Diet Variability of Steelhead/Rainbow Trout in a Coastal Basin in Central California: Relative Importance of Seasonal, Spatial, and Ontogenetic Variation

Abstract

Diets of stream-dwelling salmonids vary at multiple spatial and temporal scales, but the relative importance of different sources of variation has rarely been evaluated for a population. In a small coastal basin in central California, we sampled diets of steelhead/Rainbow Trout Oncorhynchus mykiss monthly for a year from two reaches that differed in the presence of streambed travertine (calcium carbonate) to determine the relative magnitude of spatial, seasonal, and ontogenetic variation in consumption and diet composition. Seasonal parameters (cosine function terms for months in periodic regression and multivariate models) and fish size (FL) were greater sources of variation than differences between the two study reaches, and seasonal and ontogenetic patterns were evident despite high interindividual variability. Seasonal parameters explained the most variation in diet composition, which varied in an orderly pattern over the year. Consumption was highest in spring and summer for both aquatic and terrestrial prey, and seasonal parameters accounted for more variation than fish size for consumption of aquatic prey but this was reversed for terrestrial prey and total consumption. The main ontogenetic trend was increased consumption of terrestrial invertebrates: from 15-20% of the energy consumed by 60-100-mm fish to 60% of the energy consumed by fish larger than 160 mm. In addition, diet breadth (taxonomic richness and prey size) and the degree of variation among individuals also increased with fish size. Diet differences between sites were relatively small and included higher consumption of aquatic invertebrates at the site without travertine in 4 months and modest differences in the relative abundances of common prey taxa. Although seasonal and ontogenetic patterns were evident, interindividual variation was the dominant scale of diet variation, and diets of individual fish ranged from specialized to generalized relative to the population. Based on these findings, O. mykiss appeared to be opportunistic feeders that took advantage of diverse aquatic and terrestrial prey available in these study reaches.

Importance of non‐native isopods and other terrestrial prey resources to steelhead/rainbow trout Oncorhynchus mykiss in coastal streams in Big Sur, California

“…Comparable quadrat‐based estimates are not available for riparian areas, although non‐native isopods dominated arthropod samples from pitfall traps and numbered in up to tens per trap in New Mexico (Cartron et al, 2003; Ellis et al, 2001) and an inland basin in northern California (Holway, 1998). However, similar to other studies, peak abundance and surface activity of isopods in Big Sur appears to be in spring (March–May) based on our diet samples and observations in Big Creek (Rundio & Lindley, 2019). Consequently, the diet and density data presented here based on sampling in late June probably underestimate the abundance of isopods compared to the annual maximum.…”

“…Although the energetic content and taxonomic composition of O. mykiss diets varied among sites, there was no clear spatial pattern or trend to the variation and overall diets were generally similar across sites. The importance of terrestrial prey resources, including isopods, to O. mykiss populations across the Big Sur coast broadens our previous findings from a single basin (Rundio & Lindley, 2008, 2019) and highlights that non‐native terrestrial invertebrates can be an important prey subsidy to salmonids in some systems.…”

“…Sites were selected non‐randomly, dictated by land ownership and physical access in often steep stream canyons; however, sites were well distributed across the region (Figure 1) and represented a range of geographic/landscape variables such as drainage area, elevation, and gradient (Table S1), so this did not appear to bias the study. Single sites were sampled in 10 of the basins, and three sites were sampled in Big Creek where we have conducted intensive studies on O. mykiss and the stream food web (Rundio & Lindley, 2008, 2012, 2019; Rundio et al, 2012). At each site, we selected a 100‐m reach that was representative of habitat conditions of the surrounding area.…”

“…All field sampling was conducted during early summer from June 22–30, 2006. Terrestrial invertebrate subsidies in this region are highest from early summer through fall, and surface activity by non‐native isopods, and their abundance in the diet, is highest in spring through early summer before declining as the dry season progresses (Rundio & Lindley, 2008, 2019). Diet samples were collected from 25 fish per site.…”

“…Endangered Species Act, our secondary objective was to evaluate basic aspects of energy content and composition of the diet. This included differences related to fish size, which is often a large source of diet variation in stream salmonids and in particular may influence the use of terrestrial prey (Dineen et al, 2007; Gustafsson et al, 2010; Syrjänen et al, 2011) including non‐native isopods (Rundio & Lindley, 2019). Finally, although landscape conditions across the Big Sur coast are relatively similar in terms of major features such as landform, forest type, low level of development, and climate, the region is large (>100 km from north to south) and there is some local variation in geology and other environmental conditions (Rundio, 2009), so we also sought to determine whether there were large‐scale geographic patterns or trends in diet or riparian isopod abundance.…”