Using Landsat and ECOSTRESS Thermal Remote Sensing to Examine Habitat Conditions for the Endangered Delta Smelt

Abstract:

The San Francisco Bay Delta is critical both to the world’s biodiversity and to California’s water supply, with a pressing need for water quality data to support sustainable water resource management. Of particular concern is the impact that water management decisions in the Delta have on the endangered Delta Smelt (Hypomesus transpacificus) and to invasive species of fish that may occupy the same habitat. Water temperature is a critical factor governing habitat suitability in aquatic and estuarine systems. With temperatures continuing to increase under climate change projections, it is expected that the number of days of the year in which water temperatures are lethal to smelt will increase. Understanding thermal stress on threatened and endangered fish populations is an important aspect of water management and conservation in the Bay Delta. This study uses Landsat 5, 7, and 8 to examine trends in water surface temperature spanning 1985-2019, relative to the thermal tolerance of the endangered Delta Smelt (Hypomesus transpacificus) and two non-native fish, the Largemouth Bass (Micropterus salmoides) and Mississippi Silverside (Menidia beryllina). We also compare diurnal water surface temperature from ECOSTRESS to bulk water temperature measurements from the Department of Water Resources California Data Exchange Center (CDEC). The accuracy of the Landsat water surface temperature (WST) products were validated using two in-situ stations instrumented with thermal radiometers on Lake Tahoe and the Salton Sea (r = 0.98, R² = 0.96, p < 1%, n = 1849) with a bias of -0.03 from 1999-2018. Thermally unsuitable habitat, defined as WST pixels exceeding critical thermal maximum for each species, were assessed for each Landsat acquisition, compared inter-annually by maximum. This study found that the changes in area deemed thermally unsuitable for the Delta Smelt (> 28.6°C) increased at the fastest rate of the species considered here at 2.6 km²yr^-1. We also found an increasing trend of 1.8 km²yr^-1 in area of habitat that exceeded Delta Smelt thermal tolerance but was within the tolerance of the competing non-native species, suggesting an advantage to non-native species with higher thermal tolerances under warming conditions. This study also observed a relationship between increases in thermally unsuitable area to decreases in the smelt abundance index derived from the CA Department of Water Resources Fall Midwater Trawl (r = -0.52, ⍴ = -0.52, 𝛕 = -0.37, R² = 0.25, p < 1%, n = 33). These findings provide a potential metric for leveraging remote sensing information as a first order proxy for thermal habitat suitabilities in support of management and restoration actions.