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Summary Traditional techniques for stable isotope analysis (SIA) generally require sacrificing animals to collect tissue samples; this can be problematic when studying diets of endangered species such as the pallid sturgeon Scaphirhynchus albus. Our objectives were to (i) determine if pectoral fin tissue (non-lethal) could be a substitute for muscle tissue (lethal) in SIA of juvenile pallid sturgeon, and (ii) evaluate the influence of preservation techniques on stable isotope values. In the laboratory, individual juvenile pallid sturgeon were held for up to 186 day and fed chironomids, fish, or a commercially available pellet diet. Significant, positive relationships (r² ≥ 0.8) were observed between fin and muscle tissues for both δ15N and δ13C; in all samples isotopes were enriched in fins compared to muscle tissue. Chironomid and fish based diets of juvenile pallid sturgeon were distinguishable for fast growing fish (0.3 mm day−1) using stable δ15N and δ13C isotopes. Frozen and preserved fin tissue δ15N isotopes were strongly related (r2 = 0.89) but δ13C isotopes were weakly related (r2 = 0.16). Therefore, freezing is recommended for preservation of fin clips to avoid the confounding effect of enrichment by ethanol. This study demonstrates the utility of a non-lethal technique to assess time integrated food habits of juvenile pallid sturgeon and should be applicable to other threatened or endangered species.

Underwater strobe lights can influence the behavior and distribution of fishes and are increasingly used as a technique to divert fish away from water intake structures on dams. However, few studies examine how strobe lights may affect organisms other than targeted species. To gain insight on strobe lighting effects on nontarget invertebrates, we investigated whether underwater strobe lights influence zooplankton distributions and abundance in Lake Oahe, South Dakota. Zooplankton were collected using vertical tows at 3 discrete distances from an underwater strobe light to quantify the influence of light intensity on zooplankton density. Samples were collected from 3 different depth ranges (0–10 m, 10–20 m and 20–30 m) at

Carey MP, Maloney KO, Chipps SR, Wahl DH. Effects of littoral habitat complexity and sunfish composition on fish production. Ecology of Freshwater Fish 2010: 19: 466–476. © 2010 John Wiley & Sons A/S Abstract – Habitat complexity is a key driver of food web dynamics because physical structure dictates resource availability to a community. Changes in fish diversity can also alter trophic interactions and energy pathways in food webs. Few studies have examined the direct, indirect, and interactive effects of biodiversity and habitat complexity on fish production. We explored the effects of habitat complexity (simulated vegetation), sunfish diversity (intra- vs. inter-specific sunfish), and their interaction using a mesocosm experiment. Total fish production was examined across two levels of habitat complexity (low: 161 strands m−2 and high: 714 strands m−2) and two sunfish diversity treatments: bluegill only (Lepomis macrochirus) and bluegill, redear sunfish (Lepomis microlophus), and green sunfish (Lepomis cyanellus) combination. We also measured changes in total phosphorus, phytoplankton, periphyton, and invertebrates to explain patterns in fish production. Bluegill and total fish production were unaffected by the sunfish treatments. Habitat complexity had a large influence on food web structure by shifting primary productivity from pelagic to a more littoral pathway in the high habitat treatments. Periphyton was higher with dense vegetation, leading to reductions in total phosphorus, phytoplankton, cladoceran abundance and fish biomass. In tanks with low vegetation, bluegill exhibited increased growth. Habitat complexity can alter energy flow through food webs ultimately influencing higher trophic levels. The lack of an effect of sunfish diversity on fish production does not imply that conserving biodiversity is unimportant; rather, we suggest that understanding the context in which biodiversity is important to food web dynamics is critical to conservation planning.

Didymosphenia geminata, a relatively new aquatic nuisance species that can form extensive, mucilaginous mats on stream substrates, was reported from Rapid Creek, South Dakota in 2002. To examine the association between D. geminata and the invertebrate community in Rapid Creek, macroinvertebrates were quantified using three gear types in the fall of 2006. D. geminata was present at two of four sites sampled (range = 5.53 to 809.68 g m−2 dry mass). At each site, invertebrates were collected using dip nets, Surber samplers, and drift nets. The combined percentage of Ephemeroptera, Plecoptera, and Trichoptera in areas with D. geminata was lower (41%) than in areas without D. geminata (76%). Diptera abundance was higher at sites with D. geminata than in sites where D. geminata was absent.

We evaluated the influence of drought conditions on the biomass of brown trout Salmo trutta in Spearfish Creek, upper Rapid Creek, and lower Rapid Creek in the Black Hills of western South Dakota. Stream discharge, mean summer water temperature, the biomass of juvenile and adult brown trout, and brown trout size structure were compared between two time periods: early (2000–2002) and late drought (2005–2007). Mean summer water temperatures were similar between the early- and late-drought periods in Spearfish Creek (12.4°C versus 11.5°C), lower Rapid Creek (19.2°C versus 19.3°C), and upper Rapid Creek (9.8°C in both periods). In contrast, mean annual discharge differed significantly between the two time periods in Spearfish Creek (1.95 versus 1.50 m3/s), lower Rapid Creek (2.01 versus 0.94 m3/s), and upper Rapid Creek (1.41 versus 0.84 m3/s). The mean biomass of adult brown trout in all three stream sections was significantly higher in the early-drought than in the late-drought period (238 versus 6...

In the absence of visual cues, information gained from chemically-mediated inspection behavior may serve to reduce predation risk by informing minnow shoals of the proximity of potential predators. Behavioral inspection of predator chemical cues implies that sensory modes other than visual detection may be important for assessing predation risk in shoaling fishes. To assess the potential for inspection behavior by fathead minnow (Pimephales promelas; FHM), we quantified the responses of FHM to chemical cues of a natural predator, northern pike, and chemical cues of conspecifics. The presence of northern pike odor elicited inspection-like behavior among FHM. In control treatments, FHM remained within vegetated cover, and no differences were observed in the number of individuals in open water areas. When northern pike odor was added, significantly more FHM were observed in the open water habitat, although FHM activity was similar to that of controls. This suggests that FHM respond to northern pike ...

Walleyes Sander vitreus are the most popular fish among South Dakota anglers, but smallmouth bass Micropterus dolomieu were introduced to provide new angling opportunities. Some walleye anglers have reported reductions in the quality of walleye fisheries since the introduction of smallmouth bass and attribute this to the consumption of young walleyes by smallmouth bass and competition for shared prey resources. We quantified the diets of walleyes and smallmouth bass in the lower reaches of Lake Sharpe (a Missouri River reservoir), calculated the diet overlap between the two predators, and determined whether they partitioned shared prey based on size. We also quantified walleye diets in the upper reach of the reservoir, which has a different prey base and allowed us to compare the growth rates of walleyes within Lake Sharpe. Age-0 gizzard shad Dorosoma cepedianum composed a substantial proportion of the diets of both predators, regardless of location, for most of the growing season; the patterns in shad vulnerability appeared to drive the observed patterns in diet overlap. Smallmouth bass appeared to consume a smaller size range of gizzard shad than did walleyes, which consumed a wide range. Smallmouth bass consumed Sander spp. in some months, but in very low quantities. Given that global climate change is expected to alter the population and community dynamics in Great Plains reservoirs, we also used a bioenergetics approach to predict the potential effects of limiting prey availability (specifically, the absence of gizzard shad and rainbow smelt Osmerus mordax) and increased water temperatures (as projected from global climate change models) on walleye and smallmouth bass growth. The models indicated that the absence of rainbow smelt from the diets of walleyes in upper Lake Sharpe would reduce growth but that the absence of gizzard shad would have a more marked negative effect on both predators at both locations. The models also indicated that higher water temperatures would have an even greater negative influence on walleye growth; however, smallmouth bass growth was predicted to increase with higher temperatures. Fisheries managers should consider strategies to enhance the prey base or mitigate the effects of increased water temperatures that may occur in the future as a result of global climate change. Such proactive actions may alleviate potential future competition between walleyes and smallmouth bass resulting from changes in the fish community.

Summary Stocking is a commonly employed conservation strategy for endangered species such as the pallid sturgeon, Scaphirhynchus albus. However, decisions about when, where and at what size pallid sturgeon should be stocked are hindered because vulnerability of pallid sturgeon to fish predation is not known. The objective of this study was to evaluate the vulnerability of age-0 pallid sturgeon to predation by two Missouri River predators under different flow regimes, and in combination with alternative prey. To document vulnerability, age-0 pallid sturgeon (

Summary The pallid sturgeon (Scaphirhynchus albus) is an endangered species native to the Missouri and Mississippi rivers. To date, recovery efforts have focused on stocking juvenile fish, but little is known about ontogenetic changes in diet composition. Although diet composition for pallid sturgeon is believed to change from macroinvertebrates to fish, it is unclear at what size and/or age these ontogenetic diet shifts occur. To evaluate diet composition, 29 hatchery-stocked pallid sturgeon (range 356–720 mm fork length [FL]; mean = 549; SE = 23) were collected from the Missouri River downstream of Fort Randall Dam, South Dakota and Nebraska during summer 2006. The majority of pallid sturgeon (72%) were captured within a large delta region formed by the Niobrara River in the headwaters of Lewis and Clark Lake. Predominant prey of pallid sturgeon based on percent occurrence was Ceratopogonidae (81%), Isonychiidae (67%), Chironomidae (52%), and fishes (24%). Percent composition by wet weight showed that diets were composed of fishes (68%), Ephemeroptera (23%), Decapoda (6%), and Diptera (3%). Graphical analysis of combined data showed that mayflies, particularly Isonychiidae, were an important component of pallid sturgeon diets. Nonetheless, the percent composition of fishes in the diet increased with pallid sturgeon body size; for fish > 600 mm FL (5–7 years of age) diets were composed primarily of fish prey (66%, mostly johnny darters Etheostoma nigrum). These findings highlight the importance of ontogenetic changes in diet composition for pallid sturgeon. Moreover, the unique habitat formed in the delta region is characterized by higher fish and invertebrate densities that may enhance foraging opportunities and thus improve recovery efforts for stocked pallid sturgeon.

Summary Movement and distribution of the endangered pallid sturgeon Scaphirhynchus albus has generally been documented using radio telemetry. However, because of the time and cost involved in tracking individual fish (i.e. small sample size), it is often difficult to evaluate spatial distribution of groups of fish over long time periods (> 3 years). Standardized sampling for pallid sturgeon, which relies on a variety of gear types, has been conducted on the Missouri River downstream of Fort Randall Dam annually since 2003. Using catch data from 2003 to 2006, the spatial distribution of juvenile pallid sturgeon was evaluated using spatial scan statistics. Presence/absence of pallid sturgeon was summarized from a variety of gear and distribution patterns were analyzed based on: (i) each gear per season, (ii) all gear pooled per season, (iii) each gear pooled across seasons, and (iv) pooled data from all gear and years combined. Three significant clusters of pallid sturgeon captures were identified when all gear and years were pooled. Distribution patterns identified using data from summer trammel nets agreed well with the overall pooled dataset and could be used to identify areas with a high probability of pallid sturgeon presence. This methodology can be used to identify areas where pallid sturgeon are likely to occur, thus improving sampling efficiency for monitoring vital statistics for this endangered species. Moreover, this approach could be applied to other reaches of the Missouri River using existing data from the Pallid Sturgeon Monitoring and Assessment Program. Once identified, these areas could then be evaluated to better understand the habitat requirements of pallid sturgeon.

We evaluated changes in wetland abundance, size, and classification between average (1979–1986) and above-average (1995–1999) precipitation periods for two physiographic regions in eastern South Dakota. Temporal shifts in wetland numbers, area, and class varied by topographic location. In high wetland density areas (> 8 wetlands/100 ha), our data suggests that larger, semipermanent wetlands expanded and absorbed nearby wetland basins into their margins, resulting in a net “loss” or disappearance of temporary and seasonal wetlands in above-average water condition years. “Losses” described here are not deemed permanent as in cases of draining or filling, and wetlands may re-form when water conditions return to normal. Nevertheless, temporary disappearance of smaller more isolated wetlands may have implications for breeding waterfowl and other fauna. Percent change of semipermanent basin numbers was positively correlated with wetland density, whereas the opposite was true for seasonal wetlands. Loss...

Feeding rate and water temperature are known to influence the accuracy of fish bioenergetics models. In an effort to improve the accuracy of a juvenile esocid bioenergetics model, we used a regression-based approach to develop a corrective equation that accounted for the prediction error associated with variable feeding rates and water temperatures. The regression model explained 58% of the variability in the consumption rate prediction error for age-0 tiger muskellunge (northern pike Esox lucius × muskellunge E. masquinongy) and included the following variables: Initial body weight (g), water temperature (°C), and relative growth rate (cal·g−1·d−1). The corrected model accurately predicted observed food consumption and had lower prediction error (mean error = 7%) than the uncorrected model (mean error = 43%). Moreover, the differences between the observed and modeled estimates of cumulative food consumption were significantly greater than zero for the uncorrected model while those between the ob...

We conducted in situ testing of strobe lights as a potential fish deterrent by examining avoidance and acclimation behaviors of rainbow smelt Osmerus mordax in Lake Oahe, South Dakota. Split-beam hydroacoustics were used to assess the effectiveness of the deterrent system by comparing proximal densities of rainbow smelt before and after the device was activated. In summer 2005, strobe lights successfully repelled rainbow smelt to a minimum horizontal distance of 21 m at both 1 h and 4 h postactivation; the model AGL-FH 920 flashhead produced a light intensity of 6,585 lumens per flash. Similarly in 2004, a model AGL-FH 901 flashhead, which produced a light intensity of 2,634 lumens per flash, repelled rainbow smelt to a horizontal distance of 15 m. A comparison of strata 10 m above and below the strobe light system showed that rainbow smelt were vertically displaced to approximately 6 m. We conclude that strobe lights elicit behavioral avoidance by rainbow smelt and may provide an effective means...

The development and application of fish bioenergetics models have flourished in recent years, due in part to the complexity of the issues being faced by fisheries biologists. As with any model, the accuracy of bioenergetics models can be hampered by uncertainty in model parameters. A review of the literature showed that field and laboratory tests of bioenergetics models often result in poor agreement between model predictions and independent data. Nonetheless, bioenergetics modeling continues to be used to make important management decisions. Recent tests of model predictions have shown that parameter uncertainty is influenced by factors such as feeding rate, physiological adaptations, and prey composition and abundance. In an attempt to reduce the uncertainty in modeling applications, we propose a framework that highlights the importance of (1) model evaluation, (2) hypothesis-based parameter testing, and (3) improved communication between model developers and model users. Adherence to this fram...

We used muscle-derived lactate dehydrogenase (LDH) to measure effects of prey size and vegetation density on anaerobic foraging activity by largemouth bass (Micropterus salmoides). Largemouth bass (240–303 mm total length, TL) were fed bluegill (Lepomis macrochirus) prey (range 33–83 mm TL) in laboratory feeding trials. Prey selectivity experiments showed that small bluegills (88%) over larger (>65 mm) individuals. Largemouth bass activity, as indexed by LDH, increased with increasing prey size and was 20% higher in fish feeding on large (mean size = 80 mm) versus small (mean size = 35 mm) bluegill. Bioenergetics modeling revealed that food consumption was appreciably underestimated (29%–34%) for largemouth bass foraging on large bluegills (65 and 80 mm), implying that activity costs vary with prey size, consistent with LDH measurements. In contrast to prey size, vegetation density had little effect on anaerobic energy expenditure of largemouth bass. For two size groups of largemouth bass (mean = 244 and 316 mm) foraging on 50 mm bluegill, mean LDH activity was similar across simulated vegetation densities ranging from 70 to 350 stems·m–2. These findings highlight the importance of prey size on foraging costs by piscivores and the difficulties of accounting for activity level in bioenergetics models.

Recent decline in the lesser scaup Aythya affinis population has been linked to changes in wetland conditions along their spring migration routes. In particular, the use of amphipod prey by lesser scaup has declined in many regions of the upper Midwest U.S.A. and has been linked to expanded fisheries, although empirical data on diet overlap are lacking. To explore patterns of prey use by lesser scaup and diet overlap with fishes, we quantified diets of scaup and fishes during the 2003 and 2004 spring migration in eastern South Dakota, U.S.A. We compared diet overlap between lesser scaup and fishes collected from Twin Lakes, South Dakota—an important stopover location for spring-migrating scaup. Plant seeds occurred in >95% of lesser scaup diets (n = 118) and represented an appreciable amount of consumed biomass (>70%). Gastropods, amphipods, and chironomids were the most abundant invertebrate prey taxa and occurred in 29–34% of lesser scaup diets. Although relatively frequent, these taxa each contributed only 4–27% of the diet by weight. Percent dry mass of amphipods, a preferred prey by lesser scaup, was low (4%) indicating that amphipod availability may be reduced during spring migration. Analysis of fish diets showed that black bullhead Ameiurus melas and yellow perch Perca flavescens had the highest diet overlap with lesser scaup at 94% and 92%, respectively. Moreover, mean size of amphipods and chironomids found in fish diets were significantly larger than that consumed by lesser scaup. Our findings support the notion that amphipod use by spring-migrating lesser scaup has declined and that size-selective predation by fishes may influence prey availability for lesser scaup.

We examined the effects of strobe lights on plasma cortisol concentrations and avoidance behavior across a broad range of fish taxa. Representative fish taxa from five families were evaluated and included: Centrarchidae (largemouth bass Micropterus salmoides), Cyprinidae (fathead minnow Pimephales promelas), Ictaluridae (channel catfish Ictalurus punctatus), Percidae (yellow perch Perca flavescens), and Salmonidae (Chinook salmon Oncorhynchus tshawytscha). Mean (±SE) plasma cortisol concentrations for channel catfish (89 ± 22 ng/mL), yellow perch (81 ± 39 ng/mL), and Chinook salmon (173 ± 27 ng/mL) increased significantly compared with those in control groups (50 ± 11, 3.0 ± 1.9, and 102 ± 35 ng/mL, respectively) after 1 h of exposure to strobe lights. After 7 h of exposure, plasma cortisol levels were similar to those in control groups for all fish taxa. Fathead minnow showed no detectable response to strobe lights at either 1 or 7 h of exposure. Behavior experiments showed that the mean distanc...

Mean stomach fullness provides a useful index for quantifying fish diets. However, estimating stomach fullness for planktivorous fishes can be time-consuming and prone to error because of small prey and unidentifiable remains. In this study we developed a predictive equation for estimating the stomach volume of yellow perch Perca flavescens as a function of total length (TL). We then used an optical plankton counter (OPC) to estimate the biovolume of invertebrate prey consumed. The OPC quickly estimated the digital size and abundance of zooplankton prey, which can then be converted to estimates of prey biovolume. Stomach volume (V (mm3)) for yellow perch (113–279 mm TL) was significantly related to body size (L (mm)) and was estimated as V = 3 × 10−7 · L2.96. Using the OPC, yellow perch stomach contents (99% Daphnia pulex) were converted to prey biovolume (mm3) and then divided by stomach volume (mm3) to estimate stomach fullness (%). This approach provided reasonable estimates of stomach fullnes...

Atmospheric deposition of mercury (Hg) has been shown to be a significant source of Hg on the landscape (Rada et al. 1989; Swain et al. 1992) and is believed to contribute to increased Hg concentrations in aquatic food webs (Sorensen et al. 1994; Edwards et al. 1999). Methylmercury (MeHg) is the organic, bioavailable form of Hg that accumulates to toxic levels in top-level predators in aquatic systems (Suedel et al. 1994). Although limnological conditions in lakes and rivers can affect Hg methylation and concentrations in fishes, these relationships often vary among water bodies and fish species (McMurtry et al. 1989; Bodaly et al. 1993). Thus, regional studies are needed to identify mechanisms of local MeHg production and factors associated with Hg contamination in fishes. The Prairie Pothole Region of North America has a unique variety of natural wetlands and glacial lakes that are important for fish, shorebirds, waterfowl, and humans. Cyclical climate, characterized by extended wet–dry periods, cause high variation in water surface area within the region (Rosenberry 2003). For example, consecutive years of high precipitation during the mid-1990s caused dramatic surface area increases in many glacial lakes and wetlands of eastern South Dakota. After lake levels increased, several fish populations were found to contain elevated Hg concentrations (>1 lg/g), prompting local officials to post fish consumption advisories (South Dakota Game, Fish and Parks, 2006). The discovery of elevated Hg concentrations in fishes was surprising because (1) there were no apparent pointsource inputs of Hg, and (2) the limnological conditions of most of these lakes (eutrophic, high pH) generally do not favor Hg methylation or bioaccumulation (Grieb et al. 1990; Pickhardt et al. 2002). Moreover, lakes that experienced large increases in surface area generally contained fast growing fish populations, a situation that usually lowers Hg concentration due to growth dilution (Rodgers and Qadri 1982; MacCrimmon et al. 1983). In this study, we document changes in lake surface area for glacial lakes in the Prairie Pothole Region and relate this to Hg concentration in adult walleye (Sander vitreus). Although a similar phenomenon is known to occur in reservoirs with fluctuating water levels (Jackson 1988; Snodgrass et al. 2000; Sorensen et al. 2005), widespread effects of surface area changes on Hg concentrations in fishes have not been documented in natural, glacial lakes. We hypothesized that increases in lake surface area enhanced T. M. Selch (&) S. R. Chipps U.S. Geological Survey South Dakota Cooperative Fish and Wildlife Research Unit, Department of Wildlife and Fisheries Sciences, South Dakota State University, Box 2140B, Brookings, SD 57007-1696, USA e-mail: Trevor.Selch@sdstate.edu

In western painted turtles (Chrysemys picta bellii), males often exhibit one of two morphs: (1) a reticulated form, characterized by an intricate network of dark markings on the carapace or (2) a non-reticulated form. Although several hypotheses have been proposed to explain the adaptive significance of reticulate melanism (RM) on western painted turtles, no attempts have been made to document whether RM is linked to habitat conditions or if the presence of melanism affects heating rates. To evaluate these questions, we compared the frequency of adult male turtles with RM across three different habitats: riverine (rivers), lacustrine (lakes) and palustrine (wetland) habitats. Using manipulative experiments, we also tested the hypothesis that body heating rates are higher in turtles with RM. Reticulate melanism occurred on 99 (31%) of 320 male turtles captured in South Dakota from 2002 to 2003. Turtles with reticulate melanism were significantly larger than non-reticulated turtles; RM was not obse...

We quantified the influence of agricultural activities on environmental and biological conditions of floodplain wetlands in the upper Missouri River basin. Seasonally-flooded wetlands were characterized as low impact (non-disturbed) or high impact (disturbed) based on local land use. Biological data collected from these wetlands were used to develop a wetland condition index (WCI). Fourteen additional wetlands were sampled to evaluate the general condition of seasonally-flooded floodplain wetlands. Structural and functional attributes of macrophyte, algae, and macroinvertebrate communities were tested as candidate metrics for assessing biotic responses. The WCI we developed used six biological metrics to discriminate between disturbed and non-disturbed wetlands: 1) biomass of Culicidae larvae, 2) abundance of Chironomidae larvae, 3) macroinvertebrate diversity, 4) total number of plant species, 5) the proportion of exotic plant species, and 6) total number of sensitive diatom species. Disturbed wetlands had less taxa richness and species diversity and more exotic and nuisance (e.g., mosquitoes) species. Environmental differences between low and high impact wetlands included measures of total potassium, total phosphorus, total nitrogen, alkalinity, conductance, and sediment phosphorus concentration. Canonical analyses showed that WCI scores were weakly correlated (P = 0.057) with environmental variables in randomly selected wetlands. In addition, mean WCI score for random wetlands was higher than that for high impact wetlands, implying that floodplain wetlands were less impacted by the types of agricultural activities affecting high impact sites. Inter-year sampling of some wetlands revealed that WCI metrics were correlated in 2000 and 2001, implying that biological metrics provided useful indicators of disturbance in floodplain wetlands.

We explored relationships among black bullhead (Ameiurus melas) population characteristics and physicochemical attributes in shallow lakes and quantified relationships between population characteristics of black bullhead and sport fishes. Lake characteristics and fisheries survey data were collected from the Sandhills region of northcentral Nebraska from May through June, 1998 and 1999. Relative abundance of black bullheads was inversely related to proportional stock density (r=-0.672, df=15, P=0.004); however, neither relative weight nor growth was significantly (P ≥ 0.20) related to black bullhead relative abundance. Population characteristics of common panfish species such as bluegill (Lepomis macrochirus), green sunfish (L. cyanellus), pumpkinseed (L. gibbosus), and yellow perch (Perca flavescens) were not correlated with black bullhead relative abundance or size structure. Rather, proportional stock density (r=0.655, df=10, P=0.029) and growth (r=0.59, df=11, P=0.04) of black bullhead were p...

We developed a bioenergetics model for the western mosquitofish Gambusia affinis by combining data on temperature-dependent feeding rates with published information on respiration, temperature tolerance, and reproduction. To corroborate the model, we compared bioenergetics estimates with independent observations from laboratory feeding and growth trials. The model was then applied to a western mosquitofish population in Illinois to estimate feeding rates on natural prey taxa, including mosquito (Culicidae) larvae. Laboratory feeding experiments revealed that western mosquitofish have a high feeding capacity. Maximum consumption rates were 0.25–1.06 g·g−1·d−1 at water temperatures between 5°C and 40°C; the highest feeding rates were observed at 32°C. Bioenergetics estimates of food consumption agreed well with laboratory measurements across a wide range of water temperatures (10–35°C). However, for fish experiencing negligible growth, model estimates of food consumption were higher than actual val...

We compared the seasonal biomass (μg dry weight/L) and body size of Daphnia pulex populations between South Dakota lakes that experienced nearly complete winterkill (n = 2) and those that did not experience fish winterkill the previous winter (n = 2). In spring (March–May), D. pulex biomass was substantially lower in winterkill lakes (0.4–1.9 μg/L) than in nonwinterkill lakes (13.8–129.4 μg/L). In summer months (June–July), D. pulex biomass increased in all lakes but was generally higher in winterkill lakes (332.1–469.3 μg/L) than in nonwinterkill lakes (88.6–204.2 μg/L). By September, D. pulex biomass was substantially higher in winterkill lakes (243.8 μg/L) than in nonwinterkill lakes (1.0 μg/L). The mean size of D. pulex increased significantly from March to August in winterkill lakes but not in nonwinterkill lakes. The seasonal differences in D. pulex biomass and size may have been related to size-selective predation by planktivorous fishes because the ratio of spine length to body length in ...

Direct approachesfor estimating the feeding rate of the opossum shrimp Mysis relicta can be hampered by variable gut residence time (evacuation rate models) and non-linear functional responses (clearance rate models). Bioenergetics modeling provides an alternative method, but the reliability of this approach needs to be evaluated using independent measures of growth and food consumption. In this study, we measured growth and food consumption for M. relicta and compared experimental results with those predicted from a Mysis bioenergetics model. For Mysis reared at 10°C, model predictions were not significantly different from observed values. Moreover, decomposition of mean square error indicated that 70% of the variation between model predictions and observed values was attributable to random error. On average, model predictions were within 12% of observed values. A sensitivity analysis revealed that Mysis respiration and prey energy density were the most sensitive parameters affecting model output. By accounting for uncertainty (95% CLs) in Mysis respiration, we observed a significant improvement in the accuracy of model output (within 5% of observed values), illustrating the importance of sensitive input parameters for model performance. These findings help corroborate the Mysis bioenergetics model and demonstrate the usefulness of this approach for estimating Mysis feeding rate.

We evaluated the performance of injectable fluorescent tags for use in mass marking of centrarchid fishes. Using age-0 largemouth bass Micropterus salmoides and bluegills Lepomis macrochirus (50–100 mm total length), we compared (1) the retention rate of injectable photonic dyes (IPDs) applied to the fin rays of largemouth bass, (2) retention rates of IPDs versus visible implant elastomers (VIEs), and (3) the effect of colorless and brightly colored marks on prey selectivity by adult largemouth bass. Retention rate of IPDs declined appreciably in age-0 largemouth bass (50 mm) after 120 d with only 19% of fish retaining visible marks by that time. By the end of the experiment (day 310), no marks were visible in anal fin rays of largemouth bass. For larger age-0 largemouth bass (100 mm), retention rates of IPDs after 48 d were greatest for fish marked in the anal fin (40%), followed by caudal (20%) and dorsal fin (5%) markings. Visible implant elastomers were retained at a higher rate (84.4%) than ...

We investigated zooplanktivory and nutrient regeneration by the opossum shrimp Mysis relicta and kokanee Oncorhynchus nerka to assess the relative roles of these planktivores in oligotrophic food webs. Using bioenergetic models and clearance rate estimates, we quantified phosphorus (P) excretion rates and consumption of cladoceran prey by Mysis and kokanees in Lake Pend Oreille, Idaho, from 1995 to 1996. Consumption of cladoceran prey by Mysis was 186 kg·ha−1·year−1, whereas consumption by kokanees was less than one quarter as much, at 45 kg·ha−1·year−1. Similarly, Mysis excreted approximately 0.250 kg P·ha−1·year−1 during nighttime migrations into the upper water column, whereas P excretion by kokanees was less than one third as much, at approximately 0.070 kg P·ha−1·year−1. On a volumetric basis, nocturnal excretion by Mysis ranged from 0.002 to 0.007 μg P·L−1·d−1 and accounted for less than 1% of the soluble reactive P typically measured in the upper water column of the lake. Hence, nutrient r...

Metabolic rate of age 0 muskellunge Eso.'( mosquinongy ranged from 0·10 at S' C to 0·24 mg O! g - 1 h - 1 at :so C and was significantly higher in spring and autumn than during winter months at comparable water temperatures. Reduced metabolic rate in winter was consistent with the metabolic compensation hypothesis. implying that metabolism of muskellunge varies indepen­ dently of acclimation temperature and gonadogenesis. Moreover. seasonal variation in metabolic rate has imponant implications for energy budget studies. Single-season estimates of esocid metabolism may be inadequate to describe annual energy requirements: the magnitude of errors will depend on the time of year metabolic rate was measured. As a result. it is suggested that seasonal variation in metabolic rate be incorporated into energy budget detenninations for fishes. C WOO The F"lSheries SocIeI)' of the Bntish Isles

We measured growth of age-0 tiger muskellunge as a function of ration size (25, 50, 75, and 100% Cmax) and water temperature (7.5–25°C) and compared experimental results with those predicted from a bioenergetic model. Discrepancies between actual and predicted values varied appreciably with water temperature and growth rate. On average, model output overestimated winter consumption rates at 10 and 7.5°C by 113 to 328%, respectively, whereas model predictions in summer and autumn (20–25°C) were in better agreement with actual values (4 to 58%). We postulate that variation in model performance was related to seasonal changes in esocid metabolic rate, which were not accounted for in the bioenergetic model. Moreover, accuracy of model output varied with feeding and growth rate of tiger muskellunge. The model performed poorly for fish fed low rations compared with estimates based on fish fed ad libitum rations and was attributed, in part, to the influence of growth rate on the accuracy of bioenergetic...

Maximum daily consumption was estimated for Mysis relicta fed ad libitum rations of Daphnia pulex at 4,10,15 and 18°C. Gut-residence time was also evaluated for M.relicta fed clado- ceran prey at 4, 10 and 15°C. Mean daily consumption (g dry weight of Daphnia g~' dry weight of Mysis day1) ranged from 6% at 4°C to 12% at 10°C. At 18°C, Mysis feeding rate declined to 9% day1. Mean, weight-adjusted consumption rates exhibited a 'dome-shaped' response in relation to water temperature. Consumption rate was highest at 10°C and lowest at 4°C. Estimated Qt0 was more sensitive from 4 to 10°C (Q,o= 3) than from 10 to 15°C (£?10 = 1.2). Gut-residence time for Mysis was inversely related to water temperature, implying that evacuation rate increases linearly with water temperature. Feeding and gut-evacuation rates become disassociated at water temperatures >10°C. As water temperature increased above 10°C, relative evacuation rate increased, whereas feeding rate declined. It is postulated that at higher water temperatures, disassociated feeding and gut-evacuation rates reduce the scope for growth of vertically migrating Mysis and impose a physiological constraint that isolates Mysis from warm, epilimnetic water during thermal stratification.

Elevated mercury concentration has been documented in a variety of fish and is a growing concern for human consumption. Here, we explore the influence of physiochemical and watershed attributes on mercury concentration in walleye (Sander vitreus, M.) from natural, glacial lakes in South Dakota. Regression analysis showed that water quality attributes were poor predictors of walleye mercury concentration (R² = 0.57, p = 0.13). In contrast, models based on watershed features (e.g., lake level changes, watershed slope, agricultural land, wetlands) and local habitat features (i.e., substrate composition, maximum lake depth) explained 81% (p = 0.001) and 80% (p = 0.002) of the variation in walleye mercury concentration. Using an information theoretic approach we evaluated hypotheses related to water quality, physical habitat and watershed features. The best model explaining variation in walleye mercury concentration included local habitat features (W(i) = 0.991). These results show that physical habitat and watershed features were better predictors of walleye mercury concentration than water chemistry in glacial lakes of the Northern Great Plains.

High concentrations of some hepatic elements might be contributing to the decline of the continental lesser scaup (Aythya affinis) population. We evaluated hepatic element concentrations of male and female lesser scaup collected from the upper Midwest (Iowa, Minnesota, North Dakota, and South Dakota) during the 2003 and 2004 spring migrations. We measured concentrations of 24 elements in livers of 117 lesser scaup. We found that only selenium concentrations were at levels (>3.0 μg/g wet weight [ww)]) proposed to adversely affect reproduction. Approximately 49% of females (n = 61) had individual hepatic concentrations >3.0 μg/g ww selenium (Se). Our observed hepatic concentration of Se was similar to that reported in lesser scaup collected from the mid-continental United States but less than Se concentrations reported from the Great Lakes region. We found that the liver cadmium (Cd) concentration for males was significantly higher than that for females. Gender differences in hepatic Cd concentrations have not been previously reported for lesser scaup, but Cd is known to have negative impacts on male reproduction. Our results indicate that lesser scaup migrating through the upper Midwest in spring have elevated Se levels and that males carry a significantly greater Cd burden than females. Moreover, elemental concentrations might be high enough to affect reproduction in both male and female lesser scaup, but controlled laboratory studies are needed to adequately assess the effects of Se and Cd on lesser scaup reproduction.

Bluegill sunfish (Lepomis macrochirus) are known to diversify into two forms specialized for foraging on either limnetic or littoral prey. Because juvenile bluegills seek vegetative cover in the presence of largemouth bass (Micropterus salmoides) predators, natural selection should favor the littoral body design at size ranges most vulnerable to predation. Yet within bluegill populations, both limnetic and littoral forms occur where vegetation and predators are present. While adaptive for foraging in different environments, does habitat-linked phenotypic variation also influence predator evasiveness for juvenile bluegills? We evaluate this question by quantifying susceptibility to predation for two groups of morphologically distinct bluegills; a limnetic form characteristic of bluegills inhabiting open water areas (limnetic bluegill) and a littoral form characteristic of bluegills inhabiting dense vegetation (littoral bluegill). In a series of predation trials, we found that bluegill behaviors differed in open water habitat but not in simulated vegetation. In open water habitat, limnetic bluegills formed more dense shoaling aggregations, maintained a larger distance from the predator, and required longer amounts of time to capture than littoral bluegill. When provided with simulated vegetation, largemouth bass spent longer amounts of time pursuing littoral bluegill and captured significantly fewer littoral bluegills than limnetic fish. Hence, morphological and behavioral variation in bluegills was linked to differential susceptibility to predation in open water and vegetated environments. Combined with previous studies, these findings show that morphological and behavioral adaptations enhance both foraging performance and predator evasiveness in different lake habitats.

We described the distribution of cladoceran zooplankton under the ice in a natural, glacial lake. Local light availability apparently altered the spatial distribution of cladocerans. Light levels measured under snow-covered areas (0.178 lux) were an order of magnitude less than those measured at the same depth under clear ice (1.750 lux). Cladoceran density under snow-covered areas was significantly higher (Bosmina spp.=3.34/L; Daphnia spp.=0.61/L) than cladoceran abundance under clear ice (Bosmina spp.=O.91/L; Daphnia spp.= O.19/L).