Seasonal Changes in Condition of Appalachian Brook Trout

Reliable fish condition estimates help managers better understand ecosystem population dynamics. Therefore, our study objectives were to measure seasonal changes in percent dry weight and energy density (physiological-based measures of condition) of stream-dwelling Appalachian Brook Trout Salvelinus fontinalis, field-validate bioelectrical impedance analysis (BIA) models, and compare reliability of BIA and morphometric-based estimates of condition. Percent dry weight was highly correlated to energy density (R = 0.93; J/g wet weight = −1,803.5 + 286.43 ∙ [percent dry weight]), and the relationship was clearly different from those published previously for other salmonids. Significant seasonal changes in adult Brook Trout conditionwere observed and likely related to energy depletion from reproduction and changes in terrestrial invertebrate consumption. Adult percent dry weight peaked in early September and was lowest in February. Age-0 fish did not have large changes in measured condition between summer and winter. Bioelectrical impedance analysis was able to estimate adult mean monthly percent dry weight reliably; however, it did not appear to outperform results from relative weight (Wr). NeitherWr nor predicted percent dry weight from BIA was a reliable estimator of condition for individual adult fish. The BIA model for age-0 fish was unable to provide reliable predictions for either individual fish or monthly mean estimates, due in large part to the small range in measured condition. The BIA estimated monthly mean energy density more reliably than didWr. Overall results of this study indicate that BIA did not perform appreciably better thanWr, which required much less effort to collect. Potential BIA model improvements may be possible by accounting for changes in skin temperature. Until improvements in the Brook Trout BIA models occur, their use should be limited to estimating mean energy density. Researchers and fisheries biologists often seek reliable estimates of fish condition. Detailed information about fish condition can provide insight to fisheries researchers and managers about the overall health of an ecosystem (Karr 1981). Fish condition can be correlated to environmental variables such as flow (Weisberg and Burton 1993), sedimentation (Sullivan and Watzin 2010), pH (Suns and Hitchin 1990), dissolved oxygen level (Benejam et al. 2008), and concentration of heavy metals (Suns and Hitchin 1990; Clements and Rees 1997). Because changes in these important variables often occur due to environmental degradation, reliable estimates of fish condition can be used to monitor and ultimately protect aquatic ecosystems and, in many circumstances, the human populations using them. Fish condition also varies naturally over time with changes in food abundance (Jackson et al. 2002; Yamamura et al. 2002), temperature (McClendon and Rabeni 1987), flow (Oliva-Paterna et al. 2003), and reproductive cycles (Kortet et al. 2003). Cunjak and Power (1986) demonstrated that lipid levels in Brook Trout Salvelinus fontinalis decreased and water content increased over winter in a subarctic river system. Decreases in the condition of stream-dwelling Brook Trout from summer to winter appear to be related to both reproduction and *Corresponding author: [email protected] Received August 16, 2016; accepted October 20, 2016 196 North American Journal of Fisheries Management 37:196–206, 2017 © American Fisheries Society 2017 ISSN: 0275-5947 print / 1548-8675 online DOI: 10.1080/02755947.2016.1254130 insufficient energy intake in relation to metabolic costs (Cunjak et al. 1987). Survival, reproduction, and overall abundance will likely be influenced in stream systems where large seasonal changes in Brook Trout condition occur. Therefore, by closely monitoring condition we can better explain changes in the population dynamics of Brook Trout and their role in the ecological function of aquatic ecosystems. With this detailed knowledge we can make better management decisions that will help improve damaged aquatic ecosystems and prevent degradation to the healthy ones. Fish condition has historically been estimated using morphometric-based indices such as Fulton’s condition factor (see Nash et al. 2006), relative condition (Le Cren 1951), or relative weight (Wege and Anderson 1978). In addition to these morphometric-based approaches, several studies have recently been published where bioelectrical impedance analysis (BIA) was used to estimate fish condition (see Hartman et al. 2015). Although techniques such as BIA have been used frequently over the past decade, it is still unclear how well resulting estimates of condition perform compared with traditional morphometric-based approaches (Hartman et al. 2015). The potential field applications of BIA range from providing estimates of fish condition to estimating seasonal changes in energy density in a cost-effective manner without having to euthanize the fish. However, BIA field studies are needed to validate previously developed BIA models by collecting data that are completely independent from model development and are also from a large temporal and temperature range. Because taking BIA measures in the field is more time consuming than measuring length and weight alone, a direct comparison of BIA and morphometric-based estimates of condition is warranted. Bioelectrical impedance analysis models and temperature corrections established in the laboratory are currently available for Brook Trout (Hafs and Hartman 2015) and provide an excellent starting point for a BIA field validation study. Furthermore, Utz and Hartman (2009) sampled Brook Trout at nine different sites that spanned a wide range of stream sizes and Brook Trout densities. They were able to provide evidence of density dependence for both age-0 and adult Brook Trout in Appalachian streams. By sampling the same sites previously looked at by Utz and Hartman (2009) over a large temporal scale, we should be able to sample Brook Trout over a large range of body conditions and water temperatures, thereby creating a favorable setting for a BIA field validation study. Based on results of previous research, the objectives of this study were to (1) accurately measure seasonal changes in percent dry weight and energy density (physiological-based measures of fish condition) of stream-dwelling Appalachian Brook Trout, (2) field-validate previous BIA Brook Trout models, and (3) provide a direct comparison of BIA and morphometric-based estimates of Brook Trout condition. METHODS Fish sampling.—Brook Trout were collected at nine different sample sites on six different Appalachian headwater streams (Strahler order 1 or 2), all within the Middle Fork River watershed, West Virginia. Kittle Creek contained three sites; Rocky Run had two sites; and Light Run, Brush Run, Sugar Drain, and Mitchell Lick Fork each contained one site. When a stream contained multiple sites, the sites were separated by ≥2 km. Basin area at sample sites ranged from 0.83 to 15.38 km, while mean riffle depth and pool depth of sample sites ranged from 7.46 to 15.21 cm and from 15.53 to 32.52 cm, respectively (Utz and Hartman 2009). Previous research (Utz and Hartman 2009) indicated that these nine sites had a wide range of Brook Trout densities (0.028–0.237 fish/m); therefore, fish from these sites were also considered to have been in a wide range of condition and thus suitable for BIA model validation. Each 200-m-long site was sampled on the first weekend of every month from May 2010 to April 2011, except in February. Snow-covered roads prevented us sampling in Mitchell Lick Fork, upper Kittle Creek, Brush Run, and middle Kittle Creek during that month. Two passes with backpack electrofishing equipment were done at each site to capture trout. Upon capture, trout were anesthetized with tricaine methanesulfonate (MS-222) and BIA analysis was done. Data collection for BIA.—Resistance and reactance were measured on all adult Brook Trout (>100 mm TL; Hakala 2000; Sweka 2003), with both external rods and subdermal needle electrodes using a Quantum II bioelectrical body composition analyzer (RJL Systems, Clinton Township, Michigan). Only external rod electrodes were used for age-0 trout because age-0 Brook Trout are more sensitive and we wanted to limit sampling time. Electrode specifications were the same as those in Hafs and Hartman (2011) for adult fish and Hafs and Hartman (2014) for age-0 Brook Trout. Measurements for BIA were taken at two locations for both adult and age-0 trout based on the recommendations of Hafs and Hartman (2011, 2014). For adult Brook Trout, the first measurement was taken by placing the electrodes along the dorsal midline of the fish, with one electrode located immediately posterior to the head while the other electrode was positioned immediately anterior to the adipose fin (DML). For age-0 trout, the first measurement was taken by placing one electrode parallel to the lateral line halfway between the lateral line and the dorsal midline directly above where the lateral line intersects the opercular flap. The other electrode was placed halfway between the lateral line and the dorsal midline directly below the adipose fin (DTL). The second set of measurements for both adult and age-0 trout were taken by placing one electrode along the dorsal midline just anterior to the dorsal fin while the other electrode was placed along the ventral midline below the other electrode (DTV). Electrode placements are shown in Figure 1. Following the directions of Hafs and Hartman (2011), the distance between each of the SEASONAL CHANGES IN BROOK TROUT CONDITION 197