Dynamics and extent of larval lake sturgeon Acipenser fulvescens drift in the Upper Black River, Michigan

Lake sturgeon larval drift is not uniform in time or space and subsequent efforts to determine the relative abundance have suffered because of the lack of information during this early life history period. The purpose of this study was to obtain information about the early life history of lake sturgeon, determine the extent and duration of lake sturgeon larval drift, and examine this relationship to water flow and temperature in the Upper Black River, Michigan. This study also compares the results of other studies to further evaluate the dispersion of larvae. Larval production was quantified using drift nets anchored to the stream bottom from May to June in 2000–2002. Larval drift nets captured 780 larvae in 2000; 2975 larvae in 2001; and 2041 larvae in 2002. For the 2000, 2001, and 2002 spawning season, we estimated that 7107 (95% CL: ± 1470), 17 409 (95% CL: ± 5163), and 15 820 (95% CL: ± 3168) larval lake sturgeon were produced in the Upper Black River (UBR), respectively. Catch per unit effort values of drifting larvae were greatest after peak water flows, with most larvae captured in the middle of the river channel. A mean daily water temperature above 16°C was an important environmental stimulus that influenced peak larval dispersion away from spawning sites. The results of this study suggested that natural reproduction was still occurring in the Black Lake system.     

The drift of zooplankton and microzoobenthos in the river Strandaelva,western Norway

The drift of zooplankton (rotifers, cladocerans, cyclopoid copepods) and microscopical zoobenthos (mainly bdelloid rotifers and small chironomid larvae) was investigated by filtering samples of river water. The number of drifting benthic rotifers varied between 1 000 and 6 000 ind. m^–3 in the lake inlet, and between 30 and 500 ind. m^–3 in the lake outlet, without any seasonal trend. The number of drifting insect larvae was approx. equal in the lake inlet and outlet, with a maximum in summer (250–300 ind. m^–3) and minimum in winter (ca. 10 ind. m^–3). Increasing water flow resulted in an increasing number of drifting zoobenthos. Downstream from the lake, the number of drifting benthic rotifers was increasing from approx. 300 ind. m^–3 in the outlet to 6 500 ind. m^–3 3.4 km downstream, while the number of insect larvae was ca. 100 ind. m^–3 in the outlet and leveled off at approx. 300 ind. m^–3 after 200 m. The number of drifting zooplankton in the lake outlet varied between 20 and 2 000 ind. m^–3 for crustaceans, and between 300 and 20 000 ind. m^–3 for rotifers, both with a maximum in late summer/autumn and a minimum in winter. The number of drifting zooplankton decreased by some 45% in the first 200 m from the lake outlet, but some zooplankton was still found in the drift 3.4 km downstream. The largest species was removed first from the drift. The diurnal variation in the number of drifting zooplankton in lake outlets appear to be related to the vertical migration in the lake, i.e. the largest number drifting when most animals are in the upper water layers.Contribution from the Voss Project, University of OsloContribution from the Voss Project, University of Oslo     

Effects of structural habitat on drift distance and benthic settlement of the caddisfly,Ceratopsyche sparna

We conducted two experiments in flow-through, artificial streams to examine how habitat structure affected drift and benthic resettlement of larval hydropsychid caddisflies (Ceratopsyche sparna). In the first experiment, we quantified drift distance and the number of times larvae re-entered the drift in 9.0 × 0.51-m channels with contiguous patches (ea. 2.5-m long) of biofilm-covered gravel, biofilm-covered cobbles (– Cladophora), and Cladophora-covered cobbles (+ Cladophora). In the second experiment, we tracked nocturnal movements of larvae after benthic settlement in 2.8 × 0.1-m channels, each containing one of the three habitat types. In experiment 1, drift distance was (1) greatest in gravel and lowest in cobbles + Cladophora, (2) inversely related to hydraulic roughness of habitats, (3) independent of body size, and (4) similar for live and dead larvae. Average drift distance was relatively short (<2.5 m), regardless of habitat type. Number of drift re-entries also varied among habitats, being greatest in gravel and lowest in cobbles + Cladophora. No larvae re-entered the drift after settling in Cladophora patches. Results from experiment 2 revealed that drift propensities were higher for larvae in biofilm-covered gravel and cobbles than in cobbles + Cladophora. Larvae remaining in substrate patches (i.e. not drifting) laid fewer draglines in biofilm-covered stones than in Cladophora patches. Extent of benthic movement (i.e., crawling) by non-drifting larvae did not differ significantly among habitats. However, distance moved did differ with flow direction, being 4× greater downstream than upstream. These results highlight how local substrate and hydraulic conditions interact to affect small-scale movements of caddisfly larvae.     

Seasonal variation in the diet of juvenile lake sturgeon,Acipenser fulvescens,Rafinesque, 1817, in the Winnipeg River,Manitoba, Canada

The lake sturgeon, Acipenser fulvescens, is a threatened species across most of its range. To understand any potential shifts in diet across season or habitat, stomach contents were examined from juvenile lake sturgeon caught in the Winnipeg River, Manitoba, Canada. This information will aid in assessment of environmental impacts and conservation initiatives for this threatened species. From 2006 to 2008 gut contents were collected from juvenile lake sturgeon using a non‐lethal gastric lavage methodology. Juvenile lake sturgeon were sampled from four discrete deep water habitat types (13.7–27.4 m depths), during the months of May, June, July and October. In total, 13 066 prey items belonging to 14 prey groups were collected from 345 juvenile lake sturgeon (251–835 mm TL). Overall, juvenile lake sturgeon consumed a low diversity of organisms; insect larvae from three invertebrate orders, Trichoptera, Diptera and Ephemeroptera, accounted for 97.4% of the total numeric abundance of prey items recovered. A relationship between size of juvenile lake sturgeon and prey quantity or type was not observed. Diet of juvenile lake sturgeon varied in relation to season, with Diptera (77.6%) being the most abundant prey in May, Trichoptera the most abundant in June (n = 3,056, 60.4%) and July (n = 2,055, 52.6%). During October, 96% of stomachs examined were empty. In terms of habitat type, the standardized abundance of invertebrate prey items was highest in deep water habitats characterized by medium water velocity (depths > 13.7 m) and coarse substrate (particle sizes > 0.063 mm) in May and June when compared to deep water habitats characterized by low water velocity (<0.2 m.s^?1) and fine substrate (particle sizes < 0.063 mm). However, in July, the opposite was observed and prey abundance in juvenile lake sturgeon stomachs decreased with increasing particle size. Results suggest a high degree of dietary overlap among juvenile lake sturgeon from multiple size/age classes occupying deep‐water habitats of the Winnipeg River.     

Biological attributes of age-0 lake sturgeon in the lower Peshtigo River, Wisconsin

Lake sturgeon Acipenser fulvescens are imperiled throughout the Laurentian Great Lakes basin. Efforts to restore this species to former population levels have been ineffective due in part to limited information regarding its early life history. The objectives of this study were to characterize the larval drift and biological attributes of age‐0 lake sturgeon in the lower Peshtigo River, Wisconsin. Lake sturgeon larvae were captured from May to June 2002 and 2003 using drift nets, while age‐0 juveniles were captured from June through October 2002 and 2003 using wading, snorkeling, backpack electrofishing, and haul‐seine surveys. Larval drift occurred within 14 days of adult spawning and extended from 1 to 3 weeks in duration, with two peaks in the number of fish drifting downstream each year. Larvae had a median total length (TL) of 19 mm (range: 13–23; N = 159) in 2002 and 18 mm (range: 13–24; N = 652) in 2003. Catch‐per‐unit‐effort for larvae was 0.18 fish h^?1 m² and 0.94 fish h^?1 m² in 2002 and 2003, respectively. Age‐0 juvenile lake sturgeon exhibited rapid growth (i.e. 2.57 mm day^?1 in TL and 0.66 g day^?1 in wet weight) throughout summer and fall months; relative condition of fish in both years was approximately 100, indicating good condition. Absolute abundance of age‐0 juveniles in 2003 was estimated at 261 fish using the Schnabel estimator. The results from this study indicate that the lower Peshtigo River contains important nursery habitats suitable for age‐0 lake sturgeon.     

Effect of water velocity and temperature on energy use,behaviour and mortality of pallid sturgeon Scaphirhynchus albus larvae

Natural reproduction of pallid sturgeon Scaphirhynchus albus has been limited for decades and a recruitment bottleneck is hypothesized to occur during the larval stage of development. In this study, we evaluated the effects of water velocity and temperature on the swimming activity, energy use, settling behaviour and mortality of endogenously feeding larvae. The swimming activity of drifting sturgeon larvae (i.e., fish exhibiting negative rheotaxis) increased at low water velocity. In subsequent experiments, we observed greater energy depletion and resultant mortality of larvae in no-flow environments (0 cm s⁻¹) compared to tanks with water velocity ranging from 3.5 to 8.3 cm s⁻¹. The growth rate of drifting larvae was positively related to water temperature (18.7–23.3°C), but reduced growth rate at low water temperature (18.7°C) resulted in protracted development that extended average drift duration by ~4 days compared to larvae reared at 23.3°C. This study provides evidence that cooler summer water temperatures, characteristic of present-day conditions in the upper Missouri River, can reduce larval development and extend both the drift duration and distance requirements of S. albus. Moreover, if dispersed into low velocity environments, such as in reservoir headwaters, larvae may experience increased mortality owing to a mismatch between early life stage drift requirements and habitat conditions in the river. Manipulation of water releases to increase seasonal water temperature below dams may aid survival of S. albus larvae by shortening the time and distance spent drifting.     

Assessment of spatio‐temporal variation in larval abundance of lake sturgeon (Acipenser fulvescens) in the Rupert River (Quebec,Canada), using drift nets

The Rupert River is one of the largest tributaries on the east coast of James Bay. Lake sturgeon (Acipenser fulvescens) is present all along the main stem where several spawning grounds have been located, four of which are major spawning grounds that have been studied at km 216, 281, 290 and 362. The total number of drifting larvae was estimated with drift nets set along transverse transects at km 212, 276, and km 287 from 2007 to 2009, and at km 361 in 2008 and 2009, using a new technique, namely, a Doppler current meter to measure water velocity within transect sub‐sections corresponding to Voronoï polygons. There was a substantial, persistent difference in the number of larvae produced by the four main spawning areas. On average, the most productive site (km 276) produced over five times more larvae than the least productive site (km 361). Average estimated numbers were 41,194 at km 212, 176,840 at km 276, 106,212 at km 287, and 30,642 at km 361. Temporal variations were of much less amplitude than spatial differences. Between 2007 and 2009, interannual variations were not significant, except at km 212, despite differences in river flow during incubation and larval drift. The number of gravid females and the quality of spawning grounds would likely be the main factors influencing the total number of larvae. Vertical distribution of larvae is variable between sites and years, and shows a slight tendency for larvae to be more surface oriented. Higher flow near the surface would partly explain larger surface drifting of larvae. Transverse distribution is uneven and often associated with the location of the spawning grounds and the river flow. Given the uneven vertical and transverse distribution of larvae, an effective sampling strategy should cover the complete water column and full river width. Where depth exceeds 3 m, at least two stacked nets are recommended. In large rivers, filtering close to 1% of total river flow should result in acceptable confidence intervals, allowing a good comparison of the number of larvae in space and time.     

The seasonal dynamics and distribution of Chaoborus flavicans larvae in adjacent lake basins of different morphometry and degree of eutrophication

1.  Seasonal dynamics, spatial distribution and population size of the phantom midge Chaoborus flavicans in different parts of the eutrophic Lake Hiidenvesi (30.3 km²) were studied.
2.  Density of larvae was low in the shallow, most eutrophic parts of the lake, while the deep Kiihkelyksenselkä basin was inhabited by a dense population. In the deepest part of Kiihkelyksenselkä (33 m) density was 13 989 ± 3542 m^–2 in May, declined to 1102 ± 274 m^–2 in July and recovered to 7225 ± 1314 m^–2 by October. In spring and autumn the majority of larvae were benthic while, during high summer, few larvae were found in the sediment.
3.  Horizontal distribution fluctuated seasonally. On 3 June < 5% of the population inhabited areas shallower than 10 m. On 6 July the limnetic fraction was still restricted to regions deeper than 10 m, but 43% of benthic larvae were found between 6 and 10 m depths. In October both limnetic and benthic larvae were concentrated in areas deeper than 20 m.
4.  Within the lake, distribution was mainly regulated by stratification characteristics, degree of eutrophy being less important. The seasonal horizontal movements were probably induced by food shortage. Larvae could not meet their energetic demands in stratified areas and dispersed to shallower water, reducing predation risk by use of the benthic habitat.     

Vertical migrations of fish larvae: Eulerian and Lagrangian observations in the Eastern English Channel

The Eastern English Channel is known for its strong hydrodynamics.Tidal and residual currents are reinforced by the south-westerndominant wind and drift waters from the English Channel to theNorth Sea. Previous spatial studies have shown that the advectionof larvae could differ from one species to another. Flounder(Pleuronectes flesus) larvae were found offshore, drifting tothe north until the fins were formed; then they were found nearthe coast. However, sole (Solea solea) larvae remained in coastalwaters during their development. The difference in larval spatialdistribution is assumed to be related to the interaction betweenvertical migration and advection by alternative tidal currents,leading to a selective tidal stream transport. To describe thevertical distribution of these larvae, two strategies were used.First, a Eulerian study was carried out with samples taken atthe same geographical location every 1.5 h for 41 h. Ichthyoplanktonwere collected in the water mass using a Bongo net and witha suprabenthic multi-net sledge, at four layers above the seabed, between 0.1 and 1.4 m. Secondly, to enable water movementto be disregarded, a Lagrangian study was carried out by usinga Bongo net every 3 h, following a drifting buoy for 3 days.The results show that even during the youngest stages, solelarvae are able to perform tidal and diel vertical migration.We assume that they may limit their advection to the North Seabecause of their upward migration during ebb and at night, whichmay enable them to remain in the same area dealing with thecurrents. Flounder larvae begin their vertical migration atthe stage of notochord flexion, which ends their drift to thenorth. The larvae reach the bottom of the water column, particularlyduring ebb when they are concentrated in the first 40 cm abovethe bottom. This behaviour favours their advection during flow,leading to efficient and fast transport towards the coast.     

Where to stay by night and day: Size-specific and seasonal differences in horizontal and vertical distribution of Chaoborus flavicans larvae

1. Data on the distributions of pelagic and benthic Chaoborus flavicans larvae were gathered in 1994 and tested for their agreement with the predator avoidance hypotheses. The development of all Chaoborus life stages, as well as the horizontal and vertical distribution in the four larval instars, was followed from May until October. We expected the largest larvae to dwell deeper by day, thus avoiding predation by visually foraging fish.
2. In agreement with this prediction body size increased with daytime depth, and this was true both between and within instars. The migration amplitude consequently increased with larval instar.
3. There was also evidence for horizontal migration, mainly in the third but also in the fourth instar.
4. Along a horizontal transect with increasing depth, locations with many benthic larvae had fewer pelagic larvae. Oxygen concentration was a good predictor of maximum benthic larval depth for most of the season but failed to predict their distribution in autumn.     

Fish drift in a Danube sidearm‐system: II. Seasonal and diurnal patterns

Seasonal and diurnal patterns of larval and juvenile fish drift were investigated in the Marchfeldkanal, a man‐made side branch of the Danube River near Vienna, Austria. A clear seasonal pattern with peak densities in mid‐June was found. Species composition varied over time, showed a site specific pattern and was dominated by tubenose goby Proterorhinus marmoratus . Water temperature was the main factor responsible for the increase of drift densities until the median drift date and repeated occurrence of early larval stages in drift indicated repeated spawning for many species. Significant differences in drift densities between different time periods of the day (day, dusk, night and dawn) were found for common bream Abramis brama , barbel Barbus barbus , chub Leuciscus cephalus , tubenose goby and roach Rutilus rutilus . The highest drift rates occurred at night (2200–0400 hours), with 86% of all larvae drifting during the hours of darkness. Fish larvae of different lengths drifting at different phases of the day were found for common bream, bleak Alburnus alburnus and chub, with largest larvae drifting during dusk (chub) and day (bleak and common bream). For bleak, all gudgeon species Gobio spp., tubenose goby, roach and for all cyprinid species combined, one 2 h night sample was found to be sufficient to predict the total 24 h drift.     

Relative larval loss among females during dispersal of Lake Sturgeon (<Emphasis Type="Italic">Acipenser fulvescens</Emphasis>)

Mortality that occurs during larval dispersal as a consequence of environmental, maternal, and genetic effects and their interactions can affect annual recruitment in fish populations. We studied larval lake sturgeon (Acipenser fulvescens) drift for two consecutive nights to examine whether larvae from different females exposed to the same environmental conditions during dispersal differed in relative levels of mortality. We estimated proportional contributions of females to larval collections and relative larval loss among females as larvae dispersed downstream between two sampling sites based on genetically determined parentage. Larval collections were composed of unequal proportions of offspring from different females that spawned at upstream and downstream locations (~0.8 km apart). Hourly dispersal patterns of larvae produced from females spawning at both locations were similar, with the largest number of larvae observed during 22:00–23:00 h. Estimated relative larval loss did not differ significantly among females as larvae were sampled at two sites approximately 0.15 and 1.5 km from the last section downstream of spawning locations. High inter- and intra-female variation in larval contributions and relative larval loss between nights may be a common feature of lake sturgeon and other migratory fish species, and likely is a source of inter-annual and intra-annual variation in fish recruitment.     

Rainbow Darter (Etheostoma caeruleum,Storer, 1845) predation on early ontogenetic stages of Lake Sturgeon (Acipenser fulvescens,Rafinesque, 1817)

Previous molecular diet analysis identified lake sturgeon (Acipenser fulvescens, Rafinesque, 1817) DNA in the gastrointestinal tracts of stream-resident rainbow darters (Etheostoma caeruleum, Storer, 1845) during the egg incubation, free embryo, and larval drift stages. The objectives of this experimental study were to: (a) quantify levels of predation by rainbow darters on lake sturgeon at the egg and free-embryo stages; and (b) evaluate whether predation varied as a function of substrate size and rainbow darter body size. We conducted experimental trials in 23-L polycarbonate tanks 0.41 m (L) × 0.33 m (W) × 0.30 m (D) with a standardized benthic area of 0.14 m². The tanks were randomly assigned one of two different substrate size classes: large rock (51.35 mm ± 0.91 mm) or small rock (27.68 mm ± 0.57 mm). We stocked individual rainbow darter, which were deprived of feed for 48 hr, with lake sturgeon (133 individuals/m²) in each of 12 replicates per ontogenetic stage and substrate type. The number of surviving lake sturgeon was quantified following a 24-hr predation exposure period. We used a generalized linear model with a binomial distribution to assess the influence of ontogenetic stage, substrate size, and rainbow darter body size on proportional lake sturgeon survival. Predation on lake sturgeon occurred at both egg (6.25 ± 1.16 individuals, mean ± 2SE) and free embryo (3.08 ± 1.08 individuals, mean ± 2SE) stages. Egg proportional survival was generally lower than at the free embryo stage in both substrate sizes; however, free embryo proportional survival was greater in small substrate trials. Rainbow darter total length did not affect the probability of lake sturgeon survival at either developmental stage. Results demonstrate that rainbow darters prey on early ontogenetic stages of lake sturgeon, corroborating previous results based on genetic diet analysis. Results fill a major knowledge gap concerning the vulnerability of pre-drift sturgeon to predation by an abundant river resident species that was previously discounted as a predator for early ontogenetic stages of lake sturgeon due to its small body size.     

Lasting effects of maternal behaviour on the distribution of a dispersive stream insect

1. Predicting population dynamics at large spatial scales requires integrating information about spatial distribution patterns, inter-patch movement rates and within-patch processes. Advective dispersal of aquatic species by water movement is considered paramount to understanding their population dynamics. Rivers are model advective systems, and the larvae of baetid mayflies are considered quintessential dispersers. Egg laying of baetids along channels is patchy and reflects the distribution of oviposition sites, but larvae are assumed to drift frequently and far, thereby erasing patterns created during oviposition. Dispersal kernels are often overestimated, however, and empirical tests of such assumptions are warranted because of the pivotal role distribution patterns can have on populations. 2. We tested empirically whether the egg distribution patterns arising from oviposition behaviours persisted and were reflected in the distribution patterns of larval Baetis rhodani. In field surveys, we tested for associations between egg mass and larval densities over 1 km lengths of four streams. A control species, the mayfly Ephemerella ignita, was employed to test for covarying environmental factors. We estimated drift rates directly to test whether larvae dispersed between riffles (patches of high egg mass density) and whether drift rates were density-dependent or density-related - expected outcomes if drift erases patterns established by maternal behaviours. 3. Positive associations between egg masses and larval benthic densities were found for neonate and mid-stage larvae of Baetis, but not the control species, suggesting persistence of the patchy distribution patterns established at oviposition. Drift rates were high, and riffles were net exporters of neonate and mid-stage larvae, but drift rates were unrelated to benthic densities and few drifters reached the next riffle. Riffles were sinks for large larvae, suggesting ontogenetic shifts in habitat use, but little long-distance dispersal. 4. Overall, the results suggest that most neonate and mid-stage larvae of B. rhodani remain close to the natal riffle, and late-stage larvae disperse shorter distances than routinely assumed. The persistence of maternal effects on distribution patterns well into juvenile life of an allegedly iconic disperser suggests that traditional models of how dispersal influences the population dynamics of many lotic invertebrates may be incorrect.     

Causes of larval drift of the fire salamander,Salamandra salamandra terrestris,and its effects on population dynamics

Summary The larval drift of the fire salamander was investigated over a period of three years in a mountain brook (Niederbergisches Land, F.R. Germany), as well in a laboratory water channel. The rate of larval drift fluctuated between 19% and 41% of the total population of larvae in a defined section of the brook during these three years. Most (83%) of the drifting larvae were hatchlings or very young stages. The drift was dependent on the strength of the current, the number of spawning females, the presence of suitable hiding places, sufficient space and adequate food. Hungry larvae drifted more often than satiated animals. The drift behaviour of hatchlings differed distinctly from that of older larvae. The significance of ecological factors on larval drift is discussed. It is evidently a more important factor in selection than has hitherto been recognized.     

Fine-Scale Distribution and Spatial Variability of Benthic Invertebrate Larvae in an Open Coastal Embayment in Nova Scotia,Canada

This study quantified the fine- scale (0.5 km) of variability in the horizontal distributions of benthic invertebrate larvae and related this variability to that in physical and biological variables, such as density, temperature, salinity, fluorescence and current velocity. Larvae were sampled in contiguous 500-m transects along two perpendicular 10-km transects with a 200-µm plankton ring net (0.75-m diameter) in St. George’s Bay, Nova Scotia, Canada, in Aug 2009. Temperature, conductivity, pressure and fluorescence were measured with a CTD cast at each station, and currents were measured with an Acoustic Doppler Current Profiler moored at the intersection of the 2 transects. Gastropod, bivalve and, to a lesser extent, bryozoan larvae had very similar spatial distributions, but the distribution of decapod larvae had a different pattern. These findings suggest that taxonomic groups with functionally similar larvae have similar dispersive properties such as distribution and spatial variability, while the opposite is true for groups with functionally dissimilar larvae. The spatial variability in larval distributions was anisotropic and matched the temporal/spatial variability in the current velocity. We postulate that in a system with no strong oceanographic features, the scale of spatially coherent physical forcing (e.g. tidal periodicity) can regulate the formation or maintenance of larval patches; however, swimming ability may modulate it.     

Using spatial, seasonal, and diel drift patterns of larval Lost River suckers Deltistes luxatus (Cypriniformes: Catostomidae) and shortnose suckers Chasmistes brevirostris (Cypriniformes: Catostomidae) to help identify a site for a water withdrawal structure on the Williamson River, Oregon

A small irrigation diversion dam near Chiloquin, Oregon, was removed and replaced with a pump station to improve fish passage for Lost River suckers (Deltistes luxatus) and shortnose suckers (Chasmistes brevirostris) entering the Sprague River on their spawning migrations. During the developmental phase of the pump station, a need was identified to better understand the larval drift characteristics of these endangered catostomids in order to reduce entrainment into the irrigation system. The spatial, seasonal, and diel distribution of drifting larvae was measured during the 2004 spawning season at two proposed sites on the Williamson River where the pump station could be located. Larval drift for both species coincided with the irrigation season making them subject to entrainment into the irrigation system. Drift occurred almost exclusively at night with larvae entering the drift at sunset and exiting the drift at sunrise. Nighttime larval densities were concentrated near the surface and at midchannel at both sites. Densities were generally greater on the side of mid-channel with greater flow. During early morning sampling we detected a general shift in larval drift from surface to subsurface drift. We also observed an increase in larval densities towards the shore opposite from the proposed pump station at the upper site whereas larval densities remained high at midchannel at the lower site. During daytime sampling, the few larvae that were collected were distributed throughout the water column at both pump sites. This study found that larvae drifting during all time periods were generally distributed further across the cross section, deeper in the water column, and closer to where the proposed water withdrawal structure would be built at the downstream site when compared to the upstream site. Recommendations were provided to locate the withdrawal facility at the upstream site and operate it in a manner such that larval entrainment would likely be minimized.     

Downstream drift of the larvae of Chironomidae (Diptera) in the River Chew,S.W. England

The drift of larval Chironomidae (Diptera) was investigated at two sites on the River Chew, using a pump filtration system with 50 µm mesh-aperture aerial nets, situated on the river bank.Chironomid larvae were found to drift in far greater numbers than previously reported, even in slow-flowing water. First and second instar larvae dominated the drift.Variation in drifting behaviour between taxa was observed, where Orthocladiinae drifted in all instars and Chironominae predominantly as first and second instars. Proportionally different rates of drift were observed between these taxa from the benthos at the two sites.It is suggested that the majority of chironomid drift represents an active dispersal and colonisation mechanism by which population redistribution and habitat selection occurs.     

Noncalcifying larvae in a changing ocean: warming,not acidification/hypercapnia,is the dominant stressor on development of the sea star Meridiastra calcar

Climate change driven ocean warming and acidification is potentially detrimental to the sensitive planktonic life stages of benthic marine invertebrates. Research has focused on the effects of acidification on calcifying larvae with a paucity of data on species with alternate developmental strategies and on the interactive effects of warming and acidification. To determine the impact of climate change on a conspicuous component of the intertidal fauna of southeast Australia, the development of the noncalcifying lecithotrophic larvae of the sea star Meridiastra calcar was investigated in the setting of predicted ocean warming (+2 to 4 ^°C) and acidification (?0.4 to 0.6 pH units) for 2100 and beyond in all combinations of stressors. Temperature and pH were monitored in the habitat of M. calcar to place experiments in context with current environmental conditions. There was no effect of temperature or pH on cleavage stage embryos but later development (gastrula‐larvae) was negatively effected by a +2 to 4 ^°C warming and there was a negative effect of ?0.6 pH units on embryos reaching the hatched gastrula stage. Mortality and abnormal development in larvae increased significantly even with +2 ^°C warming and larval growth was impaired at +4 ^°C. For the range of temperature and pH conditions tested, there were no interactive effects of stressors across all stages monitored. For M. calcar, warming not acidification was the dominant stressor. A regression model incorporating data from this study and projected increasing SST for the region suggests an increase in larval mortality to 70% for M. calcar by 2100 in the absence of acclimation and adaptation. The broad distribution of this species in eastern Australia encompassing subtropical to cold temperate thermal regimes provides the possibility that local M. calcar populations may be sustained in a warming world through poleward migration of thermotolerant propagules, facilitated by the strong southward flow of the East Australian Current.     

Drift behaviour and microhabitat selection in the preimaginal stages of Simulium chutteri (Diptera Simuliidae)

The aquatic stages of the cattle biting pest, Simulium chutteri Lewis utilize river flow in their dispersal and colonization behaviour. Peaks of drifting activity in S. chutteri larvae occurred in the early morning and late afternoon. It is deduced that female flies scatter eggs onto slow flowing waters upstream of rapids. Larvae hatch from drifting eggs and colonize substrates in slower flowing regions upstream of rapids, while later stage instars move into faster flowing regions within rapids where they complete their development. This microhabitat selection by S. chutteri leads to rapid attainment of large population sizes in suitable sections of river and reduces competition between different stage larvae. In their utilization of a variety of microhabitats the larvae of this species differ from co-existing simuliid species which restrict developmental stages to single habitats.Catastrophic drift was recorded for S. chutteri larvae and could be a mechanism to regulate population size.Drift of simuliid larvae off rapids was not related to benthic population densities in the rapids and was therefore not due to excessive production. It is concluded that larval drift off rapids is related to habitat disturbances associated with water flow fluctuations and the activity of aquatic predators and other animals.The distribution of S. chutteri in the Vaal River is restricted by oviposition requirements of the adult female. Knowledge of drift behaviour and water flow requirements of Simuliidae have been applied to manage the population size of S. chutteri in the Vaal River.