Patterns and predictors of condition indices in a critically endangered fish

Abstract

Condition indices are key predictors of health and fitness in wild fish populations. Variation in body condition, therefore, can be used to identify stressful conditions that may impact endangered species, such as California's endemic Delta Smelt (Hypomesus transpacificus McAllister, 1963). Here, we examined spatiotemporal variation in the condition indices of [ 1600 Delta Smelt collected over nine years (2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018)(2019), a period characterized by tremendous variability in hydrodynamic and water quality conditions. The population exhibited low hepatosomatic index (HSI) and condition factor (CF) during September/October/November (fall), and both condition indices declined over the nine-year study during fall. HSI was positively correlated with indicators of pelagic productivity (e.g., Chlorophyll a, zooplankton biomass, and proximity to tidal wetlands), whereas CF was negatively correlated with temperature, peaking at a relatively cool 10-13 °C. In sum, seasonal and interannual variation in body condition corresponded most strongly with pelagic productivity and water temperature, with little correlation to freshwater outflow. Management actions that increase pelagic productivity, restore and freshen productive wetlands during late summer-fall, and reduce water Handling editor: Daniele Nizzoli

Hydrodynamics structure plankton communities and interactions in a freshwater tidal estuary

“…Though our study was conducted in a simplified, artificial channel, our results suggest that terminal estuarine habitats or other habitats with low exchange can support high zooplankton biomass. In impaired estuaries, both artificial and natural terminal habitats with long residence times may contribute high quality food resources to the pelagic food web (Frantzich et al, 2018), and help sustain imperiled populations of native fishes subject to food limitation (Hammock et al, 2021). Future work aimed at understanding the contribution of long-residence time habitats to pelagic food webs, or comparing trophic relationships across estuaries could prioritize (1) characterizing community composition in the context of hydrodynamic variation; (2) sampling with sufficient taxonomic resolution to quantify key trophic interactions and responses to abiotic drivers (Ward & Paerl, 2016), and (3) sampling phytoplankton at higher temporal frequency to match timescales of responses to abiotic drivers such as nutrients.…”

“…Our goal was to characterize how hydrodynamic processes (e.g., water exchange) mediates drivers of temporal variation in phytoplankton and zooplankton biomass in the northern Sacramento-San Joaquin River Delta (hereafter, the "Delta"), a freshwater tidal portion of the San Francisco Estuary (SFE). Understanding these dynamics in the SFE is necessary to quantify food availability for threatened native fishes such as the Delta Smelt (Hypomesus transpacificus), the health and fitness of which are positively correlated with chlorophyll a and zooplankton biomass (Hammock et al, 2021). We conducted longitudinal sampling along a terminal channel in the north Delta (Figure 1), characterized by spatial gradients in water residence time (Downing et al, 2016;Stumpner, Burau, & Forrest, 2020), turbidity (Feyrer et al, 2017), and nutrient concentrations (Loken et al, 2022).…”

Polygenic discrimination of migratory phenotypes in an estuarine forage fish

“…For example, variation in freshwater outflow is known to affect dispersal, habitat suitability, and population dynamics of several species ( Jassby et al 1995 ; Kimmerer 2002 ). Laboratory studies ( Swanson et al 2000 ; Feyrer et al 2007 ; Nobriga et al 2008 ; Komoroske et al 2014 , 2015 ; Jeffries et al 2016 ) and observations of the wild population of Delta Smelt ( Lewis et al 2021 ; Hammock et al 2022 ) indicate strong sensitivity to variation in temperature. In addition to longitudinal studies of life history traits, studies examining the degree to which genotypic and phenotypic variation corresponds with variation in the sensitivity of Delta Smelt to environmental variation remains an unexplored direction of inquiry ( Lewis et al 2022 ).…”

Liver glycogen as a sensitive indicator of food limitation in Delta Smelt

“…Hepatosomatic index was the most sensitive measure of food limitation examined, declining signi cantly after four days of fasting at 16°C, followed by condition factor at seven days. This was the rationale for subsequently modeling hepatosomatic index and condition factor of Delta Smelt as a function of environmental variables such as water temperature, salinity, and zooplankton abundance (Hammock et al 2021). However, Delta Smelt could conceivably swim a considerable distance in four days given their estimated swimming speed of 0.72 km/h (Swanson et al 1998), especially if aided by currents (Bennett and Burau 2015).…”