Elizabeth A. Marschall

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

Abstract 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...

RIVER DISCHARGE DRIVES SURVIVAL OF LARVAL WALLEYE

This work was funded by Federal Aid in Sport Fish Restoration Program project F-69-P, administered jointly by the U.S. Fish and Wildlife Service and the Ohio Division of Wildlife.

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 (Phj~sa, 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 Phj~sa 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 Hellsoma. 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 Phj~sa 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.

From Star Charts to Stoneflies: Detecting Relationships in Continuous Bivariate Data

Within many ecological systems, relationships between controlling factors and associated response variables are complex. In many cases, the response should vary little when the controlling factor exerts strong effects. Conversely, when the effect of the controlling factor is weak or absent, the response may vary greatly with effects of other factors. Correlation or regression analyses often may not be appropriate for testing these relationships, because variance of the response changes with values of the controlling factor. We suggest using a technique from the astronomy literature, a two-dimensional Kolmogorov-Smirnov (2DKS) test, to detect relationships in bivariate data with these patterns of variance. This technique successfully identified simulated bivariate data composed of paired independent values as having nonsignificant relationships and simulated bivariate data in which mean and variance of y was constrained at high levels of x as having significant relationships. Using these simulations and examples from aquatic and terrestrial systems, we demonstrate that the 2DKS is a robust test for detecting nonrandom patterns in bivariate distributions that commonly arise in many ecological systems.

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

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).

How Non-native Species in Lake Erie Influence Trophic Transfer of Mercury and Lead to Top Predators

ABSTRACT Lake Eries food web has been dramatically modified by exotic species. Both exotic dreissenid mussels and the round goby Neogobius melanastomus have shifted the food web from a pelagic-based to a benthic-based one, potentially creating a new pathway for contaminant transfer to top predators. Before the invasion of round gobies, few predators of dreissenids occurred in Lake Erie, allowing contaminants to be confined to these benthic organisms. The invasion of the round goby has produced a new pathway through which these contaminants can enter the food web. To characterize heavy-metal transfer through this new food web and to assess risk to humans, water, surficial sediment, dreissenid, round goby, and smallmouth bass Micropterus dolomieui samples were collected at three sites during summers, 2002 and 2003, and analyzed for total lead (Pb), total mercury (Hg), and methyl mercury (MeHg). In addition, we compared smallmouth bass Pb and Hg concentrations to those measured in 1993/1994, before round gobies were prevalent. Pb biodiminished and MeHg biomagnified through the food web to smallmouth bass; patterns were similar among our three sites. Total Pb concentrations in smallmouth bass were higher before the incorporation of round gobies into their diet. We attributed this decline to changes in food web structure, changes in contaminant burdens in prey, or declines in sediment Pb concentrations in Lake Erie. By comparison, Hg concentrations in smallmouth bass changed little, before and after the round goby invasion, possibly due to a shift in diet that increased growth. Despite a decline in sediment Hg concentrations in Lake Erie, smallmouth bass continued to accumulate Hg at historical rates possibly because of their high consumption rates of benthivorous round gobies. As smallmouth bass continue to consume round gobies during their lives, their Hg concentrations may well continue to increase, potentially increasing the risk of Hg contamination to humans.

Adult Survival and Imperfect Assessment of Parentage: Effects on Male Parenting Decisions

Applications of molecular methods to assess parentage have revealed that the distribution of reproductive success among individuals often differs, sometimes dramatically, from expectation based on observation of behavioral association. Much theory exists on whether and when males should reduce parental care in response to level of paternity. Life‐history theory predicts that trade‐offs in reproductive effort should be influenced by adult survival. We used a dynamic programming approach to address how level of paternity, ability to assess paternity, and adult survival rate interact to affect male tolerance of reduced parentage in a given brood. Adult survival has the greatest influence on male decisions such that, for any given cost of reproduction and value of male care, tolerance of extrapair fertilizations (EPFs) decreases as adult survival increases. An unexpected result of these models is that an optimal response also depends on a males ability to predict probability of parentage (i.e., uncertainty). These models better characterize the nature of paternity uncertainty and its effect on EPF tolerance than have previous models and add to our understanding of the complex relationship between uncertainty, mating strategies, and adult survival.

High Growth Rate of Young-of-the-year Smallmouth Bass in Lake Erie: a Result of the Round Goby Invasion?

The round goby (Neogobius melanostomus) first appeared in Lake Erie in 1993, quickly reaching extremely high densities (> 100/m2) in nearshore habitats frequented by juvenile smallmouth bass (Micropterus dolomieu). As round gobies are both potential competitors and prey for smallmouth bass, we investigated changes in young-of-the-year (YOY) smallmouth bass diet and growth between preand post-round goby invasion in the Bass Islands, Lake Erie. During 1999–2001, YOY smallmouth bass fed more frequently on fish than in either 1919 or 1975. Fish comprised 21% of YOY smallmouth bass (21–100 mm total length) diets by weight in 1975 and 51% of the diet during 1999–2001, but increased to 90% for YOY > 100 mm during 1999–2001. Of fish consumed by first-year smallmouth bass during 1999–2001, 75% were round gobies. Abundant round gobies may facilitate the transition to piscivory for YOY smallmouth bass, resulting in higher YOY growth rates during 1999–2001 (1.2 mm/d) than in the 1940s, 1950s, or 1970s (0.58–0.85 mm/d). By consuming round gobies, juvenile smallmouth bass growth rate has increased, which has possible consequences for survival, reproduction, and age at maturity. Although the round goby invasion may have increased smallmouth bass growth rate, there are negative effects: round gobies are also predators, changing energy and contaminant transfer through the Lake Erie food web.

Predation on juvenile fishes: dynamic interactions between size-structured predators and prey

Predation is a major source of mortality for most larval and juvenile fishes (Houde, 1987; Bailey and Houde, 1989). The magnitude and nature of this mortality can substantially affect cohort survival and size distribution, and selectively influence which individuals survive (Nielsen, 1980; Zaret, 1980; Rice et al., 1987, 1993b). These dynamics during the early life history are often important in determining the population dynamics and community structure that we observe at the adult stage (Tonn and Paszkowski, 1986; Kerfoot and Sih, 1987). A better mechanistic understanding of the predation process may help us understand and predict these consequences.

Bluegill growth as modified by plant density: an exploration of underlying mechanisms

SummaryBluegill (Lepomis macrochira) growth varies inconsistently with plant density. In laboratory and field experiments, we explored mechanisms underlying bluegill growth as a function of plant and invertebrate density. In the laboratory, bluegills captured more chironomids (Chironomus riparius) than damselflies (Enallagma spp. and Ischnura spp.), but energy intake per time spent searching did not differ between damselfly and chironomid treatments. From laboratory data, we described prey encounter rates as functions of plant and invertebrate density. In Clark Lake, Ohio, we created 0.05-ha mesocosms of inshore vegetation to generate macrophyte densities of 125, 270, and 385 stems/m2 of Potamogeton and Ceratophyllum and added 46-mm bluegill (1/m2). In these mesocosms, invertebrate density increased as a function of macrophyte density. Combining this function with encounter rate functions derived from laboratory data, we predicted that bluegill growth should peak at a high macrophyte density, greater than 1000 stems/m2, even though growth should change only slightly beyond 100 stems/m2. Consistent with our predictions, bluegills did not grow differentially, nor did their use of different prey taxa differ, across macrophyte densities in the field. Bluegills preferred chironomid pupae, which were relatively few in numbers but vulnerable to predation, whereas more cryptic, chironomid larvae, which were associated with vegetation but were relatively abundant, were eaten as encountered. Bluegills avoided physid snails, which were abundant. Contrary to previous work, vegetation did not influence growth or diet of bluegill beyond relatively low densities owing to the interaction between capture probabilities and macroinvertebrate densities.