bluuebunny/crossref_metadata_2025 huggingface dataset:
Laboratory Study of Foraging by Atlantic Mackerel, Scomber scombrus, on Natural Zooplankton Assemblages
Authors: Pepin P. , Koslow J. A. , Pearre Jr. S.
Published: July 2008

This study reports on the behavioural response of captive Atlantic mackerel, Scomber scombrus, to changes in composition and abundance of natural zooplankton assemblages. Filter-feeding activity and swimming speed increase with increasing prey concentration. Swimming speed also increases with increasing prey size. Prey preference shows a size selective pattern that is slightly influenced by the size of the largest available prey, but not by variations in total prey abundance or density of individual size categories. Feeding rates under average natural conditions of prey abundance (0.1 g wet weight∙m−3) indicate that mackerel would be unable to satiate during a foraging period restricted to daylight hours. Although prey aggregation or night feeding may enable mackerel to attain their maximum ration in the wild, a review of the literature suggests that pelagic fish seldom satiate. The dynamic response of mackerel to changes in prey abundance can have important consequences to the impact that mackerel and other planktivorous fish may have on the dynamics of multispecies zooplankton and ichthyoplankton communities.
Alternative Harvest Strategies for Pacific Herring (Clupea harengus pallasi)
Authors: Hall D. L. , Hilborn R. , Stocker M. , Walters C. J.
Published: July 2008

A simulated Pacific herring (Clupea harengus pallasi) population is used to evaluate alternative management strategies of constant escapement versus constant harvest rate for a roe herring fishery. The biological parameters of the model are derived from data on the Strait of Georgia herring stock. The management strategies are evaluated using three criteria: average catch, catch variance, and risk. The constant escapement strategy provides highest average catches, but at the expense of increased catch variance. The harvest rate strategy is favored for its reduced variance in catch and only a slight decrease in mean catch relative to the fixed escapement strategy. The analysis is extended to include the effects of persistent recruitment patterns. Stock–recruitment analysis suggests that recruitment deviations are autocorrelated. Correlated deviations may cause bias in regression estimates of stock–recruitment parameters (overestimation of stock productivity) and increase in variation of spawning stock biomass. The latter effect favors the constant escapement strategy, which fully uses persistent positive recruitment fluctuations. Mean catch is depressed for the harvest rate strategy, since the spawning biomass is less often located in the productive region of the stock–recruitment relationship. The model is used to evaluate the current management strategy for Strait of Georgia herring. The strategy of maintaining a minimum spawning biomass reserve combines the safety of the constant escapement strategy and the catch variance reducing features of the harvest rate strategy.
Juvenile Atlantic Cod (Gadus morhua) Can Be More Freeze Resistant than Adults
Authors: Kao Ming H. , Fletcher Garth L.
Published: July 2008

Adult Atlantic cod (Gadus morhua) produce antifreeze glycopeptides during the winter which help to depress the freezing point of their blood plasma to approximately −1.2 °C. Since the temperature at which fish can freeze and die approximates the freezing point of their blood plasma, it is evident that adult cod do not have the capacity to survive in ice-laden waters when the water temperatures drop to −1.5 °C. Templeman and Fleming hypothesized that small cod are more adapted to cold water than adults. The present study documents the fact that the blood plasma freezing points of juvenile cod (mean length = 33 cm, 3-yr-olds) were significantly lower than those of adult cod (> 45 cm) (juvenile, −1.55 °C; adult, −1.23 °C). This difference was largely attributable to the doubling of plasma antifreeze glycopeptide levels in the juvenile cod. These results directly support Templeman and Fleming's hypothesis and indicate that juvenile cod, unlike adults, are capable of surviving the icy marine conditions which prevail along the northeast coast of Newfoundland during the winter.
Comparison of in situ Estimates of ChlorophyllaObtained with Whatman GF/F and GF/C Glass-Fiber Filters in Mesotrophic to Hypereutrophic Lakes
Authors: Prepas E. E. , Dunnigan M. E. , Trimbee A. M.
Published: July 2008

In response to the recent attention given to picoplankton, many researchers have switched from GF/C to GF/F glass-fiber filters for chlorophyll a (Chl a) analyses; GF/F filters have a smaller pore size and are more expensive than GF/C filters. In this study, Chl a was measured with both Whatman GF/F and GF/C glass-fiber filters on euphotic zone water from 28 mesotrophic to hypereutrophic lakes (Chl a ranged from 2 to 175 μg∙L−1) in central Alberta. The differences between paired Chl a estimates with the two grades of filter paper were remarkably small; there was no evidence that GF/F filters collected more Chl a than GF/C filters (P > 0.2). Hence there is no justification for switching from GF/C to GF/F filters for routine Chl a analyses on productive lakes.
Leaky Filters: A Warning to Aquatic Ecologists
Authors: Stockner John G. , Klut M. Emilia , Cochlan William P.
Published: July 2008

We examined the retention characteristics of commonly used 0.2-μm nucleation-track (Nuclepore®, Poretics®), polymer (Millipore®, Sartorius®) and inorganic membrane (Anopore®) filters. Scanning electron micrographs of the filter surface showed many to contain large holes or pores, some 5 × larger than the manufacturer's stated nominal pore diameter. Electron micrographs of filtrates from both fresh and seawater samples contained a variety of organisms, including viruses, ultramicrobacteria, bacteria, phototrophic picoplankton, and larger nano- and microphytoplankton (e.g. diatoms), which we believe passed through the large openings. Total particles in the size range 1.0–12.8 μm passing through the various 0.2-μm filters tested ranged from 2.2 to 14.3%, with 'best' retention (>97%) by the Anopore®. Average particle retention for all 0.2-μm filters tested was 92.5%. Higher passage rates (lower retention) are predicted for even smaller (<1.0 μm) particles (prochlorophytes, bacteria, viruses) and investigators are urged to use care when using polycarbonate or matrix type filters for partitioning size fractions for chemical, physiological, and ecological work, and caution when interpreting results, especially if separations have not been verified by microscopy, incubation, or culture.
A Unified Approach to the Analysis of Fish Growth, Maturity, and Survivorship Data
Authors: Schnute Jon T. , Richards Laura J.
Published: July 2008

This paper presents an approach to the analysis of growth, maturity, and survivorship data that unifies an extensive biometrics literature, particularly in fisheries and forestry. The model involves a probabilistic response y[Formula: see text] to a positive factor x[Formula: see text]. Because the relationship is asymptotic, the model can also be interpreted as a descriptor of biological growth, in which the asymptotic size is scaled to 1. The model generalizes earlier work by Gompertz, von Bertalanffy, Weibull, Richards, Chapman, and Schnute; it also extends the logit method introduced by Berkson. We identify reasonable curve families for the model, in which a transform parameter associated with x or y determines the curve shape, and we provide a complete analysis of curve types. We describe likelihood functions associated with two classes of data, typified by (1) size-at-maturity data and (2) time–mortality data. We illustrate the model's application to both data classes and show that it provides a useful extension to past methods of analysis.
Diet Selection and the Contribution of Detritus to the Diet of the Juvenile White Sucker (Catostomus commersoni)
Authors: Ahlgren Molly O.
Published: July 2008

The ash-free dry mass (AFDM) of detritus, invertebrates, and algae in the diet of juvenile white sucker (Catostomus commersoni) was determined by quantitative microscopy. Fish were collected from a northern Michigan pond from January through October 1986 and their seasonal diet was compared with benthc invertebrate abundance. The quantity of detritus in sucker foreguts was inversely related to benthic microcrustacean densities. In July, microcrustacean densities were high and they comprised 95% of the AFDM in foregut contents. By October, microcrustacean densities had declined to 13% of their maximum density and detritus comprised over 90% of the sucker's diet AFDM. In laboratory aquaria, sucker that were fed detritus mixed with four different densities of Artemia ingested significantly more detritus from diets that provided lower Artemia densities. In the presence of high Artemia densities, sucker completely rejected detritus and ingested only Artemia, The fact that juvenile sucker can separate detritus from invertebrates that they swallow demonstrates that detritus is not ingested incidentally. Both laboratory and field data support the hypothesis that detritus is ingested intentionally when preferred invertebrate prey are scarce.
Nutritional Significance of Facultative Detritivory to the Juvenile White Sucker (Catostomus commetsoni)
Authors: Ahlgren Molly O.
Published: July 2008

The nutritional significance of detritus in the diet of the juvenile white sucker (Catostomus commersoni) was evaluated by experiments designed to assess assimilation efficiency and growth of sucker fed detritus and other foods. Sucker in laboratory aquaria assimilated detritus with the following efficiencies: dry mass = 19.6%, ash-free dry mass (AFDM) = 59.7%, total amino acids = 68.9%, energy = 69.4%. Assimilation efficiencies calculated for invertebrates and natural field diets were slightly higher. Diatom assimilation was low (5.5%). In growth experiments, sucker fed detritus ad libitum lost weight, while those fed Artemia ad libitum grew rapidly. However, detritus increased the growth rate of sucker fed a limited invertebrate ration and reduced the rate of weight loss relative to unfed fish. The ratio of digestible protein to digestible energy calculated for natural detritus diets (3.2 mg amino acid/kj) indicates that the protein content of detritus is too low to support growth. Detritus provides energy which complements limited invertebrate protein to enhance growth or reduce the rate of weight loss when invertebrate prey are not available.
Spatial and Temporal Distribution of Sub-Ice Macrofauna in the Barrow Strait Area, Northwest Territories
Authors: Pike Daniel G. , Welch Harold E.
Published: July 2008

The distribution of sub-ice macrofauna was investigated in the Barrow Strait area near Resolute, N.W.T. from April–June, 1985, 1986, and 1987, and in January 1986. Invertebrates were sampled using remotely deployed net and video camera systems. Ten species of amphipods and two species of mysids were found, but over 90% of the biomass was made up of only four species of amphipods: Weyprechtia pinguis, Onisimus litoralis, Gammarus setosus, and Onisimus spp. juveniles. Species composition varied annually, with W. pinguis dominating in spring 1985, O. litoralis in 1986, and G. setosus in 1987. There were also areal differences in species composition over distances of 50–200 km. Underlying water depth was the major environmental correlate of both total biomass and species composition; total biomass decreased with increasing underlying water depth, being near zero over >100 m depth. We hypothesize that spring sub-ice distribution is a reflection of summertime benthic distribution. Total biomass decreased with increasing snow depth early in the spring, probably because the attenuation of incident light by snow reduces the concentration of sub-ice algae. Total biomass was lower within 1 km of the ice edge than in areas further from the ice edge.
Wood Dynamics in Coastal Plain Blackwater Streams
Authors: Benke Arthur C. , Wallace J. Bruce
Published: July 2008

We quantified woody debris in the river swamps of the sixth order Ogeechee River and several smaller tributaries in the Coastal Plain of the southeastern USA, compared swamp wood with woody debris in the channel, and studied wood movement in the swamp and main channel over 20 mo. Woody debris in the Ogeechee River swamps was relatively low (0.362–0.880 kg ash-free dry mass (AFDM)/m2) in comparison to several mixed temperate deciduous forests. Similarly, wood in the tributary swamps was low (mean = 0.82 kg AFDM/m2), and there were no trends along the river continuum. Wood in the channels of both the Ogeechee (6.46 kg AFDM/m2) and a fourth order tributary (2.24 kg AFDM/m2) were significantly higher than found in their adjacent floodplains. Woody debris appeared to increase in stream channels from smaller tributaries to the sixth order river, opposite of that observed in other river systems. Tagging of logs showed that only 17% of wood in the Ogeechee channel had moved after 3 major floods, much less than in the swamps (21–84%). The abundance and stability of woody debris in the main channel allows it to be a major habitat type and source of food for both riverine invertebrates and fishes. The fate of most swamp wood appears to be decomposition and fragmentation, rather than import to the river channel.
Recycling of Elements Transported Upstream by Runs of Pacific Salmon: I, δ15N and δ13C Evidence in Sashin Creek, Southeastern Alaska
Authors: Kline Jr. Thomas C. , Goering John J. , Mathisen Ole A. , Poe Patrick H. , Parker Patrick L.
Published: July 2008

Values of δ15N and δ13C (the per mil deviation from the recognized isotope standard) from biota of a southeastern Alaska stream (Sashin Creek) that receives an annual run of 30 000 anadromous pink salmon (Oncorhynchus gorbuscha) were measured to determine sources of nitrogen (N) and carbon (C). Marine-derived nitrogen (MDN) is the predominant source of N for food webs found in the lower 1200 m of the stream which, due to a waterfall, is the only portion of the stream available to salmon returning to spawn. Comparable spawning section biota were enriched by about 5 per mil relative to the salmon-free control section, corresponding to the difference between 0 and 100% MDN in a mixing model. Food webs of resident rainbow trout (O. mykiss), at the outlet of one of the source lakes, Sashin Lake, have very low δ13C, suggesting the importance of a respired C pool in the lake. The source of C in the remainder of the stream is C fixation by autochthonous producers and marine organic C (within the salmon spawning section). Resident fishes in the salmon spawning section depend on MDN and some of the C delivered by the annual run of returning salmon.
Effects of Fish Removal on the Limnetic Ecosystem of a Eutrophic Lake
Authors: Reinertsen Helge , Jensen Arne , Koksvik Jan Ivar , Langeland Arnfinn , Olsen Yngvar
Published: July 2008

The effects of fish elimination in the eutrophic Lake Haugatjern, Sør-Trøndelag County, Norway, were studied during 1979–84. Total elimination of the fish populations by rotenone in late 1980 resulted in a fourfold decrease in the algal biomass. The species composition changed from a dominance by the large-sized Anabaena flos-aquae and Staurastrum luetkemuelleri to smaller, fast-growing species and gelatinous green algae. The total zooplankton biomass remained at the same level in all years, but while the rotifers almost disappeared after the rotenone treatment, the daphnids increased their share of the biomass from 49–63% during 1979–80 to 74–90% during 1982–84. The mean individual size of the adult daphnids increased in the same period from 1.3 to 1.8 mm. A 30% drop in the total phosphorus concentration in the lake after the biomanipulation was explained by increased sedimentation of zooplankton and reduced phosphorus release from the epilimnetic sediments because of the lowered pH. The fish elimination also resulted in a lower yield of biomass per unit of phosphorus in the lake.
Nighttime Pond Respiration Rate: Oxygen or Temperature Dependent?
Authors: Madenjian Charles P.
Published: July 2008

Two different models for the nighttime dissolved oxygen concentration (DO) dynamics in ponds were compared to determine which was more accurate. DO and water temperature were monitored overnight in marine shrimp (Penaeus vannamei and P. monodon) ponds; wind speed was also measured. The Whole Pond Respiration–Diffusion (WPRD) model, in which night respiration rate of the pond system is a function of water temperature, was fitted to observations of DO at dusk and approximately 3 h after dusk to generate predictions of pond DO at dawn The Oláh model, which is based on respiration rate varying in direct proportion to DO, was also applied to the same set of DO observations to generate predictions of dawn DO. The mean absolute values of the residuals (the residual is equal to observed dawn DO minus predicted dawn DO) from applications of the WPRD and Oláh models were 0.39 and 0.77 mg O2∙L−1, respectively, and the Oláh model showed consistent bias. The respiration rate of the pond system at night was therefore better described as a function of water temperature than as dependent on pond DO.
Diel Feeding Chronology and Diet Selection of Rainbow Trout (Oncorhynchus mykiss) in the Henry's Fork of the Snake River, Idaho
Authors: Angradi T. R. , Griffith J. S.
Published: July 2008

Concurrent rainbow trout (Oncorhynchus mykiss) stomach contents and invertebrate drift samples were collected during three 24-h periods in summer 1987. Feeding was discontinuous through the day on all dates. Mean stomach content weight was minimal after 0400 MST and sharply increased between 1000 and 1200 MST on all three dates. Feeding apparently did not occur after twilight. Mean stomach content weight was correlated with water temperature on two dates and was never correlated with invertebrate drift density for non-age-0 trout. Daily ration (wet weight) was 7% of live weight for non-age-0 trout and 21% for age-0 trout. Trichoptera, Ephemeroptera, and Diptera were most important in the diet; terrestrial insects and aquatic vertebrates were rare. The degree of selectivity varied through 24 h and the interpretation depended on the method of analysis used. Occurrence of low-drift cased Trichoptera larvae in stomachs was correlated with amount of filamentous algae ingested, indicating a degree of epibenthic foraging, although no diel pattern could be reliably discerned. Mean length of prey items in stomachs was larger that the drift in 83% of the samples. Our findings support experimentally derived decision rules-of-thumb for foraging trout: select larger prey items, select vulnerable prey, and relax selectivity when hungry.
Electrophoretic Assessment of the Contribution of Transplanted Scottish Atlantic Salmon (Salmo salar) to the Esva River (Northern Spain)
Authors: Morán Paloma , Pendás Alberto M. , Garcia-Vázquez Eva , Izquierdo Jorge T. , Rutherford Dennis T.
Published: July 2008

The genetic influence of stocking on the Atlantic salmon (Salmo salar) population from the Spanish Esva River was analysed by electrophoresis of protein loci. Genetic variation within parr and mature male parr samples demonstrated that Scottish-origin parr stocked in 1990 survived the first summer. However, samples collected from returning adults in 1990 and 1991 did not indicate any contribution from previous stocking programs. Altered MEP-2* frequencies indicate that the native gene pool may have been disrupted by foreign genomes from precocious males or grilse.
Growth Models with Corrections for the Retardative Effects of Tagging
Authors: Xiao Yongshun
Published: July 2008

Length increment data from mark–recapture experiments are commonly used to obtain information on animal growth, assuming that tagging does not affect the growth of marked animals. The assumption is violated in many studies, but the effects of tagging on growth and estimates of growth parameters have not been and cannot be examined without appropriate models. This paper describes a model allowing quantification and estimation of the retarding effects of tagging on animal growth simultaneously with growth parameters in all existing growth models, reduction or elimination of biases in growth parameters induced by tagging, and relaxation of a key assumption in growth analysis using length increment data. A special case of this model was applied to simulated data and to tagging data from a centropomid perch (Lates calcarifer) to demonstrate its general utility. Tagging was inferred to have stopped the fish growth for 36.44 d (ASE = 12.70 d) if von Bertalanffy growth is assumed, but the period of recovery from tagging seemed size or age independent within the size range studied. If tagging retards animal growth, L∞ is slightly overestimated and K underestimated for unbiased data. Potential applications and limitations of the model are also discussed.
Changes in the Diet of Demersal Fish due to Eutrophication-Induced Hypoxia in the Kattegat, Sweden
Authors: Pihl Leif
Published: July 2008

The diets of five dominant bottom-feeding fish species, Atlantic cod (Gadus morhua), whiting (Merlangius merlangus), plaice (Pleuronectes platessa), dab (Limanda limanda), and American plaice (Hippoglossoides platessoides), were analysed during spring and autumn between May 1984 and April 1988 in the southeast Kattegat. A general long-term change in the diet of bottom-feeding fish in the Kattegat has taken place since the beginning of the century and, for some of the species, the feeding pattern was different from that in adjacent areas. The observed dietary shift appeared to be due to changes in species composition of benthic macrofauna. Prey species favored by increased organic enrichment were dominant in the diet. Several infaunal species increased in dietary importance during recent hypoxia. In contrast, during hypoxia, a reduction of epibenthic crustaceans was observed in the fish diet. This study emphasizes the potential effects of organic enrichment and hypoxia (due to eutrophication) on trophic interactions in marine benthic communities. Repeated stress from hypoxia might favor small-sized prey species with a short life cycle, which would in turn favor small-sized fishes. Thus, altered food resources and the direct effects of hypoxia might result in shift in dominance among demersal fish species.
Fourier Analysis of Sea Scallop (Placopecten magellanicus) Shells in Determining Population Structure
Authors: Kenchington E. L. , Full W. E.
Published: July 2008

Sea scallops (Placopecten magellanicus) were sampled from each of four scallop beds: the northeast peak of Georges Bank, Western Bank, St. Pierre Bank, and the Bay of Fundy near Digby, N.S. The outlines of scallop top shells were captured by video imagery using a semiautomated approach. Fourier shape descriptors were calculated and analyzed. Age and year-class effects were shown to be important factors influencing scallop shell shape. Allometric effects on Fourier harmonics also have a significant influence on shell shape in this species. Comparisons of Fourier harmonics between scallop beds separated by age and sampling year, and corrected for allometry, revealed significant differences in the majority of harmonic numbers. Post hoc analyses identified St. Pierre Bank and the Bay of Fundy as being significantly different from the other beds in a suite of separate analyses.
Acute and Chronic Toxicity of Waterborne Arsenite to Rainbow Trout (Oncorhynchus mykiss)
Authors: Rankin M. G. , Dixon D. G.
Published: July 2008

The 144-h LC50 (95% fiducial limits) of arsenite for 4.5-g rainbow trout (Oncorhynchus mykiss) was 18.5 (17.9–19.1) mg∙L−1. In a pair-fed growth study which exposed trout to 0.0, 0.76, 2.48, or 9.64 mg arsenite∙L−1 over 17 wk, growth was significantly reduced (by 55%) only at 9.64 mg∙L−1. The reduction was attributable to both reduced appetite (primarily) and direct metabolic impact (marginally). Fish at 9.64 mg∙L−1 suffered 10% mortality, usually associated with necrotic erosion of the mandibular and olfactory regions of the head. All fish exposed to 9.64 mg∙L−1 showed inflammation of the gallbladder wall, a lesion absent at lower exposure concentrations. There were no arsenite impacts on hepato- and splenosomatic index, hematocrit, hemoglobin, total plasma lipid, cholesterol, and protein or brain concentrations of norepinephrine, dopamine, and serotonin. Exposure to 0.0, 0.76, 2.48, and 9.64 mg arsenite∙L−1 for 26 wk resulted in mean (SE) equilibrium whole-body As concentrations of 0.3 (0.02), 0.2 (0.02), 0.4 (0.10), and 1.7 (0.40) μg As∙g wet weight−1, respectively, No depuration below these concentrations occurred during a 12-d period in arsenite-free water. The threshold of chronic toxicity was estimated to be 4.9 mg∙L−1.
Pelagic Food Chain Structure in Ontario Lakes: A Determinant of Mercury Levels in Lake Trout (Salvelinus namaycush)
Authors: Cabana Gilbert , Tremblay Alain , Kalff Jacob , Rasmussen Joseph B.
Published: July 2008

The trophic structure of pelagic communities in glaciated regions is highly variable due to restricted dispersal of glacial relict taxa and recent species introduction. Much of the between-lake variation in Hg in lake trout (Salvelinus namaycush) flesh from the St. Lawrence system (non-point-source contaminated lakes), which spans more than two orders of magnitude (0.03–3.96 μg/g), results from differences in the length of pelagic food chains. Hg levels from the longest food chains where pelagic forage fish and the crustacean Mysis relicta were present were about 3.6-fold higher than those from the shortest food chains where these last two trophic components were missing. A lack of correlation between Hg levels in small-mouth bass (Micropterus dolomieu), which do not exploit the pelagic food chain, and the length of the pelagic food chain confirmed that the positive relationship observed in lake trout could not be attributed to the confounding effect of some physicochemical factors covarying with our pelagic food chain classification. Our results confirmed that the food chain biomagnification model developed by Rasmussen et al. (1990. Can. J. Fish. Aquat. Sci. 47: 2030–2038) to predict PCB levels in lake trout can also be applied to Hg biomagnification.
Phosphorus–Chlorophyll Relationships under Contrasting Herbivory and Thermal Stratification: Predictions and Patterns
Authors: Mazumder Asit
Published: July 2008

Analyses of the relationships between total phosphorus (TP) and chlorophyll a (Chl) among a large number of temperate lake ecosystems having contrasting herbivore communities and thermal stratification reveal that systems lacking large Daphnia (SH systems) exhibit four times more Chl yields than systems having large Daphnia (LH systems) in oligotrophic as well as eutrophic and hypereutrophic systems. Mixed (MIX) systems exhibit greater Chl yields than stratified (STR) systems. Within each group of stratified and mixed systems, greater Chl yields to constant TP are observed in systems lacking large Daphnia (MIX-SH and STR-SH) than those having large Daphnia (MIX-LH and STR-LH). Consequently, a hierarchy of Chl yield patterns is produced; STR-LH and MIX-SH systems exhibit the lowest and highest Chl yields, respectively, at constant TP. These patterns may be a reflection of variable chemical (nutrients), biological (herbivory), and physical (thermal stratification) characteristics among aquatic ecosystems. Sigmoid patterns of TP–Chl relationships appear to be related to the transition from stratified to mixed systems along the TP gradient. The finding that highly variable Chl yields to TP can be summarized into a set of TP–Chl trajectories for specific types of lake ecosystems may have strong implications for lake management.
Phosphorus–Chlorophyll Relationships under Contrasting Zooplankton Community Structure: Potential Mechanisms
Authors: Mazumder Asit
Published: July 2008

Data from experimental enclosures and natural and manipulated lakes were used to test whether the variable chlorophyll a (Chl) yields to total phosphorus (TP) can be explained by the variable contributions of dissolved (TDP) and zooplankton phosphorus (ZP; P > 200 μm) to TP. Results indicate that low Chl yields to TP in systems with abundant large Daphnia are closely related to greater contributions of TDP and ZP to TP. An opposite pattern is produced by systems lacking large Daphnia. These patterns seem to be consistent among mesoeutrophic to hypereutrophic systems; Chl yields to TP decline with increasing contributions of TDP to TP. In the large-Daphnia-dominated systems, both high grazing and reduced phosphate demand allow a greater sequestering of P into zooplankton and dissolved pools rather than in algae, which generate a lower observed Chl yields to TP. Conversely, lack of large Daphnia and low grazing rates allow proliferation of small algae, and an associated intense phosphate demand allows greater sequestering of P into small algae rather than into zooplankton and dissolved pools, and consequently, a higher Chl yield to TP is produced.
Constrained Least Squares Estimation of Mixed Population Stock Composition from mtDNA Haplotype Frequency Data
Authors: Xu Shizhong , Kobak Carol J. , Smouse Peter E.
Published: July 2008

Nuclear allozymes have provided the markers of choice for genetic stock identification, especially for Pacific salmonids. For other species, a dearth of allozyme variation has led workers to pursue mtDNA markers. Haploid mtDNA markers obviate methods based on Hardy–Weinberg equilibrium; all mtDNA markers are rigidly linked, violating the independence assumption; and mtDNA sample sizes are small. Based on a conditional weighted least squares approach for the case of known source population genotype frequencies, we develop an unconditional method for estimated genotype frequencies. We compare the methods with Monte Carlo simulation, evaluating effects of source population and mixed harvest sample sizes and those of source population divergence and mixture composition. When genetic resolution is poor or sample sizes are inadequate, the conditional method works better, but with larger sample sizes and reasonable population resolution, the unconditional method is superior.
Computer Simulations of the Influence of Ocean Currents on Fraser River Sockeye Salmon (Oncorhynchus nerka) Return Times
Authors: Thomson Keith A. , Ingraham Jr. W. James , Healey Michael C. , LeBlond Paul H. , Groot Cornelius , Healey Christopher G.
Published: July 2008

We hypothesized that the interannual variability of the northeast Pacific Ocean circulation affects the return times of Fraser River sockeye salmon (Oncorhynchus nerka). Homeward migrations were simulated for 1982 (with a relatively weak Alaska Gyre circulation) and 1983 (with a relatively strong circulation) in the context of three sequential return migration phases: a nondirected oceanic phase, a directed oceanic phase, and a directed coastal phase. Passive drifters were simulated to examine the influence of ocean currents during the nondirected oceanic phase: model fish south of 48°N were advected closer to Vancouver Island in 1983 compared with 1982; those north of 48°N were advected closer to Vancouver Island in 1982 than in 1983. Fish were simulated during the directed oceanic phase using a variety of behaviour scenarios: model fish starting south of 50°N had earlier return times in 1983 than in 1982; those starting north of 50°N had return times in 1983 that were generally the same as or later than in 1982. We inferred that ocean currents would modulate the environmental influences on return times during the directed coastal migration phase, by deflecting sockeye salmon into different oceanographic domains along the British Columbia coast.
Spatial and Temporal Variations in Chemical Contamination of American Eels, Anguilla rostrata, Captured in the Estuary of the St, Lawrence River
Authors: Hodson P. V. , Castonguay M. , Couillard C. M. , Desjardins C. , Pelletier E. , McLeod R.
Published: July 2008

Levels of polychlorinated biphenyls (PCBs), mirex, and pesticides were 10–100 times higher in migratory adult American eels, Anguilla rostrata, sampled at Kamouraska, Quebec, in 1990 than in eels from an uncontaminated reference tributary; concentrations in eels within the estuary varied little among sites. In contrast, mercury levels were the same at estuarine and reference sites, suggesting natural sources or atmospheric deposition of mercury. Dioxins, furans, and polynuclear aromatic hydrocarbons were virtually absent at all sites. During the 7 wk of migration, levels of PCBs, mirex, and pesticides in eels increased, while mercury did not. Since 1982, levels of PCBs and mirex have declined by 68 and 56%, respectively, and the percentage of eels exceeding human health guidelines for PCBs and mirex was about twofold lower in 1990 than in 1982. Levels of other pesticides have also declined, except that dieldrin is unchanged since 1982. While chemical concentrations are declining, levels of specific, highly toxic PCB congeners are sufficiently high that eel consumption by beluga whales (Delphinapterus leucas) is still likely to be hazardous. The hazard to the eels themselves is unknown due to a scarcity of toxicity data, but the highest concentrations of chemicals were observed in gonads.
Why Is Recruitment of the American Eel, Anguilla rostrata, Declining in the St. Lawrence River and Gulf?
Authors: Castonguay M. , Hodson P. V. , Couillard C. M. , Eckersley M. J. , Dutil J.-D. , Verreault G.
Published: July 2008

We critically review four potential causes of a drastic decline in juvenile American eels, Anguilla rostrata, recruiting to Lake Ontario (81-fold decline from 1985 to 1992) and in juvenile eel densities in tributaries to the Gulf of St. Lawrence. Silver eels from the St. Lawrence River were much more contaminated and had a high prevalence of deformities and lesions than a reference stock although they were on average less contaminated in 1990 than in 1982 (1.12 versus 4.54 μgg−1 for PCB; 0.025 versus 0.07 μg∙g−1 for mirex). Lethal toxicity from chemical contamination has been known to occur in St. Lawrence River eels for the past 25–30 yr. Major habitat modifications in the St. Lawrence took place in the 1950's (St. Lawrence Seaway and hydroelectric dams), about 30 yr before recruitment started declining; this long delay argues against these perturbations being primary causes of the decline. There is little evidence that commercial fishing and oceanographic changes caused the decline. Overall, we conclude that we do not know what caused the pronounced recruitment decline. We propose research avenues and hypotheses that may advance understanding and emphasize that because of panmixia, the recruitment decline could be species wide.
Significance of Organic Detritus in the Diet of Larval Lampreys in the Great Lakes Basin
Authors: Sutton Trent M. , Bowen Stephen H.
Published: July 2008

Larval sea lamprey (Petromyzon marinus) and northern brook lamprey (Ichthyomyzon fossor) were collected monthly from three streams in the Upper Peninsula of Michigan from May 1992 through May 1993 and larval sea lampreys were collected during summer months from sites throughout the Great Lakes basin. Organic detritus made up most of the diet ash-free-dry-mass (AFDM) throughout the year, averaging 97.79%, with algae (2.12%) and bacteria (0.09%) making up the remainder of the diet AFDM. Assimilation efficiency for AFDM averaged 72% during warmer months and 53% during cooler months (annual mean = 61%). Gut fullness (amount of AFDM in the anterior one-tenth of the intestine) was low (mean = 0.10 mg diet AFDM∙g−1ammocoete). There were no significant differences in these measures between ammocoetes collected from the Upper Peninsula and those collected throughout the Great Lakes basin. From a laboratory-determined relationship between gut fullness and feeding rate, feeding rate in the field was estimated to be extremely slow, ranging from 4.2 to 5.5 mg diet AFDM∙g−1ammocoete∙d−1. These observations indicate that larval lampreys efficiently utilize a diet of organic detritus during warmer months when stream temperatures and food quality are more favorable for feeding, digestion, and growth.
Bioaccumulation of Radiocesium by Fish: the Influence of Physicochemical Factors and Trophic Structure
Authors: Rowan David J. , Rasmussen Joseph B.
Published: July 2008

Although many measurements have been made on radiocesium levels in water and aquatic biota, no agreement has been reached regarding the factors affecting bioaccumulation of these radionuclides. With monitoring data from countries that operate nuclear facilities and data from the primary literature, we explored the chemical and ecological factors that determine the bioaccumulation of radiocesium. Using log–linear regression we found that the bioaccumulation of 137Cs by fish was a negative function of both dissolved potassium and suspended sediment concentration, and a positive function of temperature. Important ecological factors were the trophic level of the fish (piscivores bioaccumulate more than plank-tivores and benthivores), and the length of the food chain as reflected by the ratio of piscivore yield relative to net primary production. Fish from softwater drainages, which make up a large portion of northern Europe and Canada, are more vulnerable to radiocesium contamination than fish from hardwater sedimentary drainages, because these waters are extremely low in potassium and suspended sediment, and their watersheds are less efficient in retaining radiocesium. High dissolved potassium, short food chains, and the much greater volume and mixing potential of the ocean make marine fish less vulnerable to releases of radiocesium.
Crustacean Zooplankton Communities and Lake Morphometry in Precambrian Shield Lakes
Authors: Keller W. , Conlon M.
Published: July 2008

To investigate relationships between crustacean zooplankton communities and lake morphometry we sampled 60 near-neutral Precambrian Shield lakes, including many small, shallow lakes. Morphometry-related patterns in the distributions of many zooplankton species were evident. Lake depth determined the occurrence of hypolimnetic species, and lake depth, lake area, and watershed area were positively related to the species richness of zooplankton communities. Among lakes with fish, deeper (maximum depth >8 m) lakes were characterized by greater species richness, higher abundances of a number of species including Daphnia pulex, Daphnia galeata mendotae, Daphnia dubia, and Diacyclops bicuspidatus thomasi, and lower abundances of Leptodiaptomus minutus, than shallower (maximum depth <8 m) lakes. Increased predation pressure by small fish species in smaller, shallower lakes probably influenced the observed species distributions. Lakes inferred to be fishless based on the presence of Chaoborus americanus typically had relatively high abundances of D. pulex, Diaphanosoma brachyurum, and Aglaodiaptomus leptopus, and absences of D. g. mendotae and D. birgei, patterns attributable to intense invertebrate predation on zooplankton.
Physiological Response of American Plaice (Hippoglossoides platessoides) Exposed to Low Salinity
Authors: Munro J. , Audet C. , Besner M. , Dutil J.-D.
Published: July 2008

American plaice (Hippogiossoides platessoides) maintained under natural conditions of temperature (0–10 °C), salinity (26–28 g∙L−1) and photoperiod (48°N) were transferred in four consecutive seasons directly to seawater tanks at different salinities (28 (control), 21, 14, and 7 g∙L−1). During each season, a major response to salinity was observed at 7 g∙L−1 and a minor response at 14 g∙L−1. After 42 d at the lowest salinity (7 g∙L−1), plasma sodium concentrations had dropped by 28% below those of the controls, muscle water content had increased by 3%, and plasma potassium had decreased by 13%. In winter and summer, fish in the lowest salinity also had elevated plasma cortisol concentrations compared with controls. No mortality occurred at 7 g∙L−1 following three 96-h survival tests conducted in October, December, and February, whereas fish transferred to fresh water died within 24 h. These results suggest that American plaice is moderately tolerant to changes in salinity and that it should not be held for aquaculture purposes in estuarine conditions at a salinity below 14 g∙L−1.
A 6-Yr Record of Nutrient Element Sedimentation and Recycling in Three North Temperate Lakes
Authors: Poister David , Armstrong David E. , Hurley James P.
Published: July 2008

Sedimentation of C, N, and P from the water column was assessed during the ice-free seasons of 1986–91 in three northern Wisconsin lakes. Seasonal trends in mass sedimentation were different among lakes but consistent from year to year within each lake. High rates of nutrient sedimentation were associated with spring and fall blooms of large siliceous algae. Nutrient recycling, calculated as the difference between uptake during photosynthesis and loss to sedimentation, showed seasonal trends that were related to sedimentation. Recycling within the water column was the most important source of nutrients to primary producers, accounting for 85–90% of P demand during the summer stratified period.
Biological Events during Ice Breakup in the Great Whale River (Hudson Bay)
Authors: Hudon Christiane
Published: July 2008

The physical events and changes of abundance of freshwater and marine organisms were documented daily at the mouth of the Great Whale River to a distance of about 5 km in the freshwater plume extending in Hudson Bay. For the 2-wk period before, during, and after the ice breakup, water flow and concentration of coarse (>500 μm) particulate organic matter (CPOM) decreased, while transparency and temperature increased. The Great Whale River outflow to coastal Hudson Bay for that period was estimated to contain 8.8 t of CPOM and 1021 million insects (primarily simulids). Insect drift was positively related to river flow during daylight hours, whereas night drift intensified with receding flow values. The maximum drift of freshwater–anadromous fish larvae (coregonids and Lota lota) coincided with peak flow at ice breakup. A stratified under-ice freshwater plume isolated the freshwater drift material (CPOM, insect larvae, and fish larvae) from the marine community and increased the distance at which terrigenous material was exported into Hudson Bay. During the short period of ice breakup, the Great Whale River contributes substantial amounts of material, which probably stimulates biological production on the coast of Hudson Bay.
The Harmful PhytoplankterChaetoceros concavicornisCauses High Mortalities and Leucopenia in Chinook Salmon (Oncorhynchus tshawytscha) and Coho Salmon (O.kisutch)
Authors: Yang C. Z. , Albright L. J.
Published: July 2008

Chaetoceros concavicornis is a harmful phytoplankter that occurs in many temperate coastal seawaters and can cause fin fish mortalities when present at concentrations as low as 5 cells∙mL−1. At even lower concentrations, this diatom can stress salmonids to such an extent that they may express a disease to which they are most prone at the time of C. concavicornis exposure. We report mortality rates of coho salmon (Oncorhynchus kisutch) and chinook salmon (O. tshawytscha) exposed to different concentrations of C. concavicornis. Our data indicate that in the presence of harmful concentrations of C. concavicornis, blood hematocrit and erythrocyte, glucose, and lactate concentrations of yearling chinook salmon (O. tshawytscha) increase. The microridges of the primary lamellae decreased in prominence in the presence of harmful concentrations of this phytoplankter, while the goblet cells became more prominent and more numerous. Neutrophil, lymphocyte, and thrombocyte concentrations in the blood became depleted. These data suggest that suppression of a portion of the Chinook's immune system is occurring which may partially explain the earlier observation that salmonids cultured in the presence of harmful C. concavicornis phytoplankton became more susceptible to disease, including Vibrio infections.
A Niche Definition for Lake Trout (Salvelinus namaycush) and Its Use to Identify Populations at Risk
Authors: Ryan Phil A. , Marshall Terry R.
Published: July 2008

Lake trout (Salvelinus namaycush) populations require cold water with high dissolved oxygen content for survival. We developed models that predict the availability of such habitat, using lake mean depth, which describes both the thermal regime and the initial oxygen reserves, and using a measure of primary productivity, which defines the subsequent oxygen demand of the sediments and water column. Measures of primary production include either phosphorus concentration, chlorophyll a concentration, or Secchi disk transparency. The models are presented as a series of seasonal oxygen depletion isopleths that predict the extent to which the oxygen content of thermally suitable habitat is lost during the period of thermal stratification. The presence or absence of native lake trout in lakes of northwestern Ontario superimposed on these graphs indicated that natural populations seldom occur in lakes in which the seasonal oxygen depletion exceeds 40%. This isopleth is a niche boundary in its representation of adverse temperature and oxygen conditions for lake trout. The delineation of this boundary permits the identification of lakes where lake trout populations could be seriously affected by cultural eutrophication, overfishing, or climate warming.
Regulation of Rotifers by Predatory Calanoid Copepods (Subgenus Hesperodiaptomus) in Lakes of the Canadian Rocky Mountains
Authors: Paul Andrew J. , Schindler David W.
Published: July 2008

The abundance of planktonic rotifers in alpine lakes is regulated by predation from Diaptomus (subgenus Hesperodiaptomus) arcticus (a large calanoid copepod) regardless of nutrient conditions. We manipulated both predators and nutrient levels in large in situ enclosure experiments (2250 L). Hesperodiaptomus arcticus suppressed densities of the soft-bodied rotifer Polyarthra dolichoptera significantly (two to three orders of magnitude) and the loricate rotifer Keratella quadrata less so (up to one order of magnitude) relative to populations in predator-free enclosures. Keratella quadrata's armoured lorica may limit predation by Hesperodiaptomus. Nutrient addition resulted in higher phytoplankton standing crops and rotifer densities but did not permit rotifers to exceed predator consumption. In 69 lakes from the Canadian Rocky Mountains, the presence of Keratella or Polyarthra was negatively correlated with Hesperodiaptomus density. A maximum-likelihood logistic regression model predicts that as Hesperodiaptomus densities increase the probability of occurrence for both rotifer genera decreases, with Polyarthra declining more rapidly than Keratella.
Microbial Activity in Acidified and Limed Humic Lakes
Authors: Tranvik Lars J. , Granéli Wilhelm , Gahnström Gunnar
Published: July 2008

To test the hypothesis that acidification negatively affects microbial processes in water and sediment, five acidified (pH of surface water 5.2 ± 0.3) and five limed (pH 6.9 ± 0.2) humic (water colour 91 ± 15 and 77 ± 28 mg Pt/L, respectively) lakes in southern Sweden were studied. Sediment pH of the acidified lakes was significantly lower than in limed lakes down to a sediment depth of at least 3 cm. In spite of this, there was no difference between acid and limed lakes with respect to bacterial abundance in the sediment, sediment oxygen uptake, or utilization of amino acids by sediment bacteria. Similarly, there was no difference in biological water column parameters, i.e., chlorophyll a, gross and net oxygen production, respiration, bacterial abundance, and the utilization of amino acids by pelagic bacteria. Assuming that the restoration of pH by liming also restores pre-acidification conditions of microbial life, this study suggests that the level of acidification typical for most areas does not influence microbial activity negatively.