Stability in reproductive timing and habitat usage of Chinook salmon across six years of varying environmental conditions and abundance

The spatial and temporal distribution of spawning activity by autumn‐run Chinook salmon Oncorhynchus tshawytscha (Walbaum) was examined across multiple years. The study period included two years of extreme drought conditions when water temperatures in the spawning reach of the Stanislaus River were considered sub‐optimal for spawning and egg incubation. Despite varying levels of superimposition, redd counts and densities remained stable and positively associated with river location, indicating that superimposition may be driven by habitat preference rather than the absence of suitable spawning locations. Spawning occurred slightly later during drought years (6–10 days later compared with wetter years). This delay was attributable to deferred migration instead of deferred spawning, and the majority of redds were constructed at water temperatures exceeding the optimal temperature range. As a consequence, estimates of juvenile production during 2014 and 2015 were among the lowest on record. These findings may be related in part to the high hatchery contribution to the population, above‐average temperatures during spawning and incubation, and superimposition rates. Management recommendations include adequate cold‐water storage in the upstream reservoir, refined spawning habitat restoration techniques in the light of superimposition rates and, on a broader system scale, actions that reduce the amount of stray hatchery Chinook salmon.     

Obstacles to migration constrain nest distribution of Atlantic salmon

Abstract – Programmes of habitat restoration usually assume that the targeted populations will readily colonise the newly provided habitat. However, this assumption may not always hold, and the success of restoration may be impaired if the individuals are driven to aggregate in areas of the habitat already available instead of spreading to new ones. We investigated how weirs situated along a river could drive Atlantic salmon (Salmo salar) to aggregate their redds and tested whether aggregation intensified at low fish density. For this, we applied spatial point pattern analysis to a data set consisting on the distribution of salmon redds in a small river over 15 years (1992–2006). Within the habitat suitable for spawning, redds were significantly aggregated, especially in the first 15 morphodynamic units below weirs. Our results suggest that the constraint imposed by obstacles on redd distribution should be considered when conducting habitat restoration.     

Brown trout redd superimposition in relation to spawning habitat availability

Abstract – The relationship between redd superimposition and spawning habitat availability was investigated in the brown trout (Salmo trutta L.) population inhabiting the river Castril (Granada, Spain). Redd surveys were conducted in 24 river sections to estimate the rate of redd superimposition. Used and available microhabitat was evaluated to compute the suitable spawning habitat (SSH) for brown trout. After analysing the microhabitat characteristics positively selected by females, SSH was defined as an area that met all the following five requirements: water depth between 10 and 50 cm, mean water velocity between 30 and 60 cm s^?1, bottom water velocity between 15 and 60 cm s^?1, substrate size between 4 and 30 mm and no embeddedness. Simple regression analyses showed that redd superimposition was not correlated with redd numbers, SSH or redd density. A simulation‐based analysis was performed to estimate the superimposition rate if redds were randomly placed inside the SSH. This analysis revealed that the observed superimposition rate was higher than expected in 23 of 24 instances, this difference being significant (P < 0.05) in eight instances and right at the limit of statistical significance (P = 0.05) in another eight instances. Redd superimposition was high in sections with high redd density. High superimposition however was not exclusive to sections with high redd density and was found in moderate‐ and low‐redd‐density sections. This suggests that factors other than habitat availability are also responsible for redd superimposition. We argue that female preference for spawning over previously excavated redds may be the most likely explanation for high superimposition at lower densities.     

Influence of European sculpin, Cottus gobio, on Atlantic salmon Salmo salar, recruitment and the effect of gravel size on egg predation – implications for spawning habitat restoration

Abstract  The study examined if recruitment of juvenile Atlantic salmon, Salmo salar L., was influenced by the presence of European sculpin, Cottus gobio L., and if the spawning substrate size used by salmon influences sculpin predation on salmon eggs. A combination of information on spawning site selection by female salmon and associated densities of juvenile salmon indicated that recruitment of juvenile salmon was 10 times lower in areas where sculpin was present than areas without sculpin. Predation rate on salmon eggs was found to be dependent on substrate size in artificial redds created in stream aquaria using four different sizes of substrate (13, 23, 37 and 62 mm). Predation rate averaged 83% in the aquaria with the largest substrate size, whereas a rate of only 2–3% was observed using smaller substrate sizes. Sculpin may thus be an important factor influencing the recruitment of juvenile salmon. Selecting small enough gravel sizes during restoration of salmon spawning habitat could therefore be important to minimise egg predation.     

Above parr: Lowland river habitat characteristics associated with higher juvenile Atlantic salmon (Salmo salar) and brown trout (S. trutta) densities

Understanding juvenile salmonid habitat requirements is critical for their effective management, but little is known about these requirements in lowland rivers, which include important but unique salmonid habitats. We compared the relative influence of in-stream Ranunculus cover, water depth, prey abundance, distance upstream and two previously unexplored factors (water velocity heterogeneity and site colonisation potential) on summer densities of juvenile Atlantic salmon and brown trout. We applied electrofishing, habitat surveys and macroinvertebrate kick sampling, and calculated the site colonisation potential from salmon redd surveys across 18–22 sites in a lowland river in 2015–2017. Due to a recruitment crash in 2016, models including and excluding this unusual year were explored. Excluding 2016 data, juvenile salmon densities showed a positive association with Ranunculus cover and numbers of nearby upstream redds, and a negative association with distance upstream from the tidal limit. Trout densities were positively associated with velocity heterogeneity, indicating a potential indirect influence of Ranunculus mediated by water velocity. When including 2016, year had the largest effect on densities of both species, highlighting the impact of the recruitment failure. These findings uncover interspecific differences in the habitat requirements of juvenile salmonids in lowland rivers. Velocity heterogeneity and site colonisation potential had high explanatory power, highlighting that they should be considered in future studies of habitat use. These findings demonstrate that temporal replication and recruitment dynamics are important considerations when exploring species–habitat associations. We discuss potential management implications and argue that Ranunculus cover could be an important management tool in conservation of lowland salmonids.     

A comparative study of juvenile salmon density in 20 streams throughout a very large river system in northern Norway

Abstract – The objective was to compare juvenile salmon density in 20 streams throughout the very large River Tana, northern Norway, and to relate variation in density to a suite of environmental factors. Four sampling sites were electrofished in each stream (one at the mouth of the stream and three within the stream) in August and October 2000, 2001, 2002. 0+ salmon parr were absent from seven streams, present at the mouth of 11 streams, and present within only two streams, both of which were probably spawning streams. Older parr migrated upstream into most streams and their highest densities were usually found in streams flowing directly into the spawning habitat in the three largest tributaries of the Tana or the river itself. Juvenile salmon were sparse or absent in streams flowing into smaller tributaries. Most streams with high parr densities were those of dense riparian vegetation that provided terrestrial invertebrates as drift food for the salmon parr, cover for fish, cooler stream temperatures in summer, and food for benthic stream invertebrates that were also a source of food for the parr.     

A retrospective assessment of the flow regulation of the West Salmon River, Newfoundland, Canada

The Upper Salmon Hydroelectric Project in central Newfoundland, Canada, constructed in the early 1980s, affected the hydrology of the West Salmon River, a major spawning and juvenile rearing river for landlocked Atlantic salmon, Salmo salar L., and brook trout, Salvelinus fontinalis (Mitchill). A controlled flow release strategy, based on Tennant's Montana method, was developed to protect this habitat and prescribed a release of 40% of the mean annual flow (MAF) (2.6 m³ s^–1) between 1 June and 30 November and 20% of the MAF (1.3 m³ s^–1) from 1 December to 31 May. Studies were conducted to assess the impact of river regulation including: (1) a post-impoundment evaluation of the anticipated geomorphic and sedimentary characteristics; (2) monitoring of juvenile fish populations under regulation; and (3) a retrospective IFIM (instream flow incremental methodology) assessment. Studies provided evidence of the initial stages of river aggradation. Biological monitoring found no apparent effects of sediment deposition on spawning and egg incubation. However, densities of older age classes (parr, 1+ and greater) declined under regulation, possibly related to poor overwintering conditions under the lower winter flow. IFIM study results supported these observations and indicated that the prescribed flow regimen provided habitat conditions that would benefit salmon fry more than older age classes. This retrospective assessment suggested that future proposals for flow regulation in Newfoundland should consider the need for more dynamic flow management as well as to provide overwintering habitat for resident fish. Habitat–hydraulic models are preferred to standard setting approaches owing to more detailed analysis of habitat trade-offs as related to flow regulation.     

Potential negative impacts of introduced rainbow trout on endangered Sakhalin taimen through redd disturbance in an agricultural stream, eastern Hokkaido

Nomoto K, Omiya H, Sugimoto T, Akiba K, Edo K, Higashi S. Potential negative impacts of introduced rainbow trout on endangered Sakhalin taimen through redd disturbance in an agricultural stream, eastern Hokkaido.
Ecology of Freshwater Fish 2010: 19: 116–126. © 2009 John Wiley & Sons A/S
Abstract –  Sakhalin taimen ( Hucho perryi ) populations have decreased in Hokkaido, northernmost Japan, primarily because of overexploitation and habitat degradation. Here we document another threat to this species, that of spawning redd superimposition by artificially introduced rainbow trout ( Oncorhynchus mykiss ). Sakhalin taimen and rainbow trout are the only spring-spawning salmonid species in Hokkaido. In 2006–2008, spawning activities of both these species were observed in a Hokkaido stream, and it was determined that their spawning periods overlapped during mid-late April. They also spawned at similar water velocities, depths and substrate compositions. Although female Sakhalin taimen were larger than female rainbow trout, their egg burial depths were nearly identical. During the observation period, rainbow trout redds were approximately five times more abundant than Sakhalin taimen redds, and about 30% of the observed Sakhalin taimen redds were superimposed by rainbow trout redds. The high degree of spatial and temporal overlap in spawning, the similar egg burial depths of both species, and the high proportion of superimposed redds suggest that the introduced rainbow trout impact the endangered Sakhalin taimen in Hokkaido, and possibly, in other areas where the two species occur together.     

Brook trout spawning areas in lakes

Abstract— Nine lakes containing self-sustaining populations of brook trout ( Salvelinus fontinalis ) were surveyed to provide quantitative information on the abundance and distribution of redd sites. We measured rates of groundwater flow at redds in four of these lakes, and in one of these lakes we also compared the frequency of site use over four years from spawning observations. The number of redd sites in each lake ranged from 1 to 53 (mean=30) with most found relatively close to shore (∼ 1 m deep). Spawning areas per lake (16.9–829.4 m²) were not related to lake size and were often not continuous areas of littoral zone habitat. Rates of groundwater flow in lakes, averaged across redd sites in each lake, ranged from 20.0 to 107.9 ml. m.⁻² min^−1. The intensive survey of redd site use in one lake demonstrated that only 11% of sites (10 of 92) were used in all four years of observation. The mean rates of groundwater flow at sites used in all four years were significantly greater than at sites used intermittently. Data from this and other studies point to the conservation of subcatchments associated with groundwater discharge areas used by spawning brook trout as a means of maintaining self-sustaining populations.     

Linking habitat characteristics with juvenile density to quantify Salmo salar and Salmo trutta smolt production in the river Sävarån,Sweden

Abstract Habitat mapping along 85 km of river was related to juvenile (15 years of electric fishing) and smolt (3 years of screw‐trapping) abundance data to estimate salmon, Salmo salar L., and sea trout, Salmo trutta L., smolt production in the River Sävarån, northern Sweden. Spawning site selection by radio‐tagged salmon (n = 12) and sea trout (n = 4) was also assessed. Fifty‐one hectares of potential spawning and nursery habitat was found in the main stem river, representing 25% of the total river area. These areas were estimated to yield 1300–7580 salmon and 630–3540 sea trout smolts based on juvenile densities, equating with 3 years of screw‐trap data (2990–5080 salmon and 680–2520 trout smolts, respectively). A hypothetical maximum production of about 19 900 salmon smolts was predicted for the river at a density of 40, 0+ salmon 100 m^?2. Tracking adults during the spawning period identified optimal and potential reproductive areas.     

The effects of fluvial processes and habitat heterogeneity on distribution, growth and densities of juvenile Atlantic salmon (Salmo salar L.), with consequences on abundance of the adult fish

Abstract – The required freshwater habitats of juvenile Atlantic salmon ( Salmo salar ) are, in general, well known, but vary in quality, related to interacting effects of several variables, which may depend on different parts of a river system. Examples are given of ranges of densities and growth that can be found at various sites in eastern Canada, illustrating the biological and physico-chemical factors affecting production of juvenile salmon. Relative growth rates can indicate habitat quality and population densities. Salmon parr have negative effects on brook trout in riffle habitats. The effects of migrations within the river and of changes with stream succession on juvenile salmon production are illustrated with examples from a Newfoundland river. Migration of age-classes can be quantified from 'self-thinning' curves. Lakes have enhancing effects on downstream fluvial habitats, and, at least in Newfoundland, and probably in many boreal areas, the lacustrine proportion of the basin can be used as an index for deriving estimates of required spawning escapement. The factors described should be taken into account for more refined estimates of river production and management of the salmon resource.     

Factors affecting habitat selection by a small spawning charr population, bull trout, Salvelinus confluentus: implications for recovery of an endangered species

Bull trout, Salvelinus confluentus (Suckley), populations are declining in many streams of North America and are listed under the Endangered Species Act in the United States. Many small populations are isolated in fragmented habitats where spawning conditions and success are not well understood. Factors affecting habitats selected for redds by spawning bull trout and redd habitat characteristics within Gold Creek, a headwater stream in the Yakima River within the Columbia River basin, Washington State, USA, were evaluated. Most spawning (>80% of the redds) occurred in upstream habitats after dewatering of downstream channels isolated fish. Habitats were selected or avoided in proportions different to their availability. For example, most bull trout selected pools and glides and avoided riffles despite the latter being more readily available. Although preferences suggest influences of prolonged fish entrapment, site fidelity could be important. A habitat with redds commonly contained abundant cover, gravel substratum and higher stream flows. The major factors influencing habitat selection by spawning fish and their persistence in streams of the Yakima and Columbia River regions include entrapment of fish by dewatering of channels and geographical isolation by dams. The goal of the US Government's recovery plan is ‘to ensure the long‐term persistence of self‐sustaining bull trout populations’. Recovery plans linked to provisions for protecting and conserving bull trout populations and their habitats were recommended. Landscape approaches are needed that provide networks of refuge habitats and greater connectivity between populations. Concurrent recovery efforts are encouraged to focus on protecting small populations and minimizing dangers of hybridization.     

Modelling the reestablishment of coho salmon (Oncorhynchus kisutch) in Klamath River tributaries after dam removal

The planned removal of four dams on the Klamath River (anticipated 2024) will be the largest river restoration effort ever undertaken on the planet. Dam removal will restore access to >50 km of the Klamath River mainstem for coho salmon, but mainstem habitat may not be suitable for rearing juvenile coho salmon. Instead, small tributaries may provide most rearing habitat for reestablishing coho salmon. We used four approaches to evaluate six Klamath River tributaries above existing dams to assess their potential to support juvenile coho salmon: (1) We measured summer temperature regimes and evaluated thermal suitability. (2) We applied an Intrinsic Potential (IP) model to evaluate large-scale geomorphological constraints on coho salmon habitat. (3) We used the Habitat Limiting Factors Model (HLFM) to estimate rearing capacity for juveniles given current habitat conditions. (4) We developed an occupancy model using data from reference tributaries to predict coho salmon rearing distribution. All six streams had summer temperatures cooler than the mainstem Klamath River. However, five of the streams have barriers that will restrict coho salmon to within 5 km of the confluence with the Klamath River and two were disconnected mid-summer. Despite these constraints, the tributaries will likely produce coho salmon. Most streams had high IP in their lower reaches, the HLFM model estimated a total capacity of 105,000 juvenile coho salmon, and the occupancy model predicted juvenile coho salmon will rear throughout the accessible reaches. Protection and habitat enhancement for these tributaries will be important for coho salmon reestablishment post-dam removal.     

Importance of instream cover for young masu salmon, Oncorhynchus masou, in autumn and winter

Densities of wild masu salmon, Oncorhynchus masou Brevoort, were investigated before (late-September) and after (mid-November) autumnal habitat shifts in a small river in Hokkaido, northern Japan. Abundance of instream cover habitat formed by coarse woody debris, submerged vegetation and undercut banks was quantified. Density of 1+ and older masu salmon was significantly correlated with cover abundance in autumn, although cover was not significant for density of 0+ fish, indicating that clear-cut reaches were utilized by 0+ masu salmon in autumn. In early winter, densities of 0+ and 1+ and older masu salmon were both correlated with cover availability. These results suggest that the amount of cover habitat is an important regulator for densities of juvenile masu salmon during winter.     

Spawning period of a southern brown trout population in a highly unpredictable stream

Abstract –  The spawning period of brown trout ( Salmo trutta L.) was studied in the river Castril, southern Spain, by means of redd counts. This mountain stream is located near to the southern limit of the species' natural distribution range and it shows a highly unpredictable flow regime. The spawning period extended from December to mid-April and the maximum reproductive activity was in February. These results represent the latest reproduction date and the longest spawning period reported in the literature along the natural distribution range of the species. However, belated spawning in the Castril is congruent with the known latitudinal cline: the lower the latitude, the later the spawning period. Our results, along with a review of the literature on natural populations, also showed that the duration of reproduction is the longer, the lower the latitude. Spawning lasted twice as long in the main stem of the river, which is connected with a reservoir, than in the isolated reaches. These differences may be linked to the influence of the reservoir and to habitat fragmentation. We discuss and support the hypothesis that a long spawning period is an advantage for survival in unpredictable habitats. The belated and protracted spawning period found in river Castril has important implications in fisheries management. A strong research effort is needed in order to fill the critical lack of data on southern brown trout populations.     

Current status of natural spawning of chum salmon <Emphasis Type="Italic">Oncorhynchus keta</Emphasis> in rivers with or without hatchery stocking on the Japan Sea side of northern Honshu,Japan

Information on the status of natural spawning is needed on the Japan Sea side of northern Honshu, Japan for ecosystem-based sustainable management of chum salmon resources. We conducted on-site visual surveys in October–December of 2015 and 2016 that targeted spawning chum salmon redds in all rivers?>?5 km long (total 94 rivers) in Akita, Yamagata, Niigata (including Sado Island), and Toyama prefectures. The ratio of rivers found to host natural reproduction to the total number of surveyed rivers was 93.6% (44/47) in stocked rivers and 74.5% (35/47) in non-stocked rivers. These results show that there is a wide occurrence of natural reproduction of chum salmon in these rivers, regardless of the history of hatchery stocking. The density of spawning redds (number of redds/1000 m²) as an indicator of chum salmon escapements did not differ (P?=?0.54) between stocked rivers (mean 3.5, N ?=?49) and non-stocked rivers (mean 2.4, N? =?36),when rivers where no redds were observed were excluded from the analysis. These results suggest that chum salmon escapements into non-stocked rivers may not be negligible. Conservation measures for wild fish are needed in stocked and non-stocked rivers to promote enhancement programs based on natural reproduction.     

Distribution and population genetic structure of river- and sea-type sockeye salmon in western North America

Abstract– In contrast to the well-known "lake-type" sockeye salmon, two additional anadromous life-history types have been recognized within the species: 'river-type' sockeye salmon whose juveniles spend 1 or 2 years in off-channel river habitats prior to migrating to sea, and "sea-type" sockeye salmon that initially rear in similar river habitats yet migrate to sea as underyearlings. Persistent populations of river-/sea-type sockeye salmon occur in small numbers throughout the species'range in North America but are usually associated with glacier-fed rivers. We found published and unpublished records showing that riverine-spawning sockeye salmon occur in 11 rivers in western Washington, USA, that don't have access to juvenile lake-rearing habitat. Evidence of persistent spawning was strongest for the Nooksack and Skagit rivers in northern Puget Sound. We analyzed allozyme frequency differentiation in 26 laketype and 12 river-/sea-type populations of sockeye salmon in North America, ranging from northern Puget Sound, Washington (including 3 in the Nooksack and Skagit rivers) to northern Southeast Alaska. Across this 2000 km range, river-/sea-type sockeye salmon showed very little genetic differentiation between populations, much less than that displayed by the highly divergent lake-type sockeye salmon. Genetic similarity among river-/sea-type sockeye salmon in this study is likely a result of common ancestry and a high level of historical gene flow among river-/sea-type sockeye salmon populations.     

Reproductive success of Atlantic salmon (Salmo salar) mature male parr in a small river,the Nivelle: influence of shelters

Grimardias D, Merchermek N, Manicki A, Garnier J, Gaudin P, Jarry M, Beall E. Reproductive success of Atlantic salmon (Salmo salar) mature male parr in a small river, the Nivelle: influence of shelters.
Ecology of Freshwater Fish 2010: 19: 510–519. © 2010 John Wiley & Sons A/S Abstract – The breeding activity of a small population of Atlantic salmon (Salmo salar L.) was monitored on the Nivelle River in southwest France to estimate the overall contribution of mature male parr and the numbers involved in egg fertilisation. In the twelve redds sampled, 563 eggs were collected for parental assignment and the physical habitat was characterised to investigate the possible relationship between parr reproductive success and habitat complexity. The overall contribution of mature male parr was particularly high (87%, one of the highest estimates ever reported). Concerning habitat complexity, granulometry and shelter diversity were correlated with the number of parr breeders contributing to egg fertilisation in each redd. A complex habitat increased multiple paternity, which could affect the effective size and genetic variability of small salmon populations.     

An assessment of the potential for the application of two simple models to Atlantic salmon, Salmo salar, stock management in chalk rivers

Many salmon populations in the UK are under threat, which is assumed to result from a combination of anthropogenic and natural factors. Positive management is required to bring them back to their former productivity levels. An attempt has been made to use two simple models to assess the suitability of several management approaches for use on chalk rivers. One model is based on the survival rates at each stage of the salmon life cycle and the other estimates the predation of young salmon by other fish. This work highlights the paucity of data available to calibrate even the simplest salmon management models. For those variables where data are available, the variability is very high. Despite the poor quality of input data, the outputs are close to observed numbers in the River Frome, a chalk river in Dorset, UK. The models were used to assess the effect of managing the fishery using reduced rod catches, pike culling, gravel cleaning and trout stocking on adult salmon and smolt numbers.