Fish are ectotherms that rely on the surrounding water to regulate their body temperature. They can move around and find colder thermal refuge in shaded or deeper areas of water, however sometimes these thermal refuges are limited or nonexistent. When the surrounding water begins to warm it can alter a fish’s respiration and metabolism. At cooler temperature fish tend to grow slowly due to a reduced metabolic rate, while at warmer temperatures fish can grow faster due to an increased metabolic rate. Faster growth can be good for fish, particularly in their larval stages when they are poor swimmers and prone to predation. However, too much heat is a bad thing. Once the surrounding water becomes too warm it can begin to induce stress, inhibit proper cellular function, and at too high of temperatures it can lead to death. This lethal temperature is known as a fish’s critical thermal maxima.

                Not all species are the same, and most species have different ideal water temperatures, thermal tolerances, and thermal maxima. Longfin Smelt prefer fairly cool water temperatures compared to most fish that inhabit freshwater. Longfin smelt can comfortably inhabit water temperatures ranging from 9-20 °C. At temperatures greater than 20 °C individuals can become stressed and may be at risk of death if exposure continues for too long. Longfin smelt’s critical thermal maxima is considered 24.8 °C (Jeffries et al. 2016). Eggs and larval longfin smelt appear to require cooler temperatures than adults and juveniles, indicating that these thermal tolerances can change with life stage.

                In the San Francisco Estuary water temperature is largely influenced by freshwater inflow from the Sacramento and San Joaquin rivers. Greater inflow results in cooler temperatures. We covered how greater freshwater flows appear to be beneficial in the last blogpost, and perhaps one of the primary reasons why are these cooler temperatures the freshwater brings into the estuary.

                After examining existing literature, I have begun to summarize temperature data into variables we believe are meaningful for the species. Some research indicates that at temperatures of 15 °C and above the hatch rate success of fertilized eggs can sharply decrease (Yanagitsuru et al. 2021). Additionally, larval growth rates appear to be influenced by temperature where larva grow at faster rates at 12 °C compared to 9 °C and 15 °C.

                So, thanks to prior laboratory and field research we can identify water temperatures that are important to this species. Numerous water quality monitoring stations exist in the estuary and take hourly temperature data, and some of these stations have been recording data since 1983. Using the data collected at some of these stations that are operated by the California Department of Water Resources, we can make summaries of this data at meaningful temperatures at meaningful times in the species life history. By taking this hourly data we can summarize it into daily averages, then monthly averages. We can also look at how many days a year or in a particular month (say, January an important month for spawning) water temperatures exceeded 15 °C.

                Knowing the thermal tolerances of the species we can create variables that may help explain growth rates, occupancy patterns, and abundance trends. This is my current task, and it is an exciting opportunity to aid in our understanding and the conservation of this species.

Photo Credit: Steve Culberson, USFWS

Agency: U.S. Fish and Wildlife Service

Program: US Fish & Wildlife Service - DFP

Location: San Francisco Bay National Wildlife Refuge Complex