Evaluating Novel Warmwater Sportfish Monitoring Techniques (Hydroacoustics, Age and Growth Methods) in Southwestern Reservoirs

Since the American Fisheries Society (AFS) published Standard Methods for Sampling North American Freshwater Fishes, many fisheries management organizations have adopted standard fish sampling techniques that can be used to effectively evaluate fish populations over time and across space. I evaluated hydroacoustics and age and growth methods, two techniques that are not routinely used by the Arizona Game and Fish Department (AZGFD) to monitor fisheries in Southwestern reservoirs. Hydroacoustics, the use of sound navigation and ranging (SONAR) to detect fish in water, is growing in popularity but is only discussed as a standard method for coldwater systems. My goal was to determine whether hydroacoustics could add new, important information to warmwater fish monitoring in Southwestern reservoirs, or if current methods (i.e., benthic gill nets and boat electrofishing) sufficiently characterize fish assemblages in these systems. I compared fish relative abundance, species composition, and length distribution information measured with hydroacoustic sampling to AFS standard benthic gillnetting and electrofishing in 4 Arizona reservoirs (Apache, Bartlett, Pleasant, Roosevelt). All surveys occurred between October 2020 and May 2021. There were significant, but weak relationships between hydroacoustic estimates of fish density and benthic gill net and electrofishing Catch Per Unit Effort (CPUE). Weak relationships may have been caused by hydroacoustics sampling a wider range of reservoir depths and zones than other gear types. Hydroacoustics sampled more small-bodied fish than benthic gill nets and electrofishing, especially fishes inhabiting pelagic reservoir zones such as Gizzard Shad Dorosoma cepedianum. I recommend the use of hydroacoustics for standard fish monitoring in Southwestern reservoirs that targets small-bodied pelagic forage fishes such as Gizzard Shad and Threadfin Shad Dorosoma petenense. Age and growth data are frequently used to monitor and manage important North American sport fishes like Largemouth Bass Micropterus salmoides. Continental and regional growth standards have been developed for the species to assess fish growth over time and across space. However, age and growth of Largemouth Bass are typically not collected in Arizona and evaluating growth estimation methods for the species benefits fisheries management. My objectives were to 1) estimate Largemouth Bass growth in several Southwestern warmwater reservoirs using otoliths and 2) compare precision and bias of age estimates from scales and otoliths. I collected Largemouth Bass from 3 reservoirs in Arizona (Alamo, Peña Blanca, and Roosevelt) using boat electrofishing in spring 2021. Age structures were removed from bass, prepared, and aged by three individual readers. Differences in precision and bias between scales and otoliths were compared in terms of reader agreement percentages, confidence ratings, average coefficient of variation (ACV), and age-bias plot analysis. I used age estimates from bass otoliths and age-length keys to fit von Bertalanffy growth functions (VBGFs) to age and length data in each reservoir. Von Bertalanffy growth functions were used to estimate growth parameters, predicted lengths-at-age, and relative growth indices (RGIs) based on growth standards. Largemouth Bass scale age estimates were less precise, higher at younger ages, and lower at older ages compared to otoliths. Growth was lower in Peña Blanca Lake than in the other two reservoirs according to VBGF parameter estimates and estimated mean lengths-at-age. Relative growth indices suggested that Largemouth growth in all three reservoirs was above average at younger ages and average to below average at older ages. The results from this study build on a growing body of literature supporting the standardized use of otoliths for Largemouth Bass age and growth estimation