The retention of larval rainbow smelt Osmerus mordax in the well-mixed, turbid upper estuary of the St. Lawrence River is achieved by active tidal vertical migrations. Time-series analysis of their vertical distribution obtained during two 98 h sampling series conducted in June and July 1986 revealed that larvae were near the surface during flood tides and nearer the bottom during ebbs. The nature of vertical migration changes with larval size. In June, young larvae concentrated at the surface during flood. Their subsequent descent appears to be a passive sinking and dispersal throughout the water column beginning after the flooding tide attains maximum speed. In July, older larvae concentrated during both flood and ebb and the amplitude of their vertical migration increased. Thus, older larvae use currents more efficiently as reflected by their average position in the estuary, which was farther upstream In July than in June, and by the dfferential larval size distribution along the estuary, larger larvae being located farther upstream than smaller larvae in July. Passive vertical transport was not responsible for the observed vertical movements as larvae concentrated in surface or bottom layers when mixing of the water column was maximal. Series of hourly average densities calculated from discrete depth samples revealed cyclic variations associated with tidal advection. As smelt larvae are advected by water masses, their longitudinal distribution was influenced by both active vertical migrations and hdal circulation in the estuary

We investigated the magnitude of short-term variations in the flux of several species of fish larvae in Conception Bay, Newfoundland, determined the contribution of these fluxes to changes in the abundance of these species, and assessed the factors that may be causing the variations in these fluxes. The net effect of transport ranged from a daily influx of 5.8% to a daily efflux of 6.2% from the population. Short-term variations in transport were associated with variations in wind stress. The contribution of transport to variations in population abundance ranged from 12 to more than 75% of the average daily rate of change in numbers for the different species. The contribution of physical processes to estimated mortality rates is variable and can have a significant impact on the investigation and interpretation of potential biological processes (e.g., predation and starvation) influencing the survival of larval fish. We predicted that the average correction to the instantaneous mortality rate due to advection and diffusion should be proportional to A−1/2, where A is the area of system under study. This highlights the susceptibility of small systems to estimation error of mortality rates caused by short-term variations in physical forcing.

Distribution patterns, abundance and species composition of the assemblages of fish eggs and larvae in Conception Bay, Newfoundland. Canada. during the early summers of 1990 and 1991 are described. Several species appear to use Conception Bay as a spawning ground. lnterannual and seasonal differences in both species composition and abundance in the bay appear to be associated with differences in environmental conditions, particularly with temperature. Both egg and larval abundances were lower by an order of magnitude in 1991 corresponding to the coldest and less saline waters. Temperature and wind were the environmental factors most associated with the spatial distribution of eggs. In both years, highest abundances of all species were usually found at the head and the eastern side of the bay, corresponding to the warmest waters under dominant westerly winds. On one occasion, greatest abundances and warmer waters were found on the western side of the bay, concomitant with a southerly wind episode. Clustering samples by their relative species composition revealed well-defined assemblages of fish larvae. The variability in relative species composition was strongly associated with the physico-chemical conditions of the surface layer and was consistent with patterns of seasonal successions observed in other ecosystems. We propose that temperature is the principal factor controlling spatio-temporal occurrence of fish eggs and larvae in Conception Bay and, most probably, in coastal waters of Newfoundland and Labrador, Canada. This association may be critical in ecosystems characterized by an extremely short growing season. Conception Bay may play an important role in the early-life history of fish inhabiting the northeast Newfoundland/Labrador shelf. Even if a smaller proportion of fish reproduced in the coastal reglon, their offspring may contribute signiflcantly to the number of individuals recruiting to the population if growth and survival are considerably higher in coastal areas than in offshore areas due to the presence of more favourable habitats for early-life stages of fish.

We compared the short-term variability in salinity, temperature and concentration of suspended particulate matter experienced by benthic and pelagic animals in the middle St. Lawrence Estuary on both horizontal and vertical axes. In addition, we examined the effect of 3 patterns of vertical distribution (no vertical migrations, die1 and tidal vertical migrations) on the environmental variability experienced by pelagic animals. Results demonstrated that the nature of environmental variability is different for the 2 types of animal. For benthic animals, environmental variability is experienced on the horizontal axis, whereas the vertical axis is more important for pelagic animals. The magnitude and frequency of abiotic fluctuations experienced by benthic animals depend on the environmental factors that control the longitudinal position of the estuarine gradient, whereas for pelagic animals they mostly depend on the animals' behaviour on the vertical axis. Consequently. favourable habitats for benthic and pelagic animals in the estuary are different. The most upstream section of the estuary is the most variable for benthic animals because the tides of greatest range generate the highest abiotic fluctuations on the horizontal axis. In contrast, the same section is the most abiotically stable for pelagic animals because tides of highest energy generate the strongest vertical mixing. The highest abiotic fluctuations experienced by these animals are in the most vertically stratified waters downstream in the estuary. As a result, minimum numbers of species of benthic and pelagic animals are reached at different locations in the estuary corresponding to the areas which are the most abiotically variable for each type of animal. We propose that well-mixed parts of estuaries should be considered as abiotically stable habitats for the pelagic fauna, whereas vertically stratified areas should be considered as abiotically variable except in the case where vertical rnigratlons are not necessary for the retention of animals.

SynopsisWe tested the hypothesis that the mechanism of retention of tomcod, Microgadus tomcod, larvae and juveniles in the well-mixed part of the St. Lawrence Estuary is similar to that of sympatric smelt, Osmerus mordax, larvae who actively migrate to the surface during flood tides and to the bottom during ebb tides so as to minimize net downstream displacement. The vertical distribution of tomcod larvae and juveniles was documented during two 98-h sampling series at 2 anchor stations in June and July, 1986. An hourly index of the center of mass of fish in the water column calculated to take into account daytime net avoidance in surface waters suggested that tomcod remained deep in the water column and that their accumulation at the head of the estuary was the result of passive upstream transport by net residual circulation rather than active tidal migrations. For both series, depth of fish was inversely related to density of the water suggesting that the buoyancy of fish influenced their vertical distribution. Tomcod larvae and juveniles were advected by tidal currents. In June, larger larvae were found at low slack water indicating that they were located upstream of smaller larvae. In July, larger juveniles were located downstream of smaller juveniles, the difference in mean length between low and high slack water attaining 20 mm. Ontogenetic buoyancy changes may be responsible for these differences in the vertical distribution of tomcod. Comparisons of the early life-history stages of tomcod and smelt retained in the same area under the same hydrodynamical conditions indicate that more than one mechanism permits retention in a well-mixed estuary and that the observed species-specific patterns of vertical distribution are not simply interpretable as adaptations to retention.

The effects of habitat structure and time of day on predation success of Moses perch, Lufjanus russelli, and blue-spotted trevally, Caranx bucculentus, feeding on juvenile brown tiger prawn, Penaeus esculentus, were tested in the laboratory. Our experiments demonstrated that both softsubstratum, in which prawns can burrow, and seagrass habitats reduce predation rates by fish during daytime when compared to hard-substratum habitats. However, the protection offered by burrowing in a soft substratum during daytime disappears at night when prawns become active and are more available to fish. Seagrass structure offers the same protection during both night and day. This is attributed to the structural complexity of the seagrass habitat, as well as the behdviour and disruptive coloration of the prawns. The results suggest that fish predation on juvenile P. esculentus may not be very important in seagrass nurseries but could be very high in other habitats.

The relationship between vertical migration, estuarine retention and species-specific patterns of longitudinal distribution of the pelagic larvae of rainbow smelt, Osmerus mordux, and tomcod, Microgadus romcod, was investigated in the upper section of the St Lawrence Middle Estuary. We hypothesized that the species-specific use of the vertical pattern of current exhibited by the two species results in the partitioning of the estuarine habitat in the longitudinal plane while assuring retention. Important differences in the longitudinal distribution of tomcod and smelt larvae were related to the ontogeny of their vertical distribution in the water column. In June, small tomcod and smelt larvae are generally associated with waters of salinities less than 5%0. As the larvae grow. their vertical distribution patterns change, leading to a horizontal separation of the two species. Tomcod juveniles migrate downstream into colder, more saline waters, whereas larger smelt larvae migrate upstream into warmer, tidal fresh waters. Ontogenetic changes in vertical distribution serve to concentrate larvae in specific conditions that may optimize physiological conditions and also permit population persistence.