M. dolomieu. Assuming a capture efficiency of 8% for our predator sampling, losses of stocked fish to predation were as high as 27.5% within 12 h of stocking, and losses generally increased with predator density. In comparison, the mortality of stocked fish held in predator‐free enclosures was only 3.5% at 84 h poststocking, indicating that mortality resulting from transport and other variables was low and similar among stocking sites (P = 0.17). This study suggests that predation was the primary factor affecting the poststocking survival of fingerling Florida largemouth bass, and we recommend that managers consider predator densities when selecting stocking sites. Future efforts should examine the relationships between predation and habitat variables such as habitat composition and habitat complexity. Additional research should evaluate alternative stocking strategies such as using larger fish and habituating stocked fish before release.
As a test of the match-mismatch hypothesis, we examined the effects of prey availability and water temperature on growth and survival of weekly cohorts of larval threadfin shad Dorosoma petenense in J. Slrom Thurmond Reservoir, Georgia-South Carolina. Hatching dates were estimated from otolith increments, and availability of prey was estimated from the abundance of zooplankton size-classes commonly eaten by larval threadfin shad. Growth rates of 31 cohorts ranged from 0.39 to 0.78 mm/d, demonstrating the potential for stage-duration effects on cohort survival. Daily growth rate was related to water temperature and prey availability for larvae up to 21 d old. Growth increased linearly with water temperature up to 28°C, but the relation between growth and prey availability was more complex. Growth rate increased with prey density up to 160-290 organisms/L; at higher densities, growth rate decreased or was unchanged. Cohort survival ranged from 0.65 to 0.96 (per millimeter increase in length) and was significantly correlated with water temperature and growth rate but not with prey availability.
Stocking fish into preexisting fish communities is not always successful. Although there may be many reasons why stockings fail, predation is perhaps the most likely reason. Reducing predation by allowing fish to become accustomed to their new environment before release (i.e., habituation) may improve stocking efficiency. We evaluated the effect of habituation on predation of stocked, hatchery-reared fingerling Florida largemouth bass Micropterus salmoides floridanus (30-64 mm total length) in simple (i.e., open-water) and structurally complex (i.e., with vegetation and cobble) habitats in indoor tanks. We found that fingerling largemouth bass survival significantly increased (P Ͻ 0.004) from 26% to 46% when fish were habituated in a predator-free enclosure for at least 15 min. Surviving fish spent most (95%) of their time in the structurally complex habitat. However, the survival of all fish stocked in structurally complex habitat did not differ from that of all fish stocked in open water (P ϭ 0.61). Although predation on nonhabituated, hatchery-reared largemouth bass approached 75%, we found that short-term habituation can enhance survival of stocked largemouth bass, as manifested by improved predator avoidance. This suggests that habituating fingerling largemouth bass before stocking may result in substantial increases in survival.
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