Seasonal growth potential of Oncorhynchus mykiss in streams with contrasting prey phenology and streamflow

Abstract

The growth of any organism depends on habitat conditions, food availability, and their seasonal interactions. Yet in the vast literature on Pacific salmon (Oncorhynchus), the seasonal interaction between habitat conditions and food availability has received relatively little attention. We examined juvenile Oncorhynchus mykiss rearing, physical habitat, and resource phenologies in two Mediterranean coastal streams-one perennial, cool, and shaded and the other intermittent, seasonally warm, and sunny. We used a bioenergetic model to investigate the timing and magnitude of growth potential for drift-foraging O. mykiss during the spring and summer in both systems. Growth potential peaked at least 2 months earlier in the intermittent stream than in the perennial stream. By early summer (June), growth potential had declined in the intermittent stream, whereas growth rates were peaking in the perennial stream. However, the mid-July lipid content of juvenile O. mykiss in the intermittent stream was nearly twice that of fish in the perennial stream. By late summer (August), foraging profitability declined in both streams, as abiotic conditions in the intermittent stream approached lethal. In contrast, the perennial stream maintained suitable abiotic conditions even though the growth rate was low. We suggest that the divergent resource phenologies and seasonal mortality risks experienced by anadromous O. mykiss rearing in these streams could drive diversification of traits governing size, age, and timing of outmigration.

Flow augmentation from off-channel storage improves salmonid habitat and survival

“…The biomass of invertebrate drift increased relative to control reaches following the onset of augmentation, consistent with our hypothesis (H5) and previous studies, which have shown that increased wetted area and riffle velocity can increase the production, hydraulic transport, and behavioral drift of invertebrate species (Annear et al 2004, Svendsen et al 2004Naman et al 2016). However, the natural phenology of invertebrate drift in Porter Creek was near its annual minimum by during mid-summer (Rossi et al 2022) (Kelson et al 2019, Rossi et al 2022. Thus, we suspect that the benefits to growth and survival from this level of augmentation (13.9 L/s) in mid-summer are more likely due to decreased metabolic stress from increased dissolved oxygen, or perhaps increased mobility of fish, than this modestly increased drift rate.…”

The salmonid and the subsurface: Hillslope storage capacity determines the quality and distribution of fish habitat

“…The watershed is very lightly grazed seasonally, but satellite records show that long-standing fence lines do not result in major differences in plant biomass or biome type (see video in megaboulder-pinned knickpoint (e.g., Roering et al, 2015) just downstream of the Dry Creek outlet. Salmonids are nevertheless commonly found in other small melange streams (e.g., Rossi et al, 2022) and throughout the Eel River watershed, which integrates flow and temperature signals from both Central and Coastal Belt streams.…”

“…However, the slower rate of flow recession in spring and higher mean summer baseflows can support fish that oversummer in the stream and rear for at least one year before out-migration (Kelson & Carlson, 2019). Secondary production increases later than in intermittent streams, which along with sustained recessions and greater channel shading leads to a later peak in growth potential (Rossi et al, 2022). Perennial flow creates a greater extent of wetted channel and sustained summer rearing environment (both space and water quality) and reduces summer mortality relative to intermittent streams.…”

“…All else being equal, streams fed by CZs with low storage capacity will have early, faster recessions and water temperature warming, which will lead to earlier spring increase in juvenile salmonid growth potential, but also an earlier decrease in summer growth potential because very low flows decrease drifting prey and warm water to stressful levels. Whereas in perennial streams fed by CZs with high storage, growth potential for juvenile salmonids will peak later and be sustained longer into the dry season (Rossi et al, 2022).…”

“…Warmer water would also directly accelerate egg development in nonperennial streams (Figure 9), so both earlier spawn timing and faster incubation should result in earlier emergence of juveniles. Earlier increases in food availability in sunlit channels support an earlier spring peak in growth potential in intermittent versus perennial streams (Ebersole et al, 2006; Rossi et al, 2022). When the wetted channel dries completely or conditions become lethal, out‐migration before the stream dries is the only option for survival.…”