Diel Habitat Partitioning by Bull Charr and Cutthroat Trout During Fall and Winter in Rocky Mountain Streams

We used underwater observation to determine diel habitat partitioning between bull charr, Salvelinus confluentus, and cutthroat trout, Oncorhynchus clarki, during fall and winter (0.1–8.3°C) in two Rocky Mountain streams that differed in habitat availability. The majority (>70%) of both species emerged from concealment cover at night, though bull charr exhibited a greater tendency for nocturnal behavior than cutthroat trout. Differences in day and night counts were most pronounced at temperatures <3°C, when very few fish of either species were observed in the water column during the day, but both species were common at night. Both species used concealment cover of large woody debris and boulder substrate crevices in deep pools during the day. At night, fish emerged from cover and habitat use shifted to shallow water with low cover. Microhabitat partitioning among species and size classes occurred at night, cutthroat trout moving into shallower, faster water that was farther from cover compared to bull charr. Smaller fish of both species occupied focal positions in slower, shallower water closer to the substrate than larger fish. Large, mixed-species aggregations also were common in beaver ponds both day and night. High variation in diel and site-specific winter habitat use suggests the need for caution in developing habitat suitability criteria for salmonids based solely on daytime observations or on observations from a few sites. Our results support the need to incorporate nocturnal habitat use and partitioning in studies of salmonid ecology.     

Growth, survival, and starvation resistance of Colorado squawfish larvae

Synopsis Growth and survival of Colorado squawfish, Ptychocheilus lucius, larvae under fluctuating 18, 22, and 26° C (5° C diel fluctuations) and constant 18, 22, 26° C, and 30° C temperature conditions and ration size corresponding to 12.5, 28,64,142, 320 brine shrimp nauplii fish^–1 day^–1 determined from laboratory experiments. Growth was optimal at 31° C and high at temperatures of 26° C to 30° C, at the highest food abundance. Lowest growth was under lowest food rations and highest temperatures. Growth of Colorado squawfish larvae declined substantially at temperatures < 22° C. Neither growth nor survival was significantly different between fluctuating or constant regimes. Survival of Colorado squawfish larvae was highest (95%) at 26.2° C and 235 nauplii fish^–1 day^–1 and high at temperatures of 20 to 30° C with food abundance > 180 nauplii fish^–1 day^–1. Survival was lowest when food abundance was low and temperature was high. Highest mortality occurred more than 20 days after experiments began and mortalities occurred sooner in higher than lower temperatures. Colorado squawfish larvae denied food for 5, 10, or 15 d after first feeding could have begun (6 d), had survival greater than 87 % which was equivalent to continuously fed controls. Survival of fish denied food for 17.5 d after feeding could have begun declined from 84% before feeding to 57% after feeding. Point of no return was estimated between 17.5 and 20 d. Colorado squawfish have relatively high starvation resistance. Low, stable flows that simulate natural hydrographs may enhance growth, survival, and recruitment of early life stages of Colorado squawfish by increasing water temperature and food abundance in regulated rivers of the Colorado River basin.     

Activity patterns in northern rock sole are mediated by temperature and feeding history

Studies of activity patterns of fishes rarely consider the impact of environmental conditions or organism state. In this paper we demonstrate the influence of temperature and feeding history on the diel activity patterns of juvenile (age 1+) northern rock sole Lepidopsetta polyxystra (Orr and Matarese). Activity type (benthic vs. water column) and level were determined from hourly video recordings of fish in laboratory tanks with the aid of infrared illumination. Groups of rock sole (n = 4 per group) were observed at 2, 5, 9 and 13 °C without food and at 9 °C with food offered once in the morning, at dusk or at night. In all experiments, rock sole exhibited peak activity levels during the dusk and dawn transition periods and a temporal segregation of activity mode. Daytime and crepuscular activity was predominantly benthic, occurring within a few centimeters of the tank bottom, whereas a significant fraction of nighttime activity (up to 90%) occurred with rock sole swimming in the water column or at the water surface. The primary effect of water temperature on rock sole behavior was a reduction in daytime and crepuscular activity at low temperatures. Conversely, nighttime activity levels were independent of water temperature, resulting in a thermally induced shift in the primary activity period. Morning and dusk feeding produced a short-term (1 h) elevation in activity followed by activity suppression until the subsequent crepuscular phase. Nighttime feeding resulted in a longer period of elevated activity (2 h) and did not suppress further benthic or water column activity. These observations suggest a temporal segregation of activity between daytime foraging and nighttime movement that balances energetic demands with predation risk.     

Seasonal variation in diel behaviour and habitat use by age 1+ Steelhead (<Emphasis Type="Italic">Oncorhynchus mykiss</Emphasis>) in Coast and Cascade Range streams in Oregon,U.S.A.

The seasonal diel behaviour of age 1+ steelhead from Coast and Cascade Range streams in Oregon was examined in the field and in laboratory streams. During the summer, fish from both areas were active during the day in natural streams: they held position in the water column in moderate velocities and depths. At night, fish were in slower water, closer to the bottom above smaller substrates. In winter, diel behaviour differed between the two groups. Coastal fish exhibited behaviour similar to that observed in the summer. Cascade Range fish were not observed during the day, but were found at night, holding positions close to the bottom in slower water. In laboratory streams, fish from both regions were subjected to a decreasing temperature regime from 16°C to 2°C. Use of cover for concealment during the day was negatively correlated with water temperatures for both groups. However, the shelter-seeking response to declining water temperatures was significantly greater for Cascade fish than it was for coastal fish. Field and laboratory observations of diel behaviour support the hypothesis that steelhead from the two geographic regions have different adaptive strategies for winter conditions and that these differences, because they persisted even in laboratory conditions, are probably genetically based.     

Ontogenetic Behavior and Migration of Gulf of Mexico Sturgeon, Acipenser oxyrinchus desotoi, with Notes on Body Color and Development

We observed Suwannee River Gulf sturgeon, Acipenser oxyrinchus desotoi, in the laboratory and found free embryos (first interval after hatching) hid under rocks and did not migrate. Thus, wild embryos should be at the spawning area. Larvae (first interval feeding exogenously) initiated a slow downstream migration, and some juveniles (interval with adult features) continued to migrate slowly for at least 5 months, e.g., a 1-step long larva-juvenile migration. No other population of sturgeon yet studied has this migration style. A conceptual model using this result suggests wild year-0 sturgeon have a variable downstream migration style with short-duration (short distance) migrants and long-duration (long distance) migrants. This migration style should widely disperse wild fish. The model is supported by field studies that found year-0 juveniles are widely dispersed in fresh water to river km 10. Thus, laboratory and field data agree that the entire freshwater reach of river downstream of spawning is nursery habitat. Foraging position of larvae and early juveniles was mostly on the bottom, but fish also spent hours holding position in the water column, an unusual feeding location for sturgeons. The holding position of fish above the bottom suggests benthic forage in the river is scarce and fish have evolved drift feeding. The unusual migration and foraging styles may be adaptations to rear in a river at the southern limit of the species range with poor rearing habitat (low abundance of benthic forage and high summer water temperatures). Suwannee River Gulf sturgeon and Hudson River Atlantic sturgeon, A. o. oxyrinchus, are similar for initiation of migration, early habitat preference, and diel migration. The two subspecies differ greatly for migration and foraging styles, which is likely related to major differences in the quality of rearing habitat. The differences between Atlantic sturgeon populations show the need for geographical studies to represent the behavior of an entire species.     

Seasonal and Diel Changes in Behaviour,Microhabitat use and Preferences by Young Pool-dwelling Atlantic Salmon,Salmo salar,and Brown Trout,Salmo trutta

There was a pronounced decline in activity of young pool-dwelling Atlantic salmon, Salmo salar, and brown trout, Salmo trutta, as the water temperatures dropped in the autumn and early winter, and the fish switched from a predominantly diurnal towards a nocturnal activity pattern. Such a switch in activity pattern has previously been observed in young brown trout, but the present study is the first documentation for juvenile Atlantic salmon under natural conditions. Juvenile fish fed actively even when water temperatures were below 0°C, although foraging behaviour at near-freezing temperatures was recorded exclusively during night surveys. This indicates that other proximate factors, in addition to water temperature, affect the activity of young salmon and trout in rivers. Trout kept feeding positions significantly higher above bottom than salmon in August and September, but both species reduced the height above bottom at the onset of winter, possibly due to reduced swimming performance and lowered food availability in the upper part of the water column.     

Space use and feeding patterns of an offshore fish assemblage in a shallow mesotrophic lake

Synopsis Diel and spring/summer space-use and feeding patterns were investigated in an assemblage dominated by five fish species occupying the offshore waters of Lake Opinicon, a shallow mesotrophic lake in southeastern Ontario. We assessed fish distribution and diel movement in May and July through the use of gill nets set at various depths in 1.5–7.0 m depth contour zones, supplemented by observations of fish reaction to the nets. Golden shiners and alewives occupied the upper part of the water column, with the former concentrated at the littoral zone-open water interface, and the latter in the open water. Yellow perch occupied the lower part of the water column in all depth contours. Bluegills were abundant in the upper to midwater depths in all contour zones; black crappies were concentrated in the 2.5–3.5 m zones. All of these species showed either a diel or a spring-summer change in distribution pattern. Bluegills were more abundant in offshore locations in July, whereas golden shiners and yellow perch were more abundant onshore in May. Alewives and black crappies showed distinct diel movements in July, as they were largely absent from the study area during the day, but returned at night to feed. In general, there was more spatial separation among the five species in July than in May.Patterns of spatial distribution among the species generally corresponded with the type and variety of prey consumed, and with diel movement of prey in the case of water column feeders. Other factors that apparently affected spatial distribution and seasonal shifts in this assemblage were risk of predation (golden shiner), spawning activity (alewife), and a decline in prey abundance from spring to summer (bluegill and yellow perch).     

Diurnal changes in the vertical distribution of cladocerans in a biomanipulated lake

The diel vertical migration of cladoceran zooplankton in LakeVesijärvi was studied after a distinct improvement of the waterquality following mass removal of fish. Four out of seven speciesshowed diurnal changes in vertical distribution. The two speciesdominating at 0–10 m, Bosmina crassicornis and Daphniacucullata, migrated reverse, while hypolimnetic Bosminalongispina and Ceriodaphnia quadrangula ascended at night.The migration did not cross the thermocline, suggesting thatzooplankton had a minor role in restricting the availability ofphosphorus for phytoplankton. The reverse migration in theepilimnion was likely connected to the feeding behaviour of thedominating planktivores. During night, smelt (Osmeruseperlanus (L.)) migrated from the hypolimnion to the surface tofeed on daphnids, while roach (Rutilus rutilus (L.)) thatinhabited the 5–15 m depth, was day-active and fed mainly on Bosmina. In the hypolimnion, the cladocerans used the low oxygenconcentration as a refuge and at night migrated to layers moreprofitable for feeding. This suggests that the predation by fishwas the ultimate factor of the vertical migration, but otherenvironmental factors determined its magnitude.     

Near- bottom depletion of zooplankton over coral reefs: I: diurnal dynamics and size distribution

Changes in the near-bottom abundance of zooplankton on scales of centimeters to meters and hours to seasons are of great importance to corals and other benthic zooplanktivores. Our objective was to characterize such spatio-temporal changes over several coral reefs in the Gulf of Aqaba, Red Sea. Using arrays of underwater pumps, we found a substantial depletion of zooplankton near the bottom. Vertical gradients in zooplankton abundance were steeper during the night than day, mostly due to a greater nocturnal increase in zooplankton biomass higher in the water column. On average, the layer <1 m above bottom (mab) was depleted by 2.6±2.2 mg m^–3 (46±35%) and 1.4±1.4 mg m^–3 (37±43%) during night and day, respectively. A long time series of bi-weekly samples at 0.5 mab, lasting 1.5 years, indicated a doubling of the biomass during the night with no apparent seasonality. The diel change was due to an increase in the abundance of only large (>200 m) zooplankters around dusk and their disappearance in the morning. Diurnal predation by zooplanktivorous fish, sediment resuspension by benthivorous fish and zooplankton behavior appear to control the dynamics of suspended particles over the reef, creating sharp vertical gradients and a remarkable diel cycle in the ratio between nutritious plankton and inorganic particles.     

Predator-dependent diel migration by <Emphasis Type="Italic">Halocaridina rubra</Emphasis> shrimp (Malacostraca: Atyidae) in Hawaiian anchialine pools

Diel migration is a common predator avoidance mechanism commonly found in temperate water bodies and increasingly in tropical systems. Previous research with only single day and night samples suggested that the endemic shrimp, Halocaridina rubra, may exhibit diel migration in Hawaiian anchialine pools to avoid predation by introduced mosquito fish, Gambusia affinis, and perhaps reverse migration to avoid the predatory invasive Tahitian prawn, Macrobrachium lar. To examine this phenomenon in greater detail, we conducted a diel study of H. rubra relative abundance and size at 2-h intervals in three anchialine pools that varied in predation regime on the Kona-Kohala Coast of Hawai‘i Island. We found two distinct patterns of diel migration. In two pools dominated by visually feeding G. affinis, the abundance of H. rubra present on the pool bottom or swimming in the water column was very low during the day, increased markedly at sunset and remained high until dawn. In contrast, in a pool dominated by the nocturnal predator M. lar, H. rubra density was significantly lower during the night than during the day (i.e., a pattern opposite to that of shrimp in pools containing fish). In addition, we observed that the mean body size of the shrimp populations varied among pools depending upon predator type and abundance, but did not vary between day and night in any pools. Our results are consistent with the hypothesis that H. rubra diel migratory behavior and size distributions are influenced by predation regime and suggest that diel migration may be a flexible strategy for predator avoidance in tropical pools where it may be a significant adaptive response of endemic species to introduced predators.     

Environmental Biology of an Endemic Cyprinid,Varicorhinus alticorpus,in a Subtropical Mountain Stream of Taiwan

An endemic freshwater fish, Varicorhinus alticorpus, was studied from 1990 to 1993 in a fast flowing mountain stream in southern Taiwan (22°30'N, 120°30'E). The analysis of environmental conditions suggested that the fish could tolerate water temperatures between 19–24°C, a pH of 8.0–8.8, and flow velocity from 20–100cmsec^–1. The fish occupied riffles and pools. They foraged and schooled during daytime and aggregated in crevices between rocks at night. Most juveniles stayed at the shallow sandy-pebble flat by the pool where flow was slow. The species feed primarily on the periphyton growing on the rocks, and leave unique scars after grazing. One peak of juvenile recruitment was observed right after the rainy season, suggesting that reproduction of the fish has adapted to the climate. The change of abundance of adults along the habitats (at 150 to 800 m altitude) also indicated that the fish might migrate to low altitudes for spawning and disperse back to higher altitudes for exploiting new resources. The environmental biology of this fish shows an example of a cyprinid adapted to habitats in a subtropical mountain stream.     

Diel activity and thermoregulatory behavior of a fully aquatic frog: Xenopus laevis

Ten adult Xenopus laevis were tested individually for 48-hr periods, following an initial 24-hr introductory period, in electronic shuttleboxes which allowed them to control water temperatures without operant conditioning. Locomotor activity was recorded via photocell-monitored light beams. The frogs were nocturnal, being nearly twice as active at night as during the day. The mean preferred temperature was 22.4°C, with no significant difference between night (22.5°C) and day (22.3°C), although the modal preferendum shifted from 24°C by day to 22°C at night, with a corresponding change in skewness. The range of voluntarily occupied temperatures was 14–32°C by day and 14–29°C at night. The median thermal preferendum was 22°C both day and night.     

Ovigerity,selective predation,and variable diel vertical migration in Euchaeta elongata (Copepoda: Calanoida)

Summary We present a statistical analysis of a previously published (Yen, 1983) but heretofore unanalyzed data set on the vertical distributions and diel vertical migration (DVM) of adult females of the marine planktonic copepod Euchaeta elongata in Dabob Bay, Washington, USA. Non-ovigerous females were strongly migratory on all four dates sampled, residing between 75–175 m during the day and at shallower depths during the night, commonly entering the upper 50 m of the water column. In contrast, ovigerous females were non-migratory or weakly migratory, largely remaining between 100–175 m both day and night, and entering the upper 50 m of the water column only rarely. Thus non-ovigerous females always migrated much more strongly, as measured by both amplitude of migration and the proportion of animals migrating, than did ovigerous females. These results led us to hypothesize that differential susceptibility to visually orienting predators was the cause of these differences in DVM behavior in female E. elongata, and we subsequently undertook an experimental study of the feeding selectivity of the copepod's natural predator, Pacific herring (Clupea harengus pallasi). Pacific herring exhibited a highly significant preference for ovigerous over nonovigerous adult female E. elongata. The demographic consequences of variable DVM in adult female E. elongata were investigated by way of life table analyses. Results indicated that under conditions of thermal stratification of the water column there is a distinct demographic disadvantage (reduced rate of realized population growth) incurred by non-migratory or weakly migratory ovigerous females due to delayed egg development at cooler subsurface temperatures. We conclude that ovigerous female E. elongata remain at depth both day and night to avoid visually orienting predators, and that such behavior must afford the copepod a demographic advantage of no less than a 26% reduction in adult mortality to offset the demographic cost of delayed egg development.     

Ontogenetic Behavior and Migration of Volga River Russian sturgeon, Acipenser gueldenstaedtii, with a Note on Adaptive Significance of Body Color

We conducted laboratory experiments with Volga River Russian sturgeon, Acipenser gueldenstaedtii, to develop a conceptual model of early behavior. We daily observed fish from day-0 (embryos, first life interval after hatching) to day-29 feeding larvae for preference of bright habitat and cover, swimming distance above the bottom, up- and downstream movement, and diel activity. Hatchling embryos initiated a downstream migration, which suggests that predation risk of embryos at spawning sites is high. Migration peaked on days 0–5 and ceased on day 7 (8-day migration). Migrants preferred bright, open habitat and early migrants swam-up far above the bottom (maximum daily median, 140cm) in a vertical swim tube. Post-migrant embryos did not prefer bright illumination but continued to prefer white substrate, increased use of cover habitat, and swam on the bottom. Larvae initiated feeding on day 10 after 170.6 cumulative temperature degree-days. Larvae did not migrate, weakly preferred bright illumination, preferred white substrate and open habitat, and swam near the bottom (daily median 5–78cm). The lack of a strong preference by larvae for bright illumination suggests foraging relies more on olfaction than vision for locating prey. A short migration by embryos would disperse wild sturgeon from a spawning area, but larvae did not migrate, so a second later migration by juveniles disperses young sturgeon to the sea (2-step migration). Embryo and larva body color was light tan and tail color was black. The migration, behavior, and light body color of Russian sturgeon embryos was similar to species of Acipenser and Scaphirhynchus in North America and to Acipenser in Asia that migrate after hatching as embryos. The similarity in migration style and body color among species with diverse phylogenies likely reflects convergence for common adaptations across biogeographic regions.     

Temporal plasticity in thermal‐habitat selection of burbot Lota lota a diel‐migrating winter‐specialist

In this study, animal‐borne telemetry with temperature sensors was coupled with extensive habitat temperature monitoring in a dimictic reservoir, to test the following hypotheses: behavioural thermoregulation occurs throughout the year and temperature selection varies on a diel and seasonal basis, in a winter‐specialist diel‐migrating fish. Burbot Lota lota demonstrated nightly behavioural thermoregulation throughout the year, with a large seasonal shift between selection for very cold temperatures (<2° C) optimal for reproduction during the spawning period and selection for warmer temperatures (12–14° C) optimal for hunting and feeding during non‐reproductive periods. During daylight hours, while L. lota avoided habitats warmer than optimal for reproduction and feeding during the spawning and non‐reproductive periods, respectively, active selection was limited to selection for 4–6° C habitat during the prespawning period. Although behavioural thermoregulation explained the night‐time migration, behavioural thermoregulation only partially explained daytime behaviour, indicating that diel migration is best explained by a combination of factors. Thus, thermal‐habitat selection was a good predictor of night‐time habitat occupancy in a diel‐migrating species. Together, these results show that thermal‐habitat selection by fishes may be important throughout the year and a more seasonally plastic behaviour than previously recognized.     

Low Temperature Tolerance of Pacu, Piaractus mesopotamicus

Pacu, Piaractus mesopotamicus (=Colossoma mitrei), is a South American warm water fish species found in the temperature range of 15–35°C. The culture of a warm water species in temperate regions demands knowledge on its temperature requirements. Pacu introduction into the Israeli fish culture system is being considered. Temperature range in the region is 8–33°C, thus the minimum winter water temperatures might be a limiting factor. To determine what is the minimum temperature pacu would tolerate, and hence which overwintering operations in warm-temperate regions are required for this warm water species, low temperature tolerance tests in the laboratory and observations in the field were carried out. Laboratory experiments reducing temperature by 1–3°C per day were carried out with fish of 150–200g, about the size pacu reach after one culture season. The field observations compared survival of two-year-old pacu of 1.3kg mean weight overwintered in outdoors and in greenhouse ponds. For one-year-old fish 7.5°C was found to be the lower temperature tolerance limit. Two-year-old fish withstand short exposures to this temperature rather well and their lower tolerance limit might be lower. This indicates that in warm-temperate regions pacu should survive in outdoors ponds. In this case some loss of weight should be expected, and suspension of feeding when temperature drops below 16–18°C is recommended to avoid wasting feed that the fish will not consume anyway. To be in the safe side, inflow of the warmest available water into the ponds is recommended if maximum water temperature drops to 10°C or below. Overwintering in greenhouses or other heated facilities would be recommended if an exceptionally cold winter is expected and for regions with lower winter minimum temperatures.     

Respiration of juvenile Arctic cod (Boreogadus saida): effects of acclimation,temperature, and food intake

Oxygen consumption (VO₂) of juvenile Arctic cod (Boreogadus saida) was investigated at low tempera tures (six temperatures; range -0.5 to 2.7°C). Small (mean wt. 6–8 g) and large (mean wt. 14 g) fish were acclimated, or adjusted to a constant temperature (0.4°C), for 5 months and then tested for metabolic cold adaptation (elevated metabolic rates in polar fishes). Short-term (2 weeks) acclimated fish showed elevated VO₂ similar to previously established values for polar fishes, but there was no such evidence after longterm acclimation. Long-term acclimation caused VO₂ values to drop significantly (from 86.0 to 46.5 mg O₂·kg^–1·h^–1, at 0.4°C), which showed that metabolic cold adaptation was a phenomenon caused by insufficien: acclimation time for fish in respiration experiments. We also measured the effects of temperature and feeding on VO₂. A temperature increase of 2.3°C resulted in relatively large increases in VO₂ for both longand short-term acclimated fish (Q₁₀ = 6.7 and 7.1, respectively), which suggests that metabolic processes are strongly influenced by temperature when it is close to zero. Feeding individuals to satiation caused significant increases in VO₂ above pre-fed values (34–60% within 1–2 days after feeding). Respiration budgets of starved and fed Arctic cod at ambient temperatures in Resolute Bay N.W.T., Canada, were used to model annual respiration costs and potential weight loss. Low respiration costs for Arctic cod at ambient temperatures result in high growth efficiency during periods of feeding and low weight loss during periods of starvation.     

Diel Periodicity and Environmental Influence on the Smolt Migration of Arctic charr, Salvelinus alpinus, Atlantic salmon, Salmo salar, and Brown Trout, Salmo trutta, in Northern Norway

We caught smolts of Arctic charr, Salvelinus alpinus (L.), Atlantic salmon, Salmo salar L., and brown trout, Salmo trutta L., in a trap situated at the mouth of the river Halselva (70° N 23° E), northern Norway during a 5-year period. Salmon and charr were the first to leave freshwater at the end of May, while most trout left freshwater about 14 days later. Whereas the midnight sun shines continuously during the downstream migration period, the light intensity has a diel intensity pattern. The majority of the descending migrants were recorded during the night. The number of descending fish was relatively low at water temperatures below 3°C. The increase in water level was largely caused by snowmelt and thus correlated with lower water temperatures. The number of migrants of all three species increased with increasing water level and decreased with increasing water temperature, with the exception of trout, which increased with water temperature. Notably, the increase in number of migrants was also correlated with the increase in water level the following day, indicating that fish movements represent an early response to a later spate. There was no significant relationship between the number of migrants and the daily change in water level or temperature. The three species were highly synchronised in their daily number of migrants. The strongest synchronisation was found between Arctic charr and Atlantic salmon, followed by Arctic charr and brown trout.     

Ontogenetic Behavior and Migration of Dabry's Sturgeon, Acipenser Dabryanus, from the Yangtze River, With Notes on Body Color and Development Rate

We conducted laboratory experiments with Dabry's sturgeon, Acipenser dabryanus, from the upper Yangtze River to develop a conceptual model of early behavior. We daily observed fish from day-0 (embryo, first life interval after hatching) to day-30 feeding larva for preference of bright habitat and cover, swimming distance above the bottom, up- and down-stream movement, and diel activity. Hatchling to day-12 embryos and days 13–24 larvae were similar for ontogenetic behavior, i.e., neither initiated a dispersal migration, both swam within 15cm of the bottom, both preferred bright habitat, and neither strongly preferred cover or open habitat. Embryos and larvae were weakly active day and night. Days 72–76 juveniles had a weak nocturnal downstream migration, indicating wild juveniles disperse from a spawning site. In other sturgeon species yet studied representing three genera on three continents, Dabry's sturgeon is the first that does not disperse as an embryo or larva. Development of Dabry's sturgeon is slow, requiring more cumulative temperature degree days per millimeter of larvae TL than is required for other sturgeons to develop into larvae. Thus, a dispersal migration that diverts energy from development may not be adaptive. The available information suggests the initial dispersal of early life intervals is likely done by females, which spawn in a dispersed spawning style, not the usual aggregated spawning style. Juvenile migrants had a black body and tail with a light line along the lateral scutes. The color of juvenile migrants shows that a dark body and tail is characteristic of Acipenser that migrate downstream as larvae or juveniles.     

The biology of pygmy whitefish, Prosopium coulterii, in a closed sub-boreal lake: spatial distribution and diel movements

Using gillnets and trap nets, we examined the spatial distribution, diel movements, and environmental tolerances of pygmy whitefish, Prosopium coulterii, in a small boreal lake in north-central British Columbia. Most gillnets were set below the thermocline but we also fished a shore net in the littoral zone. During the ice-free season (May to November) there was a strong diel onshore–offshore movement: during the day pygmy whitefish were offshore and below the thermocline (water temperatures of 4–6°C) but at night they were inshore and above the thermocline (water temperatures of 12–18°C). This onshore–offshore movement occurred close to the bottom and, regardless of where they were caught, most fish were <4 m off the bottom. Oxygen concentrations in most of the hypolimnion dropped to <5.0 mg l⁻¹ in June and by late August to <1.0 mg l⁻¹; indicating pygmy whitefish can tolerate low oxygen conditions. The catch of pygmy whitefish in gillnets set below the thermocline was highly skewed: 53% of the nets were empty, 37% caught 18 or less fish, and 10% caught 70% of the total catch (742 fish). Trap nets produced similarly skewed results: most trap net sets caught no pygmy whitefish but one set caught over 2,000 individuals. Our catch data suggest that in Dina Lake #1 some pygmy whitefish aggregate.