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USGS2024

Network connectivity contributes to native small-bodied fish assemblages in the upper Mississippi River system

Effective management and conservation of fishes requires understanding habitat use across multiple life stages while ensuring necessary habitats are both available and accessible. Tributary habitats may play an important role in recruitment and dispersal of fishes in anthropogenically modified rivers such as the Mississippi and Illinois Rivers of the Midwest U.S.A. Identifying source locations that contribute to recruitment of fish populations can determine the importance of connectivity within river networks and pinpoint critical habitats that sustain fish populations. In the Upper Mississippi River System (UMRS), the environments that fish use in early life stages (i.e., natal origin) can be identified using otolith trace element analysis due to stability and distinctness in water chemistry (strontium: calcium [Sr:Ca]) among water bodies that is reflected in otoliths. Here, we used trace element analysis to determine natal origin of six small-bodied fishes including bullhead minnow ( Pimephales vigilax ), emerald shiner ( Notropis atherinoides ), gizzard shad ( Dorosoma cepedianum ), bluegill ( Lepomis macrochirus ), orangespotted sunfish ( Lepomis humilis ), and yellow perch ( Perca flavescens ) across six reaches of the UMRS (Pools 4, 8, 13, 26, the Open River of the Middle Mississippi River, and the La Grange Pool of the Illinois River). Otolith core Sr:Ca for fishes was quantified using laser ablation inductively coupled plasma mass spectrometry. Using the resulting Sr:Ca chemical signatures of otolith cores, natal origin (tributary, immigrant, or potential resident) was determined for individual fish based on family-specific relationships between otolith and water chemistries. We found that all species originated from tributaries and other reaches (i.e., were immigrants) to varying extents, which acted as evidence for network connectivity. Specifically, tributaries contributed up to 48% of individuals at a given reach. In certain reaches, Pool 26 and the Open River reach, up to 80% of individuals in a species immigrated from another mainstem river reach. Network connectivity was also important in both upstream and downstream directions. Contributions from network connectivity varied among species: bullhead minnow used less whereas orangespotted sunfish used more network connectivity than when all species were combined. Further, the use of network connectivity varied spatially where individuals captured in Pool 8 and the La Grange Pool less often and those from Pool 26 and the Open River more often originated from network connectivity compared to the whole assemblage across reaches. These results indicate that species' life history traits probably interacted with the physical environment, which differs spatially, to yield observed recruitment source patterns. Our results show that network connectivity contributes to established assemblages of native small-bodied fishes throughout the UMRS and underscore the role of interjurisdictional management in maintaining network connectivity to sustain fish populations.

Illinois, Iowa, Minnesota, Missouri, Wisconsin↗

USGS2011

Preference of redear sunfish on zebra mussels and rams-horn snails

We tested prey preferences of adult (200- to 222-mm long) redear sunfish ( Lepomis microlophus ) on two size classes of zebra mussels ( Dreissena polymorpha ) and two-ridge rams-horns ( Helisoma anceps ) in experimental aquaria. We also tested physical limitations on consuming these mollusks and determined prey bioenergetic profitability. Redear sunfish strongly preferred rams-horns over zebra mussels, but they displayed no size preference for either prey. Ingestion was not physically limited since both prey species up to 15-mm long fit within the pharyngeal gapes of redear sunfish. Rams-horns were more bioenergetically profitable than zebra mussels and ingestion of rams-horn shell fragments was about three times less than zebra mussels. Rams-horns were somewhat more resistant to shell-crushing, but all size ranges of both prey species tested were crushable by redear sunfish. These studies suggested that the redear sunfish should not be considered a panacea for biological control of zebra mussels.

Journal of Freshwater Ecology↗

USGS2016

Effects of energy development on wetland plants and macroinvertebrate communities in Prairie Pothole Region wetlands

Energy production in the Williston Basin, USA, results in the coproduction of highly saline, sodium chloride-dominated water (brine). The Prairie Pothole Region (PPR) overlies the northeastern portion of the Williston Basin. Although PPR wetlands span a range of salinity, the dominant salt is sodium sulfate, and salinities are much lower than brine. Introduction of brine to wetlands can result in pronounced water-quality changes; however, the ecological effects of such contamination are poorly understood. We examined the effects of brine contamination on primary productivity, emergent macrophyte tissue chemistry, and invertebrate communities from 10 wetlands in the PPR. Based on a recognized Contamination Index (CI) used to identify brine contamination in the PPR, water-quality samples indicated that six wetlands were uncontaminated while four were contaminated. Across this gradient, we observed a significant decrease in above-ground biomass and a significant increase in tissue chloride concentrations of hardstem bulrush ( Schoenoplectus acutus ) with increased CI values. Additionally, a significant decrease in macroinvertebrate taxonomic richness with increased CI values was observed. These findings provide needed insight on the biological effects of brine contamination on PPR wetlands.

Journal of Freshwater Ecology↗

USGS2008

Evidence for an apical Na-Cl cotransporter involved in ion uptake in a teleost fish

Cation-chloride cotransporters, such as the Na+/K +/2Cl- cotransporter (NKCC) and Na+/Cl - cotransporter (NCC), are localized to the apical or basolateral plasma membranes of epithelial cells and are involved in active ion absorption or secretion. The objectives of this study were to clone and identify 'freshwater-type' and 'seawater-type' cation-chloride cotransporters of euryhaline Mozambique tilapia (Oreochromis mossambicus) and to determine their intracellular localization patterns within mitochondria-rich cells (MRCs). From tilapia gills, we cloned four full-length cDNAs homologous to human cation-chloride cotransporters and designated them as tilapia NKCC1a, NKCC1b, NKCC2 and NCC. Out of the four candidates, the mRNA encoding NKCC1a was highly expressed in the yolk-sac membrane and gills (sites of the MRC localization) of seawater-acclimatized fish, whereas the mRNA encoding NCC was exclusively expressed in the yolk-sac membrane and gills of freshwater-acclimatized fish. We then generated antibodies specific for tilapia NKCC1a and NCC and conducted whole-mount immunofluorescence staining for NKCC1a and NCC, together with Na+/K+-ATPase, cystic fibrosis transmembrane conductance regulator (CFTR) and Na+/H+ exchanger 3 (NHE3), on the yolk-sac membrane of tilapia embryos acclimatized to freshwater or seawater. The simultaneous quintuple-color immunofluorescence staining allowed us to classify MRCs clearly into four types: types I, II, III and IV. The NKCC1a immunoreactivity was localized to the basolateral membrane of seawater-specific type-IV MRCs, whereas the NCC immunoreactivity was restricted to the apical membrane of freshwater-specific type-II MRCs. Taking account of these data at the level of both mRNA and protein, we deduce that NKCC1a is the seawater-type cotransporter involved in ion secretion by type-IV MRCs and that NCC is the freshwater-type cotransporter involved in ion absorption by type-II MRCs. We propose a novel ion-uptake model by MRCs in freshwater that incorporates apically located NCC. We also reevaluate a traditional ion-uptake model incorporating NHE3; the mRNA was highly expressed in freshwater, and the immunoreactivity was found at the apical membrane of other freshwater-specific MRCs.

Journal of Experimental Biology↗

USGS2020

The freshwater mysid Mysis diluviana (Audzijonyte and Väinölä, 2005) (Mysida: Mysidae) consumes detritus in the presence of Daphnia (Cladocera: Daphniidae)

Freshwater mysids of the Mysis relicta group are omnivorous macroinvertebrates that form an important link between fishes and lower trophic levels in many north temperate to Arctic lakes, where they exhibit diel vertical migration (DVM) to exploit subsurface food-rich layers at night. Benthic food resources have been assumed to be less important for mysid diets than pelagic zooplankton. Studies have nevertheless indicated that mysids consume benthic sedimented detritus, calling this assumption into question. We conducted a food-choice experiment to evaluate the feeding preferences of Mysis diluviana (Audzijonyte & Vainola, 2005) by presenting field-caught specimens in individual foraging arenas with multiple choices of food. Experimental food treatments included a preferred pelagic prey (Daphnia), a presumed less desirable benthic resource (detritus), and a combination of both. We hypothesized that M. diluviana 1) prefers Daphnia over detritus and consumes only Daphnia in combination treatments, and 2) would not consume detritus except when detritus was the only food source available. Contrary to our hypothesis, M. diluviana readily consumed detritus in the presence of Daphnia. M. diluviana unexpectedly consumed more individuals of Daphnia in the presence rather than in the absence of detritus. Our results demonstrate that mysids take advantage of benthic food resources even in the presence of a presumably preferred zooplankton prey, calling to question the long-held assumption that benthic resources are unimportant when considering the trophic role of freshwater mysids of the M. relicta group.

Journal of Crustacean Biology↗

USGS2014

Book review: Ecology of North American freshwater fishes

This book will be important in courses for upper undergraduates studying fish ecology or for graduate students. However, it will also be an excellent reference for the fishery manager who asks ‘Why does this fish do that?’. With the wealth of great information contained in Ross’ book, chances are an answer will be found. Review info: Ecology of North American freshwater fishes. Edited by Stephen T. Ross, 2013. ISBN: 978-0520249455, 408 pp.

Journal of Fish Biology↗

USGS1995

Size limitation on zebra mussels consumed by freshwater drum may preclude the effectiveness of drum as a biological controller

The septa lengths of bivalve shells were used to estimate shell lengths of the largest zebra mussels ( Dreissena polymorpha ) crushed and consumed by freshwater drum ( Aplodinotus grunniens ) to determine if size limitation could preclude the effectiveness of drum as a biological controller of the zebra mussel. We examined gut samples of drum (273 to 542 mm long) collected from western Lake Erie in 1991, found the largest mussel (shell length = 21.4 mm) in the 11th largest drum (TL = 405 mm), and observed a reduction of mussel size in larger drum. The lack of a relationship between mussel size and drum size for larger specimens suggests that either drum prefer smaller mussels or the gape between the upper and lower pharyngeal teeth restricts drum feeding to zebra mussels of limited size. Although drum may reduce zebra mussel populations, because of the apparent size limitation of prey it is unlikely that drum would be fully effective as a biological controller; thus, this fish should not be introduced beyond its native range for that purpose.

Journal of Freshwater Ecology↗

USGS2021

Evaluating establishment success of non-native fishes introduced to inland aquatic habitats of tropical Pacific islands

An information-theoretic approach was used to evaluate non-native freshwater fish species introduced to insular habitats of Hawaii and Guam comparing successful establishments vs. failures. Since the late 1800s, as many as 81 non-native freshwater fish species have been recorded as introduced to Hawaii and Guam (combined) and 50 (62%) of these are documented as having one or more established populations. We examined eleven independent variables to investigate establishment success by creating 21 a priori logistic regression models ranked using Akaike's Information Criterion adjusted for small sample size. An additional eight post-hoc models were included that comprised the best a priori model and various combinations of individual variables. The best overall model of establishment probability included effects of taxonomic affinity (family membership), prior establishment success on other tropical islands, and hypoxia tolerance. Establishment success in Hawaii and Guam was highest for those species established on many other islands, and according to our best model air-breathing fishes were more likely to become established. Six fish families, each with from three to 18 species introduced to Pacific islands, were highly successful at establishment: Cichlidae (16 established of 18 species introduced), Poeciliidae (seven of eight), Cyprinidae (four of seven), Centrarchidae (four of four), Clariidae (three of three), and Loricariidae (three of four). Those that successfully established include both small and moderately large-bodied taxa, while representing a diverse array of other morphological and life-history traits. Pathways and motives associated with fish introductions in the Pacific have been linked to desires to develop aquaculture, enhance wild stocks of food, sport, and bait fishes, for use as biological control agents, or are linked to the ornamental fish trade. We found that many established species were introduced via multiple pathways (up to eight) and our analyses suggest that the combination of prior establishment success on other tropical islands and presence of non-native fishes in multiple pathways was indicative of high propagule pressure. Our study results and conclusions on Pacific tropical island introductions are in general agreement with previous studies on non-native freshwater fishes in other regions of the world and similar to observations in continental ecosystems and temperate zones.

Hawaii↗

USGS2001

Survey protocol for assessment of endangered freshwater mussels the Allegheny River, Pennsylvania

The United States Endangered Species Act (ESA) requires a biological assessment of any activity that is authorized, funded, or carried out by a federal agency and likely to affect a federally listed endangered species or its critical habitat. We developed a standardized survey protocol for biological assessments of the effects of bridge replacements on 2 federally listed endangered freshwater mussels, Epioblasma torulosa rangiana and Pleurobema clava, found in the Allegheny River, Pennsylvania. The protocol combines qualitative sampling to determine species present with quantitative sampling to estimate density. Data on species present satisfy the minimum requirement of a biological assessment, whereas estimates of density are needed to assess the number of individuals that would die as a result of bridge replacement. Some excavation of substrate is necessary for unbiased population estimates because of species and sex-specific differences in detection at the substrate surface. We reduced the amount of excavation and cost of the survey by using a statistical sampling technique called double sampling, which uses counts from excavating a subset of quadrats to calibrate counts from searching the substrate surface of all quadrats. We applied the survey protocol to the Allegheny River at West Hickory where E. t. rangiana was the 3 rd and P. clava was the 4 th most abundant mussel at the site. Only 31% of P. clava and 52% of E. t. rangiana (80% of females, 45% of males) were detected at the substrate surface. We estimated that 9173 (95% CI: 6309–13,336) E. t. rangiana and 7010 (95% CI: 4462–11,013) P. clava lived within 50 m of the existing bridge and would be affected immediately by bridge construction. (Population estimates did not include mussels too small to be retained on a 6.35-mm-mesh sieve.) Application of the protocol is not limited to biological assessment under the ESA, but is appropriate where site-specific status of freshwater mussel populations is required.

Pennsylvania↗

USGS2025

Evidence for size‐based predation risk during Atlantic salmon (Salmo salar) smolt migration.

Hatchery supplementation is frequently employed during the conservation and recovery of imperilled salmon populations. At the smolt stage, hatchery rearing practices often produce individuals that are larger than wild conspecifics. Under this ‘bigger is better’ strategy, it is assumed that larger fish are less susceptible to predation during migration. We tested this hypothesis on hatchery-reared Atlantic salmon ( Salmo salar ) smolts with fork lengths representative of those of natural and hatchery origins, allowing us to isolate the influence of size from rearing history. From May to June 2023 we characterized predation risk for acoustic-tagged ( n = 50) and tethered ( n = 192) smolts of various sizes through a mostly free-flowing section of the Penobscot River, Maine, USA. Across both methods, more than 50% of smolts were predated, with the majority of predation events being attributed to smallmouth bass ( Micropterus dolomieu ). Tethered smolts of all sizes experienced similar predation risk. In the acoustic telemetry component of this study, smaller, wild-sized smolts incurred greater overall mortality relative to standard hatchery sizes (95% vs. 75%), the majority of which occurred within 3 km of the release site. Collectively, these results allude to a strong predation influence imposed by smallmouth bass on smolts in freshwater sections of the Penobscot River and small-bodied migrants may incur greater predation risk, particularly near stocking sites.

Journal of Fish Biology↗

USGS2007

Variation in salinity tolerance, gill Na+/K+-ATPase, Na+/K+/2Cl– cotransporter and mitochondria-rich cell distribution in three salmonids Salvelinus namaycush, Salvelinus fontinalis and Salmo salar

We compared seawater tolerance, gill Na + /K + -ATPase and Na + /K + /2Cl – cotransporter (NKCC) abundance, and mitochondria-rich cell (MRC) morphology of three salmonids, lake trout Salvelinus namaycush , brook trout Salvelinus fontinalis and Atlantic salmon Salmo salar . They were transferred directly from 0 p.p.t. (parts per thousand; freshwater) to 30 p.p.t. seawater, or transferred gradually from 0 to 10, 20 and 30 p.p.t. at 1-week intervals and kept in 30 p.p.t. for 3 weeks. The survival rates of lake trout, brook trout and Atlantic salmon were 80%, 50% and 100% following direct transfer, and 80%, 100% and 100% during gradual transfer, respectively. Plasma Na + , K + and Cl – concentrations in surviving lake trout increased rapidly and remained at high levels in 30 p.p.t. of both direct and gradual transfer, whereas those in brook trout showed a transient increase following direct transfer but did not change significantly during gradual transfer. Only minor changes in plasma ions were observed in Atlantic salmon smolts in both direct and gradual transfer. These results suggest that lake trout retains some degree of euryhalinity and that brook trout possesses intermediate euryhalinity between lake trout and Atlantic salmon smolts. Gill Na + /K + -ATPase activity of lake trout and brook trout increased in seawater, whereas that of Atlantic salmon smolts was already upregulated in freshwater and remained high after seawater exposure. NKCC abundance was upregulated in parallel with gill Na + /K + -ATPase activity in each species. Immunocytochemistry with anti-Na + /K + -ATPaseα -subunit and anti-NKCC revealed that the two ion transporters were colocalized on the basolateral membrane of gill MRCs. Immunopositive MRCs were distributed on both primary filaments and secondary lamellae in all three species kept in freshwater; following transfer to seawater this pattern did not change in lake trout and brook trout but lamellar MRCs disappeared in Atlantic salmon. Previous studies on several teleost species have suggested that filament and lamellar MRCs would be involved in seawater and freshwater acclimation, respectively. However, our results in lake trout and brook trout suggest that lamellar MRCs could be also functional during seawater acclimation.

Journal of Experimental Biology↗

USGS2003

Effects of life history variation on size and growth in stream-dwelling Atlantic salmon

A large size variation amongst life histories for stream-dwelling Atlantic salmon Salmo salar was found and the relative effect of life histories on size varied over time. As early as December (age 0+ years), fish that later smolted at age 2+ years were significantly larger than fish that did not smolt at age 2+ years. In contrast, there were no mass differences at age 0+ years between fish that would mature or not at age 1+ years (October). The mass differences between smolts and non-smolts persisted until smolting, and differences between mature and immature fish first appeared in May (age 1+ years). Following September (age 1+ years), there was also a significant interaction between smolting and maturity. Previously mature and immature age 2+ year smolts were not significantly different in size, but immature age 2+ year non-smolts were much lighter than mature age 2+ year non-smolts. Based on mass differences, the apparent 'decision' to smolt occurred c. 5 months before (winter, age 0+ years) the decision to mature (late spring, age 1+ years). In addition to strong seasonal growth variation, sizes of freshwater Atlantic salmon were largely structured by the complex interaction between smolt-age and maturity. ?? 2003 The fisheries Society of the British Isles.

Journal of Fish Biology↗

USGS2012

Occurrence of triclosan, triclocarban, and its lesser chlorinated congeners in Minnesota freshwater sediments collected near wastewater treatment plants

The antimicrobial agents triclosan (TCS), triclocarban (TCC) and their associated transformation products are of increasing concern as environmental pollutants due to their potential adverse effects on humans and wildlife, including bioaccumulation and endocrine-disrupting activity. Analysis by tandem mass spectrometry of 24 paired freshwater bed sediment samples (top 10 cm) collected by the U.S. Geological Survey near 12 wastewater treatment plants (WWTPs) in Minnesota revealed TCS and TCC concentrations of up to 85 and 822 ng/g dry weight (dw), respectively. Concentrations of TCS and TCC in bed sediments collected downstream of WWTPs were significantly greater than upstream concentrations in 58% and 42% of the sites, respectively. Dichloro- and non-chlorinated carbanilides (DCC and NCC) were detected in sediments collected at all sites at concentrations of up to 160 and 1.1 ng/g dw, respectively. Overall, antimicrobial concentrations were significantly higher in lakes than in rivers and creeks, with relative abundances decreasing from TCC > TCS > DCC > NCC. This is the first statewide report on the occurrence of TCS, TCC and TCC transformation products in freshwater sediments. Moreover, the results suggest biological or chemical TCC dechlorination products to be ubiquitous in freshwater environments of Minnesota, but whether this transformation occurs in the WWTP or bed sediment remains to be determined.

Minnesota↗

USGS2009

Distinct freshwater and seawater isoforms of Na+/K+-ATPase in gill chloride cells of Atlantic salmon

Gill Na(+)/K(+)-ATPase (NKA) in teleost fishes is involved in ion regulation in both freshwater and seawater. We have developed and validated rabbit polyclonal antibodies specific to the NKA alpha1a and alpha1b protein isoforms of Atlantic salmon (Salmo salar Linnaeus), and used western blots and immunohistochemistry to characterize their size, abundance and localization. The relative molecular mass of NKA alpha1a is slightly less than that for NKA beta1b. The abundance of gill NKA alpha1a was high in freshwater and became nearly undetectable after seawater acclimation. NKA alpha1b was present in small amounts in freshwater and increased 13-fold after seawater acclimation. Both NKA isoforms were detected only in chloride cells. NKA alpha1a was located in both filamental and lamellar chloride cells in freshwater, whereas in seawater it was present only as a faint background in filamental chloride cells. In freshwater, NKA alpha1b was found in a small number of filamental chloride cells, and after seawater acclimation it was found in all chloride cells on the filament and lamellae. Double simultaneous immunofluorescence indicated that NKA alpha1a and alpha1b are located in different chloride cells in freshwater. In many chloride cells in seawater, NKA alpha1b was present in greater amounts in the subapical region than elsewhere in the cell. The combined patterns in abundance and immunolocalization of these two isoforms can explain the salinity-related changes in total NKA and chloride cell abundance. The results indicate that there is a freshwater and a seawater isoform of NKA alpha-subunit in the gills of Atlantic salmon and that they are present in distinct chloride cells.

Journal of Experimental Biology↗

USGS2024

Groundwater-Surface water interactions research: Past trends and future directions

Interactions between groundwater and surface water sustain groundwater-dependent ecosystems and regulate river temperature and biogeochemical cycles, amongst many other processes. These interactions occur in freshwater environments including rivers, springs, lakes, and wetlands, and in coastal environments via tidal pumping, submarine groundwater discharge, and seawater intrusion. Here, we explore groundwater-surface water interactions research using bibliometric analyses of titles, abstracts, and keywords from 20,275 journal papers published between 1970 and 2023 extracted from Scopus. Analyses show that research into groundwater-surface water interactions is highly multi-disciplinary, with growing contributions from the social and biological sciences. The number of groundwater-surface water interactions papers is rapidly increasing with over 1200 papers published per year since 2020. Drawing on our data-driven approach and expert knowledge, we synthesise current research trends and identify critical future research directions. Despite the thousands of papers on groundwater-surface water interactions, important processes are still difficult to quantify or predict at meaningful spatial scales to inform water-resources management. We see benefits in future groundwater-surface water interactions research focusing on: (1) using new technologies including internet-of-things-based sensors, uncrewed vehicles, and remote-sensing approaches for data collection to inform groundwater-surface water interactions at large scales, (2) seeking approaches to upscale site-specific findings to better inform management, and (3) continuing the movement towards multi-disciplinary investigations to better inform the understanding of groundwater-surface water interactions and processes that will enable better management outcomes.

Journal of Hydrology↗

USGS1975

Polypodium sp. (Coelenterata) infection of paddlefish (Polyodon spathula) eggs

Parasitism of fish by coelenterates is rare and the first North American infection was reported in sturgeon (Hoffman et al., 1974, J Parasitol 60: 548-550). In the USSR parasitism of sturgeon (Acipenser sp.) eggs by the coelenterate Polypodium hydriforme Ussov 1885 has long been reported and investigated [Raikova, 1959, in Parasites of Freshwater Fish and the Biological Basis for their Control. Bull State Sci Res Inst Lake and River Fish. XLIX. Eng. transl. No. TT61-31056, O. T. S., U. S. Dep. Commerce, Washington, D. C., 1962, 235 p.: Raikova, 1973, (Proc Second Internat Symp on Cnidaria), Seto Marine Biological Laboratory 20: 165-173]. This report represents the first known infection of Polypodium in the paddlefish (Polyodon spathula).

Journal of Parasitology↗

USGS2015

Mottled duck (Anas fulvigula) movements in the Texas Chenier Plain Region

As a surrogate species for Strategic Habitat Conservation, the mottled duck (Anas fulgivula) is an indicator species to coastal marsh health and function. Currently, biologists have a relatively poor understanding of regional mottled duck movements. We outfitted adult female mottled ducks with solar satellite transmitters during summer 2009–2011. Movement patterns were measured among years and phenology, in relation to available habitat at the landscape level, and in association to potential disturbance. Movement distances were measured in ArcGIS and then evaluated using analysis of variance for independent variables of year, month, biological time period, and season. Average weekly distances traveled by mottled ducks were relatively short (<5,000m) compared to other waterfowl. Movement occurrence and distance were linked to biological season with longest distances documented during the molt period. Movements also differed among years, with drought conditions associated with longer movement distances. Magnitude of movements may be an indicator of habitat quality for mottled ducks in the Texas Chenier Plain Region. By focusing on providing large freshwater pools and fresh/intermediate marsh during the molt period, managers could positively impact mottled ducks.

Texas↗

USGS2025

Expression of corticoid-regulatory genes in the gills of Atlantic salmon (Salmo salar) parr and smolt and during salinity acclimation

In teleost fishes, cortisol is the major corticoid and has both glucocorticoid and mineralocorticoid actions. However, how fish tissues discriminate between these distinct corticosteroid actions is unclear. In mammals, the major factors responsible for intracellular corticosteroid regulation are glucocorticoid receptors ( gr s) and the mineralocorticoid receptor ( mr ), but their role in osmoregulation of fish is unclear. 11β-hydroxysteroid dehydrogenases ( hsd11b s) control the levels of intracellular corticosteroids by converting from bioactive forms to inert forms. To investigate how Atlantic salmon ( Salmo salar ) respond to cortisol in different physiological or environmental conditions, we performed comparisons of parr and smolt, and osmotic challenge experiments to examine the physiological responses and gill transcript levels of genes underlying cortisol-signalling, including gr1 , gr2 , mr , hsd11b2 and hsd11b3 . Because cortisol may interact with growth hormone and prolactin during salinity changes, transcript levels encoding growth hormone receptors ( ghr1 , ghr2 ) and the prolactin receptor ( prlr ) were also examined. Hsd11b2 transcript levels in seawater-acclimated fish were consistently lower compared to fish acclimated to fresh water. After transfer to seawater, prlr transcript levels in fish significantly decreased and transcript levels of ghr1 , ghr2 and hsd11b3 showed no change or were slightly higher than those of freshwater control groups. Gr1 , gr2 and mr transcript levels were slightly but consistently higher in fish acclimated to fresh water relative to seawater. Our results indicate that changes in corticosteroid receptor and hsd11b2 transcript levels in the gills may be important mechanisms that regulate corticoid signals to achieve ion homeostasis in Atlantic salmon.

Journal of Fish Biology↗