Effects of vitamin B1 (thiamine) deficiency in lake trout (Salvelinus namaycush) alevins at hatching stage

The objectives of this study were to examine the relationship between thiamine concentrations in unfertilized eggs and yolksac individuals of lake trout (Salvelinus namaycush), along with any associated histopathological changes in the tissues of alevins at the hatching stage. We address these questions in a lake trout population from different spawning grounds of Lake Michigan (North and South), known for compromised survival due to early mortality syndrome (EMS). However, a dichotomous forage base of lake trout spawning stocks, with a dietary thiaminase-rich alewife in the North, and dietary low-thiaminase round goby in the South, provides the basis for the assumption that different diets may lead to differences in severity of EMS between different stocks. Lake trout eggs of 18 females were collected and fertilized individually with the sperm of several males. The eggs, eyed embryos and newly-hatched alevins were sampled to examine thiamine utilization during embryogenesis. Progenies of females with low (< 0.73 nmol/g) and high (> 0.85 nmol/g) levels of thiamine were chosen for histological studies. The obtained results showed that total thiamine levels in the body and yolk of eyed embryos and alevins at hatching were influenced by thiamine levels of unfertilized eggs and it decreased during embryogenesis (to 51% in eyed embryos and 28% in newly-hatched alevins in comparison to unfertilized eggs). The survival of lake trout until hatching stage does not correlate with the thiamine level, however it was affected by collection site and was significantly higher in fish from the South site (Julian's Reef). At the hatching stage, no pathological changes were observed in the brain, olfactory lobe, retina or liver in embryos regardless of thiamine concentrations in unfertilized eggs. It has been concluded that an enhanced thiamine requirement for the fast muscle mass growth near the swim-up stage is responsible for overt and histopathological signs of EMS. Current study confirms earlier findings that lake trout suffering from EMS can be successfully treated by immersion in thiamine solution as late as at the swim-up stage.     

Spatial and Temporal Relations between Fluvial and Allacustrine Yellowstone Cutthroat Trout, Oncorhynchus clarki bouvieri, Spawning in the Yellowstone River, Outlet Stream of Yellowstone Lake

Yellowstone cutthroat trout (YCT), Oncorhynchus clarki bouvieri, that spawn in the outlet of Yellowstone Lake show two potamodromous migration patterns, fluvial and allacustrine. The main purpose of this study was to determine whether those fluvial and allacustrine YCT represent reproductively isolated stocks. Redd surveys indicated spawning occurred during about 5 consecutive weeks between late May and mid-July 1993–1995. Lake fish (N=6), defined as radiotagged YCT that entered Yellowstone Lake after the spawning period (i.e. allacustrine pattern), were found in the river between the lake outlet (river kilometer [Rkm] 0) and Rkm 20.0 during spawning. Probable lake fish (N=28; tagged YCT that were last detected near the lake outlet) were found between Rkm 0 and Rkm 22.5 during spawning. River fish (N=4; tagged YCT that remained in the river when annual tracking concluded in fall, i.e. fluvial pattern) were found between Rkm 1.1 and Rkm 18.0 during spawning. Fidelity to spawning areas used between consecutive years was suggested by one of five lake fish and the single river fish for which data were available. Spatial overlap in spawning and a lack of temporal separation between the life-history types during spawning suggested that fluvial and allacustrine YCT were not reproductively isolated. Radiotagging, as well as visual observations made annually from boats during April and May, indicated fluvial YCT overwintered downstream from Rkm 14 and were few, probably on the order of 10% of all YCT that spawned in the Yellowstone River.     

Use of space and food by resident and migrant brown trout,Salmo trutta

Synopsis Parr and resident forms of brown trout,Salmo trutta, from Vangsvatnet Lake, Norway live in freshwater, while migrant forms live in coastal waters during summer and in freshwater during winter. About 80% of parr and residents live at depths <5 m, smolts and migrants are more confined to near-surface water. Brown trout partly segregate by size, age and sex from spring through autumn. More than 90% of parr and residents in the tributaries are 0–2 years old, 2–14 cm in length, in the littoral zone 0–3 years old, 7–24 cm in length, and in the pelagic zone 2–6 years old, 18–32 cm in length. The mean body length of equal-aged fish increases from tributaries through littoral to pelagic zones in the lake. Smolts (2–7 years, 14–29 cm) leave the lake from April through August and return during September–October. Migrants (2–11 years, 23–67 cm) leave the take in April–May and return during August–September; sexually mature fish return earlier than immatures. Female brown trout are less stream-dwelling, but more migrant and pelagic than males. Most individuals in the lake population spend the winter in the littoral zone. In the tributaries, diet differs significantly between age-groups of parr; young fish feed on smaller food items than do older fish. In the lake, parr and residents living in the same habitats feed on the same food items. Littoral brown trout feed mainly on insect larvae and chironomid pupae, pelagic brown trout feed on zooplankton and surface insects. Migrants feed little while staying in freshwater, except for matures which feed on young salmonids and surface arthropods during the 2 first months after they had returned from coastal waters. The results are discussed in relation to growth possibilities and mortality risks of the different habitats.Reprint request to B. Jonsson.     

Integration of acoustic telemetry and GIS to identify potential spawning areas for lake trout (Salvelinus namaycush)

Locations of potential spawning areas for lake trout (Salvelinus namaycush) were predicted in Lake Opeongo, Ontario, Canada using information gained via acoustic telemetry and geographic information system (GIS) technologies. From 1998 to 2000, 18 adult lake trout (mean fork length 553 mm) implanted with acoustic transmitters (battery life 2 years) were manually tracked. For evening fall locations within the erosive zone of the lake (determined using an existing sedimentation model), habitat variables (slope, depth, and effective fetch) were summarised using GIS. Sites selected by lake trout during the spawning window were in areas of mean fetch equal to 1.5 km and mean slope of 10.6% (n = 50 fixes). We used GIS to identify areas that matched the mean habitat criteria and thus locate potential spawning areas. This model correctly identified 19 of 21 known spawning sites, as well as additional sites used by spawning females in an earlier telemetry study. Depths of traditional fall netting sites are shallow compared to areas in which telemetered lake trout were found during evenings of the spawning period (means 3.1 vs. 5.1 m, respectively). Through the use of information on spawning habitat selection gained through telemetry and knowledge of the physical characteristics of the lake, we provide an alternative means of identifying potential spawning habitat for lake trout.     

Comparison of brook trout reproductive success and recruitment in an acidic adirondack lake following whole lake liming and watershed liming

Limestone applications to the catchment of one tributary to Woods Lake were highly effective in reducing stream acidity and stabilizing seasonal fluctuations in pH. The resulting improvement in stream water quality also led to a dramatic shift in reproductive strategy of the Woods Lake brook trout population. Prior to catchment liming, brook trout in Woods Lake were restricted to spawning on poor quality near shore substrate with limited ground water seepage. Reproductive success was limited by high mortality of eggs and larvae and recruitment from in lake spawning was not successful. Spawning brook trout did not utilize the tributary for spawning prior to watershed liming. Mitigation of acidity in the tributary, by catchment liming, effectively extended the spawning habitat available to the Woods Lake brook trout population and one year following treatment brook trout spawned successfully in the tributary for the first time in 6 years of observation. Significant recruitment of young trout into the lake population occurred from 1991 through 1993, although the absolute number of fish captured was relatively small. In the fall of 1993, four year classes of naturally spawned brook trout were present in the lake. Although reproductive success was enhanced by improving tributary spawning habitat in the Woods Lake basin, self maintenance of the population may be limited by low recruitment rates of young trout, due to high levels of summer mortality resulting from predation. Mitigation of this constraint would require substantially higher levels of fry production than were observed in Woods Lake and/or enhanced refugia for young trout. The results of this experiment suggest that re-establishment of tributary spawning populations of brook trout may be possible, with future reductions in acidic deposition, in acidic Adirondack lakes with limited in-lake spawning habitat.     

Spawning habitat selection and suitability for Japanese dace, Tribolodon hakonensis

We have investigated the effects of three current velocities and three substrate sizes (gravel to cobble) on the spawning behavior of Japanese dace, Tribolodon hakonensis, in spawning-induction and habitat-selection experiments. In the spawning-induction experiment under laboratory conditions the number of females induced to spawn was significantly fewer at low current velocity (ca. 5 cm s^–1) than at medium (ca. 30 cm s^–1) or high (ca. 50 cm s^–1) current velocity. Females spawned independently of substrate size in the experimental tank, and did not bury their eggs in the substrate bed. In the habitat-selection experiment under field conditions, females selected the substrate microhabitat and spawned more frequently at a site with medium substrate size (very coarse gravel; major axis ca. 40 mm) than at sites with small (fine gravel; ca. 8 mm) or large (cobble; ca. 70 mm) substrate size. We conclude that Japanese dace select spawning sites at least partly on the basis of current velocity and substrate size, which affects the survival rate of eggs.     

Factors affecting seasonal mortality of rosy bitterling (Rhodeus ocellatus kurumeus) embryos on the gills of their host mussel

I investigated the seasonal change in factors affecting embryonic mortality in the rosy bitterling, Rhodeus ocellatus kurumeus, a freshwater fish that spawns on the gills of living unionid mussels. Research was conducted in a small pond during 1999 and 2001 in which bitterling were provided with Anodonta sp. mussels for spawning. Bitterling spawned between April and July, peaking mid–late May. Seasonal survival rate of bitterling embryos in their mussel hosts was unimodal, with a peak between late April and mid May (about 70% of total spawnings). In mid April, survival was about 50%. The lowest survival was from late May to July (0%). Losses of bitterling embryos from mussels were identified by ejections from the mussel host. Ejections were categorized as either ejections of live embryos, or ejections of embryos that died in the mussel and were subsequently expelled from the mussel. Ejection rates of live embryos were higher in the earlier part of the spawning period (early–mid April) and dead embryo ejections in the later period (after June). The ejection rate of live embryos was higher among younger embryos earlier in the season, probably because of the incomplete development of morphological and behavioural traits associated with maintaining the embryo inside the mussel gill chambers, and as a consequence of a more protracted developmental period at low temperatures making them more susceptible to ejection. The ejection rate of dead embryos was higher in older embryos later in the season, and in larger mussels and at high embryo densities. The survival of embryos in mussels was probably related to oxygen availability, with mortalities probably caused by asphyxiation. Increased embryo mortalities may arise through competition among embryos, between embryos and mussel, and ambient dissolved oxygen levels. The optimal period for bitterling to spawn may represent a balance between two opposing factors; with positive and negative effects of a seasonal rise in temperature directly affecting embryonic growth rate and oxygen availability.An erratum to this article can be found at This revised version was published online January 2005 with the correction of the authors name.     

Effect of incubation temperature on green sturgeon embryos, <Emphasis Type="Italic">Acipenser medirostris</Emphasis>

Regulation of river flow and the amount of winter rainfall are the major factors affecting the water temperature of the spawning grounds, for green sturgeon in the Klamath River. During the primary spawning period of green sturgeon, mid-April to June, the water temperature may vary from 8 to 21°C. To estimate the potential implications of this modified thermal regime, we examined the survival and development in three progeny groups of green sturgeon embryos from zygote to hatch, at constant incubation temperatures (11–26°C). Temperatures 23–26°C affected cleavage and gastrulation and all died before hatch. Temperatures 17.5–22°C were suboptimal as an increasing number of embryos developed abnormally and hatching success decreased at 20.5–22°C, although the tolerance to these temperatures varied between progenies. The lower temperature limit was not evident from this study, although hatching rate decreased at 11°C and hatched embryos were shorter, compared to 14°C. The mean total length of hatched embryos decreased with increasing temperature, although their wet and dry weight remained relatively constant. We concluded that temperatures 17–18°C may be the upper limit of the thermal optima for green sturgeon embryos, and that the river thermal regime during dry years may affect green sturgeon reproduction.     

Balancing competing life‐stage requirements in salmon habitat rehabilitation: between a rock and a hard place

Gravel augmentation is often applied to rivers and streams to rehabilitate salmonid spawning and incubation habitat. However, the effect of gravel size on salmon spawning utilization and embryo survival during incubation is not well understood. We conducted an experiment on a regulated and previously mined Northern California salmonid‐bearing stream in which different sized gravel (small, medium, and large) patches were placed into the stream's degraded spawning reach. We documented Oncorhynchus tshawytscha (Chinook salmon) spawning activity within the three gravel sizes for two seasons. In addition, we deployed Chinook salmon embryos into each gravel size patch and allowed them to incubate until estimated emergence time. Although all experimental gravel sizes were predicted to be within the spawning population's mobilization capabilities, model results indicated the probability of salmon building redds decreased as substrate size increased. Conversely, embryo survival increased as gravel size increased. A possible mechanism of disparate Chinook salmon embryo survival is provided by an observed decrease in embryo survival correlating with greater presence of embryo predators (leeches), which are associated with smaller gravel. Our results indicate a parent‐offspring conflict in optimal spawning gravel size for Chinook salmon, and suggest that an intermediate gravel size would maximize overall reproductive success across both spawning and incubation life stages.     

Use of cover habitat by bull trout, <Emphasis Type="Italic">Salvelinus confluentus</Emphasis>, and lake trout, <Emphasis Type="Italic">Salvelinus namaycush</Emphasis>, in a laboratory environment

Lacustrine-adfluvial bull trout, Salvelinus confluentus, migrate from spawning and rearing streams to lacustrine environments as early as age 0. Within lacustrine environments, cover habitat provides refuge from potential predators and is a resource that is competed for if limiting. Competitive interactions between bull trout and other species could result in bull trout being displaced from cover habitat, and bull trout may lack evolutionary adaptations to compete with introduced species, such as lake trout, Salvelinus namaycush. A laboratory experiment was performed to examine habitat use and interactions for cover by juvenile (i.e., <80 mm total length) bull trout and lake trout. Differences were observed between bull trout and lake trout in the proportion of time using cover (F _1,22.6 = 20.08, P < 0.001) and bottom (F _1,23.7 = 37.01, P < 0.001) habitat, with bull trout using cover and bottom habitats more than lake trout. Habitat selection ratios indicated that bull trout avoided water column habitat in the presence of lake trout and that lake trout avoided bottom habitat. Intraspecific and interspecific agonistic interactions were infrequent, but approximately 10 times greater for intraspecific interactions between lake trout. Results from this study provide little evidence that juvenile bull trout and lake trout compete for cover, and that species-specific differences in habitat use and selection likely result in habitat partitioning between these species.     

Field activity of lake trout during the reproductive period monitored by electromyogram radiotelemetry

Many lake trout Salvelinus namaycush spawn in shallow areas along windswept shores. However, precise determination of time and location of spawning is limited by its nocturnal occurrence, and possibly by some postulated, but unproven, spawning in deep water by certain populations. In White Pine Lake, Ontario, a well-studied experimental lake, lake trout spawn in two shallow locations where some direct observation is possible. Radiotelemetry apparatus was used in 1991 to obtain records of the electromyograms (EMGs) produced by muscle activity in one adult male and one adult female lake trout, captured from the lake before their reproductive period and returned to the lake at the beginning of the spawning period. Both fish survived, though only the male was detected as active over the main spawning site, where it showed evening EMG activity patterns indicating considerable activity. At other times of day, when not located over the spawning site, but present elsewhere in the lake, the male's EMG (i.e. muscle) values were considerably reduced. At the cessation of the spawning period, the transmitter-equipped male's EMG record showed no further pattern indicative of high activity during what had formerly been the daily spawning period. The female was tracked as she moved around the lake, but based on both her location and level of EMG activity, is thought not to have spawned. In 1992, a second male was captured, equipped with a transmitter and released again. This fish failed to show spawning activity but, as with the 1991 female, moved fairly considerable distances in the lake. In one instance, in a swim of short duration (6 min), well away from the spawning grounds, this fish displayed EMG activity levels resembling those of the high activity of spawning. The results obtained appear to demonstrate that EMG telemetry could be used to assess the level of muscular activity occurring in lake trout, especially in relation to reproductive behaviour, and when they cannot be directly observed.     

Suitability of habitat for spawning lake trout

We provide further insight into the reproductive ecology and spawning requirements of lake trout. New comparative information about substrate characteristics, sediment transport, quality of interstitial water at spawning substrates, and the role of temperature in site selection and time of spawning is given for lakes Simcoe and Manitou (Ontario) and Seneca Lake (New York). Spawning lake trout commonly use stable lag deposits derived from glacial sediments, or relict features such as fans, bars or submerged talus slopes. Artificial breakwaters of broken material may also provide suitable substrates. Optimal particle sizes range from 4 to 10 cm diameter but larger materials to 30 cm are also successfully utilized for spawning. The transport of finer particulates by wind generated water movements may limit the suitability of some substrates and successful spawning sites are usually remote from depositional effects. Successful embryo development is associated with low nutrient conditions, with high dissolved oxygen (>7 mg L^-1) and with low un-ionized ammonia (<12.5 g L^-1) in the interstitial water of spawning substrates. Shallow-water spawning appears to be the common strategy of colonizing lake trout. Some deepwater spawning in the Great Lakes may reflect initial colonization in shallow-water and adaptation to later increases in water level, but some may also reflect unique behavioural and physiological adaptations. Temperature is an important cue, and many wild and hatchery stocks spawn at 8 to 13 °C with latitudinal shifts in the actual time of spawning. These requirements are summarized as a dichotomous key for evaluation of approaches to restoration of lost or damaged lake trout stocks.Presented at the Conference on Rehabilitation of Lake Trout in the Great Lakes: A Critical Assessment (sponsored by the Great Lakes Fishery Commission, Ann Arbor, Michigan, January 10–14, 1994).     

Life history and ecological characteristics of humper and lean ecotypes of lake trout stocked in Lake Erie

Through combined effects of over-exploitation and predation by invasive non-native sea lamprey Petromyzon marinus (Linneaus 1958), the lake trout Salvelinus namaycush (Walbaum 1972) population in Lake Erie collapsed and was considered extirpated by 1950; annual stocking occurred in 1969 to bolster and maintain this population as natural recruitment ceased. While various hatchery strains of lean ecotype lake trout remain the focus of Lake Erie’s stocking, between 2004 and 2011 a humper-like ecotype was stocked to aid re-establishment efforts. We assessed ecotype performance via adult fish collected during the summer-stratified period (2004–2017) and evaluated if life history and ecological characteristics varied between stocked ecotypes. Ecotype-specific differences in survival, length-at-age, accumulation of weight with length, and diet were observed. Lean lake trout grew faster (length-at-age) and matured at larger sizes than the humper ecotype. Humper lake trout exhibited higher sea lamprey wounding and lower survival rates than lean lake trout. Humpers also consumed more benthic prey fish than lean lake trout, suggesting this ecotype may exhibit a more benthic orientation than leans. Our study represents the first evaluation of how a non-lean ecotype lake trout performs outside their native ecosystem and provides fishery managers with valuable information relevant to re-establishing lake trout populations in the Great Lakes.

     

Mercury accumulation in five species of freshwater fish in Lake Tyrifjorden,south-east Norway,with emphasis on their suitability as test organisms

Synopsis Mercury concentration in axial muscle of brown trout, Salmo trutta, whitefish, Coregonus lavaretus, Arctic charr, Salvelinus alpinus, smelt, Osmerus eperlanus and pike, Esox lucius, were studied in Lake Tyrifjorden during 1978–1982. Our data demonstrate that older and bigger fish on an average have higher mercury concentration than smaller and younger. Further, complex life histories as in brown trout influence the pattern of mercury accumulation. During young stages accumulation in brown trout is moderate, while accumulation in older stages is highly correlated to lake residency time. Based on our data we suggest the following requirements for a test organism and the collecting procedure; (1) life history should be simple with small sexual differences, (2) ageing should be easy and reliable, and (3) large representative samples should be easily obtained during (4) a fixed biological period i.e. the spawning period. We consider smelt as an appropriate test organism based on these criteria.     

Use of artificial eggs in studies of washout depth and drift distance for salmonid eggs

Colour-coded artificial trout eggs were used in investigations of washout depth in a natural stream and of drift distance relative to water velocity in an experimental channel and in a section of natural stream.Washout depth was studied in a spawning riffle of a stream whose bankful discharge is 5.6 m³ s^–1. During an experiment when spates never exceeded 6.5 m³ s^–1 egg washout was severe at 5 cm depth within the gravel, variable at 10 cm and negligible at 15 cm. During another experiment when a spate of 9.0 m³ s^–1 (return period 10–20 years) occurred, washout was severe at 5 and 10 cm depth and variable at 15 cm. There was also evidence that some eggs were moved short distances downstream within the gravel without being washed out.Within experimental channels, drift distance could be predicted from multiple regressions relating logarithms of water velocity, percentage of eggs settled and distance from point of release. At a water velocity of 100 cm s^–1 at 0.6 depth, 50% of eggs would settle within 8 m of the point of release. At water velocities of 75 to 100 cm s^–1 drifting eggs would, on average, travel at c. 60% of water velocity and make 1 to 2 bed contacts m^–1 of travel.A similar multiple regression can be applied to data from a natural stream channel. It predicts much larger drift distances (50% settled in 42 m at 100 cm s ^–1 ). However, in the natural channel, settlement appears aggregated and the validity of the concept of permanent settlement is in doubt.     

Temporal constraints on the potential role of fry odors as cues of past reproductive success for spawning lake trout

Deciding where to reproduce is a major challenge for most animals. Many select habitats based upon cues of successful reproduction by conspecifics, such as the presence of offspring from past reproductive events. For example, some fishes select spawning habitat following odors released by juveniles whose rearing habitat overlaps with spawning habitat. However, juveniles may emigrate before adults begin to search for spawning habitat; hence, the efficacy of juvenile cues could be constrained by degradation or dissipation rates. In lake trout (Salvelinus namaycush), odors deposited by the previous year's offspring have been hypothesized to guide adults to spawning reefs. However, in most extant populations, lake trout fry emigrate from spawning reefs during the spring and adults spawn during the fall. Therefore, we postulated that the role of fry odors in guiding habitat selection might be constrained by the time between fry emigration and adult spawning. Time course chemical, physiological, and behavioral assays indicated that the odors deposited by fry likely degrade or dissipate before adults select spawning habitats. Furthermore, fry feces did not attract wild lake trout to constructed spawning reefs in Lake Huron. Taken together, our results indicate fry odors are unlikely to act as cues for lake trout searching for spawning reefs in populations whose juveniles emigrate before the spawning season, and underscore the importance of environmental constraints on social cues.     

Behavioural response of Kootenai white sturgeon (Acipenser transmontanus,Richardson, 1836) early life stages to gravel,pebble, and rubble substrates: guidelines for rearing substrate size

Guidelines for creating rearing substrate for sturgeon early life stages are needed for restoration programmes creating habitats for spawning and rearing of early life stages. To determine the effects of rock size on motile early life stages, experiments were conducted in artificial streams to observe the behaviour of free embryos and larvae of Kootenai River white sturgeon (Acipenser transmontanus) relative to rock size. Most (≥90%) of the free embryos in replicate test streams with 100% gravel, 100% pebble, or 100% rubble hid under rocks, with few moving downstream. There was no difference in downstream movement of free embryos among rock treatments, therefore all rock types provided cover habitat. Similarly, in rock mixture tests, with a variable percentage of pebble, small rubble, or large rubble in different tanks, even fewer free embryos moved downstream. With increasing age, larvae increasingly used the open bottom and velocity refuges downstream of or alongside rocks of any size while drift feeding. Downstream movement of larvae in both rock regime tests was affected by rock size, with significantly reduced movement relative to increasing abundance of large rock (rubble). However, in all rock mixtures, free embryos (and later, larvae when they stopped dispersing) preferred the smallest rock size available (pebble; P = 0.0001). This suggests a strong innate preference of both life stages for small substrate that is likely related to increased survival. A rock mixture of 10% gravel (16–32 mm diameter) and 30–40% pebble (diameter, 30–60 mm) should provide adequate rearing substrate for free embryos and early‐larvae. The remaining 50–60% should be mixed rubble and boulders for spawning and egg rearing.     

Spawning time and early life history of Murray cod, <Emphasis Type="Italic">Maccullochella peelii</Emphasis> (Mitchell) in an Australian river

Synopsis I investigated aspects of the early life history of Murray cod in the Broken River, southern Murray-Darling Basin, Australia. I documented patterns in abundance, length, age and the amount of yolk of drifting free embryos and estimated spawning periods for adult Murray cod in two reaches throughout each breeding season between mid-October and mid-December from 1997 to 2001. Free embryos began drifting in late-October and continued until mid- to late-December. Abundances of drifting free embryos showed no obvious peak in most years and were unrelated to discharge. Length, amount of yolk and age typically varied with sampling date, but only age showed strong and negative correlations with temperature. Thus, it appears that temperature affected rates of development, and that developmental stage, not length or age, was likely to be the determinant for when free embryos left the paternal nest. Most free embryos were estimated to have spent between 5 and 7 days drifting. Spawning of Murray cod in the Broken River usually commenced in mid-October and continued until at least early December. Initiation of spawning was associated with temperatures of 15°C and above, but discharge was highly variable, and no other environmental variables were consistent across years. The mid-point of spawning – usually in the first week of November – is considered a more significant time, because it likely coincides with peak spawning, and conditions during and immediately after this time are expected to be optimal for the survival of eggs and free embryos.     

Resident spawning of Japanese smelt,Hypomesus nipponensis,along gravel shorelines in Lake Nojiri,Central Japan

In Lake Nojiri, the Japanese smelt, Hypomesus nipponensis McAllister, 1963, population has been thought to be maintained by artificial spawning, and stocking efforts as natural spawning in in-flowing streams is unlikely due to the lack of inflowing stream habitat. In this study, novel resident Japanese smelt spawning was observed along the lakeshore in shallow areas with clean gravel and flow. Spawning occurred at night during early March to mid-April and eggs, confirmed on gravel substrate, progressed to the eyed state in 5–6 weeks. Although natural spawning was observed, the ratio of eggs that progressed to the eyed stage was minimal, possibly due to thick algae cover in areas with low flow velocity. Therefore, resident Japanese smelt spawning along the lake shoreline is physically possible and potentially contributes to the overall population in Lake Nojiri, but in-lake spawning alone presumably is not a biologically viable method for population sustainability due to low survival rates and egg density at present. Environmental improvements such as construction of fish ladders to suitable in-flowing spawning habitat, introduction of clean gravel to shorelines, and water level management adjusted to the smelt spawning run would contribute to higher recruitment by natural smelt reproduction, and consequently enhance the smelt production in Lake Nojiri.

     

Elevated summer temperatures delay spawning and reduce redd construction for resident brook trout (Salvelinus fontinalis)

Redd (nest) surveys for resident brook trout (Salvelinus fontinalis) were conducted annually in a mountain lake in northern New York for 11 years with multiple surveys conducted during the spawning season in eight of those years. Repeated surveys throughout the spawning season allowed us to fit an individually based parametric model and estimate the day of year on which spawning was initiated, reached its midpoint, and ended during each year. Spawning phenology was then assessed relative to (1) mean of maximum daily air temperature and (2) mean of maximum daily water temperature at the lake bottom during summer in each year using a linear model. Elevated temperatures in summer were correlated with a delay in spawning and a reduction in the total number of redds constructed. Increasing the summer mean of maximum daily air temperatures by 1 °C delayed spawning by approximately 1 week and decreased the total number of redds constructed by nearly 65. Lake spawning brook trout select redd sites based on the presence of discharging groundwater that is relatively constant in temperature within and across years, leading to relatively consistent egg incubation times. Therefore, delayed spawning is likely to delay fry emergence, which could influence emergence synchrony with prey items. This work highlights non‐lethal and sub‐lethal effects of elevated summer temperatures on native resident salmonids in aquatic environments with limited thermal refugia.