Timing of paddlefish spawning in the Upper Missouri River,Montana, USA in relation to river conditions

Spawning activity of paddlefish Polyodon spathula in the Missouri River, Montana in 2008–2009 was examined to delineate spawning sites and times in relation to discharge, water temperature and turbidity. One hundred thirty‐six eggs were collected at water temperatures ranging from 12.0 to 20.7°C (mean, 16.3°C; SD, 2.5). Only 12 of 89 (13%) congregations of radio‐tagged adults observed during the spawning period coincided with egg captures. Six larvae were collected at water temperatures ranging from 19.1 to 21.7°C (mean, 20.5°C; SD, 0.86). Peak discharge in 2008 (903 m³ s^?1 on 14 June) was approximately 30% greater in magnitude and occurred 11 days later than peak discharge in 2009 (612 m³ s^?1 on 3 June). Despite these differences in the hydrograph, no significant differences in egg CPUE were found between years (anova , F = 0.69, P = 0.56). Logistic regression identified no significant river condition variables associated with the presence or absence of eggs (P > 0.14 for all variables). However, in both years maximum egg CPUE was recorded within 3 days of the hydrograph peak and at similar water temperatures (17.5°C in 2008, 16.8°C in 2009). These results suggest an overall association of peaking discharge and seasonally warming water temperatures with egg deposition. Higher catches of eggs and larvae than observed in this study may be necessary to clarify short‐term (day‐to‐day) effects of environmental changes on spawning activity. Continued investigation of the relationship between short‐term changes in river conditions and paddlefish spawning activity is needed to understand the mechanics underlying the reproductive success of this species.     

The post‐spawning movements of migratory brown trout Salmo trutta L.

The post spawning behaviour of sea trout Salmo trutta was studied over a 2 year period in the river and estuary of the River Fowey, south‐west England. Forty‐five sea trout kelts were trapped immediately after spawning in December and intraperitoneally tagged with miniature acoustic transmitters. The subsequent emigration into coastal waters was monitored using acoustic receivers deployed throughout the river catchment. The levels of gill Na⁺K⁺ATPase activity in sea trout kelts sampled at the same time as the tagged fish were within the range of 2·5 to 4·5 μmol Pi per mg protein per h indicating that the post‐spawning fish were not physiologically adapted to salt water. The tagged kelts were resident in fresh water between 4 and 70 days before entering the estuary. Sixty two per cent of the tagged kelts subsequently migrated successfully into coastal waters, with a higher success rate for male fish (75%) than females (58%). There was a significant size related difference in the run‐timing of the kelts with the larger fish moving more quickly into coastal waters after spawning than smaller fish. Seaward migration within fresh water was predominantly nocturnal and generally occurred in conjunction with increasing river discharge and rising water temperature. Migration through the estuary continued to be predominantly nocturnal and occurred during an ebbing tide. Residency within the estuary varied amongst individuals although it was invariably short, with most fish moving out into coastal waters within one to two tidal cycles. Five tagged kelts returned from the coastal zone and re‐entered fresh water during April and June. Marine residence time varied between 89 and 145 days (mean 118 days) and the minimum estimated marine survival was c. 18%. One of these sea trout was subsequently recaptured after successfully spawning in the vicinity where it had been previously tagged demonstrating a degree of spawning site fidelity.     

Experimental effects of immersion time and water temperature on body condition,burying depth and timing of spawning of the tellinid bivalve <Emphasis Type="Italic">Macoma balthica</Emphasis>

The burying depth of many bivalve molluscs on intertidal mudflats varies throughout the year and differs between places. Many factors are known to influence burying depth on a seasonal or spatial scale, with temperature and tidal regime probably being very important. Burying depth, body condition and gonadal development of Macoma balthica were followed throughout winter and spring in an experiment in which water temperature and immersion time were manipulated. Unexpectedly, relative water temperature, in contrast to the prediction, did not generally affect body condition or burying depth. This was probably a consequence of the exceptionally overall low water temperatures during the experimental winter. Differences in temperature did, however, result in different timing of spawning: M. balthica spawned earlier at higher spring temperatures. Longer immersion times led to higher body condition only late in spring, but led to deeper burying throughout almost the whole period. There was no effect of immersion time on the timing of spawning. We conclude that a longer immersion time leads to deeper burying, independent of body condition. We also conclude that burying behaviour of M. balthica is not determined by the moment of spawning. Electronic Publication     

Spatial and temporal patterns of movement and migration at spawning aggregations of red hind, <Emphasis Type="Italic">Epinephelus guttatus</Emphasis>, in the U.S. Virgin Islands

This study compared the spatial and temporal patterns of red hind, Epinephelus guttatus, movement and migration from annual spawning aggregations on St Thomas (STT) and St Croix (STX), United States Virgin Islands. Around STT E. guttatus migrated 6–33 km from a functional spawning migration area of 500 km² and around STX E. guttatus migrated 5–18 km from an area of 90 km². Similarities between sites were found in regards to timing of movement, temporal and spatial changes in sex ratios, annual and lunar predictability and were synchronized with environmental cues. E. guttatus spawning aggregations in the Virgin Islands occur between the winter solstice (i.e., after December 20) and about February 20 of any year and show a distinctive peak 20–40 days after the winter solstice. Spawning typically occurred during periods of declining seawater temperature and slacking currents within a temperature range of 26–27.5°C and current speed of 2.5–3.5 cm s⁻¹. Males arrived early to spawning sites and stayed longer than females. These gender-based behavioral patterns are important to E. guttatus reproductive dynamics and must be factored into future studies and the design of fisheries regulations to ensure sustainability of spawning aggregation sites. The predictability of E. guttatus spawning aggregations relative to the winter solstice will be extremely beneficial for defining the temporal and spatial aspects of area closures. The consistency and synchrony of movement and migration will improve both the efficiency of planning research and monitoring programs and directing enforcement activities during critical time periods. Applying this knowledge strategically will maximize the limited resources available for research and enforcement and lead to greater protection of spawning aggregations.     

Biotic and abiotic effects on cohort size distributions in fish

Intraspecific variation in body size is common in animals and plants. Body size affects trophic interactions like foraging ability and vulnerability to predation, which in turn affect individual fitness as well as population stability and extinction risk. Experimental and theoretical work has shown that the size distribution of individuals within cohorts is strongly influenced by intraspecific competition for resources, often leading to skewed frequency distributions. However, little is known about the effects of environmental factors such as climate and eutrophication on the cohort size‐structure of natural populations. We use a long‐term time series of scientific monitoring of a freshwater fish (European perch Perca fluviatilis) to investigate the effects of density dependence, predation, nutrient availability, climate and the timing of spawning on the cohort size distributions. We find that the mean length of the fish is best predicted by the extrinsic factors phosphorus concentration and summer temperature, and the densities of the different age‐classes, whereas the skewness of the length distribution is best predicted by phosphorus concentration, summer temperature, abundance of small fish, and the timing of spawning. Higher nutrient levels, temperatures and densities of small fish increase food availability and thus reduce competition, which is reflected in increased mean length and decreased skewness. The timing of spawning affects skewness presumably through changes in the initial size variation of the cohort and the length of the first growth season. Our results indicate that higher temperatures increase the mean length and decrease skewness due to the concurrent eutrophication of the lake. The study thereby highlights the potential impact of human‐induced environmental change on the size structure of fish populations. More studies are needed to understand better the complex mechanisms through which these factors alter the intensity of intraspecific competition in fish communities.     

Reproductive ecology and movement of pallid sturgeon in the upper Missouri River,Montana

Successful recruitment of endangered pallid sturgeon has not been documented in the upper Missouri River basin for decades, and research on the reproductive ecology of pallid sturgeon has been hindered by low sample size. A conservation propagation program was initiated in the 1990s, and the oldest age class of hatchery‐origin pallid sturgeon are becoming sexually mature increasing the number of reproductively‐active fish in the system. However, it is currently unknown how the reproductive ecology of hatchery‐origin pallid sturgeon relates to the few remaining wild fish. Following spring reproductive assessments, weekly relocations were recorded for each individual from late‐May to mid‐July to facilitate comparisons of spawning season movements among reproductive classifications and between spring hydrographs (2015 and 2016) for male pallid sturgeon. Mean total movement distances (±SE) were 104.5 km (18.9) for reproductively‐active wild males, 116.0 km (18.1) for reproductively‐active 1997‐year class males, and 20.6 km (3.0) for non‐reproductively‐active fish of unconfirmed sex. Movement characteristics of reproductively‐active males did not differ between 2015 and 2016 despite a difference of eight days in the timing of peak discharge and a difference of 79 m³/s (16.7%) in magnitude. Male aggregations were observed on the descending limb of the hydrograph in 2016 during temperatures suitable for spawning, but female pallid sturgeon underwent follicular atresia, similar to the other years of the study. Hatchery‐origin pallid sturgeon from the conservation propagation program appear to have retained reproductive characteristics from the wild broodstock, a key finding for a population where local extirpation of the wild stock is imminent.     

Effects of climate and demography on reproductive phenology of a harvested marine fish population

Shifts in phenology are a well‐documented ecological response to changes in climate, which may or may not be adaptive for a species depending on the climate sensitivity of other ecosystem processes. Furthermore, phenology may be affected by factors in addition to climate, which may accentuate or dampen climate‐driven phenological responses. In this study, we investigate how climate and population demographic structure jointly affect spawning phenology of a fish species of major commercial importance: walleye pollock (Gadus chalcogrammus). We use 32 years of data from ichthyoplankton surveys to reconstruct timing of pollock reproduction in the Gulf of Alaska and find that the mean date of spawning has varied by over 3 weeks throughout the last >3 decades. Climate clearly drives variation in spawn timing, with warmer temperatures leading to an earlier and more protracted spawning period, consistent with expectations of advanced spring phenology under warming. However, the effects of temperature were nonlinear, such that additional warming above a threshold value had no additional effect on phenology. Population demographics were equally as important as temperature: An older and more age‐diverse spawning stock tended to spawn earlier and over a longer duration than a younger stock. Our models suggest that demographic shifts associated with sustainable harvest rates could shift the mean spawning date 7 days later and shorten the spawning season by 9 days relative to an unfished population, independent of thermal conditions. Projections under climate change suggest that spawn timing will become more stable for walleye pollock in the future, but it is unknown what the consequences of this stabilization will be for the synchrony of first‐feeding larvae with production of zooplankton prey in spring. With ongoing warming in the world’s oceans, knowledge of the mechanisms underlying reproductive phenology can improve our ability to monitor and manage species under changing climate conditions.     

Reproductive guilds (maturation,spawning frequency and fecundity) in the black pomfret,Parastromateus niger (Carangidae) in the Kuwaiti waters of the Arabian Gulf

Oogenesis, oocyte maturation pattern, spawning rhythm, spawning frequency, batch fecundity and oocyte diameter–frequency distribution of the black pomfret, Parastromateus niger (Bloch, 1795) in Kuwaiti waters were investigated from October 2003 to September 2005, using histological and morphological methods. The process of development is divided into four major phases: (i) primary growth phase; (ii) secondary growth phase; (iii) maturation phase; and (iv) spawning phase, followed by the regressed phase. Development of the yolky oocyte is an asynchronous process resulting, by the time of oocyte maturation, in a clear differentiation between a ready batch of oocytes (ready for spawning) and a reserve pool. Consequently, P. niger is capable of spawning multiple times throughout the reproductive season. Spawning frequency estimates, based on final oocyte maturation (FOM) method indicated that the species spawns once every 2.8 days during an 8‐month spawning season lasting from February to September, with a potential annual number of 22.4 spawns. Batch fecundity (BF) (2132–2001 648, mean 406 010 eggs), was significantly positively related to both standard length (SL) (P < 0.05) and ovary‐free body weight (OFBW) (P < 0.05), both parameters being good predictors of BF (r² = 30.8% for SL, from 22 cm onwards, and r² = 29.6% for OFBW, from 129.5 g onwards). No significant differences in monthly BF were found throughout the spawning season. Relative batch fecundity was 336 eggs/g OFBW; thus, estimate for potential annual relative batch fecundity was 7526 eggs g^?1 OFBW. The oocyte diameter–frequency distribution analysis revealed a multimodal distribution (at 100–200, 300–400 and 500–700 μm), confirming the evidence of multiple spawning.     

Differences in reproductive strategies between obscure puffer Takifugu obscurus and ocellated puffer Takifugu ocellatus during their spawning migration

Reproductive strategies were compared between obscure puffer Takifugu obscurus and ocellated puffer Takifugu ocellatus captured in waters near Yangzhong Island in the lower reaches of the Yangtze River during the spawning migration season from February to June in two consecutive years (1995 and 1996). Results showed that obscure puffer and ocellated puffer have adopted different reproductive strategies, including different spawning times, different size at maturity, and different number and size of oocytes, resulting in two different larval sizes. In detail, the timing of the spawning migration and status of gonad development of obscure puffer was about 1 month earlier than that of ocellated puffer; the obscure puffer was obviously longer and heavier than ocellated puffer in both mature male and female fish; mean GSI of obscure puffer females (15.8%) was significantly higher than that of ocellated puffer females (14.6%); the average diameter of ocellated puffer eggs (1.49 ± 0.12 mm) was significantly larger than that of the obscure puffer (1.22 ± 0.08 mm); and obscure puffer females (320.8 oocytes mg^?1 somatic wet weight) had significantly higher relative fecundity than ocellated puffer females (125.2 oocytes mg^?1 somatic wet weight). These differences in reproductive strategies between two closely related species of the Takifugu genus indicate that both obscure puffer and ocellated puffer fit the r/K dichotomy. Obscure puffer shows K‐selected characters with maturity at relatively large size and r‐selected characters with relatively many and small offspring, whereas ocellated puffer shows r‐selected characters with maturity at a relatively small size and K‐selected characters with relatively few and large offspring.     

Spawning activity of the Australian lungfish Neoceratodus forsteri in an impoundment

This study assessed the spawning activity of the threatened Australian lungfish Neoceratodus forsteri by measuring egg densities within the artificial habitat of a large impoundment (Lake Wivenhoe, Australia). Eggs were sampled (August to November 2009) from multiple locations across the impoundment, but occurred at highest densities in water shallower than 40 cm along shorelines with a dense cover of submerged terrestrial vegetation. The numbers of eggs declined over the study period and all samples were dominated by early developmental stages and high proportions of unviable eggs. The quality of the littoral spawning habitats declined over the study as flooded terrestrial grasses decomposed and filamentous algae coverage increased. Water temperatures at the spawning site exhibited extreme variations, ranging over 20·4° C in water shallower than 5 cm. Dissolved oxygen concentrations regularly declined to <1 mg l^?1 at 40 and 80 cm water depth. Spawning habitats utilised by N. forsteri within impoundments expose embryos to increased risk of desiccation or excessive submergence through water‐level variations, and extremes in temperature and dissolved oxygen concentration that present numerous challenges for successful spawning and recruitment of N. forsteri in large impoundment environments.     

The pre‐spawning migratory behaviour of Atlantic salmon Salmo salar in a large lacustrine catchment

The movements of adult Atlantic salmon Salmo salar were determined as they migrated to spawning habitats in a large lacustrine catchment, Lough Neagh, in Northern Ireland. The minimum average ground speed of S. salar through the lake was 2·1 km day^?1 and the mean residence time was 11 days. Tagged S. salar tended to actively migrate through the lake which represented a transitory habitat for adult S. salar. Migration time from the release site, through the lake, to a spawning tributary decreased during the migratory period. During the 4 year study period between 20·5 and 41·6% of tagged S. salar which entered the lake each year, explored at least one other channel before ascending the final spawning tributary. Exploratory behaviour was more likely in S. salar which spawned in the tributaries furthest from the sea. Exploratory behaviour was also more likely to occur during periods of reduced discharge in the natal stream. The fishery management implications of complex pre‐spawning behaviour in a mixed stock lacustrine system, are discussed.     

The Role of Temperature and Embryo Development Time in the Diel Timing of Spawning in a Coral-reef Damselfish with High-frequency Spawning Synchrony

We studied the diel timing of spawning in the demersally spawning Hawaiian damselfish, Dascyllus albisella, from mid-June to late-September 1997 at two small patch reefs in Hawaii. Our objectives were to elucidate daily timing of spawning in relation to water temperature, diel timing of hatching, and short-period spawning synchrony. Spawning occurred every 5–7 days at both reefs, with all spawning on a reef concluded either within a single day (1-day spawning) or within two successive days (2-day spawning). Spawning began in early morning and continued for most of the day. There was a significant, positive linear relationship between mean daily average water temperature (= daily average temperature averaged over the period starting from the day following the last spawning day of the preceding nest cycle till the day before the first spawning day of the current cycle) and peak spawning hour of day, for 1-day spawning, and the first and second days of 2-day spawning at both reefs. The relationship between mean daily average water temperature and peak spawning hour of day was comparable among all spawning-day classes and reefs. Hatching occurred on the fourth day of development throughout the study despite the 26.5–29.1°C change in water temperature during the study period, and hatching was restricted to within two hours after sunset. We propose that D. albisella's peak spawning time is positively correlated with increased water temperature because it maintains the benefits of synchronous spawning within two constraints: the narrow daily period of hatching, and the inverse relationship between water temperature and embryo developmental time.     

Mismatch between fishway operation and timing of fish movements: a risk for cascading effects in partial migration systems

Habitat fragmentation is a growing problem worldwide. Particularly in river systems, numerous dams and weirs hamper the movement of a wide variety of species. With the aim to preserve connectivity for fish, many barriers in river systems are equipped with fishways (also called fish passages or fish ladders). However, few fishways provide full connectivity. Here we hypothesized that restricted seasonal opening times of fishways can importantly reduce their effectiveness by interfering with the timing of fish migration, for both spring‐ and autumn‐spawning species. We empirically tested our hypothesis, and discuss the possible eco‐evolutionary consequences of affected migration timing. We analyzed movements of two salmonid fishes, spring‐spawning European grayling (Thymallus thymallus) and autumn‐spawning brown trout (Salmo trutta), in Norway's two largest river systems. We compared their timing of upstream passage through four fishways collected over 28 years with the timing of fish movements in unfragmented river sections as monitored by radiotelemetry. Confirming our hypothesis, late opening of fishways delayed the migration of European grayling in spring, and early closure of fishways blocked migration for brown trout on their way to spawning locations during late autumn. We show in a theoretical framework how restricted opening times of fishways can induce shifts from migratory to resident behavior in potamodromous partial migration systems, and propose that this can induce density‐dependent effects among fish accumulating in lower regions of rivers. Hence, fragmentation may not only directly affect the migratory individuals in the population, but may also have effects that cascade downstream and alter circumstances for resident fish. Fishway functionality is inadequate if there is a mismatch between natural fish movements and fishway opening times in the same river system, with ecological and possibly evolutionary consequences for fish populations.     

Environmental cues for natural reproduction of the Chinese sturgeon,Acipenser sinensis Gray, 1835, in the Yangtze River,China

For effective conservation, it is important to explore the environmental cues initiating the spawning activities of a fish species. Based on monitoring data gathered between 1998 and 2011, the relationships between spawning activities of the Chinese sturgeon, Acipenser sinensis, and several environmental cues were analyzed using the rare events logistic regression ‘Relogit’ method, which indicated that water temperature, 1‐day ?‐discharge, and atmospheric pressure were among the key spawning cues for A. sinensis (P < 0.05). It is suggested that Chinese sturgeon might have an optimal environment window of 17–20°C water temperature, high day‐to‐day discharge increase, and low atmospheric pressure for spawning. In support of Chinese sturgeon reproduction, suggested modifications to the operational procedures for the Three Gorges Reservoir (TGR) to trigger spawning are: lowering the downstream water temperature to below 20°C before mid‐October and expanding the period with water temperatures of between 17 and 20°C; to create a day‐to‐day intermittent increase in the discharge to an optimal spawning water temperature; and to regulate flow at nights with a low atmospheric pressure.     

Time scale matters: genetic analysis does not support adaptation‐by‐time as the mechanism for adaptive seasonal declines in kokanee reproductive life span

Seasonal declines of fitness‐related traits are often attributed to environmental effects or individual‐level decisions about reproductive timing and effort, but genetic variation may also play a role. In populations of Pacific salmon (Oncorhynchus spp.), seasonal declines in reproductive life span have been attributed to adaptation‐by‐time, in which divergent selection for different traits occurs among reproductively isolated temporal components of a population. We evaluated this hypothesis in kokanee (freshwater obligate Oncorhynchus nerka) by testing for temporal genetic structure in neutral and circadian‐linked loci. We detected no genetic differences in presumably neutral loci among kokanee with different arrival and maturation dates within a spawning season. Similarly, we detected no temporal genetic structure in OtsClock1b, Omy1009uw, or OmyFbxw11, candidate loci associated with circadian function. The genetic evidence from this study and others indicates a lack of support for adaptation‐by‐time as an important evolutionary mechanism underlying seasonal declines in reproductive life span and a need for greater consideration of other mechanisms such as time‐dependent, adaptive adjustment of reproductive effort.     

Embryo and early larval stages of the Humboldt Current krill Euphausia mucronata (Crustacea: Euphausiacea)

The Humboldt Current krill, Euphausia mucronata (Crustacea: Euphausiacea), is an endemic and keystone species in the food web of this highly productive eastern border current ecosystem. The morphology and ontogeny of E. mucronata is known from calyptopis I to the adult phase, but the embryonic and early‐life stages (nauplius and metanauplius) of this broadcast spawning species are unknown. We describe the morphology and development time of these life stages to complete the knowledge of its life cycle. Embryos were obtained from purple‐gonad gravid females collected off Dichato, central Chile, during November 2012. Eight gravid females (mean=16 mm total length) were incubated in seawater at 12°C under laboratory conditions. The average development time from single‐cell embryos to the metanauplius stage was 2.2 d and hatching occurred between 20 and 25 h. The average growth rate was 0.35 mm d^?1 from the late limb bud to the metanauplius stage (range=0.21–0.48 mm d^?1). Embryos of E. mucronata had a mean chorion diameter of 0.460 mm, embryo diameter of 0.343 mm, and perivitelline space of 0.056 mm. Our biological information reported here constitutes a baseline for future ecological studies on distribution and temporal variability of spawning activity and reproductive strategies of E. mucronata in the highly variable and productive Humboldt Current ecosystem.     

Variation in the significant environmental factors affecting larval abundance of four major Chinese carp species: fish spawning response to the Three Gorges Dam

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Habitat use and population characteristics of potentially spawning shovelnose sturgeon Scaphirhynchus platorynchus (Rafinesque, 1820), blue sucker (Cycleptus elongatus (Lesueur, 1817), and associated species in the lower Wisconsin River, USA

The goal of this study was to compare the possible locations, timing, and characteristics of potentially spawning shovelnose sturgeon (Scaphirhynchus platorynchus), blue sucker (Cycleptus elongatus), and associated species during the spring of 2007–2015 in the 149‐km‐long lower Wisconsin River, Wisconsin, USA, a large, shallow, sand‐dominated Mississippi River tributary. A 5‐km index station of two pairs of rocky shoals surrounded by sandy areas was electrofished for shovelnose sturgeon and blue sucker in a standardized fashion a total of 40 times from late March through mid‐June, the presumed spawning period. On one date in 2008 and two dates in 2012, all rocky shoals and adjacent sandy areas in the lowermost 149 km of the river were also electrofished for both species. Shovelnose sturgeon and blue sucker appeared to spawn in the limited rocky areas of the river along with at least four other species: mooneye (Hiodon tergisus), quillback (Carpiodes cyprinus), smallmouth buffalo (Ictiobus bubalus), and shorthead redhorse (Moxostoma macrolepidotum), usually at depths of 0.8–2.0 m and surface velocities of 0.4–1.0 m/s. However, apparently spawning shovelnose sturgeon were found only on mid‐channel cobble and coarse gravel shoals within a single 7‐km segment that included the 5‐km index station, whereas apparently spawning blue suckers were encountered on these same shoals but also more widely throughout the river on eroding bluff shorelines of bedrock and boulder and on artificial boulder wing dams and shoreline rip‐rap. Both species showed evidence of homing to the same mid‐channel shoal complexes across years. Blue sucker tended to concentrate on the shoals earlier in the spring than shovelnose sturgeon, usually from late April through mid‐May at water temperatures of 8.0–15.5°C along with quillback and shorthead redhorse. In comparison, shovelnose sturgeon usually concentrated on the shoals from mid‐May through early June at 13.5–21.8°C along with mooneye and smallmouth buffalo. Based on recaptures of tagged fish, at least some shovelnose sturgeon and blue sucker returned to the shoals at one‐year intervals, although there was evidence that female blue sucker may have been more likely to return at two‐year intervals. Most shovelnose sturgeon could not be reliably sexed based on external characteristics. Spawning shovelnose sturgeon ranged from 487 to 788 mm fork length, 500–2400 g weight, and 5–20 years of age, whereas spawning blue sucker ranged from 495 to 822 mm total length, 900–5100 g weight, and 5–34 years of age, although age estimates were uncertain. Females were significantly larger than males for both species although there was overlap. Growth in length was negligible for tagged and recaptured presumably spawning shovelnose sturgeon and low (3.5 mm/y) for blue sucker, suggesting that nearly all growth may have occurred prior to maturity and that fish may have matured at a wide range of sizes.     

Effects of Pacific salmon spawning and carcass availability on the caddisfly Ecclisomyia conspersa (Trichoptera: Limnephilidae)

1. The effects of spawning coho (Oncorhynchus kisutch) and chum salmon (Oncorhynchus keta) on the limnephilid caddisfly Ecclisomyia conspersa were evaluated by experimentally excluding salmon from the upper 14‐m stretch of a spawning channel by a wire‐meshed fence. Density, and development and growth rates, of larvae upstream of the fence (without salmon) were compared with those downstream (with salmon). 2. Larval density in the stretch with salmon declined during spawning, but increased again after spawning subsided and the carcasses of dead fish became available. In the stretch with salmon, larval density on salmon carcasses was seven to 37 times greater than on the adjacent channel substratum. The rate of larval development in the stretch with salmon was greater than that in the stretch without salmon. Two months after carcasses became available, 98% of larvae sampled from the stretch with salmon were in the fifth instar, compared to only 23% from the stretch without salmon. Body weight of E. conspersa in the stretches with and without salmon increased by an average of 3.04 and 2.38 mg, respectively, over a 6‐month period. 3. ¹⁵N values of larvae from the stretch with salmon increased following the arrival of the fish, suggesting that the larvae were feeding on salmon‐derived material, such as eggs and carcasses, which contain a high proportion of the heavier stable isotope. In contrast, ¹⁵N values of larvae from the stretch without salmon remained relatively constant throughout the experiment. The availability of salmon carcasses as a high‐quality food source late in larval development may increase survival and fecundity of E. conspersa. 4. These substantial differences were consistent with the view that they were due to the experimental exclusion of salmon and salmon carcasses from the upstream stretch, though the study was un‐replicated and thus precludes ascribing causation more definitely.     

Small larvae in large rivers: observations on downstream movement of European grayling Thymallus thymallus during early life stages

Behaviour of early life stages of the salmonid European grayling Thymallus thymallus was investigated by assessing the timing of larval downstream movement from spawning areas, the depth at which larvae moved and the distribution of juvenile fish during summer in two large connected river systems in Norway. Trapping of larvae moving downstream and electrofishing surveys revealed that T. thymallus larvae emerging from the spawning gravel moved downstream predominantly during the night, despite light levels sufficient for orientation in the high‐latitude study area. Larvae moved in the water mostly at the bottom layer close to the substratum, while drifting debris was caught in all layers of the water column. Few young‐of‐the‐year still resided close to the spawning areas in autumn, suggesting large‐scale movement (several km). Together, these observations show that there may be a deliberate, active component to downstream movement of T. thymallus during early life stages. This research signifies the importance of longitudinal connectivity for T. thymallus in Nordic large river systems. Human alterations of flow regimes and the construction of reservoirs for hydropower may not only affect the movement of adult fish, but may already interfere with active movement behaviour of fish during early life stages.