We investigated the influence of variations in the size of prey (Mallotus villosus) and a vertebrate predator (Gasterosteus aculeatus) on larval fish mortality rates during the period of yolk absorption using mid-size mesocosms (2.7 m".Increasing predator size increased mortality rates of capelin larvae.Variations in larval capelin size resulted in 2 distinct patterns.Between experimental trials, greater mean size of larval capelin in the mesocosm reduced mortality due to predation.Within an experimental mesocosm, larger larvae suffered higher mortality than smaller individuals.Contrasting patterns of size-dependent vulnerability to predation reflect the hierarchy of processes that determine the probability that a larval fish will be preyed upon.The broad scale response of the predator was determined by the mean relative sizes of prey and predator which govern the average probabilities of encounter, attack and capture.Within the search ambit of a predator (e.g area or volume searched within a complete diurnal foraging cycle) active prey selection for larger prey due to either greater visibility or higher energy reward was an important factor.A comparison of our results with estimated predation rates by the jellyfish Aurelia aurita indicates that at a similar size a gelatinous zooplankter consumes fewer larvae than a stickleback and is a less efficient predator as measured by the energy ingested relative to energy demands.For both vertebrate and invertebrate predators, the ratio of prey to predator lengths was a strong predictor of the daily mortality rate due to predation.Relative prey-predator sizes may provide a useful perspective to assess changes in larval fish vulnerability as they grow through a predator field.
Une anemie microcytaire avec de tres nombreuses anomalies erythrocytaires est decouverte chez un jeune homme sans particularite clinique, faisant evoquer une hemoglobinopathie ou une myelodysplasie de l’adulte jeune. L’electrophorese de l’hemoglobine montre une hemoglobine S a 67 %, une hemoglobine F a 29 % et une hemoglobine A2 a 4 %. L’etude moleculaire des genes de globine portera le diagnostic de drepanocytose/β 0 thalassemie associee a une α thalassemie. Nous rappelons la maladie drepanocytaire et ses differentes formes lorsqu’elle est associee a des thalassemies.
The study of tumors in fish has been hampered by the lack of suitable bioindicators especially because the ultimate diagnosis is not achieved until the fish are dead or killed. The present study was undertaken to develop simple, reliable and minimally invasive procedures to detect neoplasms in lake whitefish (Coregonus clupeaformis) and investigate their applicability as fish tumor biomarkers. Ethoxyresorufin-o-deethylase (EROD) assays have been widely used as an indicator of the activity of cytochrome P450-1A1, an isozyme located in a variety of tissues and in measurable amounts in the liver of many vertebrates. A fluorometric protocol using very small sized samples such as those from biopsies was used to measure EROD activity. Although no direct correlation could be made between EROD activity and liver tumor occurrence, EROD activity was significantly higher in livers with high melanomacrophage aggregates (P<0.001). Fibronectin (Fn) is a plasma and cellular protein that can be conveniently measured from biological fluids and has been used as tumor biomonitor in humans. Changes in Fn within serum and tissue samples of whitefish were monitored. Mean Fn levels in serum samples (n=65) was 2.03% of total serum proteins. Among the serologically evaluated fish, three had hepatic neoplasia as diagnosed by histological means. Fn levels in two of these were reduced at 1.89 and 1.22% of total serum proteins, however, no statistical correlations could be made with such small sample size, and further analysis is in progress.
Large in situ enclosures were used to investigate the mortality of yolk-sac larval capelin (Mallotus villosus) by four species of jellyfish predators. Daily instantaneous larval mortality increased linearly with predator density but was independent of initial larval density, indicating a linear functional response of the four jellyfish species. The addition of alternative prey did not modify the linearity of the functional response and did not significantly reduce or increase larval mortality imposed by two jellyfish species. The evidence of a linear response in large enclosures contrasts with the curvilinear (Type II) response previously reported in studies conducted in small laboratory containers. Larval mortality was strongly dependent on predator size within jellyfish species and increased with initial larval size. Larval capelin appeared more susceptible to predation at the time of transition from endogenous to exogenous feeding. Daily larval mortality rates due to predation varied between 0.015 and 1.58 depending on jellyfish species and appeared more strongly determined by body mass than differences in feeding appendages of jellyfish. Our results indicate that reduced larval mortality to jellyfish predation is achievable only by timing larval release in water mass where jellyfish numbers are naturally reduced. We estimated that macro invertebrate predation has the potential of being the primary regulator of larval survival and we hypothesized that large intra- and interannual variation in larval survival may be due to the variability in the emergence timing of capelin larvae relative to the growth trajectory of jellyfish.
To evaluate patterns of population genetic structure among freshwater and estuarine amphipods, we review available literature and summarize new data from two species within the Great Lakes-St. Lawrence River drainage basin. A total of 24 published studies and 3 presently unpublished studies were included covering 10 amphipod genera and 28 currently recognized species. Studies were subdivided according to habitat type (continuous or discrete) to examine the effect of habitat fragmentation on the degree of genetic variation and differentiation within and among amphipod populations. Populations of aquatic amphipods appear to be highly structured genetically, particularly among discrete habitats. Of the 28 currently recognized species, 57% showed moderate to great levels of differentiation among populations. Six of these taxa from four genera (Gammarus, Hyalella, Para-corophium, and Paramelita), showed evidence of genetic differentiation at the species level, suggesting that our current taxonomic inventories may be considerably underestimated for amphipods. Passive dispersal among habitats may be responsible for the high levels of differentiation found among populations, particularly for taxa from discrete or discontinuous habitats (e.g., Hyalella azteca). Taxa from continuous habitats had lower levels of differentiation among populations, but surprisingly, also had lower levels of overall genetic variability. Accordingly, the evolutionary response of widespread, continuously distributed taxa to environmental change may be limited (e.g., Gammarus fasciatus in the Great Lakes-St. Lawrence River). Our review indicates that there is an urgent need to determine the causes and consequences of reduced genetic variability in continuous populations, and conversely, increased genetic differentiation and variation of amphipods in discrete habitats. Such information will be critical for understanding patterns of species diversity among discrete and continuous habitats. Furthermore, it will provide insight into the possible responses of aquatic amphipods to large-scale environmental changes (e.g., global warming, exotic species introductions), particularly with respect to the increasing anthropogenic fragmentation of natural habitats.
In situ experiments were conducted to assess the use of multi-plate samplers for antifouling experiments and to test the antifouling effectiveness of a chitin-based coating against zebra mussels (Dreissena polymorpha). A series of multi-plate samplers consisting of three parallel plates coated with chitin, and untreated control samplers, were submerged horizontally or vertically in a marina of the St Lawrence River for 3 1/2 months (July-October) in 1998 and 1999. Mussels attached to the chitin-treated substrates were on average 2.75 times more abundant than on the control samplers, indicating that chitin is not effective as an antifouling agent against zebra mussels. The abundance, size distribution and spatial dispersion of mussels on the plates varied both between plates and between top vs bottom sides of plates in horizontal collectors, but not in vertical samplers. The three plates composing each multi-plate horizontal sampler do not represent true replicates for statistical analysis. The bottom side of plates exhibited the least variability and might therefore serve as the experimental unit. Substrate heterogeneity and plate orientation were identified as confounding factors to be controlled for in future experiments. Sunlight exposure and colonization by sponges strongly influenced zebra mussel abundance and should be considered when performing in situ experiments. Because of the influence of uncontrolled factors, it is recommended that in situ pilot studies be conducted to statistically test the effectiveness of antifouling products once the threshold level of the desired effectiveness is defined.
Le fleuve Saint-Laurent est-il un milieu favorable a cette espece exotique? Serons-nous confrontes a un probleme majeur avec cette nouvelle venue? Ces deux questions sont au coeur du programme de recherche et de suivie de la moule zebree depuis sont introduction dans le fleuve Saint-Laurent. Une equipe de chercheurs du Centre Saint-Laurent d'Environnement Canada a tente d'y repondre et presente un premier constat general.
