Ambient River Temperature Research Articles

Characterizing the Flow Regime in Brook Trout Incubation Habitats and Implications for Management in a Hydro‐Regulated River

AbstractThe operation of dams, diversions, and power generation facilities unavoidably influence the ecological function of rivers. We evaluated vertical and horizontal hydraulic gradients and subsequent water temperature changes in the hyporheic zone of a spawning area for Brook Trout Salvelinus fontinalis in a managed Lake Superior tributary in Ontario, then conducted a laboratory‐based experiment to show the impact of cold shock during incubation on the timing of larval emergence and survival. Upwelling groundwater was observed at the spawning area during the monitoring period (October 28, 2016, to January 11, 2017); hyporheic water temperatures remained above 3.7°C, whereas ambient river temperature above the substrate fell to 1.5°C until water was released from the upstream control dam. After the release event, water level increased by 0.9 m over the spawning area, and within 24 h, vertical and horizontal flow gradients were reversed in the hyporheic zone; downwelling conditions were observed for 30 h between surface and hyporheic water 1.8 m below substrate, and hyporheic water temperature decreased in unison. Hyporheic temperatures at shallow inshore sites fell below 1°C for more than 53 h. In the laboratory, applying a worst‐case scenario of dam‐induced cold shock on Brook Trout redds using historic monitoring records (i.e., <1.0°C for 40 h), we found no appreciable effect on survival and development timing of incubating Brook Trout eggs compared with a control group. Survival from fertilization to hatching was high for both treatment and control groups (>90%) and lower from fertilization to emergence (55%). Overall, our results suggest that standard winter operating procedures on a Lake Superior tributary had little impact on Brook Trout egg survival and development time to hatching and emergence; however, our findings also suggest that staged winter discharges would lessen the reversal of flow in the hyporheic zone and the magnitude of temperature changes experienced at Brook Trout redds.

Egg incubation temperature influences the population-specific outmigration rate of juvenile brown trout Salmo trutta.

The present experiment tested if temperature during embryogenesis and parental heritage affected the migratory behaviour of young brown trout Salmo trutta. Two parental forms were used, a freshwater resident form and an anadromous form, both from the same river system but geographically isolated since 1993–95. Four groups of young S. trutta were produced and reared from (a) freshwater resident parents spawning in a tributary to the River Imsa, Norway, (b) anadromous parents spawning in the main stem of the same river system, (c) resident male × anadromous female parents and (d) resident female × anadromous male parents. The eggs were incubated until first exogenous feeding in River Imsa water, either unheated or heated c. 2.7°C above ambient temperature. Thereafter, all fish experienced the same ambient river temperature until release. Groups were released below an impassable waterfall 900 m upstream of the mouth of the River Imsa, either as age‐0 in October 2019 or as age‐1 in May 2020. About 7.5% of the released fish moved downstream and were captured in a trap at the outlet. For any given body size, the proportion of warm incubated trout that moved downstream was greater than the proportion of cold incubated trout. It was also found that most emigrants of the October‐released S. trutta were caught within a month of release. Also, most May‐released S. trutta emigrated in October. The offspring of the freshwater resident parents emigrated to a larger extent than offspring of anadromous parents. Thus, the difference in emigration with regard to embryonic temperature was phenotypically plastic and may be associated with an epigenetic effect of the thermal conditions during early development. The effect of parental origin suggests there may be genetic divergence between the geographically isolated populations.

Long-term effects of temperature on growth, energy density, whole-body composition and aerobic scope of age-0 Lake sturgeon (A. fulvescens)

Fish are known to show high intraspecific phenotypic variation during early life history. Specifically environmental temperature during early life can result in developmental plasticity, which will influence developmental trajectory and ultimately individual fitness. In the present study, we examined long-term effects of early rearing temperature on growth, energy density, body composition (i.e., whole body glucose, triglyceride and protein concentration) and aerobic scope of age-0 Lake Sturgeon (A. fulvescens) to determine if temperature manipulation after yolk absorption would have a long-term impact on these traits. At 58 days post hatch, fish were subjected to one of three temperature manipulations (16 °C; control, 18 °C and ambient river temperature in a range of 14.0–19.4 °C; Ambient) for 35 days after which all fish were raised in Ambient conditions until 360 days post hatch, including 4 months of natural winter temperatures. We hypothesized that temperature conditions experienced before the first winter of life would result in short-term effects of improved growth and energy reserves. Our results showed that temperature manipulation may result in a short-term, reversible effect of improved growth prior to overwintering without a longer-term effect on growth. Enhanced somatic growth of total length and body mass prior to winter did not translate into improved energy reserves, and observed differences in growth rate between treatments did not correlate with aerobic scope. Our data demonstrate that a subtle change in temperature during early life history in Lake Sturgeon may result in short term positive effects on growth rate. These data may enhance current conservation aquaculture operations to promote winter survival in fall released fingerlings without long-term implications on growth phenotypes.

Mimicking Transgenerational Signals of Future Stress: Thermal Tolerance of Juvenile Chinook Salmon Is More Sensitive to Elevated Rearing Temperature Than Exogenously Increased Egg Cortisol

Elevated temperatures resulting from climate change are expected to disproportionately affect ectotherms given their biological function has a direct link to environmental temperature. Thus, as climate change leads to rapid increases in water temperatures in rivers, aquatic ectotherms such as fish may be highly impacted. Organisms can respond to these stressors through flexible and rapid phenotypic change induced via developmental and/or transgenerational plasticity. In oviparous species, gravid females may translate environmental stress across generations via increased exposure of eggs to maternally-derived glucocorticoids (i.e., maternal stress), which has been shown to result in diverse phenotypic effects in offspring. Recent studies suggest these phenotypic changes from maternal glucocorticoids may elicit predictive adaptive responses, where offspring exposed to maternal stress may be better prepared for the stressful environment they will encounter (i.e., environmental match hypothesis). We applied the environmental match hypothesis to examine whether a prenatal exogenous increase in egg cortisol may prepare Chinook salmon offspring (Oncorhynchus tshawytscha) to cope with thermal challenges after being reared in chronically elevated temperatures. Specifically, we exposed eggs to aqueous bath of cortisol-dosed (1000ng/mL) or control (0ng/ml) solutions, and then raised both treatments at current (+0°C—contemporary ambient river temperature) or elevated (+3°C—projected future river temperature) thermal regimes. We quantified thermal performance in fish 7-9 month post fertilization using two methods: via critical thermal maximum (CTMax), and energetic responses (in plasma cortisol, glucose, and lactate) to environmentally relevant, but challenging thermal spikes over 3 days. Overall, we found that exposure to elevated rearing temperatures had a large impact on thermal tolerance, where elevated-temperature reared offspring had significantly higher CTMax. In comparison, egg cortisol treatment had little to no clear effects on CTMax and blood energetic response. Our study demonstrates the importance of elevated water temperatures as an inducer of offspring phenotypes (via early developmental cues), and highlights the significance of examining offspring performance in environments with ecologically relevant stressors.

Influence of prey condition and incubation method on mortality, growth and metabolic rate during early life history in lake sturgeon, Acipenser fulvescens

Environmental conditions during early life can have a profound impact on developmental trajectory and ultimately ecological fitness of individuals. Therefore, from a conservation perspective it is vital to understand the longer-term implications of early phenotypic development on survival. In this study, we examined the effects of temperature (maintained at 16°C or ambient river temperature), prey condition (live or dead Artemia) and incubation method (tumbling jar or natural hatching over substrate) on the routine or standard metabolic rate (RMR, SMR), maximum metabolic rate (MMR), factorial aerobic scope, energy density (ED), whole body triglyceride concentration (TG), growth and mortality rate of age-0 lake sturgeon. Our results demonstrated that fish fed live artemia had significantly lower ED, growth and high mortality rates than those fed dead artemia at 32 days post-fertilisation (dpf) (p < .001). However, at 133 dpf fish fed live artemia showed higher MMR and no difference in ED, TG and growth rate compared to those fed dead prey during early life history. The present study showed that inclusion of live prey at the onset of exogenous feeding may be considered to promote a more natural phenotypic development in larval lake sturgeon.

Characterizing physical habitat preferences and thermal refuge occupancy of brook trout (Salvelinus fontinalis) and Atlantic salmon (Salmo salar) at high river temperatures

AbstractAnthropogenic influences, including climate change, are increasing river temperatures in northern and temperate regions and threatening the thermal habitats of native salmonids. When river temperatures exceed the tolerance levels of brook trout and Atlantic salmon, individuals exhibit behavioural thermoregulation by seeking out cold‐water refugia – often created by tributaries and groundwater discharge. Thermal infrared (TIR) imagery was used to map cold‐water anomalies along a 53 km reach of the Cains River, New Brunswick. Trout and salmon parr did not use all identified thermal anomalies as refugia during higher river temperature periods (&gt;21°C). Most small‐bodied trout (8–30 cm) were observed in 80% of the thermal anomalies sampled. Large‐bodied trout (&gt;35 cm) required a more specific set of physical habitat conditions for suitable refugia, that is, 100% of observed large trout used 30% of the anomalies sampled and required water depths &gt;65 cm within or adjacent to the anomaly. Densities of trout were significantly higher within anomalies compared with areas of ambient river temperature. Salmon parr were less aligned with thermal anomalies at the observed temperatures, that is, 59% were found in 65% of the sampled anomalies; and densities were not significantly different within/ outside anomalies. Salmon parr appeared to aggregate at 27°C, and after several events over 27°C variability in aggregation behaviour was observed – some fish aggregated at 25°C, others did not. We stipulate this is due to variances of thermal fatigue. Habitat suitability curves were developed for velocity, temperature, depth, substrate, and deep water availability to characterize conditions preferred by fish during high‐temperature events. These findings are useful for managers as our climate warms, and can potentially be used as a tool to help conserve and enhance thermal refugia for brook trout and Atlantic salmon in similar systems.

Quantifying thermal refugia connectivity by combining temperature modeling, distributed temperature sensing, and thermal infrared imaging

Abstract. Watershed-scale stream temperature models are often one-dimensional because they require fewer data and are more computationally efficient than two- or three-dimensional models. However, one-dimensional models assume completely mixed reaches and ignore small-scale spatial temperature variability, which may create temperature barriers or refugia for cold-water aquatic species. Fine spatial- and temporal-resolution stream temperature monitoring provides information to identify river features with increased thermal variability. We used distributed temperature sensing (DTS) to observe small-scale stream temperature variability, measured as a temperature range through space and time, within two 400 m reaches in summer 2015 in Nevada's East Walker and main stem Walker rivers. Thermal infrared (TIR) aerial imagery collected in summer 2012 quantified the spatial temperature variability throughout the Walker Basin. We coupled both types of high-resolution measured data with simulated stream temperatures to corroborate model results and estimate the spatial distribution of thermal refugia for Lahontan cutthroat trout and other cold-water species. Temperature model estimates were within the DTS-measured temperature ranges 21 % and 70 % of the time for the East Walker River and main stem Walker River, respectively, and within TIR-measured temperatures 17 %, 5 %, and 5 % of the time for the East Walker, West Walker, and main stem Walker rivers, respectively. DTS, TIR, and modeled stream temperatures in the main stem Walker River nearly always exceeded the 21 ∘C optimal temperature threshold for adult trout, usually exceeded the 24 ∘C stress threshold, and could exceed the 28 ∘C lethal threshold for Lahontan cutthroat trout. Measured stream temperature ranges bracketed ambient river temperatures by −10.1 to +2.3 ∘C in agricultural return flows, −1.2 to +4 ∘C at diversions, −5.1 to +2 ∘C in beaver dams, and −4.2 to 0 ∘C at seeps. To better understand the role of these river features on thermal refugia during warm time periods, the respective temperature ranges were added to simulated stream temperatures at each of the identified river features. Based on this analysis, the average distance between thermal refugia in this system was 2.8 km. While simulated stream temperatures are often too warm to support Lahontan cutthroat trout and other cold-water species, thermal refugia may exist to improve habitat connectivity and facilitate trout movement between spawning and summer habitats. Overall, high-resolution DTS and TIR measurements quantify temperature ranges of refugia and augment process-based modeling.

Influence of a dynamic rearing environment on development of metabolic phenotypes in age-0 Lake Sturgeon, Acipenser fulvescens.

Environment–phenotype interactions are the most pronounced during early life stages and can strongly influence metabolism and ultimately ecological fitness. In the present study, we examined the effect of temperature [ambient river temperature (ART) vs ART+2°C], dissolved oxygen (DO; 100% vs 80%) and substrate (presence vs absence) on standard metabolic rate, forced maximum metabolic rate and metabolic scope with Fulton’s condition factor (K), energy density (ED) and critical thermal maximum (CTmax) in age-0 Lake Sturgeon, Acipenser fulvescens, before and after a simulated overwintering event. We found that all the environmental variables strongly influenced survival, K, ED and CTmax. Fish reared in elevated temperature showed higher mortality and reduced K pre-winter at 127 days post-hatch (dph). Interestingly, we did not find any significant difference in terms of metabolic rate between treatments at both sampling points of pre- and post-winter. Long-term exposure to 80% DO reduced ED in Lake Sturgeon post-winter at 272 dph. Our data suggest that substrate should be removed at the onset of exogenous feeding to enhance the survival rate of age-0 Lake Sturgeon in the first year of life. Effects of early rearing environment during larval development on survival over winter are discussed with respect to successful recruitment of stock enhanced Lake Sturgeon, a species that is at risk throughout its natural range.

Understanding summertime thermal refuge use by adult Atlantic salmon using remote sensing, river temperature monitoring, and acoustic telemetry

Adult Atlantic salmon (Salmo salar) return to natal rivers several months before spawning and during summer can be subjected to temperatures that exceed their upper temperature tolerance limits. Salmon use thermal refuges to minimize exposure to high temperatures, but little information exists regarding behavioral thermoregulation by adult Atlantic salmon. We examined behavioral thermoregulation by Atlantic salmon during summer in-river residence in a Quebec river with a novel combination of thermal infrared remote sensing, river temperature monitoring, and acoustic telemetry. Adults engaged in behavioural thermoregulation at cooler ambient river temperatures (17–19 °C) than previously recorded for this species and maintained body temperature within a narrow range (16–20 °C) via use of cool and warm refuges. Adults used large, stable, stratified pools as refuges, allowing multiple individuals to thermoregulate simultaneously without leaving the pool. Low river discharge and high temperatures can be physical barriers to salmon migration, preventing them from accessing suitable refuges (e.g., pools). Identifying and maintaining connectivity to thermal refuges may be critical for persistence of Atlantic salmon populations as climate changes and rivers warm.

Preserving, augmenting, and creating cold‐water thermal refugia in rivers: concepts derived from research on the Miramichi River, New Brunswick (Canada)

AbstractSummer water temperatures are rising in many river systems in North America, and this warming trend is projected to intensify in the coming decades. Cold‐water fish may alleviate thermal stress in summer by aggregating in discrete cold‐water plumes that provide thermal refuge from high ambient river temperatures. Reliance on cold‐water thermal refugia is expected to increase in a warming climate, and many river reaches already lack suitable thermal refugia as a result of an absence of thermal diversity. A comprehensive fish management strategy could proactively address this imminent threat to cold‐water fish populations across North America by preserving existing thermal refugia, augmenting thermal anomalies to improve performance as refugia, and creating new thermal refugia in uniformly warm river reaches. We provide practical recommendations on how these measures can be accomplished based on insight derived from recent research focused on the Miramichi River, New Brunswick. Opportunities include limiting land use change, construction aggregate extraction (e.g. sand and gravel pits), and groundwater pumping/consumption. Existing thermal anomalies can be enhanced by controlling advective thermal mixing between cold‐water tributaries and the river mainstem flow, installing riparian shading, and adding temporary structures for protection from avian predators. New refugia can be created by temporarily pumping groundwater to discrete points within the river during periods of thermal stress. These concepts are discussed in the context of a comprehensive thermal refugia management strategy. Copyright © 2014 John Wiley &amp; Sons, Ltd.

High biolability of ancient permafrost carbon upon thaw

AbstractOngoing climate warming in the Arctic will thaw permafrost and remobilize substantial terrestrial organic carbon (OC) pools. Around a quarter of northern permafrost OC resides in Siberian Yedoma deposits, the oldest form of permafrost carbon. However, our understanding of the degradation and fate of this ancient OC in coastal and fluvial environments still remains rudimentary. Here, we show that ancient dissolved OC (DOC, &gt;21,000 14C years), the oldest DOC ever reported, is mobilized in stream waters draining Yedoma outcrops. Furthermore, this DOC is highly biolabile: 34 ± 0.8% was lost during a 14 day incubation under dark, oxygenated conditions at ambient river temperatures. Mixtures of Yedoma stream DOC with mainstem river and ocean waters, mimicking in situ mixing processes, also showed high DOC losses (14 days; 17 ± 0.8% to 33 ± 1.0%). This suggests that this exceptionally old DOC is among the most biolabile DOC in any previously reported contemporary river or stream in the Arctic.

Trichodinid (Ciliophora: Trichodinidae) Infections in Perch (Perca fluviatilis) Experimentally Exposed to Pulp and Paper Mill Effluents

Wild-caught European perch (Perca fluviatilis) were exposed in the laboratory to untreated bleached pulp and paper mill effluent in two separate experiments. The first experiment was conducted at 7-8°C using effluent concentrations of 5 and 10%, and the second experiment was conducted at ambient river temperature of 4-20°C using an effluent concentration of 1%. Trichodinid ciliates were identified and enumerated at the end of the exposure using a mucus subsampling technique from gill and skin as well as a formalin immersion technique, which provided total counts on each fish. Four different trichodinid species were identified on the fish. Prevalence of infection, mean number, and mean density of Trichodina spp. decreased on fish exposed to effluents compared with controls. Prevalence of infection, mean number, and mean density of Trichodinella epizootica decreased on fish exposed to 5% and 10% effluents but increased on fish exposed to 1% effluents compared with controls. These results demonstrate that trichodinid ciliates vary in their susceptibility to at least certain types of contaminants and cautions against using trichodinids as environmental indicators without delineating species.

Resource quantity and seasonal background alter warming effects on communities of biofilm ciliates

The impacts of experimental warming on field-related communities of biofilm ciliates were studied in contrasting seasons (winter vs. summer), which incorporated both different species sets and environmental background conditions. The biofilms for the experiments were cultivated in river bypass systems that were exposed to increasing temperatures based on the ambient river temperature. Opposing effects of warming were observed for ciliate 'summer' and 'winter' communities. While winter warming resulted in both stimulation (abundance and biomass) of the ciliate communities and significant shifts in the community structure, summer warming induced a significant decline in the ciliate biomass, but did not affect the relative community composition. By the simultaneous manipulation of temperature and resource density in summer, it was demonstrated that negative warming effects on the ciliate quantity during summer could be compensated by increasing the availability of food. Taken together, our results indicate that the responses of ciliate communities towards warming are strongly coupled to the availability of resources, and that the strongest impacts of environmental warming should thus be expected in resource-rich environments.

Evidence of changing migratory patterns of wild Atlantic salmon Salmo salar smolts in the River Bush, Northern Ireland, and possible associations with climate change

The migration patterns, timing and biological characteristics of wild Atlantic salmon Salmo salar smolts in the River Bush, Northern Ireland, were examined over the period 1978-2008. A distinct change in the timing of the smolt run was detected with progressively earlier emigration periods evident across the time series. The shift in run timing ranged from 3.6 to 4.8 days 10 years(-1) for a range of standard migratory audit points. The timing of smolt emigration has been linked to ambient river temperature patterns. Distinct seasonal patterns were evident for biological characteristics of River Bush smolts with mean age and fork length decreasing throughout the emigration period. Marine survival patterns in 1 sea winter River Bush S. salar were strongly influenced by the run timing of the preceding smolt year such that later emigrating cohorts demonstrated increased survival. Possible mechanisms for this relationship based on local climatic variation have been explored, including the effect of potential thermal mismatch between freshwater and marine environments.

Evaluation of a Spawning Habitat Enhancement Site for Chinook Salmon in a Regulated California River

AbstractAn evaluation of the effectiveness of a project to enhance spawning habitat for Chinook salmon Oncorhynchus tshawytscha was conducted in the Mokelumne River, a regulated stream in California's Central Valley. Approximately 976 m3 of clean river gravel (25–150 mm) was placed in berm and gravel bar configurations along the 45‐m enhancement site. Physical measurements taken before and after gravel placement indicate that the project significantly increased channel water velocities, intergravel permeability, and dissolved oxygen; reduced channel depths; and equilibrated intergravel and ambient river temperatures. These positive benefits remained throughout the 30‐month monitoring period. Adult Chinook salmon began spawning at the previously unused site within 2 months after gravel placement and continued to use the site during the three spawning seasons encompassed by the study. Bed material movement was documented by channel bathymetry surveys over two water years. Topographical channel surveys provide a useful tool for monitoring bed material transport and layering redd locations on contour maps. Although its usefulness in restoring salmon populations is poorly understood, gravel enhancement can be an effective means for improving salmon spawning habitat in rivers where upstream dams have effected low gravel recruitment.

The trophic dynamics of riverine bacterioplankton: Relationships among substrate availability, ectoenzyme kinetics, and growth

The role of dissolved organic matter (DOM) in the trophic dynamics of aquatic systems is difficult to study. One approach is to focus on the activity of the extracellular enzyme systems that heterotrophic bacteria use to obtain nutrients from DOM. These data are easy to acquire, but difficult to interpret, because we lack general models that relate these activities to the two sides of the trophic reaction: the DOM pool and bacterial production. To address this gap, we collected water samples from the Maumee River on nine dates from May 1996 through November 1996. The water was filtered and placed in bottles to which a variety of substrates were added (final concentration 20 µM unless otherwise noted): ammonium, leucine, albumin (1 mg liter−1), glucose, cellobiose, starch (1 mg liter−1), vanillin, and tannin (2 mg liter−1). After 20 h at ambient river temperature, respiration, productivity, growth efficiency, and the activities of α‐glucosidase, β‐glucosidase, leucine aminopeptidase, phosphatase, and esterase were measured. The glucosidases were the most responsive enzymes, followed by phosphatase and peptidase. Saccharide and albumin additions produced the largest increases in production and growth efficiency. Phenolic amendments generally depressed growth efficiency. The results suggest that production was primarily carbon limited: Peptides appeared to be a major component of the diet, but high productivities were associated with the availability of saccharides. During the summer, production appeared to be fueled by lysates and exudates generated through the microbial loop. Lower ratios of productivity (e.g., enzyme activity and positive metabolic responses to phenols) suggested that allochthonous carbon may have been important during spring and autumn.

Use of shallow marginal habitat by Phoxinus phoxinus: a trade‐off between temperature and food?

Fish may alter their habitat use in accordance with the profitability of differing habitat patches. During summer, in the River Frome, minnows Phoxinus phoxinus used shallows in a selective manner, preferring shallows in which water temperature was higher than the ambient river temperature. During the morning and evening, the minnows occasionally entered the shallows but did not linger. They spent considerably longer periods in the shallows when water temperature was greater than in the main channel of the river. During these periods the minnows were inactive. Gut fullness of wild minnows varied temporally and on one occasion spatially, with gut fullness rising rapidly after dawn and remaining high throughout the day. Minnows held in enclosures in the river had significantly higher gut fullness than those held in the shallows. The minnows use shallows that are warmer than the adjacent river preferentially but must return to the river to feed. Thus, there is evidence for a trade‐off between a habitat with high prey density, but low temperature and a low prey density, warmer habitat.

Behavioural thermoregulation and homing by spring chinook salmon,Oncorhynchus tshawytscha(Walbaum), in the Yakima River

Temperature‐sensitive radio transmitters were employed to study the patterns of behavioural thermoregulation, habitat preference and movement of 19 adult spring chinook salmon,Oncurhynchus tshawytscha(Walbaum), in the Yakima River. During the 4 months prior to spawning, fish maintained an average internal temperature 2.5°C below ambient river temperature. This represented a 12 to 20% decrease in basal metabolic demand or a saving of 17.3 to 29.9 calkg−1h−1. Fish were most commonly associated with islands, pools, and rock out‐croppings along stream banks. Homing behaviour appeared to be modified to optimize temperature regimes and energy conservation. As the time of spawning approached, fish left thermal refuges and migrated to spawning grounds upstream and downstream of refuge areas. Although spring chinook salmon residing within cool‐water refuges may be capable of mitigating sub‐lethal temperature effects, cool‐water areas need to be abundant and available to the fish. The availability of suitable thermal refuges and appropriate holding habitat within mainstem rivers may affect long‐term population survival.

Seasonal Reproductive Development ofLampsilis cardium, Amblema plicata plicata, andPotamilus alatus(Pelecypoda: Unionidae) in the Upper Mississippi River

ABSTRACT Adult specimens of three species of freshwater mussels common to the upper Mississippi River were examined histologically to determine seasonal patterns of development in gametogenesis and release of glochidia. Full maturation of gonadal materials in Lampsilis cardium (formerly L. ovata ventricosa), a long-term breeder, occurred when ambient river temperatures reached 24° to 26°C, between late July and early August. By mid-August, glochidia were present in the marsupia. Glochidia were released from late May through mid-June of the following year once water temperatures reached 20°C. The long-term breeder Potamilus alatus demonstrated full gonadal maturation earlier than seen in L. cardium with reproduction completed by late July (26°C). Glochidia of P. alatus were released over a period similar to that observed for L. cardium, late May through early July of the following year. Fully mature Amblema plicata plicata, a short-term breeder, were collected from late May through early July (18° to 21°C)...

Assessment of the biological impact of point source discharges employing Asiatic clams.

Three thousandCorbicula fluminea were transplanted for six months along a stress gradient produced by the thermal discharge of a power plant to ascertain whether Asiatic clams could be employed to monitor the biological impact of point source discharges. Water temperature at the warmest site (station 1) was between 25 and 35°C, 7–13°C above that in the river. Almost complete clam mortality was recorded here within 60 days. Water temperature at the next cooler site (station 2) was elevated 0.5–2.0°C above ambient river temperature (stations 3 and 4). Adult and juvenile clam shell and tissue growth and condition index was less here (P 0.1). A one time measurement of juvenile clam scope for growth (SFG) also demonstrated a decreasing trend along the thermal gradient. SFG was positively correlated with juvenile clam shell and tissue growth (P P > 0.05).