Document Category: Sampling Techniques and Protocols

Radiotelemetry is a useful tool to describe habitat use by fish, but studies may inaccurately describe microhabitat use due to failure to account for errors when identifying fish locations. We developed a technique that identifies the extent of location errors of a hand-held radiotelemetry system. The ranges of location errors were determined among different observers, between land and water, and at an array of distances (5—125 m) between transmitters and receivers. No significant differences occurred among three observers at distances of 100 m or less or between transmitters on land or in water. Location error increased significantly with distance between transmitter and receiver; mean error was 0.5 m at 5 m and 4.5 m at 100 m. The 95th percentile of location errors was 1.5 m at 5 m and 10.5 m at 100 m. The number of trials needed to define location errors within 20% of the true mean (P ≤ 0.05) was 109 at 5 m and 46 at 100 m. Determination of location error with radiotelemetry equipment can be conducted by two people on land with a relatively small amount of effort when the results are to be applied to relatively shallow waters.

We sought to determine whether electrofishing catchability was density dependent and varied with physical and biological factors for walleyes (Sander vitreus), largemouth bass (Micropterus salmoides), smallmouth bass (M. dolomieu), northern pike (Esox lucius), and muskellunge (E. masquinongy) in Wisconsin lakes. Electrofishing catch rate (number of fish caught per shoreline mile) was linearly related to population density (number of fish per acre) in spring for largemouth bass, northern pike, and muskellunge and in fall for walleyes, largemouth bass, and smallmouth bass. In contrast, gear saturation caused the electrofishing catch rate to be nonlinearly related (hyperstable) to population density for walleyes and smallmouth bass during spring. Catchability was higher during spring than fall for walleyes, largemouth bass, and smallmouth bass. Catchability of walleyes during fall was positively related to the percentage of littoral zone, whereas catchability of largemouth bass, smallmouth bass, and muskellunge was negatively related to the shoreline development index. Other physical and chemical variables failed to describe significant residual variation in catchability of any other species in any other season. We conclude that population density cannot be accurately estimated from the electrofishing catch rates of walleyes and smallmouth bass during spring. Therefore, mark–recapture methods must still be used to estimate population density if managers choose to sample those species during spring in Wisconsin lakes.

Fisheries managers often assess fish populations using catch per unit effort (CPUE), size and age structure, growth, and condition. For many freshwater fishes and common sampling gears, CPUE, size and age structure, and condition are highest in the spring and fall, while growth commonly is fastest during the summer growing season. However, there are exceptions to these general trends, especially in populations with erratic recruitment, growth, or mortality. At the least, CPUE, size and age structure, growth, and condition of fish should be expected to change with season, given the effects of variable recruitment, growth, and mortality. However, if recruitment, growth, and mortality are relatively stable, seasonal changes in sampling data occur due to changes in fish behavior caused by many factors (e.g., changes in temperature, turbidity, food availability, photoperiod, etc.). However, these patterns of change through the seasons should not necessarily be assumed to be the same for all fish species or for species in all habitats or geographic regions. Thus, managers and researchers need to consider the seasonal patterns in sampling data for a particular species within a certain habitat in their geographic region when collecting standardized data sets, and when assessing populations and communities.

To help predict what effect potential length‐ or bag‐limit regulations might have on angler satisfaction, anglers at two Ohio lakes were asked to rate their preferences for hypothetical catches of crappies Pomoxis spp. consisting of various combinations of lengths and numbers. Anglers at Delaware and Paint Creek lakes rated catches by scoring them on a unitless semantic differentiation scale where 1 was the lowest and 5 was the highest satisfaction. Catch scenarios consisting of 5, 10, 15, or 20 fish of 127–279 mm total length in 25.4‐mm increments were described to the anglers by interviewers using verbal and visual communication. Analysis of weighted leasts‐quares estimates indicated lake, fish length, and number of fish all significantly affected variance in responses. Fish length was the most influential independent variable, Differences between lakes were most apparent for the 5‐fish scenarios. The predominant effect of fish length on angler preference justifies management actions on these two study lakes, such as length limits that would result in catches of fewer fish of larger average size.

The efficiency of gastric lavage for the removal of stomach contents of two species of fish was tested at The Pennsylvania State University in 1981. This technique was effective for 98% of the brook trout (Salvelinus fontinalis) examined, and 100% for slimy sculpins (Cottus cognatus). No mortality occurred in either species during the 3‐week post‐lavage period

To assess the effectiveness of muskellunge (Esox masquinongy) stocking, biologists conduct electrofishing survey in fall to estimate catch per effort (CPE) and relative contribution of stocked fish. Inherent assumptions of this sampling are that wild and stocked fish have equal probability of capture and that changes in CPE reflect differences in actual abundance. However, capture rates of age-0 muskellunge tend to be low for both wild and stocked fish making it difficult to determine if electrofishing catch is a meaningful predictor of actual abundance. The goal of our study was to determine the efffectiveness of electrofishing for capturing individual age-0 muskellunge by determining locations of stocked fish released into two lakes in northern Wisconsin. All stocked fish received a fin clip and at least 40 fish per lake were inserted with radio transmitters. All fish were released at a single boat ramp on each lake which followed standard Wisconsin Department of Natural Resources (DNR) protocols. Stocked muskellunge with transmitters were tracked weekly for one month after release. Fish were also sampled weekly using standard Wisconsin DNR electrofishing methods that included a single boat following a single line transect around the shoreline of each lake. Comparing known locations of radio-tagged fish to the area sampled by the electrofishing boat revealed that most sampling effort was expended outside the area where stocked fish were located. Our initial results suggest that the majority of hatchery fish remained in the vicinity of the release location with fish moving a maximum distance of 0.4-0.7 km on average away from the release location within four weeks after stocking. Based on these results, biologist will be able to improve sampling protocols to evaluate muskellunge stocking success by stratifying sampling effort so more time is spend sampling where fish are located.

Several nonlethal methods have been developed to determine the stomach contents of fish, including gastroscopes, tubes, stomach suction, stomach flushing, emetics, forceps, and chronic fistulas. By reviewing the literature on this subject, we found that the effectiveness (ability to remove all stomach contents) of the different methods depends on size, age, species of fish, and the size of the food items in the stomach. Overall, various methods of stomach flushing were the most effective method of recovering stomach items from a variety of fishes. Mechanized pressure appeared to be the most efficient method of stomach flushing for most large fishes. The use of syringes allowed stomach flushing to be performed on most young and small fishes. The use of tubes and stomach suctions, much simpler and less expensive methods than stomach flushing, were nearly as effective for some fishes such as black bass (Micropterus spp.) and salmonids.

The relationship between the temperature requirements of some fish species, using published data for growth optima, final preferences and lethal limits were examined. A good correlation was found and it is suggested that the data established gives a good estimate of the temperature promoting maximum growth. Determinations of final preferenda are easily conducted in the laboratory and could therefore be used to give rapid assessments of optimum growth temperatures of potential culture species. The practical application of such measurements is discussed.

We made concurrent collections of small (85–100) yellow perch (Perca flavescens) at the bottom of Lake Erie with an active gear (otter trawl) and a passive gear (gillnet) within a 24-h sampling period. During daylight periods when yellow perch feeding activity was most prominent, gillnetted fish possessed significantly higher median food amounts in stomachs than did trawled fish. Estimated daily food consumption for small yellow perch was 5.86% of body weight by gillnet sampling and 3.98% by trawl sampling. Passive gears tend to sample the most active fish (more likely to be feeding) and may, therefore, yield upwardly biased estimates of food consumption for individuals within a population. Active gears are considered to give more accurate estimates because quiescent and low activity, as well as the more active members of a population, are sampled.

Laboratory and field tests of an improved water flushing method for removing stomach contents of live fish were conducted on largemouth bass (Micropterus salmoides) and grass pickerel (Esox americanus vermiculatus) and compared with the suction pump, stomach flush, and induced regurgitation methods. Pulsed gastric lavage was not injurious to the fish and, of the methods tested, it enabled procurement of the most accurate samples through the entire range of predator sizes and types of prey foods.