Migration barriers are a major reason for species loss and population decline of freshwater organisms. Significant efforts have been made to remove or provide passage around these barriers; however, our understanding of the ecological effects of these efforts is minimal. Installation of a fish passage facility at the Landsburg Dam, WA, USA provided migratory fish access to habitat from which they had been excluded for over 100 years. Relying on voluntary recruitment, we examined the effectiveness of this facility in restoring coho (Oncorhynchus kisutch) salmon populations above the diversion, and whether reintroduction of native anadromous species affected the distribution and abundance of resident trout (O. mykiss and O. clarki). Before the ladder, late summer total salmonid (trout only) density increased with distance from the dam. This pattern was reversed after the ladder was opened, as total salmonid density (salmon þ trout) approximately doubled in the three reaches closest to the dam. These changes were primarily due to the addition of coho, but small trout density also increased in lower reaches and decreased in upper reaches. A nearby source population, dispersal by adults and juveniles, low density of resident trout and high quality habitat above the barrier likely promoted rapid colonization of targeted species. Our results suggest that barrier removal creates an opportunity for migratory species to re-establish populations leading to range expansion and potentially to increased population size.
Following construction of a fish ladder at Landsburg Diversion Dam on the Cedar River, Washington, USA, in fall 2003, we used DNA-based parentage to identify second generation Chinook (Oncorhynchus tshawytscha) and coho (Oncorhynchus kisutch) salmon as recruits that were produced above the dam or "strays" dispersing into the new habitat that were produced elsewhere. For both species, strays colonized immediately but decreased as a proportion of the total run over time. Chinook salmon strays were more numerous in years when the species was more abundant below the dam and included a much larger proportion of hatchery origin salmon than did coho salmon. Productivity, calculated as the ratio of female recruits sampled at the dam to female spawners, exceeded replacement in all four coho salmon cohorts but only two of five Chinook salmon cohorts, leading to more rapid population expansion of coho salmon. However, estimates of fishing mortality and recruitment into the Cedar River below the dam substantially increased Chinook salmon productivity estimates. Our results demonstrate that Pacific salmon are capable of rapidly recolonizing habitat made accessible by restoration and emphasize the importance of demographic exchange with preexisting populations during the transition from recolonization to self-sustainability.Résumé : Après la construction d'une passe à poissons au barrage de dérivation de Landsburg, sur la rivière Cedar (Washington, États-Unis), à l'automne 2003, nous avons utilisé l'ascendance basée sur l'ADN pour distinguer les saumons quinnats (Oncorhynchus tshawytscha) et coho (Oncorhynchus kisutch) de deuxième génération constituant des recrues produites en amont du barrage des individus « égarés » produits ailleurs qui s'étaient dispersés dans ce nouvel habitat. Pour les deux espèces, si les égarés ont immédiatement colonisé l'habitat, la proportion de la montaison totale qu'ils représentaient a diminué avec le temps. Les saumons quinnats égarés étaient plus nombreux les années où l'espèce était plus abondante en aval du barrage et comptaient une proportion beaucoup plus grande de saumons issus d'écloseries que les saumons cohos. La productivité, calculée comme étant le rapport du nombre de recrues femelles échantillonnées au barrage et du nombre de génitrices, dépassait le niveau de remplacement dans les quatre cohortes de saumons cohos, mais dans seulement deux cohortes de saumons quinnats sur cinq, entraînant une expansion plus rapide de la population de saumons cohos. Toutefois, les estimations de la mortalité par pêche et du recrutement dans la rivière Cedar en aval du barrage ont sensiblement accru les estimations de la productivité des saumons quinnats. Nos résultats démontrent que les saumons du Pacifique sont capables de recoloniser rapidement un habitat rendu accessible par la restauration et soulignent l'importance de l'échange démographique avec des populations préexistantes durant la transition d'une situation de recolonisation à une situation d'autosuffisance. [Traduit par la Rédaction]
Anadromous salmonid populations are particularly vulnerable to migration blockages, such as dams and culverts, because access to historic spawning and rearing habitats is prevented. The process of salmonid recolonization has not been well documented for river systems where anthropogenic migration barriers have been removed or where fish passage facilities have been constructed. In September 2003, Seattle Public Utilities completed construction of a fish passage facility that circumvented Landsburg Dam on the Cedar River, Washington. Chinook Salmon Oncorhynchus tshawytscha spawned in newly available main‐stem habitats immediately after fish passage facility construction and in all subsequent years. Further dispersal into tributary habitats occurred 5 years after construction. Redds tended to be concentrated in the downstream third of the available habitat above the dam, although some fish did utilize suitable spawning sites throughout the main stem, even in the uppermost reaches of the newly available habitat. Median spawn timing for redds observed above the dam was not significantly different from spawn timing for the source population, indicating that migration delays through the fish passage facility were minimal. Male Chinook Salmon consistently outnumbered females, with annual sex ratios ranging from 1.3:1 to 4.7:1. Chinook Salmon spawning above the dam contributed between 2.7% and 14.7% of the total annual redd count (2003–2010) for Cedar River Chinook Salmon; upstream redds as a percentage of total redds increased over time, indicating that a new, naturally reproducing population above the dam was growing. The proportion of hatchery‐origin fish spawning above the dam decreased over the duration of the study but was consistently higher than the hatchery component observed below the dam.
To examine the role of longitudinal connectivity on the spatial and temporal dynamics of mountain whitefish (Prosopium williamsoni), we quantified movement and population dynamics following installation of the Landsburg Dam fishway, Cedar River, WA, USA. Mountain whitefish is widely distributed, poorly studied and not the focus of restoration. Before the fishway, mountain whitefish were not observed above the dam. Here, we focus on snorkel counts collected at reach and mesohabitat (e.g. pools) scales over 11 summers on the 20‐km above‐dam segment following restoration. A camera within the ladder provided number, size and movement timing, thereby informing on behaviour and recolonisation. Segment‐scale abundance increased following fish passage reaching an asymptote in 7 years, and mountain whitefish were detected throughout the main stem in 10 years. Annual movement through the ladder increased over time and was positively correlated with instream abundance and discharge, but negatively correlated with water temperature. About 60% of fish movements occurred in spring and early summer, potentially for foraging opportunities. Reach‐scale abundance peaked between 7 and 10 km from the dam; deep, cool (~10.6 to 11.6°C) conditions characterised these reaches. At the mesohabitat scale, mountain whitefish detection increased with depth and velocity after accounting for distance from the dam. Our results show how restoring longitudinal connectivity allowed this nontarget species to colonise newly available habitat. Their response supports the critical roles of longitudinal connectivity and environmental conditions, that manifest at different spatial scales, in dictating how freshwater fish respond to habitat disturbance.
In the Pacific Northwest of the United States, salmon eggs incubating within streambed gravels are susceptible to scour during floods. The threat to egg‐to‐fry survival by streambed scour is mitigated, in part, by the adaptation of salmon to bury their eggs below the typical depth of scour. In regulated rivers globally, we suggest that water managers consider the effect of dam operations on scour and its impacts on species dependent on benthic habitats. We instrumented salmon‐spawning habitat with accelerometer scour monitors (ASMs) at 73 locations in 11 reaches of the Cedar River in western Washington State of the United States from Autumn 2013 through the Spring of 2014. The timing of scour was related to the discharge measured at a nearby gage and compared to previously published ASM data at 26 locations in two reaches of the Cedar River collected between Autumn 2010 and Spring 2011. Thirteen percent of the recovered ASMs recorded scour during a peak‐discharge event in March 2014 (2‐to 3‐year recurrence interval) compared to 71% of the recovered ASMs during a higher peak‐discharge event in January 2011 (10‐year recurrence interval). Of the 23 locations where ASMs recorded scour during the 2011 and 2014 deployments, 35% had scour when the discharge was ≤87.3 m3/s (3,082 ft3/s) (2‐year recurrence interval discharge) with 13% recording scour at or below the 62.3 m3/s (2,200 ft3/s) operational threshold for peak‐discharge management during the incubation of salmon eggs. Scour to the depth of salmon egg pockets was limited during peak discharges with frequent (1.25‐year or less) recurrence intervals, which managers can regulate through dam operations on the Cedar River. Pairing novel measurements of the timing of streambed scour with discharge data allows the development of peak‐discharge management strategies that protect salmon eggs incubating within streambed gravels during floods.
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