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While there is substantial information about the upstream migration of commercially and recreationally important fishes, relatively little is known about the upstream migration of small-bodied species, particularly through estuaries. In the San Francisco Estuary, there is a major need to understand the behavior of delta smelt Hypomesus transpacificus, a small pelagic fish listed under the state and federal endangered species acts. The spawning migration period may be critical as upstream movements can result in entrainment in water diversions. In general, delta smelt live in the low-salinity zone of the estuary and migrate upstream for spawning. During the fall pre-migration period, delta smelt remain primarily within the low-salinity zone in the western Sacramento–San Joaquin Delta and Suisun Bay. There were no significant upstream shifts of fish into fresher water during late fall, suggesting that delta smelt do not show pre-migration staging behavior. Following winter “first flush” flow events that appear to trigger migration, upstream movement rates are relatively rapid, averaging 3.6 km/d, a finding consistent with results from particle-tracking simulations, laboratory studies, and other fishes. Like some other native fishes, delta smelt apparently “hold” in upstream areas following migration; most do not spawn immediately. Overall, delta smelt fit the pattern of a diadromous species that is a seasonal reproductive migrant. Emerging data suggest that there is variability in the migration behavior of delta smelt, a pattern contrary to the reigning viewpoint that all smelt migrate in winter.

The Harney Basin is Oregon's largest internally draining basin and contains a unique native fish assemblage. The first and only comprehensive study of the origin and distribution of Harney Basin fishes occurred in 1971. Here, we update this study and identify potential threats to the basin's native fishes a half century later. Our assessment documents that all species of native fishes are still present, and with some exceptions distributed as originally reported. The distinctive fish assemblages in each of the 3 major subbasins within the Harney Basin (Silvies River, Silver Creek, and Donner und Blitzen River) support the hypothesis that terminal lakes in the Harney Basin (Malheur, Mud, and Harney Lakes) act as dispersal barriers, at least for native species. More than half of the fish species in the basin at present are introduced, primarily inhabiting low-elevation sites. We suggest that native non-salmonid species cohabiting these sites face the most immediate threats from climate change, water demands, and impacts from introduced fishes. Overall, our results show that native fish are still relatively widespread across the Harney Basin, but also face increasing threats despite the basin having experienced less development than many other areas in the Pacific Northwest. Improved understanding of the unique fishes of the Harney Basin, along with more detailed assessments of likely future trajectories of environmental risks are proactive measures that could improve conservation outcomes. [ABSTRACT FROM AUTHOR]

Hypoxia in coastal waters and lakes is widely recognized as a detrimental environmental issue, yet we lack a comparable understanding of hypoxia in rivers. We investigated controls on hypoxia using 118 million paired observations of dissolved oxygen (DO) concentration and water temperature in over 125,000 locations in rivers from 93 countries. We found hypoxia (DO < 2 mg L-1 ) in 12.6% of all river sites across 53 countries, but no consistent trend in prevalence since 1950. High-frequency data reveal a 3-h median duration of hypoxic events which are most likely to initiate at night. River attributes were better predictors of riverine hypoxia occurrence than watershed land cover, topography, and climate characteristics. Hypoxia was more likely to occur in warmer, smaller, and lower-gradient rivers, particularly those draining urban or wetland land cover. Our findings suggest that riverine hypoxia and the resulting impacts on ecosystems may be more pervasive than previously assumed. [ABSTRACT FROM AUTHOR]

Materials, energy, and organisms from groundwater serve as resource subsidies to lotic systems. These subsidies influence food production and post-emergent fish growth and condition through nutrient inputs and water temperature changes. To test whether post-emergent fish grew faster in gaining sites, we grew hatchery post-emergent salmon in enclosures, sampled periphyton, benthic invertebrates, and wild salmon, and modeled fish growth across a gradient of groundwater surface water exchange. Fish grew almost twice as fast in gaining (2.7% x [day.sup.-1]) than in losing (1.5% x [day.sup.-1]) sites. Fish from transient sites grew as much as gaining sites, but their condition was significantly lower (18.3% vs. 20.7%). Results suggest that groundwater surface water exchange affects fish growth and energetic condition through direct and indirect pathways. Elevated nitrogen concentrations and consistently warmer water temperature in gaining sites have a strong effect on basal production with subsequent effects on invertebrate biomass, fish growth, and condition. Findings highlight the importance of groundwater surface water exchange as a subsidy to rearing salmon and may inform strategies for restoring fish rearing habitat. Les matieres, l'energie et les organismes dans l'eau souterraine servent d'apports de ressources aux systemes lotiques. Ces apports influencent la production de nourriture et la croissance et l'embonpoint des poissons apres leur emergence par l'intermediaire d'influx de nutriments et de modifications de la temperature de l'eau. Afin de verifier si les poissons post-emergence croissent plus vite dans les sites d'ajout, nous avons fait croitre des saumons d'ecloserie post-emergence dans des enclos, echantillonne le periphyton, les invertebres benthiques et des saumons sauvages et modelise la croissance des poissons le long d'un gradient d'echange entre eau souterraine et eau de surface. La croissance des poissons etait presque deux fois plus grande dans les sites d'ajout (2,7 %-jour-1) que dans les sites de retrait (1,5 %x [jour.sup.-1]). La croissance des poissons de sites transitoires etait aussi grande que ceux de sites d'ajout, mais leur embonpoint etait significativement plus faible (18,3 % contre 20,7 %). Les resultats donnent a penser que les echanges entre eau souterraine et eau de surface ont une incidence sur la croissance et l'etat energetique des poissons par l'entremise de voies directes etindirectes. Des concentrations elevees d'azote etdes temperatures de l'eauuniformement plus elevees dans les sites d'ajout ont un effet marque sur la production de base et des effets subsequents sur la biomasse d'invertebres et la croissance et l'embonpoint des poissons. Ces constatations soulignent l'importance des echanges entre eau souterraine et eau de surface comme apport pour le grossissement des saumons et pourraient eclairer l'elaboration de strategies de restauration d'habitats de grossissement des poissons. [Traduit par la Redaction], Introduction Fluxes of materials, energy, and organisms across habitat boundaries, referred to as resource subsidies, are ubiquitous ecological phenomena that link land and water (Polis et al. 1996; Vanni et [...]

Materials, energy, and organisms from groundwater serve as resource subsidies to lotic systems. These subsidies influence food production and post-emergent fish growth and condition through nutrient inputs and water temperature changes. To test whether post-emergent fish grew faster in gaining sites, we grew hatchery post-emergent salmon in enclosures, sampled periphyton, benthic invertebrates, and wild salmon, and modeled fish growth across a gradient of groundwater - surface water exchange. Fish grew almost twice as fast in gaining (2.7%·day−1) than in losing (1.5%·day−1) sites. Fish from transient sites grew as much as gaining sites, but their condition was significantly lower (18.3% vs. 20.7%). Results suggest that groundwater - surface water exchange affects fish growth and energetic condition through direct and indirect pathways. Elevated nitrogen concentrations and consistently warmer water temperature in gaining sites have a strong effect on basal production with subsequent effects on invertebrate biomass, fish growth, and condition. Findings highlight the importance of groundwater - surface water exchange as a subsidy to rearing salmon and may inform strategies for restoring fish rearing habitat. [ABSTRACT FROM AUTHOR]

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