Allocating Sampling Effort to Equalize Precision of Electrofishing Catch per Unit Effort

We used a spatially explicit simulation model to examine the effects of lake shoreline length and lakewide fish density on electrofishing catch-per-unit-effort (CPUE) estimates of fish density. We also tested model predictions regarding the influence of shoreline length and fish density on precision of CPUE estimates by analyzing electrofishing data from Ohio reservoirs for juvenile gizzard shad Dorosoma cepedianum, which is a schooling fish, and largemouth bass Micropterus salmoides, a more solitary fish. Our goals were to estimate the impact of these factors on variability associated with population estimates derived from CPUE and to determine how these factors influence the minimum number of transects required to sample populations with a reliable degree of precision. Neither ‘‘minimum transect number’’ (number of transects sampled per lake in which all of 10 replicate simulations provided density estimates within 610% of the mean) nor ‘‘minimum variance’’ (variance among estimates given 20 transects/estimate) were affected by the size of lake being sampled. However, minimum transect number decreased with lakewide fish density, and minimum variance increased with fish density, particularly when fish were patchily distributed. Our results show that it is reasonable to choose one effort level (i.e., a constant number of transects per lake) for a variety of systems. This constant level of effort can achieve acceptable precision in systems differing in lake shoreline length, fish density, and fish patchiness, except in those systems having extremely low overall fish densities. In this case, more transects may be required.Support for this project was provided by Federal Aid in Sport Fish Restoration, project F-69-P, administered jointly by the U.S. Fish and Wildlife Service and the Ohio Division of Wildlife, and by the Department of Zoology, Ohio State University

Optimal energy allocation to ovaries after spawning

For iteroparous organisms in which fecundity is positively related to body size, a trade-off exists between allocation of energy to gonads, thus ensuring some reproductive output, and allocation to somatic growth, thus increasing potential fecundity in the future. This tradeoff can influence several life-history patterns, including when, for organisms that grow after maturity, allocation to gonads begins following the previous reproductive event. White crappie Pomoxis annularis, a spring-spawning freshwater fish, began allocating energy to ovaries in autumn at the expense of continued somatic growth and higher potential fecundity. Within five populations, the amount of early allocation varied between years. We combined dynamic programming with an individual-based model to determine how summer and spring feeding conditions interact to influence when allocation to reproduction should begin. Model results indicated that autumn allocation to ovaries was in response to future spring feeding conditions rather than recent summer feeding conditions. At least a 10% probability of poor spring feeding conditions resulted in ovary investment patterns that matched field observations. The model was unable to explain the inter-annual variation in autumn energy observed in the field. Early allocation of energy to ovaries is probably an evolutionary adaptation to the possibility of poor spring feeding conditions.This research was funded in part by Federal Aid in Sport Fish Restoration Project F69- P, administered jointly by the United States Fish and Wildlife Service and the Ohio Department of Natural Resources, Division of Wildlife

Assessing Population Responses to Multiple Anthropogenic Effects: A Case Study with Brook Trout

Population declines are often caused by multiple factors, including anthropogenic ones that can be mitigated or reversed to enhance population recovery. We used a size-classified matrix population model to examine multiple anthropogenic effects on a population and determine which factors are most (or least) important to population dynamics. We modeled brook trout (Salvelinus fontinalis) in southern Appalachian mountain streams responding to multiple anthropogenic effects including the introduction of an exotic salmonid species (rainbow trout, Oncorhynchus mykiss), a decrease in pH (through acidic deposition), an increase in siltation (from roadbuilding and logging), and an increase in fishing pressure. Potential brook trout responses to rainbow trout include a decrease in survival rate of small fish, a change in density dependence in survival of small fish, and a decrease in growth rates of all sizes. When we included these responses in the population model, we found that population size tended to decrease with an increase in small-fish growth rate (producing a population with fewer, but larger, fish). In addition, changes in patterns of density-dependent survival also had a strong impact on both population size and size structure. Brook trout respond to decreases in pH with decreased growth rate in all size classes, decreased survival rates of small fish, and decreased egg-to-larva survival rates. This combination of effects, at magnitudes documented in laboratory experiments, had severe negative impacts on the modeled population. If siltation effects were severe, the extreme increase in egg-to-larva mortality could have strong negative effects on the population. However, even very strong increases in large fish mortality associated with sport harvesting were not likely to cause a local extinction. In all of these cases, the interaction of drastic changes in population size structure with randomly occurring floods or droughts may lead to even stronger negative impacts than those predicted from the deterministic model. Because these fish can reproduce at a small size, negative impacts on survival of the largest fish were not detrimental to the persistence of the population. Because survival of small juveniles is density dependent, even moderate decreases in survival in this stage had little effect on the ultimate population size. In general, a brook trout population will respond most negatively to factors that decrease survival of large juveniles and small adults, and growth rates of small juveniles.This work was supported by the Lucas Fellowship in Biomathematics at North Carolina State University (to E. A. Marschall), the J. F. Allen Award from the American Fisheries Society (to E. A. Marschall), an Electric Power Research Institute Fellowship in Population Dynamics (to E. A. Marschall), a U.S. Forest Service Cost-Share Agreement (to L.B. Crowder and E. A. Marschall), the Department of Zoology at North Carolina State University (to L. B. Crowder), and the Department of Zoology at The Ohio State University (to E. A. Marschall)

Identifying Relationships between Catches of Spawning Condition Yellow Perch and Environmental Variables in the Western Basin of Lake Erie

Although the reproductive behavior of yellow perch Perca flavescens has been well documented in small systems, relatively little is known about the spawning preferences of yellow perch in large systems, such as the Laurentian Great Lakes. During 2006 and 2007, we compared the presence and abundance adult yellow perch during the spring spawning season with environmental variables in the western basin of Lake Erie. We also estimated the timing of yellow perch spawning by comparing the relative abundance of gravid and spent females collected in our trawls and then comparing the proportion of gravid females with environmental conditions at our sampling sites. Overall, the probability of catching adult yellow perch and the catch per unit effort increased with increasing bottom temperatures in the spring, whereas the probability of catching gravid females increased with increasing Secchi depth. However, the relationships between our catch metrics and environmental variables were not consistent across years, possibly as a result of the very strong 2003 year‐class, which became first‐year spawners in 2006. We also documented that yellow perch spawning occurred when bottom temperatures were between 11°C and 15°C in the western basin; these temperatures were reached on different dates in different parts of the basin and in different years. Thus, we suggest that management agencies consider basing the start of the commercial fishing season on prevailing bottom temperatures rather than using a set date across years and sites.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/142328/1/tafs0031.pd

Round Goby Predation on Smallmouth Bass Offspring in Nests during Simulated Catch-and-Release Angling

Round goby Neogobius melanostomus first appeared in Lake Erie in 1993 and now occur in extremely high densities in some areas. As known nest predators, round goby currently pose a threat to nest-guarding smallmouth bass Micropterus dolomieu. We conducted manipulative experiments to evaluate the combined effects of round goby predation and catch-and-release angling during 1999–2001 in the Bass Islands, Lake Erie. We quantified how many smallmouth bass offspring were consumed by round goby when nest-guarding smallmouth bass males were present, removed, and recovering from angling-related stress. In 10 h of video observations, we only saw one instance of round goby consuming smallmouth bass offspring while the nest was guarded. Upon removal of nest-guarding smallmouth bass, round goby quickly entered unguarded nests (4.3 round goby/min for nests with unhatched embryos and 1.8 round goby/min for nests with hatched embryos). During experimental catch-and-release angling, round goby consumed an average of 2,000 unhatched embryos before the guardian male returned, but postreturn offspring losses were minimal while the male recovered from angling stress. For an average smallmouth bass nest in the Bass Islands, round goby could consume all offspring from an unguarded nest in about 15 min. Round goby predation and smallmouth bass angling combined to reduce survival of smallmouth bass embryos, but we did not observe round goby consuming free-swimming larvae or juveniles. If the number of surviving smallmouth bass embryos drives adult population size, managers should consider angling regulations that reduce interference with nesting males, thus limiting the deleterious effects of round goby.This research was funded by Federal Aid in Sport Fish Restoration Project F-69-P, administered jointly by the U.S. Fish and Wildlife Service and the Ohio Department of Natural Resources, Division of Wildlife, and the Department of Evolution, Ecology, and Organismal Biology at Ohio State University

Using Time and Energetic Measures of Cost in Estimating Prey Value for Fish Predators

In the predator-prey interaction between redear sunfish (Lepomis mlcrolophus) and benthic, freshwater snails (Physa, Helisoma, and Oxytrema), we document selective predation among genera and sizes of prey and use optimal foraging theory in an attempt to explain diet selection. In experiments, sunfish strongly selected against Oxytrema and weakly discriminated between Physa and Helisoma, with Physa most often chosen; size selection within any genus did not occur. Among genera, selection results were consistent with differences in shell strength and a time cost/benefit (C/B) construct operationally defined as handling time divided by prey dry mass. Within any genus, neither shell strength (smallest snails had weakest shells) nor time C/B (largest snails had minimal C/B) provided predictions consistent with results from selective-predation experiments. To explain this discrepancy, we measured metabolic costs of handling and energy content of prey. Dividing net energy of prey (E, generated by subtracting the energetic cost of handling from prey energy content) by handling time (T) yielded values of E/T that were similar for Physa and Helisoma. Within either genus, E/T was always highest for largest snails. Thus, this construct also provided predictions inconsistent with our experimental results. Only an energetic CIB ratio, in which the energetic costs of handling were divided by the energetic value of the prey, was consistent with selection of Physa over Helisoma and no size selectivity within either genus. Whether predators add prey to their diet based on this construct is unknown at present. The lack of concurrence between theoretical predictions of how prey should be ranked by predators (i.e., according to energy gained per unit time expended) and our experimental results suggests these constructs should be re-evaluated. Unless we know the discriminatory ability of our predators (in terms of how fine a difference in prey types they can assess) and the currency used in making decisions, CIB or EIT constructs provide little insight into diet selection by predators.This research was supported by the National Science Foundation, DEB 77- 16167

Foraging in a patchy environment: prey-encounter rate and residence time distributions

Small bluegill sunfish, Lepomis macrochirus, foraging among patches in the laboratory did not search systematically within a patch; their intercapture intervals did not differ from a model of random prey encounter within a patch. Patch-residence time, number of prey eaten, and giving-up time (time between last prey capture and leaving the patch) were measured for bluegills foraging in two different three-patch 'environments' (a constant environment, in which each patch began with the same number of prey and a variable environment, in which two patches began with low prey density and one patch with high prey density). When compared with three decision rules a forager may use to determine when to leave a patch, the data most closely agreed with predictions from a 'constant residence time' rule. Bluegills responded to changes in the distribution of prey among patches, but not by using different decision rules. There was qualitative, but not quantitative, agreement with a model of random residence times. The total number of prey eaten by a bluegill during a foraging bout was similar to the number predicted from a model of random search and random residence times