Role of freshwater floodplain-tidal slough complex in the persistence of the endangered delta smelt

Abstract

Seasonal floodplain wetland is one of the most variable and diverse habitats found in coastal ecosystems, yet it is also one of the most highly altered by humans. The Yolo Bypass, the primary floodplain of the Sacramento River in California’s Central Valley, USA, has been shown to provide various benefits to native fishes when inundated. However, the Yolo Bypass exists as a tidal dead-end slough during dry periods and its value to native fishes has been less studied in this state. During the recent drought (2012–2016), we found higher abundance of the endangered Delta Smelt (Hypomesus transpacificus), than the previous 14 years of fish monitoring within the Yolo Bypass. Meanwhile, Delta Smelt abundance elsewhere in the estuary was at record lows during this time. To determine the value of the Yolo Bypass as a nursery habitat for Delta Smelt, we compared growth, hatch dates, and diets of juvenile Delta Smelt collected within the Yolo Bypass with fish collected among other putative nursery habitats in the San Francisco Estuary between 2010 and 2016. Our results indicated that when compared to other areas of the estuary, fish in the Yolo Bypass spawned earlier, and offspring experienced both higher quality feeding conditions and growth rates. The occurrence of healthy juvenile Delta Smelt in the Yolo Bypass suggested that the region may have acted as a refuge for the species during the drought years of 2012–2016. However, our results also demonstrated that no single region provided the best rearing habitat for juvenile Delta Smelt. It will likely require a mosaic of habitats that incorporates floodplain-tidal sloughs in order to promote the resilience of this declining estuarine fish species.

Otolith-based approaches indicate strong effects of environmental variation on growth of a Critically Endangered estuarine fish

“…due to reduced predation risk or higher prey availability) in turbid conditions may help facilitate growth compensation during periods of thermal stress, as has been shown for other temperate fishes (Lusardi et al 2020). If so, this may explain, in part, the ability of some freshwater resident delta smelt to survive otherwise thermally stressful conditions during warm summer conditions (Hobbs et al 2019, Mahardja et al 2019.…”

Complex life histories discovered in a critically endangered fish

“…These long-term monitoring studies have documented the persistence of Delta Smelt in the tidal freshwaters of the lower Sacramento River near the town of Rio Vista year-round, but due to their close proximity to brackish water, it was not acknowledged that Delta Smelt could complete their life cycle in freshwater in the wild 26 . Since 2002, additional surveys conducted in the North Delta have documented year-round presence and, at times, high abundance of Delta Smelt, suggesting freshwater resident fish may be important to the population 31,32,52,53 . Our otolith 87 Sr/ 86 Sr chronologies confirmed the existence of a freshwater resident life history comprising 12% of the adult population in this abundant year-class.…”

“…Two of our semi-anadromous clusters had natal 87 Sr/ 86 Sr consistent with hatching in the North Delta and their age-at-dispersal into brackish water was much older, suggesting fish hatched in this region may have found it difficult to disperse downstream into the estuary. Recently, studies have found evidence of degraded health in Delta Smelt found in the North Delta during summer months, exhibiting poor feeding, somatic condition and growth rates in addition to histological lesions consistent with contaminant exposure 53,59–61 . Collectively, these studies suggest survival may be impaired for fish in the North Delta and conditions in this region of the estuary may play a large role in the decline of Delta Smelt.…”

“…Furthermore, they found growth rates for Delta Smelt in the brackish habitats of Suisun Bay and Suisun Marsh were similar or higher than other regions in some years. Thus semi-anadromous fish continue to utilize the LSZ as evident by their larger contribution to the population 53,59 .…”

Use of a Managed Flow Pulse as Food Web Support for Estuarine Habitat

“…While food web resources have declined throughout much of the upper estuary, the Cache Slough Complex (CSC) in the north Delta region of the upper estuary (Figure 1) frequently has relatively higher levels of phytoplankton than other areas in the estuary (Downing et al 2016;Lehman et al 2010;Lehman et al 2008). The region is notable because it retains some of the more complex tidal slough channels characteristic of the historical Delta, and is connected to the Yolo Bypass, a large floodplain (Sommer et al 2001;Whipple et al 2012) with higher plankton densities (Mahardja et al 2019). The region therefore contains a much larger network of shallow habitat and long-residencetime channels, which are hypothesized to promote the development of planktonic food https://doi.org/10.15447/sfews.2021v19iss3art3 resources (Lucas et al 2009).…”