Rock Bass Research Articles

Effects of an invasive crayfish on trophic relationships in north‐temperate lake food webs

Summary1. The introduction of invasive species is one of the main threats to global biodiversity, ecosystem structure and ecosystem processes. In freshwaters, invasive crayfish alter macroinvertebrate community structure and destroy macrophyte beds. There is limited knowledge on how such invasive species‐driven changes affect consumers at higher trophic levels.2. In this study, we explore how the invasive rusty crayfish Orconectes rusticus, a benthic omnivore, affects benthic macroinvertebrates, as well as the broader consequences for ecosystem‐level trophic flows in terms of fish benthivory and trophic position (TP). We expected crayfish to decrease abundance of benthic macroinvertebrates, making most fish species less reliant on benthic resources. We expected crayfish specialists (e.g. Lepomis sp. and Micropterus sp.) to increase their benthic dependence.3. In 10 northern Wisconsin lakes, we measured rusty crayfish relative abundance (catch per unit effort, CPUE), macroinvertebrate abundance, and C and N stable isotope ratios of 11 littoral fish species. We used stable isotope data and mixing models to characterise the trophic pathways supporting each fish species, and related trophic structure to crayfish relative abundance, fish body size and abiotic predictors using hierarchical Bayesian models.4. Benthic invertebrate abundance was negatively correlated with rusty crayfish relative abundance. Fish benthivory increased with crayfish CPUE for all 11 fish species; posterior probabilities of a positive effect were >95%. TP also increased slightly with crayfish CPUE for some species, particularly smallmouth bass, largemouth bass, rock bass and Johnny darter. Moreover, both fish body size and lake abiotic variables explained variation in TP, while their effects on benthivory were small.5. Rusty crayfish abundance explained relatively little of the overall variation in fish benthivory and TP. Although rusty crayfish appear to have strong effects on abundances of benthic macroinvertebrates, energy flow pathways and trophic niches of lentic fishes were not strongly influenced by invasive rusty crayfish.

Nearshore fish assemblages associated with introduced predatory fishes in lakes

Changes to native fish assemblages in lakes are commonly associated with introduced predatory fishes. How fish assemblages change as multiple predatory species are introduced is not well understood. This study investigated the relationship between the presence of introduced large-bodied predatory fishes (largemouth bass Micropterus salmoides, pike Esox lucius, rock bass Ambloplites rupestris, smallmouth bass Micropterus dolomieu, walleye Sander vitreus) and the composition of native fish assemblages in littoral areas of 40 lakes in Algonquin Provincial Park, Ontario, Canada. Fish assemblages were compared across lakes of different predator composition, and within lakes before and after recent predator invasions. The presence of an introduced predator was associated with significantly different native fish assemblages across lakes, after controlling for environmental and spatial variables. Native fish assemblages did not significantly vary across lakes with more than one predator species. Furthermore, while declines in native species richness over time were observed in a number of lakes, these were not associated with introductions of additional predators. Several small-bodied species (brook stickleback Culea inconstans, fathead minnow Pimephales promelas, finescale dace Chrosomus neogaeus, and northern redbelly dace Chrosomus eos) consistently showed strong negative correlations with predator presence. The results suggest that predatory fish introductions alter native fish assemblages and that this impact is consistent regardless of the number of predatory species introduced. Copyright © 2011 John Wiley & Sons, Ltd and Her Majesty the Queen in Right of Canada.

River impoundment and sunfish growth

AbstractImpoundment of rivers by dams is widespread and one of the most devastating anthropogenic impacts to freshwater environments. Linking theoretical and applied research on river impoundment requires an improved capacity for predicting how varying degrees of impoundment affects a range of species. Here, growth of 14 North American sunfish species resilient to river impoundment was compared in rivers versus impoundments. Growth response to river impoundment varied widely, but consistently among taxa: five species (shadow bass, rock bass, flier, redbreast sunfish and green sunfish) showed significantly higher growth in riverine ecosystems, four species (largemouth bass, smallmouth bass, spotted bass and longear sunfish) showed significantly higher growth in impounded ecosystems, and five species (bluegill, black crappie, white crappie, redear sunfish and warmouth) displayed no difference in growth between rivers and impoundments. Furthermore, significant linear models were developed for predicting growth of two species (largemouth bass, R2 = 0.75 and warmouth, R2 = 0.44) based on a physiographically specific index of reservoir retention time. For another species (white crappie), growth could not be predicted by the retention time index in Central Lowlands rivers (R2 = 0.001), but was strongly predicted by this factor in southeastern Coastal Plain rivers (R2 = 0.76) showing how impacts of impoundment, and prediction of its consequences, can vary across river landscape types. Further analysis of fish growth in response to river impoundment, regulation and fragmentation could greatly enhance conservation biology, restoration ecology and basic land use decisions in riverine landscapes. Copyright © 2010 John Wiley & Sons, Ltd.

Generalization of predators and nonpredators by juvenile rainbow trout: learning what is and is not a threat

Learned recognition of novel predators allows prey to respond to ecologically relevant threats. Prey could minimize the costs associated with learning the identity of both predators and nonpredators by making educated guesses on the identity of a novel species based on their similarities with known predators and nonpredators, a process known as generalization. Here, we tested whether juvenile rainbow trout, Oncorhynchus mykiss, have the ability to generalize information from a known predator (experiment 1) or a known harmless species (experiment 2) to closely related but novel species. In experiment 1, we taught juvenile trout to recognize a predatory pumpkinseed sunfish, Lepomis gibbosus, by pairing pumpkinseed odour with conspecific alarm cues or a distilled water control. We then tested the trout for a response to pumpkinseeds and to novel longear sunfish, Lepomis megalotis (same genus as pumpkinseed), rock bass, Ambloplites rupestris (same family as pumpkinseed) or yellow perch, Perca flavescens (different family). Trout showed strong learned recognition of pumpkinseed and longear sunfish odour and a weak learned response to rock bass odour but no recognition of yellow perch. In experiment 2, we used latent inhibition to teach juvenile trout that pumpkinseeds were harmless. During subsequent predator learning trials, trout did not learn to recognize pumpkinseed or longear sunfish odour as potential threats, but they did learn that rock bass and yellow perch were threatening. Taken together, these results demonstrate that juvenile rainbow trout can generalize learned recognition of both predator and nonpredator odours based on the phylogenetic relatedness of predators.

Leeches (Annelida: Hirudinida) Parasitizing Fish of Lake St. Clair, Michigan, U.S.A.

Abstract In order to identify leech species prevalent in Lake St. Clair, Michigan, U.S.A,. and understand their effect on fish hosts, fish were collected during May 2006 and 2007 from Anchor Bay, Lake St. Clair in the Laurentian Great Lakes Basin. Throughout the study, 2,117 fish from 21 species were examined for the presence of leeches. Overall, 1,064 leeches were collected from 165 individual fish and identified morphologically. Hosts included the channel catfish, freshwater drum, northern pike, northern shorthead redhorse sucker, quillback sucker, rock bass, smallmouth bass, walleye, and yellow perch. Leeches attached to hosts had a prevalence of 7.79%, with a mean intensity of 6.45 leeches/infected fish and an abundance of 0.50 leeches/examined fish. Collected leeches were identified as Actinobdella pediculata (Glossiphonidae), Placobdella montifera (Glossiphonidae), and Myzobdella lugubris (Piscicolidae), which was the most commonly occurring species. The freshwater drum had the highest prevalence, m...

Predation by native sunfishes (Centrarchidae) on the invasive crayfish Orconectes rusticus in four northern Wisconsin lakes

Tetzlaff JC, Roth BM, Weidel BC, Kitchell JF. Predation by native sunfishes (Centrarchidae) on the invasive crayfish Orconectes rusticus in four northern Wisconsin lakes. Ecology of Freshwater Fish 2011: 20: 133–143. © 2010 John Wiley & Sons A/SAbstract – Introduced rusty crayfish (Orconectes rusticus) often establish abundant populations in lakes where they negatively impact native food webs. However, rusty crayfish do not become abundant in all lakes. Fish predation has been posed as one potential mechanism limiting rusty crayfish density, but there is limited empirical evidence to support this hypothesis. Using a case study approach, we quantified the extent and timing of predation on nonnative rusty crayfish by three native centrarchids in four northern Wisconsin lakes with contrasting crayfish densities. We estimated population‐level predation effects on rusty crayfish using bioenergetics models in combination with fish and crayfish abundance estimates. Bluegill (Lepomis machrochirus) and pumpkinseed (Lepomis gibbosus) consumed young‐of‐year crayfish exclusively whereas rock bass (Ambloplites ruspestris) consumed a wide size range of crayfish. Rusty crayfish were found in Lepomis diets primarily in late June and early July while rock bass consumed crayfish throughout the summer. Per individual, rock bass consumed more crayfish biomass than Lepomis, but at the population‐level, higher Lepomis densities resulted in greater numbers of crayfish and more crayfish biomass consumed by Lepomis than rock bass. Lepomis populations consumed 8 × , 14 × , 22 × and 120 × more individual crayfish than rock bass populations in the four study lakes. Bioenergetics simulations revealed that dense populations of Lepomis were capable of consuming a large percentage of the juvenile crayfish population in two of our study lakes. In systems with high Lepomis densities, Lepomis consumption of juvenile rusty crayfish may play a significant role in the invasion and establishment dynamics of rusty crayfish.

Abandoned Mine Drainage in the Swatara Creek Basin, Southern Anthracite Coalfield, Pennsylvania, USA: 1. Stream Water Quality Trends Coinciding with the Return of Fish

Acidic mine drainage (AMD) from legacy anthracite mines has contaminated Swatara Creek in eastern Pennsylvania. Intermittently collected base-flow data for 1959–1986 indicate that fish were absent immediately downstream from the mined area where pH ranged from 3.5 to 7.2 and concentrations of sulfate, dissolved iron, and dissolved aluminum were as high as 250, 2.0, and 4.7 mg/L, respectively. However, in the 1990s, fish returned to upper Swatara Creek, coinciding with the implementation of AMD treatment (limestone drains, limestone diversion wells, limestone sand, constructed wetlands) in the watershed. During 1996–2006, as many as 25 species of fish were identified in the reach downstream from the mined area, with base-flow pH from 5.8 to 7.6 and concentrations of sulfate, dissolved iron, and dissolved aluminum as high as 120, 1.2, and 0.43 mg/L, respectively. Several of the fish taxa are intolerant of pollution and low pH, such as river chub (Nocomis micropogon) and longnose dace (Rhinichthys cataractae). Cold-water species such as brook trout (Salvelinus fontinalis) and warm-water species such as rock bass (Ambloplites rupestris) varied in predominance depending on stream flow and stream temperature. Storm flow data for 1996–2007 indicated pH, alkalinity, and sulfate concentrations decreased as the stream flow and associated storm-runoff component increased, whereas iron and other metal concentrations were poorly correlated with stream flow because of hysteresis effects (greater metal concentrations during rising stage than falling stage). Prior to 1999, pH < 5.0 was recorded during several storm events; however, since the implementation of AMD treatments, pH has been maintained near neutral. Flow-adjusted trends for 1997–2006 indicated significant increases in calcium; decreases in hydrogen ion, dissolved aluminum, dissolved and total manganese, and total iron; and no change in sulfate or dissolved iron in Swatara Creek immediately downstream from the mined area. The increased pH and calcium from limestone in treatment systems can be important for mitigating toxic effects of dissolved metals. Thus, treatment of AMD during the 1990s improved pH buffering, reduced metals transport, and helped to decrease metals toxicity to fish.

Effects of a Conservative Rock Bass Length Limit on Angler Participation, Sport Fish Populations, and Crayfish Prey in a Missouri Ozark Stream

AbstractRock bass Ambloplites rupestris are popular with anglers in Missouri Ozark streams, second only to smallmouth bass Micropterus dolomieu. Historically, fishery biologists emphasized the management of smallmouth bass, but in the late 1980s they sought higher‐quality rock bass fisheries and thus the information needed to recommend harvest regulations. Using a conservative minimum length limit of 230 mm (9 in) total length (TL) in a heavily exploited rock bass fishery in the Big Piney River, Missouri, we assessed the direct and indirect effects on angler participation and attitudes (by means of an angler survey), the abundance (mark–recapture analysis) and growth rates (scale analysis) of rock bass and smallmouth bass, sport fish predation on crayfishes (order Decapoda; diet analysis), and crayfish densities (stratified quadrat sampling). These measures were evaluated for 3 years (1992–1994) before the length limit was imposed and for 6 years afterward (1995–2000) on a 17.1‐km‐long regulation section and a 10.4‐km‐long reference section. Our results indicate that the length limit (1) had substantial angler support, as shown by their attitudes and compliance with the regulation; (2) reduced the hours fished by rock bass anglers, smallmouth bass anglers, and all anglers; (3) increased the abundances of larger rock bass and smallmouth bass substantially; (4) did not reduce the growth rates of rock bass or smallmouth bass; (5) did not proportionally reduce the predation on crayfishes or increase the percentage of empty stomachs; and (6) did not reduce the densities of crayfishes. After a review of the options for long‐term management, fishery administrators and managers elected to manage Big Piney River rock bass with a minimum‐TL regulation of 205 mm (8 in) on 124 km of river. Managers on five other rivers (one with shadow bass A. ariommus) have done likewise on an additional 448 km, and the outcomes of these decisions are currently being monitored on all six rivers.

Occurrence of the Great Lake's most recent invader, Hemimysis anomala, in the diet of fishes in southeastern Lake Ontario

The Ponto-Caspian mysid, Hemimysis anomala, was first observed in southeastern Lake Ontario in May 2006. During July and August 2007, gill nets were fished in 6 to 8 m of water at two locations of known Hemimysis colonization in southeastern Lake Ontario to determine if fish that consume macroinvertebrates were beginning to include this new invasive mysid in their diets. Of nine fish species captured in August, September, and October 2007, three species had consumed Hemimysis: alewife ( Alosa pseudoharengus), rock bass ( Ambloplites rupestris), and yellow perch ( Perca flavescens); and six species had not: round goby Apollonia melanostoma, smallmouth bass Micropterus dolomieu, spottail shiner Notropis hudsonius, gizzard shad Dorosoma cepedianum, white perch Morone americana and log perch Percina caprodes. Diets of alewives from all samples were composed predominantly of Hemimysis (69.6% -100% frequency of occurrence, 46.0%–74.5% dry weight diet composition). Two of 6 rock bass stomachs sampled in August contained ≥ 98.9% Hemimysis (10 and 40 individuals each) and one of 61 yellow perch stomachs sampled in September contained 10.0% Hemimysis (6 individuals) and 90.0% fish. While Hemimysis were observed only sparsely in the diet of most nearshore fish, their predominance in alewife diets and their omnivorous feeding behavior indicated that they have the potential to alter energy flow in Great Lakes' foodwebs.

Innate Predator Recognition and the Problem of Introduced Trout

AbstractInnate predator recognition typically only occurs when there is an evolutionary history between predator and prey. Predator introductions thus can pose a substantial threat to native fauna that rely heavily on inherent identification of predators. In permanent aquatic habitats prey often encounter a variety of predatory and non‐predatory fish species, and the ability to distinguish between the two is essential to avoid wasted time and energy spent in unnecessary antipredatory efforts. Here, we present a study evaluating the ability of lab‐reared larvae of an endangered fully aquatic salamander (hellbenders: Cryptobranchus alleganiensis) to recognize chemical cues from native and introduced fish predators. We recorded responses of hellbender larvae to chemical stimuli from native and non‐native predatory fishes, a non‐predatory fish and a blank control. Eastern hellbender larvae (C. a. alleganiensis) significantly reduced activity in response to chemical stimuli from native predators (Micropterus salmoides, Micropterus dolomieu, Ambloplites rupestris, Sander vitreus, and Cottus carolinae), but responses to non‐native rainbow (Oncorhynchus mykiss) and brown (Salmo trutta) trout were not significantly different from responses to the non‐predatory control (redhorse sucker, Moxostoma spp.). Responses of larval Ozark hellbenders (C. a. bishopi) to brown trout were similar to that of the native fishes and different from the blank control, but responses to rainbow trout did not differ from the blank control. The generally weak responses of larval hellbenders to chemical cues from introduced predatory trout could lead to increased predation in the wild, which may have exacerbated the decline of hellbender populations.

Intraspecific variation in the consumption of exotic prey – a mechanism that increases biotic resistance against invasive species?

Summary1. Hyper‐successful exotic species can both displace the native prey that formerly made up a native predator’s diet and represent an abundant potential prey resource for native predators. Little is known about how this drastic change affects native predators, or their short‐ and long term potential to regulate the exotic species.2. We compared zebra mussel consumption by pumpkinseed sunfish (Lepomis gibbosus), redbreast sunfish (Lepomis auritus) and rock bass (Ambloplites rupestris) from populations that were either previously exposed to zebra mussels or naive to them.3. Fish from populations with longer exposure to zebra mussels consumed many more zebra mussels than fish from populations with shorter or no previous exposure to zebra mussels.4. Our experiment does not allow us to identify the mechanisms that underlie the patterns we found, but we discuss several plausible scenarios and their ecological implications.5. Predator adaptation to exotic prey may be an important but overlooked factor in invasion biology. The initial response to exotic prey by a native predator may be a poor estimate of its ability to present biotic resistance to the invasion over the long term.

Energy Density of Introduced Round Goby Compared with Four Native Fishes in a Lake Michigan Tributary

AbstractThe round goby Neogobius melanostomus is an invasive species that has changed Great Lakes food webs and become an important prey for many predators. We tested whether the round goby from a Lake Michigan tributary was energetically equivalent to four native fishes: Mottled sculpin Cottus bairdii, johnny darter Etheostoma nigrum, bluntnose minnow Pimephales notatus, and rock bass Ambloplites rupestris. We found positive linear relationships between energy density and the percent dry weight of a fish for each species. We also found evidence of temporal and spatial variation in round goby energy density. Energy density was lower in spring and summer than in fall. The spatial variation in energy density may be linked to the inclusion of the low‐quality prey, dreissenid mussels, in the diets of larger round goby. For a given size, the johnny darter and bluntnose minnow had the highest energy density while mottled sculpin had the lowest. Our results show that the energy density of the round goby is intermediate to those of the four native fishes, suggesting that the round goby is an energetically average prey in a Lake Michigan tributary.

Re-engineering the eastern Lake Erie littoral food web: The trophic function of non-indigenous Ponto-Caspian species

The trophic roles of key Ponto-Caspian invaders (quagga mussels Dreissena bugensis, amphipods Echinogammarus ischnus and round goby Apollonia melanostomus) within the littoral food web of eastern Lake Erie were quantified using stable isotopes (δ 13C, δ 15N). A dual stable isotope parameter search with a mass balance component was used to assess the isotopic importance of quagga mussels and amphipods as dietary items to two size classes of round goby. The utility of the mass balance simulation was also evaluated as a tool to approximate isotopic contributions of feasible prey and identify gaps incurred by “missing” prey items not included in the sampling. The mass balance dietary simulation, confirmed by stomach content data, indicated that isotopically important prey to small round goby (< 11.2 cm) were chironomids and Ponto-Caspian amphipods, while large round goby (≥ 11.2 cm) showed strong preference for quagga mussels. The dietary mass balance simulation output also supported the isotopic importance of round goby to the somatic growth of smallmouth bass, rock bass and freshwater drum. The isotopic mass balance output for yellow perch was more ambiguous, which may be in line with their known broadly omnivorous diet. The white bass output was in line with published data indicating increasing consumption of round goby for this species, while the brown trout output strongly favoured alewife isotopic contributions. However for white perch and walleye, the mass balance simulations were not in line with their known published diets in Lake Erie, probably due to a lack of key prey items in the sample set (e.g. zooplankton for white perch and shiner species for walleye). As expected, the Ponto-Caspian species have integrated themselves into the littoral food webs, and the “quagga mussel–round goby–smallmouth bass” food chain forms one of the key components within the trophodynamics of Lake Erie.

Balancing Aquatic Habitat Fragmentation and Control of Invasive Species: Enhancing Selective Fish Passage at Sea Lamprey Control Barriers

AbstractBarriers to prevent spawning migrations of sea lampreys Petromyzon marinus remain an important component of an integrated sea lamprey management program in the Laurentian Great Lakes. Concerns about effects on nontarget fishes have led to the construction of specially designed vertical‐slot trap‐and‐sort fishways to mitigate potential barrier effects. To improve passage at these fishways, we used passive integrated transponder technology to assess the performance of two fishways located on low‐head sea lamprey barriers. Fishways on the Big Carp River (which flows into Lake Superior) and Cobourg Brook (which flows into Lake Ontario) were assessed for attraction efficiency, trap attraction and retention, and passage efficiency. Based on the results of these assessments, fishways were modified by increasing the trap volume and altering the funnel characteristics to reduce escapement from the trap and then reassessed. Attraction efficiency for all tagged fish was high (≥80%) at both sites in all years. Fishway modifications improved passage from 35% in 2003 to 88% and 64% in 2004 and 2005, respectively, at the Big Carp River. As expected, white suckers Catostomus commersonii, an obligate migrant, had higher attraction and passage efficiency, fewer passage attempts, and shorter migration delay than did rock bass Ambloplites rupestris, a facultative migrant. No improvements were seen at Cobourg Brook, where passage efficiency remained low (7% in 2003, 10% in 2005), probably because of the loss of attraction flow. At both fishways, individual fish averaged 3‐10 attempts to pass through the fishways and had their migrations delayed 1‐2 weeks. The observed improvements to the Big Carp River fishway, which resulted in high fishway attraction and passage rates for white suckers, suggest that vertical‐slot fishways can help mitigate the effects of low‐head barriers for some species. Our study provides a rigorous quantifiable approach to assessing fishway performance that can be employed widely and successfully to assess initial fishway design and subsequent modifications.

Predation on emergent lake trout fry in Lake Champlain

The rehabilitation of extirpated lake trout ( Salvelinus namaycush) in the Great Lakes and Lake Champlain has been hindered by various biological and physiological impediments. Efforts to restore a lake trout fishery to Lake Champlain include hatchery stocking and sea lamprey control. Despite these management actions, there is little evidence of recruitment of naturally-produced fish in annual fall assessments. Spawning occurs at multiple sites lake-wide in Lake Champlain, with extremely high egg and fry densities, yet sampling for juvenile lake trout has only yielded fin-clipped fish. To investigate this recruitment bottleneck, we assessed predation pressure by epi-benthic fish on emergent fry on two spawning reefs and the subsequent survival and dispersal of fry in potential nursery areas. Epi-benthic predators were sampled with 2-h gillnet sets at two small, shallow sites in Lake Champlain throughout the 24-h cycle, with an emphasis on dusk and dawn hours. In total, we documented seven different species that had consumed fry, with consumption rates from 1 to 17 fry per stomach. Rock bass and yellow perch dominated the near-shore fish community and were the most common fry predators. Predator presence and consumption of fry was highest between 19:00 and 07:00. Predators only consumed fry when fry relative abundance was above a threshold of 1 fry trap − 1 day − 1 . We used an otter trawl to sample for post-emergent fry adjacent to the reef, but did not capture any age-0 lake trout. Due to the observed predation pressure by multiple littoral, species on shallow spawning reefs, lake trout restoration may be more successful at deep, offshore sites.

Systematics and Zoogeography of the Rock Basses (Centrarchidae: Ambloplites)

This study examines the role that historical events have played in the diversification of members of the genus Ambloplites by estimating divergence times of clades within the genus. DNA sequences of the mitochondrial cytochrome b gene were used to develop phylogenetic hypotheses for Ambloplites using maximum parsimony and Bayesian methods. The timing of diversification within and between species of Ambloplites was estimated using nonparametric rate-smoothing and penalized likelihood methods calibrated by fossil dates. Monophyly of three of the four species was supported. The ages of the most recent common ancestors of species of Ambloplites estimated in this study confirm earlier estimates that in most instances, their origins occurred prior to the Pleistocene. A comparison of estimated divergence times of lineages to sea level fluctuations indicates a correspondence to extremely high or low sea stands throughout the Neogene Period. Populations of Ambloplites in the Ozarks and Ouachita highlands previously assigned to A. ariommus appear to have either been introgressed by A. rupestris or were misidentified as A. ariommus. Ambloplites rupestris displays low genetic divergence throughout its range and is consistent with a re-colonization of previously glaciated areas following the Pleistocene.

Significant fish predation on zebra mussels Dreissena polymorpha in Lake Champlain, U.S.A.

The objectives of this study were to: (1) determine which fishes were consistently eating zebra mussels Dreissena polymorpha in Lake Champlain and document their feeding behaviour and (2) quantify the diet composition of the fish predators that were found to consume zebra mussels. From 2002 to 2005, freshwater drum Aplodinotus grunniens, pumpkinseed Lepomis gibbosus, yellow perch Perca flavescens and rock bass Ambloplites rupestris all consumed zebra mussels at varying frequencies and amounts. Aplodinotusgrunniens and L. gibbosus chewed clumps of zebra mussels, expelling shells, whereas P. flavescens and A. rupestris swallowed small individuals whole. Lepomis gibbosus consumed zebra mussels at the highest frequency (65–89% of prey consumed) and zebra mussels comprised a large part of this fish’s diet (up to 40% by dry mass). Zebra mussels were also an important component of the diet of A. grunniens (up to 59% of the diet by dry mass, 40–63% frequency of consumption). The percentage of the diet comprising zebra mussels in P. flavescens and A. rupestris varied significantly from year to year but never exceeded 10%. Because A. grunniens and L. gibbosus crushed zebra mussels, the nutritional return from consuming zebra mussels would be similar to other prey; for P. flavescens and A. rupestris zebra mussels were only partially digested and the nutritional return would probably be low. As predation on zebra mussels is widespread and significant, it is possible that fish predators could contribute to regulating the population of zebra mussels in Lake Champlain.

River Otter, &lt;em&gt;Lontra canadensis&lt;/em&gt;, Food Habits in the Missouri Ozarks

The reintroduction of River Otters (Lontra canadensis) between 1982 and 1992 resulted in widespread occurrence of the species throughout the Missouri Ozarks. This study examined otter diets from the vicinity of two Ozark streams in relation to seasonal and spatial trends. Otter scats (N = 4750) were collected and analyzed from the Osage Fork River and Big Piney River during the summer and winter seasons of 2001 and 2002. During the winter (January-March), fish occurred in 86% of the samples. During the summer (June–August), occurrence of fish dropped to approximately 15% for both rivers. Seven families of fish were identified in the diets, with Centrarchidae being most common regardless of river or season. Within the Centrarchidae, the genus Lepomis (mostly Longear Sunfish, Lepomis megalotis) was most common, with Micropterus (mostly Smallmouth Bass, Micropterus dolomieu) and Rock Bass (Ambloplites rupestris) also well represented. The mean age of Ambloplites consumed (mean = 3.3 years) was consistently older than that of either Micropterus (mean = 2.54 years) or Lepomis (mean = 2.78 years). Crayfish were recovered from a mean of 85.2% of scats in the winter and 99% in the summer. Smaller fish and crayfish were more common from the upper reaches of the streams while larger fish were prevalent in the lower reaches.

Fish Assemblages and Environmental Variables Associated with Gobiids in Nearshore Areas of the Lower Great Lakes

We investigated which fish species and environmental variables were associated with the invasive round goby (Neogobius melanostomus) and tubenose goby (Proterorhinus marmoratus) in nearshore Canadian waters of the Huron-Erie corridor of the lower Great Lakes. We measured a suite of environmental variables and used triplicate beach seine samples to collect fishes in summer 2006. Thirty sites were sampled in the day and a subset (n = 14) at night. Of 1,955 individuals caught in daytime samples, round goby (21.0 %), spottail shiner (17.3%) and emerald shiner (14.2%) were most abundant. Of 1,521 individuals collected at night, the most abundant species were round goby (42.3%) and emerald shiner (24.1%). Tubenose gobies represented 1% and 1.7% of all individuals caught in the day and night, respectively. Rarefaction analysis showed that overall species richness was greater in the day than night. Significantly more emerald shiner (P = 0.017), rock bass (P = 0.046) and round goby (P = 0.035) were caught at night than in the day; more logperch were caught in the day than at night (P = 0.042). Round gobies were positively associated with water temperatures up to 24°, but there was no relationship between round goby abundance and warmer temperatures. There were too few tubenose goby captured to determine their statistical association with environmental factors; however, tubenose gobies were found only where round gobies were collected. Round goby and tubenose goby were associated with yellow perch and rock bass. The benthic round goby was the most abundant species, whereas other abundant species were pelagic, schooling fishes that occupied a habitat distinct from round goby.

Influence of Summer Water-Level Variability on St. Lawrence River-Wetland Fish Assemblages

ABSTRACT Water-level and associated variability are substantial influences on wetland and shallow aquatic communities. The Akwesasne Wetland Complex is an extensive St. Lawrence River system affected by water regulation. The responses offish assemblages to short-term summer water-level variation were examined throughout this section of the St. Lawrence River and its tributaries. An influence of water-level variability was detected on abundance of three common species [bluntnose minnow (Pimephales notatus), rock bass (Amboplites rupestris), and white sucker (Catastomus commersonii)] and explained 30–44% of variation. This influence has implications for water regulation and natural resource management, and a larger scope evaluation may reveal more extensive effects.