Comparative Use of Shallow and Deepwater Habitats by Juvenile Pacific Salmon in the Columbia River Estuary Prior to Ocean Entry

Abstract

The degree to which fine‐scale habitat use by salmonid species and stocks varies within habitat types such as estuaries is not fully resolved. We sampled shallow shoreline and deeper main‐stem channel habitats in the Columbia River estuary over 3 years to compare salmon species composition, migration timing, density, size, and production type (hatchery or natural). Results indicated a high degree of spatial heterogeneity in habitat occupancy by the five salmonid species that are native to the basin. Salmonid communities at two channel habitat sites were much more similar to each other than to the community at a shoreline site. Salmonids sampled at the shoreline site were primarily subyearling Chinook Salmon Oncorhynchus tshawytscha and Chum Salmon O. keta and yearling Coho Salmon O. kisutch, with few other salmonids present. In contrast, channel habitat contained a higher diversity of salmon species, with samples representing all species of anadromous salmonids, including Sockeye Salmon O. nerka and steelhead O. mykiss. Salmonids in deeper channel habitat were generally larger than salmonids found along the shore, and the proportion of hatchery‐origin salmon was also higher in deep channel habitats. On a per‐area basis, we also found much higher densities of salmon along the shoreline than in channel habitats. For Chinook Salmon, habitat use also differed by genetic stock of origin: upper‐river stocks primarily used deeper channels, while lower‐river populations used both channel and shoreline areas. We concluded that sampling at both habitat types is required to fully encompass the migration patterns of all salmon evolutionarily significant units in the Columbia River basin. These spatial and temporal variations in salmon timing and density have ramifications for feeding, growth, and competitive interactions. This study provides information that is relevant for conservation efforts targeting specific fish populations and efforts to evaluate the potential impacts of in‐water activities in the Columbia River estuary. Received May 12, 2016; accepted August 19, 2016

Estuary habitat associations for juvenile Pacific salmon and pelagic fish: Implications for coastal planning processes

“…Juvenile salmon are known to distribute according to geospatial factors such as proximity to land (Bottom, Jones, Cornwell, Gray, & Simenstad, ; Roegner et al, ) and freshwater outflow (J. S. Macdonald et al, ). In this study, distance from the river mouth was not an important factor in describing the spatial distribution of juvenile salmon in the Skeena River estuary.…”

“…Because much of the habitat closer to shore (within <5 m) is subtidal and can be sampled with the 9 m or 5 m deep purse seines, an association with locations closer to shore suggests that fish were found in higher abundances in subtidal nearshore habitat on channel margins compared with open water between landforms. In other estuaries, larger sub‐yearlings and yearling Chinook and coho salmon are associated with deeper channel or channel margin habitats (10–15 m deep; (Gamble et al, ; Pinnix, Nelson, Stutzer, & Wright, ; Roegner et al, ), whereas smaller fry or sub‐yearling Chinook are associated with shallow nearshore habitat (<3 m deep; Bottom, Jones, et al, ; Hering et al, ; Roegner et al, ; Simenstad et al, ). As both, coho (73–242 mm) and Chinook (85–185 mm) salmon found in the Skeena River estuary are primarily larger age classes (subyearling or ≥1 age class—using designations from Roegner et al, and Weitkamp et al, ), results from this study are comparable to findings from other estuaries.…”

“…Modelling approach A investigated which combination of biophysical variables (Table ) was most important in explaining fish abundance across the estuary using multimodel inference. Since all biophysical variables were selected based on a priori hypotheses as outlined by previous studies (McNatt et al, ; Pearcy, ; Roegner, Weitkamp, & Teel, ; Semmens, ; Straty & Jaenicke, ), they were included in the modelling selection process. Owing to inconsistent depth sampling resulting from technical problems, depth was removed from the final variable list as it substantially reduced the number of sampling events with complete covariate sets.…”

A Review of Habitat Connectivity Research for Pacific Salmon in Marine, Estuary, and Freshwater Environments

“…But all estuary habitat is not the same for all salmonids: the five species of Pacific salmon present in the Columbia River estuary use shallow or deep estuary habitats at different times of the year in different ways (Roegner et al. ).…”

The Abundance and Distribution of Hatchery and Naturally Produced Chinook Salmon in Columbia River Estuary Nearshore Habitat

“…Lowerestuary purse-seine surveys found that over 90% of the Chinook and Coho salmon and steelhead in deepwater habitats originate from hatcheries (Weitkamp et al 2012). Hatchery fish also have been collected in some shallow nearshore areas (Roegner et al 2012(Roegner et al , 2016Sather et al 2016), but the combined effects of all hatchery releases on the densities, stock composition, and size characteristics of salmon in these habitats are poorly understood. It is unknown, for example, whether large size at release effectively segregates HO smolts from NP juveniles in most shallow estuarine nursery areas or whether significant overlap enables hatchery fish to dominate or displace smaller NP juveniles (e.g., Nickelson et al 1986).…”