The spatial and temporal patchiness of limnetic fishes is not well understood but has important implications for biological interactions as well as for sampling strategies. We used hydroacoustics to assess the distribution of threadfin shad Dorosoma petenense in Lake Norman, North Carolina, a large, multipurpose reservoir We sampled three transects on four consecutive days and nights in April, July, and October to enable us to characterize threadfin shad on both daily and seasonal time scales. We used a patch recognition algorithm to analyze echo-integrated hydroacoustic data in order to define patches and related patch characteristics (numbers, density, area, and mean depth). Threadfin shad in Lake Norman were patchily distributed and exhibited strong, generally consistent patterns in patch number, fish density, size, and depth. During most surveys we observed 12-16 patches/km with fish densities exceeding twice the average background density, and 1-2 patches/km with fish densities 50 times the average background density. Most patches were small (<50-100 m 2 in the vertical plane): less than 10% of the area contained 50% of the fish. In October night surveys, patches tended to be larger and less numerous and to have lower fish densities than at other times. Depth distribution of patches varied among seasons and was influenced by temperature, by oxygen concentration, and possibly by vertical distribution of predators and prey. Temporal changes in patch distribution suggested that fish composing the patches were highly mobile, spatially integrating threadfin shad impacts on their zooplankton prey. Large differences in day and night average density estimates in October suggested strong diel habitat shifts by threadfin shad during fall.
Dual‐beam and echo integration data were collected with 38‐, 120‐, 200‐, and 420‐kHz hydroacoustic systems from a single boat in August 1991 to estimate pelagic fish distribution, density, and mean lengths at three study sites of Lake Texoma, Texas–Oklahoma. Areal and volumetric densities were estimated for randomly selected transects in each of the study areas. Density varied among frequencies and sites, and between day and night sample periods. Density estimates were highest in Big Mineral and lowest in the Central Pool site, Calculation of sample sizes required to obtain coefficients of variation of the mean equal to 20% indicated that fewer samples would be required for night sampling (range, 2–14) than day sampling (8–54) and that fewer samples would be required in Big Mineral (2–3) than in Central Pool (3–6) or Little Mineral (7–14) at night. Pelagic fish in Big Mineral were closer to the surface than in Little Mineral or Central Pool. Wavelength scattering characteristics of the four frequencies resulted in inflated estimates of fish density for higher frequencies at night when Chaoborus spp. were present in the same depth strata as fish and resulted in the underestimation of densities for age‐0 pelagic fish by the 38‐kHz system. Day densities were underestimated by all frequencies because pelagic fish schooled tightly during the day, resulting in acoustic signal attenuation. Mean lengths of hydroacoustically tracked fish (4.7, 5.0, and 5.2 cm total length for Central Pool, Little Mineral, and Big Mineral, respectively) were not significantly different for 120‐ and 200‐kHz frequencies but were significantly different (P < 0.05) for the three sample sites. We recommend using hydroacoustic frequencies of 120 or 200 kHz, sampling at night when pelagic fish are not schooling, and using transects that encompass much of the variability in density within the study site.
Abstract.-Underwater acoustics is a noninvasive sampling technique that potentially reduces expense and injury to target species, but this method may be underutilized for sampling large freshwater fishes. We measured target strength (TS), developed anatomically based backscatter models, and conducted gill-net and acoustic surveys of paddlefish Polyodon spathula to explore the potential use of acoustic surveys for estimating the abundance of large freshwater fishes. Mean TS measured from two size-groups of paddlefish at 200 kHz was Ϫ37.14 decibels (dB; SD ϭ Ϫ2.36) for age-0 fish (353-406 mm) and Ϫ27.25 dB (SD ϭ Ϫ2.21) for adult fish (1,018-1,284 mm), indicating that TS could differentiate these size-groups. Backscatter models identified strong contributions of the swim bladder to TS and revealed the sensitivity of acoustic backscatter to paddlefish length, aspect, and acoustic carrier frequency. Model results were generally within one SD of measured means from individual fish of each size-group. Target strength results were used to count two populations of adult paddlefish in mobile surveys using an echo sounder with a 200-kHz, 6Њ split-beam transducer. One population was stocked in 1.6-ha Hebron Pond, where no large fish were previously present. The other population resided in 28-ha Horseshoe Lake, an Ohio River backwater. Twenty-one paddlefish stocked in Hebron Pond were accurately counted during the first of six side-looking surveys, but subsequent surveys only counted between two and seven fish. Depletion gillnetting results in Horseshoe Lake provided an estimated baseline of 130 Ϯ 55 paddlefish for comparison with abundance estimates from side-looking and down-looking acoustic surveys during day and night. Acoustic abundance estimates ranged from 187-313 fish (sidelooking) to 3,464-13,489 fish (down-looking) depending on survey time (day or night) and the approach to analysis. Ratio estimates and cluster estimates provided similar results, and the coefficient of variation of the mean (100·SE/mean) ranged from 20% to 50%. Acoustic estimates were either greater or more variable than those derived from depletion gillnetting, yet acoustic surveys required only 6 man-hours compared to 180 man-hours for the gillnetting estimate. Our study is the first to indicate that TS can be used to count adult paddlefish and that, upon refinement of survey techniques, TS can be used to estimate paddlefish abundance. The benefits of acoustic surveys may be realized sampling other large freshwater fishes when the target species can be differentiated with TS and considerations are made for transducer selection.
J. Strom Thurmond Lake (Georgia-South Carolina) strongly stratifies during the summer, producing a temperature-dissolved oxygen ''squeeze'' for landlocked blueback herring Alosa aestivalis. A coupled Eulerian-Lagrangian hybrid (CEL hybrid) model was developed to improve understanding of and better predict the in-reservoir movements of blueback herring in response to these limiting conditions. An existing application of CE-QUAL-W2, a two-dimensional, laterally averaged water quality and hydrodynamic model, was used to simulate the spatial and temporal dynamics of summer water temperature, dissolved oxygen, and hydraulics. The biological module was parameterized by using mobile hydroacoustic survey data supplemented with species composition data from gill-net surveys. The simulation accurately described the longitudinal (r 2 ϭ 0.67) and vertical (r 2 ϭ 0.93) distributions observed for blueback herring. The model results suggest that the longitudinal distribution of blueback herring in summer is best explained by a random term (57%) and horizontal water velocity (43%). The vertical distribution is best explained by the following variables (in order of their importance): water temperature (45%), a random term (44%), dissolved oxygen (6%), horizontal velocity (4%), and vertical velocity (1%). The integration of fish behavior into a flow and water quality modeling framework can be used to gain insight into the response of coolwater species to flow and water quality patterns. This insight can serve as a foundation for population and bioenergetics modeling or be used to improve the management of coolwater fish and their habitats in dynamic reservoir systems. We recommend that additional data be used to further test and refine the model.
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