Young-of-the-year Research Articles

Impact of TDG supersaturation on native fish species under different hydropower flood discharge programs.

Total dissolved gas (TDG) supersaturation caused by the operation of hydropower stations can threaten the survival and swimming performance of fish species. Different modes of hydropower flood discharges (regular vs. intermittent) from the Dagangshan hydropower station in China were studied in July and September 2017 to study the impact of TDG supersaturation on two native fish species in the downstream Dadu River. The average TDG supersaturation value was 114.3% in July under the regular discharge mode. In September, the supersaturation of TDG fluctuated in response to the intermittent discharge mode with an average TDG of 119.3%. Apparent gas bubble trauma was found on young-of-the-year (YOY) Prenant's schizothoracin and elongate loach in cages at different water depths during the flood discharge period. The mortality rate of YOY Prenant's schizothoracin and elongate loach in cages with water depths of 0-1m were 16.25% and 2.5%, respectively, in July. The fluctuating TDG levels with higher peaks in September caused higher fish mortality rates. The final mortality rates of YOY Prenant's schizothoracin and elongate loach in cages with water depths of 0-1m were 75% and 33.75%, respectively. Fish in the cages at a 0-3m water depth survived better than those in the cages at a 0-1m water depth. The critical swimming speeds (Ucrit) of YOY Prenant's schizothoracin and elongate loach without exposure to TDG supersaturation were 11.64 and 16.76 BL s-1, respectively. Ucrit decreased significantly after experiencing the flood discharge period and recovered to the normal level after experiencing the corresponding interval period.

It's a hard knock life for some: Heterogeneity in infection life history of salmonids influences parasite disease outcomes.

Heterogeneity in immunity occurs across numerous disease systems with individuals from the same population having diverse disease outcomes. Proliferative kidney disease (PKD) caused by Tetracapsuloides bryosalmonae, is a persistent parasitic disease negatively impacting both wild and farmed salmonids. Little is known of how PKD is spread or maintained within wild susceptible populations.We investigated an aspect of fish disease that has been largely overlooked, that is, the role of the host phenotypic heterogeneity in disease outcome. We examined how host susceptibility to T. bryosalmonae infection, and the disease PKD, varied across different infection life‐history stages and how it differs between naïve, re‐infected and persistently infected hosts.We investigated the response to parasite exposure in host phenotypes with (a) different ages and (b) heterogeneous infection life histories. Among (a) the age phenotypes were young‐of‐the‐year (YOY) fish and juvenile 1+ fish (fish older than one) and, for (b) juvenile 1+ infection survivors were either re‐exposed or not re‐ exposed to the parasite and response phenotypes were assigned post‐hoc dependant on infection status. In fish not re‐exposed this included fish that cleared infection (CI) or had a persistent infection (PI). In fish re‐exposed these included fish that were re‐infected (RI), or re‐exposed and uninfected (RCI). We assessed both parasite‐centric (infection prevalence, parasite burden, malacospore transmission) and host‐centric parameters (growth rates, disease severity, infection tolerance and the immune response).In (a), YOY fish, parasite success and disease severity were greater and differences in the immune response occurred, demonstrating an ontogenetic decline of susceptibility in older fish. In (b), in PI and RI fish, parasite success and disease severity were comparable. However, expression of several adaptive immunity markers was greater in RI fish, indicating concomitant immunity, as re‐exposure did not intensify infection.We demonstrate the relevance of heterogeneity in infection life history on disease outcome and describe several distinctive features of immune ontogeny and protective immunity in this model not previously reported. The relevance of such themes on a population level requires greater research in many aquatic disease systems to generate clearer framework for understanding the spread and maintenance of aquatic pathogens.

Tidally Induced Temporal Variations in Growth of Young‐of‐the‐Year Pacific Cod in the Yellow Sea

AbstractThe Yellow Sea is the southern limit of the distribution of Pacific cod (Gadus microcephalus), and the Yellow Sea Cold Water Mass (YSCWM) provides an appropriate over‐summering habitat for this species. Tidal current is the major component of the Yellow Sea hydrodynamic system, and its periodic variations influence the local ecological conditions and related ecosystems, particularly in the YSCWM, which is stratified in summer. To investigate the influence of tidal currents on Pacific cod growth, otolith daily increment in young‐of‐the‐year (YOY) Pacific cod in the Yellow Sea was analyzed in 2017. These data were integrated with a simulated tidal current using a multi‐scenario time‐delay analysis. This showed that: (1) the otolith daily increment exhibited a strong signal similar to the fortnightly tidal variation; (2) there was a lag of ∼8 days between the highest growth rate induced by the tidal current variation and monthly spring tide; and (3) the influence of the tidal current on otolith daily growth was stronger after migration of the cod into the YSCWM, particularly during the settlement stage. Given the stratified ocean structure during the over‐summering period, enhanced tidal mixing during the fortnightly spring tide would improve the exchange of nutrients and oxygen within the YSCWM, and lead to a delayed response of otolith growth due to feeding/digestion and subsequent growth. Therefore, variations in the tidal current have a marked positive ecological effect on the growth of YOY Pacific cod, and ultimately on the YSCWM ecosystem.

Spatiotemporal patterns of variability in the abundance and distribution of winter-spawned pelagic juvenile rockfish in the California Current.

Rockfish are an important component of West Coast fisheries and California Current food webs, and recruitment (cohort strength) for rockfish populations has long been characterized as highly variable for most studied populations. Research efforts and fisheries surveys have long sought to provide greater insights on both the environmental drivers, and the fisheries and ecosystem consequences, of this variability. Here, variability in the temporal and spatial abundance and distribution patterns of young-of-the-year (YOY) rockfishes are described based on midwater trawl surveys conducted throughout the coastal waters of California Current between 2001 and 2019. Results confirm that the abundance of winter-spawning rockfish taxa in particular is highly variable over space and time. Although there is considerable spatial coherence in these relative abundance patterns, there are many years in which abundance patterns are very heterogeneous over the scale of the California Current. Results also confirm that the high abundance levels of YOY rockfish observed during the 2014–2016 large marine heatwave were largely coastwide events. Species association patterns of pelagic YOY for over 20 rockfish taxa in space and time are also described. The overall results will help inform future fisheries-independent surveys, and will improve future indices of recruitment strength used to inform stock assessment models and marine ecosystem status reports.

A Hemimysis-driven novel ecosystem at a modified rubble-mound breakwater: An Engineering With Nature® Demonstration Project.

The US Army Corps of Engineers (USACE) repairs aging breakwater structures as part of routine maintenance to maintain safe navigation in Great Lakes commercial ports. A USACE repair to an existing breakwater structure in Milwaukee Harbor (WI) implementing Engineering With Nature (EWN) principles created complex rocky habitat by strategically placing cobble-sized stone over conventional 5.4 to 9.1 metric ton boulders, thus creating "control" (boulder) and "treatment" (cobble) habitats. We evaluated the resultant nature-based breakwater (NBBW) developing food web versus an adjacent referencesite on the same breakwater and determined that, unexpectedly, locally abundant Hemimysis anomala were impacting the food-web dynamics and feeding ecology of fishes occupying the structure. Fish and forage communities were sampled using gillnets, night scuba diving surveys, rock collections, and a novel trap to capture invertebrates. The resultant NBBW became home to a prolific population of nonindigenous Hemimysis, with indications that they were more abundant on cobble versus boulders,based on rainbow smelt feeding. This lithophilic/cave swelling mysid provided an important new food resource in Milwaukee Harbor for two introduced pelagic prey fishes: alewife (Alosa pseudoharengus) and rainbow smelt (Osmerus mordax). Gillnetting and night scuba diving surveys confirmed that rainbow smelt preferred to forage on the cobble section (p < 0.05). Hemimysis were also the primary food item consumed by nearshore game fishes such as young-of-the-year (YOY) yellow perch (Perca flavescens), YOY largemouth bass (Micropterus salmoides), and juvenile rock bass (Ambloplites rupestris). We propose that those breakwaters that harbor abundant Hemimysis constitute novel ecosystems (ecosystems that include both native and non-native biota) that might benefit harbor fisheries if well-managed. This project demonstrated how a low-cost design modification could be applied during the repair of rubble-mound, breakwater structures to achieve benefits beyond safe navigation. Integr Environ Assess Manag 2022;18:49-62. Published 2021. This article is a US Government work and is in the public domain in the USA.

Forecasting the response of a recovered pinniped population to sustainable harvest strategies that reduce their impact as predators

Abstract The recovery of marine mammal populations has led to increased predation on commercially valuable prey species, creating conflicts with fisheries and calls for predator control. Grey seals are important predators of Atlantic Cod and Winter Skate in the southern Gulf of St. Lawrence (sGSL), and both species are likely to be extirpated unless grey seal presence in that ecosystem is strongly reduced. We aimed to identify harvest strategies that reduced grey seal presence in the sGSL to levels that favour fish recovery while maintaining grey seal conservation goals. We fit an integrated population model to grey seal abundance, reproductive and mark-recapture data, and projected future presence in the sGSL while varying the magnitude and age-composition of the annual commercial quota. We found that both removal and conservation targets could be met with annual quotas of 6000 seals if 50% of hunted seals were young of the year (YOY), though small amounts of overhunting reduced seal abundance below limit reference levels. Harvest strategies that targeted higher proportions of YOY were less likely to trigger conservation concerns, though these strategies required much larger quotas to achieve removal targets.

Zooplanktivore fish body growth responses to browning‐induced light limitation vary over ontogeny, but not with fish density

AbstractOngoing climate change is leading to browning of many lakes and coastal areas, which can impair fish body growth and biomass production. However, whether and how effects of light limitation caused by browning on fish body growth vary over early ontogeny is unknown. In this study, we set up a mesocosm experiment to test whether roach (Rutilus rutilus) body growth responses to browning depend on body size, and if findings are robust over roach densities. We also studied a potential mechanism for size‐specific responses by conducting an aquaria experiment to test if size‐specific prey selectivity in roach changes with browning. We found that roach body growth responses to browning‐induced light limitation vary over ontogeny (independent of roach density), negatively affecting body growth of young‐of‐the‐year (YOY) but not of 1‐year‐old individuals. We also show that this difference in growth response is likely a consequence of browning‐induced alterations in zooplankton community composition and variation in prey selectivity between YOY and 1‐year‐old fish. This suggests that we should account for the diverse effects of browning over fish ontogeny, mediated via altered prey composition and ontogenetic changes in prey preference, when assessing overall impacts of browning on aquatic ecosystems.

Nursery origin and population connectivity of swordfish Xiphias gladius in the North Pacific Ocean.

Element:Ca ratios in the otolith cores of young‐of‐the‐year (YOY) swordfish, Xiphias gladius, were used as natural tracers to predict the nursery origin of subadult and adult swordfish from three foraging grounds in the North Pacific Ocean (NPO). First, the chemistry of otolith cores (proxy for nursery origin) was used to develop nursery‐specific elemental signatures in YOY swordfish. Sagittal otoliths of YOY swordfish were collected from four regional nurseries in the NPO between 2000 and 2005: (1) Central Equatorial North Pacific Ocean (CENPO), (2) Central North Pacific Ocean (CNPO), (3) Eastern Equatorial North Pacific Ocean (EENPO) and (4) Western North Pacific Ocean (WNPO). Calcium (43Ca), magnesium (24Mg), strontium (88Sr) and barium (138Ba) were quantified in the otolith cores of YOY swordfish using laser ablation inductively coupled plasma mass spectrometry. Univariate tests indicated that three element:Ca ratios (Mg:Ca, Sr:Ca and Ba:Ca) were significantly different among nurseries. Overall classification success of YOY swordfish to their nursery of collection was 72% based on quadratic discriminant analysis. Next, element:Ca ratios in the otolith cores of subadults and adults collected from three foraging grounds where targeted fisheries exist (Hawaii, California and Mexico) were examined to calculate nursery‐specific contribution estimates. Mixed‐stock analysis indicated that the CENPO nursery contributed the majority of individuals to all three foraging grounds (Hawaii 45.6 ± 13.2%, California 84.6 ± 10.8% and Mexico 64.5 ± 15.9%). The results from this study highlight the importance of the CENPO nursery and provide researchers and fisheries managers with new information on the connectivity of the swordfish population in the NPO.

Interannual Nearshore Habitat Use of Young of the Year White Sharks Off Southern California

Young-of-the-year (YOY) and juvenile-stage white sharks may use southern California nearshore beach habitats more extensively than previously known, within meters of some of the most heavily used beaches in the world. Such knowledge forms a critical component of species management and conservation plans, in addition to public safety and risk mitigation planning. We used data derived from a combination of satellite tag locations (13 animals over 3 years) and passive acoustic monitoring (34 animals over 8 years) to examine the occurrence, relative abundance, and residency patterns of YOY white sharks in southern California waters. Our results suggest that southern California contains spatiotemporally dynamic centers of primary nursery habitat. Tagged YOY white sharks formed loose aggregations at “hotspot” locations that were interannually variable, where individuals exhibited temporal fidelity, higher levels of residency, and spatially restricted movements, with multiple YOY individuals simultaneously displaying this behavior. While models of biotic and abiotic variables suggested relative abundance of tagged sharks may be predicted by sea surface temperature, salinity and productivity (chlorophyll-A), these predictors were not consistent across all years of the study. Thus, novel approaches that incorporate technologies to derive high resolution environmental data, paired with more comprehensive telemetry datasets are therefore required to better understand the extrinsic factors that drive habitat selection and residency patterns in juvenile white sharks.

Juvenile migration of Japanese dace Pseudaspius hakonensis revealed in a tributary of Lake Biwa with seasonal changes in abundance and stable isotope ratios

Migration patterns of Japanese dace Pseudaspius hakonensis in and around Lake Biwa, Japan are unknown, although fluvial–lacustrine life history has been known in most tributaries, the many mature individuals migrating upstream into tributaries in early spring. Field surveys and stable isotope analyses were used to clarify the migratory history of Japanese dace in the Ishida River, inflowing to Lake Biwa. In the field surveys, the number of young of the year (YOY) individuals caught per unit effort (CPUE) by hand net and casting net in the tributary and lake-shore area was calculated. The CPUE peaked from May to July, after the spawning season, thereafter decreasing rapidly, suggesting that YOY Japanese dace migrated downstream to Lake Biwa immediately after emerging, possibly due to the abundant food resources there. From September to November, the CPUE of YOY individuals increased in the lower reaches of the stream, although their stable nitrogen isotope values were variable, in some cases being similar to those recorded in the upper reaches and the lake-shore area. This suggests that some YOY individuals migrate upstream from the lake in autumn. In the present study, the juvenile migration pattern of fluvial–lacustrine Japanese dace in Lake Biwa was described using a CPUE survey and δ15N and δ13C analyses.

Is analysis of otolith microstructure a valid method for investigating early life history of Western Baltic cod?

Daily formation of fish otolith micro-increments is frequently assumed, however applying inferences about timing of life history events and formation of otolith macro-structures requires further validation of the periodicity of micro-increment formation. We analysed micro-increments from Western Baltic cod (WBC, Gadus morhua) otoliths marked with tetracycline-hydrochloride as part of an age validation study to test the assumption of daily formation of micro-increments. We found that the number of counted micro-increments consistently underestimated the age of cod aged 1 and older. Time at liberty was also underestimated, especially for fish at liberty during winter. In contrast, micro-increment counts of otoliths from wild-caught young-of-the-year (YOY) cod could be used to realistically estimate timing of hatch and translucent zone formation. Under ambient conditions, settlement did not correspond to any visible pattern within the otoliths, but could be inferred from the prey switch observed from stomach content analyses. We therefore conclude that micro-increments can be assumed to form on a daily basis until the first winter, and can therefore be used to investigate early life history of YOY WBC. However, the periodicity of micro-increment formation appears to vary seasonally in older individuals, with the number of micro-increments formed during the winter period being particularly low.

Temporal organochlorine profiles in young-of-the-year bluefish (Pomatomus saltatrix) in the Hudson River Estuary

The use of “chemical fingerprinting” or “profiling” has been suggested as a means to identify habitat use by young-of-the-year (YOY) bluefish (Pomatomus saltatrix). In this study, seasonal and interannual trends were examined over a 3-year period of 31 polychlorinated biphenyl (PCB) congeners and 23 chlorinated pesticides in 176 YOY bluefish collected in the Hudson River Estuary, New York State. Principal component analysis identified distinct and coherent clustering of bluefish according to sampling year. Seasonally, PCB patterns were similar among weight classes, regardless of date of capture. Throughout the study period, there was a consistent seasonal shift toward the heavier chlorinated homologs as size increased even though different congeners contributed to the overall PCB profile in Year 3. Unlike PCBs, there was no consistent pesticide accumulation pattern, which varied seasonally and interannually. The results show the first generalized interannual accumulation profiles of organochlorines during the rapid growth stage of age-0 bluefish. As knowledge of temporal changes in different ecosystems improves, this will improve an understanding on how exposure to chemicals in different ecosystems can affect the long-term health of bluefish.

Eye fluke infection changes diet composition in juvenile European perch (Perca fluviatilis)

Intraspecific diet specialization, usually driven by resource availability, competition and predation, is common in natural populations. However, the role of parasites on diet specialization of their hosts has rarely been studied. Eye flukes can impair vision ability of their hosts and have been associated with alterations of fish feeding behavior. Here it was assessed whether European perch (Perca fluviatilis) alter their diet composition as a consequence of infection with eye flukes. Young-of-the-year (YOY) perch from temperate Lake Müggelsee (Berlin, Germany) were sampled in two years, eye flukes counted and fish diet was evaluated using both stomach content and stable isotope analyses. Perch diet was dominated by zooplankton and benthic macroinvertebrates. Both methods indicated that with increasing eye fluke infection intensity fish had a more selective diet, feeding mainly on the benthic macroinvertebrate Dikerogammarus villosus, while less intensively infected fish appeared to be generalist feeders showing no preference for any particular prey type. Our results show that infection with eye flukes can indirectly affect interaction of the host with lower trophic levels by altering the diet composition and highlight the underestimated role of parasites in food web studies.

Straying of brown trout in the catchment of a large New Zealand river evaluated by otolith microchemistry

AbstractNatal homing and straying are behavioural tactics of migratory animals that return to nursery habitats to breed as adults. Straying has evolutionary and ecological importance for species flexibility, interpopulation connectivity and success of invasive species in new environments. This study aimed to estimate the relative importance of straying and homing in introduced brown trout Salmo trutta spawning in a model coastal tributary (Silverstream) of a large New Zealand river using otolith microchemistry. The data on natal homing and straying are important for the understanding of the ecological resilience of salmonid populations and metapopulations, and the development of effective management measures at stream or catchment scales. To examine brown trout homing and straying in a river catchment, otolith microchemical analysis was applied. We used linear discriminant function analysis to compare the trace element composition in the otoliths of YoY (young‐of‐the‐year) trout collected at spawning streams across the catchment and adult trout reproducing in Silverstream. The results showed that only six from a sample of 30 spawning adults in Silverstream originated from Silverstream. Another 14 fish likely originated from other Taieri tributaries further upstream, but the origin of the remaining 10 could not be determined. Contrary to published studies based on wild populations of brown trout in Europe, and several other species of salmonids, a large proportion of brown trout spawners collected in Silverstream were strays. The large number of strayers identified in Silverstream may be a consequence of the geomorphology of the Taieri catchment and highlights the adaptability of brown trout to different environmental contexts.

The Use of Baited Remote Underwater Video Systems (BRUVS) to Noninvasively Characterize a White Shark (Carcharodon carcharias) Nursery Area off Eastern Long Island, New York

AbstractUnderstanding if a discrete region fits three key criteria for shark nursery area designation is of utmost importance. Such a designation within United States territorial waters could result in the implementation of an Essential Fish Habitat (EFH) classification, a determination facilitating species protection. In this study, baited remote underwater video systems (BRUVS) were used to determine if they could effectively confirm a previously identified white shark (Carcharodon carcharias) nursery area south of Montauk, New York, so that the technique may be applied in other potential nursery areas. During experimentation, 1,348.32 h of video data were collected between Region A (Block Island Sound; 674.16 h), outside of the previously identified nursery area, and Region B (Atlantic Ocean; 674.16 h), within the identified nursery area. From the video data, 35 sightings of C. carcharias were recorded, with 31 unique individuals identified. Data associated with Region B fit all three nursery area criteria: (a) young-of-the-year (YOY) and juvenile sharks were more commonly found in this region over Region A (p &lt; 0.001), (b) several YOY and juvenile sharks (n = 4) were resighted in this region within a given year (e.g., average resighting duration: 10.5 days), and (c) YOY and juvenile sharks exhibited repeated use of this region over multiple years. We conclude that BRUVS can be used for fine-scale analyses to help designate a discrete area(s) as a shark nursery, even in turbid New York waters. Therefore, using BRUVS to identify shark nursery areas is a viable and noninvasive approach that can contribute to species conservation.

Environmental factors modify post-settlement survival and growth of winter flounder Pseudopleuronectes americanus in the absence of predation

Post-settlement mortality and growth in flatfish has the potential to profoundly impact year-class strength. However, the abiotic drivers of post-settlement mortality and growth are difficult to disentangle from the effects of predation. Young-of-the-year (YOY) winter flounder Pseudopleuronectes americanus, collected from Shinnecock Bay, NY (USA), were kept in predator-exclusion cages from May through August in 2016 and 2017, with simultaneous recording of dissolved oxygen, temperature and salinity at each caging site. Daily mortality rates in cages were 7 to 5.9 times less than mortality estimated for wild fish from the same area. We modeled the effect of 9 time-varying environmental variables, as well as the effect of cage location and depth, on survival and growth of caged fish using a Cox proportional hazards model with time-dependent covariates. The site that had the best survival in 2016 was not the site that had the best growth within that year, highlighting the potential for trade-offs between survival and growth. We found evidence that environmental conditions can impose both acute and chronic stress to YOY fish in estuarine environments, as well as potential threshold effects that warrant further exploration. Future monitoring efforts and studies of habitat suitability should take into account the degree and duration of exposure to temperature, salinity and dissolved oxygen extremes, not just the average conditions. We argue that risk can more accurately be captured by measuring aspects of the frequency, duration and intensity of both acute and chronic manifestations of environmental stressors, revealing potential thresholds and sublethal effects.

Environmental flows stimulate the upstream movement of juvenile diadromous fishes

River regulation has degraded aquatic biodiversity globally, and the effects can be pronounced for diadromous species, whose life history processes can depend on flow conditions, such as cues for adult migration, spawning, attracting recruits into coastal rivers and promoting upstream dispersal. Environmental flows are being used to mitigate the effects of river regulation, and understanding their effectiveness is required to improve management practices. This study examined the effects of targeted environmental flows on the upstream dispersal of three temperate catadromous fish species, namely common galaxias Galaxias maculatus, tupong Pseudaphritis urvillii and short-finned eel Anguilla australis. Fyke netting was used to capture fish moving upstream before and during environmental flows in summer and autumn in two coastal rivers. We found significant increases in the catch of young-of-the-year (YOY) common galaxias (6-fold higher) and juvenile short-finned eel (26-fold higher), relative to control sites, during environmental flow pulses compared with stable, regulated base-flow conditions. A significant response was not detected for YOY tupong, despite a 39% increase in the catch. These results demonstrate that environmental flows enhance the upstream movement of juvenile diadromous fishes, a critical process governing population persistence or recovery. The findings provide managers with confidence in the use of environmental flows to support populations.

Recruitment dynamics of non-native largemouth bass within the Sacramento–San Joaquin Delta

Largemouth bass (LMB; Micropterus salmoides) recruitment is limited by a critical developmental period during early life stages, but this mechanism may be less important within non-native habitats. We conducted boat electrofishing surveys in four tidal lakes of California’s Sacramento–San Joaquin Delta (SSJD) from 2010 to 2011 to describe introduced LMB recruitment dynamics. We evaluated growth, proximate composition, and health indices of young-of-the-year (YOY) LMB among tidal lakes and developed an integrated count model to determine how factors known to affect LMB recruitment shape SSJD population structure. Our results show a mismatch among growth, nutrition, and YOY abundance, where the tidal lake with the most abundant and fastest-growing LMB had the poorest nutritional status. The warm winter water temperatures and lack of a hatching-cohort growth advantage suggests overwinter starvation plays a less important role in SSJD LMB recruitment than in many native LMB habitats. Collectively, our results suggest that habitat characteristics (submerged aquatic vegetation) and not overwinter mortality shapes SSJD LMB population structure, a mechanism consistent with contemporary hypotheses about the altered fish community structure of the SSJD.

Ontogenetic shifts in mesohabitat use of young-of-year Rio Grande blue sucker in the big bend region of the Rio Grande

Alteration of flow regimes by anthropogenic activities is one of the primary environmental problems in riverine systems. Understanding how hydrologic conditions can affect ontogenetic habitat shifts of imperiled fishes is important in order to develop conservation and management strategies for each life-history stage. We examined relationships between the abundance of young-of-the-year (YOY) Rio Grande Blue Sucker and various abiotic variables in the Trans-Pecos region of the Rio Grande in Texas, USA. We used open N-mixture modeling to better understand the factors affecting ontogenetic habitat shifts of the imperiled aridland river fish. In addition, we examined differences in Rio Grande Blue Sucker total length among three mesohabitat types (pool, riffle, and run). The results of open N-mixture modeling suggested that as pool area increased, the abundance of YOY Rio Grande Blue Sucker increased. Total length of YOY Rio Grande Blue Sucker also significantly differed among the three mesohabitat types. The total lengths of YOY Rio Grande Blue Sucker in pool habitats were lower than in other mesohabitats, suggesting that YOY Rio Grande Blue Sucker undergo ontogenetic habitat shifts into greater current velocity habitats as they grow. The habitat associations we documented support the growing body of research emphasizing the importance of maintaining sufficient and appropriately timed flows to avoid prolonged low flows that limit habitat availability for native fish species during sensitive life stages in the Rio Grande and other aridland rivers.

The Impact of Hydropeaking on Juvenile Brown Trout (Salmo trutta) in a Norwegian Regulated River

The Norwegian electrical energy supply system is based on hydropower. The now deregulated energy market has led to increased use of hydropeaking production, leading to greater fluctuations in discharge and water levels below hydropower stations. The power station HOL 1, with an outlet to the Storåne River, is a large hydropeaking facility. With over 300 rapid flow increases and decreases per year since 2012, it is a river subjected to frequent hydropeaking. To quantify the stranding risk downstream of the power plant, the effect of a series of different turbine shutdown scenarios was simulated in an earlier study. The residual flow of 6 m3·s−1 and a full production of 66 m3·s−1 were considered as the baselines for the calculation of dewatered areas. A three-year study of juvenile fish density both upstream as a reference and downstream of the power plant was undertaken. There were very low densities or even an absence of brown trout (Salmo trutta) older than young-of-the-year (YoY) below the outlet of the power station, despite high densities of YoY in previous years. This is probably due to the large and rapid changes in flow below the power station. Hydropeaking has less impact on the earliest life stages of brown trout during spring and summer, as well as on spawning and egg development during winter. This is attributed spawning in late autumn occurring at a low flow seldom reached during hydropeaking. The high survival of YoY during the first summer and early autumn is likely due to a lower frequency of hydropeaking and higher residual flows, leaving a larger wetted area.