The Effects of a Prolonged Drought on Southern Steelhead Trout (Oncorhynchus mykiss) in a Coastal Creek, Los Angeles, California

Trout and invertebrate assemblages in stream pools through wildfire and drought

“…We also suspected that low DO levels played a role in the disappearance of trout from some of our study reaches during the drought, because trout disappeared from these reaches when pools became increasingly isolated and DO levels sometimes declined to lethal levels (<2-3 mg/L; this study, see also Dagit et al, 2017; Years since trout were lost Elliott, 2000). Our analyses indicated that trout were associated with DO levels >6 mg/L, which are higher than the lethal limits for juvenile and adult trout, but are required for the development of earlier life history stages and associated with the physiological performance and growth of juveniles and adults (Carter, 2005).…”

“…In our study reaches, trout may re‐distribute themselves among pools during the wet season when water levels are high; however, most trout are trapped in home pools during the summer when flows between pools are very low or non‐existent (Cooper, 1984b; Wiseman et al., 1993). Trout population sizes in our streams are highly variable through both space and time, and juvenile recruitment is sporadic (as in Bell et al., 2011; Dagit et al., 2017). The trout population in a 1.2‐km reach of one of our study streams (Rattlesnake Creek) in the summers of 1982 and 1988 was composed of 0% and 11% young‐of‐the year (<10 cm total length), respectively, with mean trout total length being 19.7 cm (range = 10–32 cm) and 15.7 cm (range = 2–36 cm) and densities in trout pools averaging 0.1/m 2 and 0.23/m 2 .…”

“…In our study reaches, trout may re-distribute themselves among pools during the wet season when water levels are high; however, most trout are trapped in home pools during the summer when flows between pools are very low or non-existent (Cooper, 1984b;Wiseman et al, 1993). Trout population sizes in our streams are highly variable through both space and time, and juvenile recruitment is sporadic (as in Bell et al, 2011;Dagit et al, 2017).…”

“…Our results show that post-fire scouring floods and prolonged droughts can extirpate sensitive species, such as the southern California steelhead trout, the top aquatic predator in southern California streams (Cooper, 1988;Cooper et al, 2015;National Marine Fisheries Service, 2012. The survival of trout and other sensitive species through these disturbances depends on protecting or restoring refuge habitats at the stream network scale, such as deep, shaded, cool stream pools with high habitat and water quality or, additionally in the case of anadromous steelhead, suitable estuarine and marine habitats (Alvarez & Dagit, 2019;Bogan et al, 2019;Cooper et al, 2021;Dagit et al, 2017;Kaylor et al, 2019;Lennox et al, 2019;National Marine Fisheries Service, 2012, 2023Robson et al, 2013). Further, the resiliency of sensitive species and stream communities to disturbances is inhibited by the construction and maintenance of barriers (e.g., dams and road crossings) that block salmonid migrations and intercept the downstream drift of stream organisms, impeding the recolonisation of disturbed habitats (Kukuła & Bylak, 2022).…”

Modelling future changes to the hydrological and thermal regime of unaltered streams using projected changes in climate to support planning for sensitive species management

“…Therefore, the ecological implication of a predicted flow change may differ depending on the recent precipitation trend. For example, a further decrease in hydroperiod in high‐elevation streams during dry and moderate periods would harm the resident rainbow trout and the non‐migrating steelhead by substantially reducing available habitat (Dagit et al, 2017). Alternatively, an increase in storms during dry years could provide additional opportunity for smolts to migrate downstream.…”