Dorsal spines may be a suitable structure for aging walleyes Sander vitreus when the otoliths are unavailable due to live release of the catch, but the specialized equipment and additional time necessary to remove and mount a readable cross section can discourage their use. I evaluated a simple method of obtaining age estimates from unsectioned dorsal spines. The basal end of each dorsal spine was sanded smooth and viewed under a dissecting microscope with side illumination. For Red Lake, Minnesota, walleyes, where consensus otolith ages by two experienced readers indicated that most fish were younger than age 7, reader agreement rates (95%) and precision (mean coefficient of variation [CV] ¼ 0.95%) of dorsal spine estimates were relatively high. Age estimates by individual readers inspecting spines generally agreed with consensus otolith ages (!95%). Reader agreement rates (70%) and precision (mean CV ¼ 2.71%) were much lower for a sample of walleyes collected from Mille Lacs Lake, Minnesota, where 94% of consensus otolith ages were greater than age 6. Furthermore, spine-based age estimates for Mille Lacs Lake walleyes exhibited relatively poor agreement with consensus otolith ages ( 40%); however, most of the discrepancies were noted for individuals of consensus otolith age 7 or more, and consistent underestimation of ages did not occur until age 13. Based on these findings, unsectioned dorsal spines offered a reasonably precise, nonlethal, and simple approach for replicating consensus otolith ages for walleyes younger than age 7. Although otoliths may still be necessary to estimate the ages of older individuals within a sample, use of unsectioned dorsal spines could allow biologists to sacrifice fewer walleyes while closely replicating otolith-based estimates of population age structure. *

Newly hatched walleye Sander vitreus (formerly Stizostedion vitreum) fry were treated by immersion in a solution of 700 mg of oxytetracycline hydrochloride (OTC)/L for 6 h to determine whether walleye fry could be marked at a younger age than previously described in the literature. The resulting marks varied in intensity but were observed on 100% of the fry inspected from groups that were treated at ages less than 24 h posthatch as well as those that were treated at ages less than 3 h posthatch. No false marks were observed on untreated controls in any of the treatment periods, and pond studies indicated no significant differences (P > 0.05) in either survival or growth between OTC‐treated fry and untreated controls. The results of this study demonstrate that walleye fry can be successfully marked immediately after hatching and that the entire annual production of walleye fry at a hatchery can be marked without additional fry‐holding facilities or significant disruption of routine hatchery operations.

In freshwater, the optimal choice between radiotelemetry or acoustic telemetry has often been unclear because the combined effects of tag power, conductivity, tag depth, and antenna type on radio tag detection distances have not been quantified. To enable more informed decisions regarding the best telemetry methods to use at particular study sites, we measured maximum detection distances of an acoustic tag and two different 48-49-MHz radio tags over a wide range of conductivities, depths, and acoustic conditions in Minnesota lakes and rivers. Radio tag detection distances increased with increasing tag power, decreased with increasing conductivity, and generally decreased with increasing depth. Detection distances measured with a Yagi antenna were typically about double those measured with a loop antenna. Maximum detection distances of the radio tags were predicted by the equation log e R ¼ 13.69 À 0.005771 C À 0.006575 D þ 0.1044 P þ 0.7275 A À 0.001302 C D À 0.00008208 C P (where R ¼ maximum detection distance, m; C ¼ surface conductivity, lS/cm, at ambient temperature; D ¼ tag depth, m; P ¼ tag output power, dB relative to 1 mW; and A ¼ antenna type [1 ¼ Yagi, 0 ¼ loop]). Acoustic detection distances were less variable overall than radio detection distances but still differed substantially between and within water bodies. We saw no consistent relationship between acoustic detection distance and tag depth, but detection distance was negatively affected by ambient noise. Our model indicates that a 48-49-MHz radio tag can be detected with a boat-mounted Yagi antenna at a depth of 50 m or more if conductivity is low enough, but the maximum detection depth decreases drastically as conductivity increases. The equation above can be used to predict whether detection distances of radio tags similar to the ones we tested will be adequate for a particular study.

Walleye Sander vitreus and Yellow Perch Perca flavescens are culturally, economically, and ecologically significant fish species in North America that are affected by drivers of global change. Here, we review and synthesize the published literature documenting the effects of ecosystem changes on Walleye and Yellow Perch. We focus on four drivers: climate (including temperature and precipitation), aquatic invasive species, land use and nutrient loading, and water clarity. We identified 1,232 tests from 370 papers, split evenly between Walleye (n = 613) and Yellow Perch (n = 619). Climate was the most frequently studied driver (n = 572), and growth or condition was the most frequently studied response (n = 297). The most commonly reported relationship was "no effect" (42% of analyses), usually because multiple variables were tested and only a few were found to be significant. Overall responses varied among studies for most species-response-driver combinations. For example, the influence of invasive species on growth of both Walleye and Yellow Perch was approximately equally likely to be positive, negative, or have no effect. Even when results were variable, important patterns emerged; for example, growth responses of both species to temperature were variable, but very few negative responses were observed. A few relationships were relatively consistent across studies. Invasive species were negatively associated with Walleye recruitment and abundance, and higher water clarity was negatively associated with Walleye abundance, biomass, and production. Some variability in responses may be due to differences in methodology or the range of variables studied; others represent true context dependence, where the effect of a driver depends on the influence of other variables. Using common metrics of impact, publishing negative results, and robust analytical approaches could facilitate comparisons among systems and provide a more comprehensive understanding of the responses of Walleye and Yellow Perch to ecosystem change.

New approaches are needed for sampling northern pike Esox lucius in the complex habitats they use as nursery areas. We evaluated the potential of Quatrefoil light traps to measure differences in density of larval and juvenile northern pike, and we monitored changes in capture probability as the fish grew in size and their swimming capabilities improved. In hatchery raceways, light traps were effective for collecting northern pike at densities that seemed most realistic relative to natural nursery habitat (4–20 fish/m2 surface area). Light‐trap catch rates differentiated among different densities in raceways, but efficiency decreased as density increased. Light traps effectively caught all sizes of fish, ranging from the stage when larvae first became active (12–13 mm total length [TL]) until the fish became too large to fit through the trap entrance (>66 mm TL). We also tried to apply the technique for sampling managed wetlands that simulated a more natural environment than the raceways. Light‐trap catches detected both increases and decreases that were attributed to different stocking rates used over 2 years in individual wetlands. Light‐trap catches showed patchy distribution of small northern pike and illustrated growth and differential survival of the fish among wetlands. Light trapping should be considered a potential tool for sampling both temporal and spatial variation in density and growth of larval and juvenile northern pike.