Walleye Sander vitreus and smallmouth bass Micropterus dolomieu interactions: an historic stable-isotope analysis approach

The carbon (δ(13) C) and nitrogen (δ(15) N) values of Sander vitreus scales differed pre and post-introduction in treatment lakes among years following the introduction of Micropterus dolomieu. No difference of δ(13) C and δ(15) N in S. vitreus scales was found in control lakes where M. dolomieu were not introduced. In treatment lakes, S. vitreus δ(15) N increased and δ(13) C decreased. No relationship was found between S. vitreus and M. dolomieu abundance in the two treatment lakes. Size structure of S. vitreus and M. dolomieu was negatively correlated and condition of the two species was positively correlated. Although S. vitreus feeding habits may have changed in the treatment lakes after M. dolomieu introductions, evidence suggests that fitness-related factors (i.e. abundance and condition) of S. vitreus remained unchanged, indicating S. vitreus and M. dolomieu may coexist where M. dolomieu have been introduced.     

Seasonal and interannual effects of hypoxia on fish habitat quality in central Lake Erie

1. Hypoxia occurs seasonally in many stratified coastal marine and freshwater ecosystems when bottom dissolved oxygen (DO) concentrations are depleted below 2–3 mg O₂ L^?1. 2. We evaluated the effects of hypoxia on fish habitat quality in the central basin of Lake Erie from 1987 to 2005, using bioenergetic growth rate potential (GRP) as a proxy for habitat quality. We compared the effect of hypoxia on habitat quality of (i) rainbow smelt, Osmerus mordax mordax Mitchill (young‐of‐year, YOY, and adult), a cold‐water planktivore, (ii) emerald shiner, Notropis atherinoides Rafinesque (adult), a warm‐water planktivore, (iii) yellow perch, Perca flavescens Mitchill (YOY and adult), a cool‐water benthopelagic omnivore and (iv) round goby Neogobius melanostomus Pallas (adult) a eurythermal benthivore. Annual thermal and DO profiles were generated from 1D thermal and DO hydrodynamics models developed for Lake Erie’s central basin. 3. Hypoxia occurred annually, typically from mid‐July to mid‐October, which spatially and temporally overlaps with otherwise high benthic habitat quality. Hypoxia reduced the habitat quality across fish species and life stages, but the magnitude of the reduction varied both among and within species because of the differences in tolerance to low DO levels and warm‐water temperatures. 4. Across years, trends in habitat quality mirrored trends in phosphorus concentration and water column oxygen demand in central Lake Erie. The per cent reduction in habitat quality owing to hypoxia was greatest for adult rainbow smelt and round goby (mean: ?35%), followed by adult emerald shiner (mean: ?12%), YOY rainbow smelt (mean: ?10%) and YOY and adult yellow perch (mean: ?8.5%). 5. Our results highlight the importance of differential spatiotemporally interactive effects of DO and temperature on relative fish habitat quality and quantity. These effects have the potential to influence the performance of individual fish species as well as population dynamics, trophic interactions and fish community structure.     

Growth, ingestion rates and metabolic activity of walleye in lakes with and without lake herring

Growth efficiencies, ingestions rates and activity levels of walleye Sander vitreus were compared in lakes with and without lake herring Coregonus artedi . Yellow perch Perca flavescens were the main prey in lakes without lake herring. Walleye were sampled in September and October from 38 lakes in Ontario in 1998 and 1999, using multimesh monofilament gillnets. Ingestion rates were estimated from annual increments in somatic mercury and body mass, and the mercury content of yellow perch and lake herring. Walleye had higher growth efficiencies, and lower ingestion and activity rates in lakes with lake herring. Lake herring grow larger than yellow perch and therefore could provide more profitable prey for larger walleye. The results are consistent with optimal foraging theory that predicts that walleye feeding on optimal prey sizes should grow more efficiently, if the ratio of feeding benefit (energy) to cost (search and seizure) is a function of the ratio of predator and prey size.     

Spawning population dynamics of smelt, Osmerus mordax

A total of 1618 Osmerus mordax were collected between 27 April and 5 May 1977 from five spawning sites along Lake Superior. The fish were measured, sexed, and aged to determine spawning population dynamics. Length/weight ratios of males vs. females varied with males' growth initially concentrated towards length, while females' growth was concentrated toward weight. These growth curves reverse after the third-year class. Male O. mordax exhibit a higher coefficient of condition.     

The spatial scale of competition from recruits on an older cohort in Atlantic salmon

Competitive effects of younger cohorts on older ones are frequently assumed to be negligible in species where older, larger individuals dominate in pairwise behavioural interactions. Here, we provide field estimates of such competition by recruits on an older age class in Atlantic salmon (Salmo salar), a species where observational studies have documented strong body size advantages which should favour older individuals in direct interactions. By creating realistic levels of spatial variation in the density of underyearling (YOY) recruits over a 1-km stretch of a stream, and obtaining accurate measurements of individual growth rates of overyearlings (parr) from capture–mark–recapture data on a fine spatial scale, we demonstrate that high YOY density can substantially decrease parr growth. Models integrating multiple spatial scales indicated that parr were influenced by YOY density within 16 m. The preferred model suggested parr daily mass increase to be reduced by 39% when increasing YOY density from 0.0 to 1.0 m⁻², which is well within the range of naturally occurring densities. Reduced juvenile growth rates will in general be expected to reduce juvenile survival (via increased length of exposure to freshwater mortality) and increase generation times (via increased age at seaward migrations). Thus, increased recruitment can significantly affect the performance of older cohorts, with important implications for population dynamics. Our results highlight that, even for the wide range of organisms that rely on defendable resources, the direction of competition among age classes cannot be assumed a priori or be inferred from behavioural observations alone.     

Growth of larval and juvenile perch: the importance of diet and fish density

In August, growth rate of young–of–the–year (YOY) Perch In lake enclosures could be explained by both YOY density and mean cladoceran biomass, suggesting that in a lake where YOY perch are dominant, growth may be density dependent in late summer and mediated through top–down control on daphnid biomass. In June, growth rate of YOY perch could not be fully explained by YOY density or by mean cladoceran biomass, suggesting that growth and survival during the first part of the summer is negatively affected by a diet of Bosmina and cyclopoid copepods only. The experiments also suggest why YOY perch have a slow growth and a low abundance in eutrophic lakes where small zooplankton dominate. The June experiment also indicated that growth of late larval or early juvenile perch improved when a larger cladoceran became available and was included in the diet.     

Food webs: experts consuming families of experts

Food webs of habitats as diverse as lakes or desert valleys are known to exhibit common "food-web patterns", but the detailed mechanisms generating these structures have remained unclear. By employing a stochastic, dynamical model, we show that many aspects of the structure of predatory food webs can be understood as the traces of an evolutionary history where newly evolving species avoid direct competition with their relatives. The tendency to avoid sharing natural enemies (apparent competition) with related species is considerably weaker. Thus, "experts consuming families of experts" can be identified as the main underlying food-web pattern. We report the results of a systematic, quantitative model validation showing that the model is surprisingly accurate.     

Structural and functional connectivity of marine fishes within a semi-enclosed Newfoundland fjord

The interplay between structural connectivity ( i.e. habitat continuity) and functional connectivity ( i.e. dispersal probability) in marine fishes was examined in a coastal fjord (Holyrood Pond, Newfoundland, Canada) that is completely isolated from the North Atlantic Ocean for most of the year. Genetic differentiation was described in three species (rainbow smelt Osmerus mordax , white hake Urophycis tenuis and Atlantic cod Gadus morhua ) with contrasting life histories using seven to 10 microsatellite loci and a protein-coding locus, Pan I ( G. morhua ). Analysis of microsatellite differentiation indicated clear genetic differences between the fjord and coastal regions; however, the magnitude of difference was no more elevated than adjacent bays and was not enhanced by the fjord's isolation. Osmerus mordax was characterized by the highest structure overall with moderate differentiation between the fjord and St Mary's Bay ( F _ST c. 0·047). In contrast, U. tenuis and G. morhua displayed weak differentiation ( F _ST < 0·01). Nonetheless, these populations did demonstrate high rates (< 75%) of Bayesian self-assignment. Furthermore, elevated differentiation was observed at the Pan I locus in G. morhua between the fjord and other coastal locations. Interestingly, locus-specific genetic differentiation and expected heterozygosity were negatively associated in O. mordax , in contrast to the positive associations observed in U. tenuis and G. morhua . Gene flow in these species is apparently unencumbered by limited structural connectivity, yet the observed differentiation suggests that population structuring exists over small scales despite high dispersal potential.     

Cost of predation avoidance in young-of-year lake trout (Salvelinus namaycush): growth differential in sub-optimal habitats

Selection of habitat to avoid predation may affect the diet of young-of-year (YOY) lake trout (Salvelinus namaycush). YOY lake trout may use inshore habitat to avoid predation; this habitat may be sub-optimal for growth. To test this, YOY lake trout were penned in nearshore and offshore pelagic areas of two arctic lakes. Toolik Lake had a lake trout population, the other lake, S6, did not. YOY lake trout in Toolik Lake lost weight, but those offshore lost less weight. The YOY lake trout in Lake S6 gained weight and those offshore gained more weight. The primary diet item of the YOY lake trout in both lakes during this experiment was the zooplankter Diaptomis probilofensis; it was also one of the most abundant species. However, its density inshore in Lake S6 was similar to inshore and offshore densities in Toolik Lake. The increased availability of alternative zooplankton prey in Lake S6 may account for the growth differential of YOY lake trout in Lake S6 relative to Toolik Lake. Bioenergetic modeling of YOY lake trout suggests that growth similar to that in the offshore of Lake S6 would be necessary for successful recruitment. If the reduced zooplankton availability in Toolik Lake leads to the reduced growth of YOY in the inshore and offshore pelagic areas, then these fish will be more susceptable to winter predation/starvation. For YOY lake trout to survive in Toolik Lake they most likely shift to feeding on benthic prey before the end of their first summer. Dept. of Chemical Engineering     

Relationship between fertilization success and the number of milt donors in rainbow smelt Osmerus mordax (Mitchell): implications for population growth rates

A series of artificial fertilizations using rainbow smelt Osmerus mordax were conducted, each designed to assess the fertilization success of a female's eggs with milt from one to five males. Mean egg fertilization success increased and variance in fertilization success decreased when more males contributed to a constant volume of milt. This may indicate the presence of an Allee effect. The results suggest that potential changes in egg fertilization success due to a decrease in the number of males, in addition to changes in egg production due to a decrease in the number of females, can affect estimates of maximum sustainable harvest rates, and recovery of depleted populations.     

Occurrence of Proteocephalus tetrastomus (Rudolphi, 1810) (Cestoda: Proteocephalidea) in larval rainbow smelt (Osmerus mordax) in North America: identification of a potential pathogen confirmed

A morphological evaluation and genetic analysis (sequencing of ITS2 region of rDNA) of proteocephalidean cestodes from rainbow smelt (Osmerus mordax) in the Saint Lawrence Estuary, Canada, has shown their conspecificity with Proteocephalus tetrastomus, a specific parasite of smelt (Osmeridae), previously known only from northern Europe, Russia, and Japan. The parasite occurs only in larval, but not adult, smelt in the Saint Lawrence Estuary. Prevalence of larval smelt infection was 42% (n = 50), mean intensity 2.1 +/- 2.4 and mean abundance 1.1 +/- 1.0.     

Mycobiota of the Saffron Cod Eleginus gracilis (Tilesius, 1810) and the Asiatic Smelt Osmerus mordaxdentex Steindachner & Kner, 1870 in the Peter the Great Bay of the Sea of Japan

Russian Journal of Marine Biology - A mycological examination of the saffron cod Eleginus gracilis (Tilesius, 1810) and the Asiatic smelt Osmerus mordax dentex Steindachner &amp; Kner, 1870,...     

Predicting the outcome of trophic manipulation in lakes—a comment on Harris (1994)

1. Harris ( Freshwater Biology , 32 , 143–160, 1994) asserts that while empirical modelling of lake ecosystem properties has yielded general predictions and useful explanations, research into the effects of food-web interactions has not. I provide an explicit example of a useful, quantitative prediction gleaned from experiments on food-web effects—the relationship between total phosphorus and the magnitude of response of algal biomass.
2. I further argue that Harris errs in asserting that food-web effects will be ineffective in eutrophic lakes. This error stems from a misconception about the relevance of ecosystem behaviour in unmanipulated systems for predictions of how an ecosystem will respond to manipulation. I cite empirical evidence demonstrating that, contrary to Harris's contention, the response of algal biomass to a change in food-web structure increases as lakes are enriched.     

Evaluating stocking of YOY pike Esox lucius as a tool in the restoration of shallow lakes

1. Stocking of piscivores in shallow, eutrophicated lakes to reduce cyprinid densities is a common approach in lake restorations. Young‐of‐the‐year (YOY) pike Esox lucius are frequently used to reduce cyprinid densities, but their effectiveness is equivocal. This study uses a simple model to assess the efficacy of pike stocking as a lake restoration tool. 2. The model evaluates YOY pike consumption in relation to different survival and growth patterns, timing of stocking, degree of piscivory of pike as well as YOY cyprinid growth patterns and productivity. The parameters used in the model are obtained from field surveys in eight study lakes stocked with YOY pike as well as from the literature. 3. Our model showed that all parameters studied were important for predicting the effects of pike stocking on cyprinids. In particular, body size at stocking, cyprinid production and pike survival were good predictors of biomanipulation success. 4. However, our model showed that only under very specific circumstances will stocking of YOY pike result in a reduction of YOY cyprinid densities large enough to affect water quality. 5. The findings of this study question the efficacy of pike stocking as an appropriate and reliable tool for restoration programmes in shallow lakes, and indicate that when used efforts should be made to optimize the timing of stocking in relation to YOY cyprinid production and to increase the stocking body size of the pike.     

Temporal genetic differentiation: continuous v. discontinuous spawning runs in anadromous rainbow smelt Osmerus mordax (Mitchill)

Genetic differentiation was greater between 'early' and 'late' spawning rainbow smelt Osmerus mordax separated by a hiatus in spawning activity compared to a continuously spawning population over a similar time period. This magnitude of temporal differentiation was comparable to that seen among spatially separated populations.     

Predicting the outcome of trophic manipulation in lakes—a comment on Harris (1994)

1. Harris ( Freshwater Biology , 32 , 143–160, 1994) asserts that while empirical modelling of lake ecosystem properties has yielded general predictions and useful explanations, research into the effects of food-web interactions has not. I provide an explicit example of a useful, quantitative prediction gleaned from experiments on food-web effects—the relationship between total phosphorus and the magnitude of response of algal biomass.
2. I further argue that Harris errs in asserting that food-web effects will be ineffective in eutrophic lakes. This error stems from a misconception about the relevance of ecosystem behaviour in unmanipulated systems for predictions of how an ecosystem will respond to manipulation. I cite empirical evidence demonstrating that, contrary to Harris's contention, the response of algal biomass to a change in food-web structure increases as lakes are enriched.     

Comparative Analysis of Mitochondrial DNA Diversity in Smelts

The polymerase chain reaction (PCR) was used to amplify a segment of the mitochondrial DNA coding for NADH-dehydrogenase subunits ND5/ND6 in five smelt species (family Osmeridae). Amplified DNA was screened for restriction fragment length polymorphism (RFLP). Nucleotide sequence divergence of mitochondrial DNA between species ranges from 11.9 (between Hypomesus nipponensis and H. japonicus) to 24.7% (between Osmerus mordax dentex and Mallotus villosus catervarius). The genetic divergence between populations of H. nipponensis, H. japonicus, and Osmerus mordax dentex was 0.32, 0.08 to 0.15, and 0.025%, respectively. The absence of common haplotypes enables differentiation of closely related smelt species and, therefore, can be used for solving current problems in the taxonomy and biogeography of this family.     

Abiotic factors,competition or predation: what determines the distribution of young crayfish in a watershed?

In the headwaters of the New River in western North Carolina, the dominant crayfish species changes with increasing stream size. One transition occurs between third- and fourth-order streams. Cambarus chasmodactylus is the dominant species in third-order tributaries. Orconectes cristavarius is the dominant species in the fourth-order South Fork of the New River. While adult C. chasmodactylus are present in the South Fork, the young-of-the-year (YOY) of this species are absent despite evidence of reproduction. In this study we evaluated the factors that may be responsible for the absence of C. chasmodactylus YOY from the South Fork. A field experiment was used to evaluate the role of abiotic factors and competition with YOY O. cristavarius. There was no significant effect of either of these factors on mortality or growth of C. chasmodactylus YOY. The growth rate of O. cristavarius was 3× faster than that of C. chasmodactylus in this experiment. Since neither abiotic factors nor competition appeared responsible for the exclusion of C. chasmodactylus YOY from the South Fork we evaluated the potential importance of selective predation by rock bass (Ambloplites rupestris), a fish species that is common in the South Fork but virtually absent in the tributaries. In a laboratory experiment, C. chasmodactylus YOY experienced significantly higher mortality than O. cristavarius YOY in the presence of rock bass. Field observations and a laboratory experiment suggest that the two crayfish species differ in their anti-predator behaviors. Cambarus chasmodactylus was less likely to swim when initially disturbed and swam shorter distances than O. cristavarius. The differences in escape behavior and growth rate may contribute to the differences in the vulnerability of the two species to rock bass predation. Our results suggest that intense predation pressure exerted by the rock bass may contribute to the virtual exclusion of C. chasmodactylus YOY from the fourth-order South Fork. Handling editor: J. Trexler     

Size-dependent resource limitation and foraging-predation risk trade-offs: growth and habitat use in young arctic char

Variation in growth and habitat use is closely connected to individual responses to habitat specific resource levels and predation risk. In three mountain lakes which differed in the density of young-of-the-year (YOY) arctic char ( Salvelinus alpinus ), we studied the growth, diets and habitat use of YOY char in relation to resource levels. With two model approaches, we further examined the extent to which YOY and 1-yr old char were resource limited and, if so, whether resource limitation was associated with habitat use and small char densities. Benthic prey dominated the diet of YOY char and YOY char habitat use was restricted to the near-shore habitat in all lakes. In all lakes were chironomid densities higher in the near-shore habitat than in the offshore benthic habitat whereas zooplankton densities were higher in the pelagic than in the near-shore habitat. Growth of YOY char in all lakes was close to predicted maximum growth despite large variation in YOY densities between lakes. Model results suggested that density dependent resource limitation in YOY char is unlikely to occur despite restricted near-shore habitat use. In contrast, strong density dependent resource limitation was predicted in 1-yr old char with a restricted habitat use to the near-shore habitat. Correspondingly, field data suggested that the habitat use of 1-yr old char was density dependent as the use of the offshore habitat increased earlier in time and to a larger extent at high densities. As small individuals are vulnerable to predation but constrained by their low food processing capacity relative to their encounter capacity, we suggest that resource limitation in small individuals should be less pronounced and habitat use should mainly depend on predation risk. A trade-off in habitat use between foraging gain and predation risk is therefore expected to be more likely for individuals, large enough to be resource limited but still small enough to be vulnerable to predation.