Two delta smelt. Photo credit: Ken Jeffries.

Two delta smelt. Photo credit: Ken Jeffries.

With the current drought in California considered to be the worst in more than a millennium and with the state desperately trying to reduce water consumption, conservationists are becoming increasingly concerned about the impact of the extreme conditions on local ecosystems. ‘Many regions of freshwater habitats have been showing an increase in water temperatures’, says Ken Jeffries from the University of California (UC), Davis, USA, and the longfin smelt and delta smelt, which reside in the inland California Delta, are at particular risk. ‘Their abundance has reached record lows’, says Jeffries, warning, ‘It has been feared that the current drought will be the final “nail in the coffin” for these species in Northern California.’ Knowing that delta smelt are vulnerable to high temperatures, Jeffries says, ‘However, we really did not know whether the temperatures have reached levels that may be detrimental to longfin smelt’. So, Jeffries teamed up with colleagues from UC Davis and the California Department of Water Resources – Richard Connon, Ted Sommer and Nann Fangue – to design a series of experiments to find out how the larvae of both species fare as the temperatures rise.

As populations of longfin and delta smelt have crashed over recent years, Jeffries, Brittany Davis and Lisa Komoroske investigated the resilience of smelt larvae bred at the UC Davis fish culture facility instead of collecting animals from the wild. Jeffries also admits that working with the longfin smelt was challenging. ‘They are extremely sensitive to handling,’ he explains.

Having learned how to work with the delicate creatures, the trio placed individual longfin smelt in small tanks and gently warmed the water until the fish toppled over to find the maximum temperature that they could tolerate. Recording that the longfins succumbed at 24.8°C, Jeffries explains that the delta smelt were already known to remain upright until 27.6°C, which corresponds well with the highest temperatures that the fish appear to bear in the wild. Jeffries warns that the longfin smelt are more vulnerable to high temperature than the delta smelt as the temperatures where the longfin smelt live in the delta are already approaching their thermal limit. And when Davis, Jeffries and Anne Todgham measured the metabolic rate of both species at 14 and 20°C, the metabolic rate of the longfin smelt changed little as the temperature rose from 14 to 20°C (29.0–21.9 μmol O2 h−1 g−1), although the metabolic rate of the delta smelt increased dramatically from 47.9 to 65.5 μmol O2 h−1 g−1 as the temperature increased.

After confirming that the threatened longfin smelt was more vulnerable to high temperatures than the already endangered delta smelt, the team warmed the two species to 20°C and collected samples of their RNA to find out which genes had been activated. Analysing the change in the gene transcript patterns with Monica Britton, Jeffries saw that the longfin smelt activated their cellular heat shock response for protection against the threatening high temperatures, in addition to expressing genes associated with growth and development. However, the delta smelt increased the expression of genes associated with metabolic processes, reflecting their increased metabolic rate.

Having identified cellular markers that can be used in the field to identify when both species are suffering the effects of temperature, Jeffries says, ‘Resource managers [in California] now have an idea of when temperatures become stressful and unsustainable for longfin smelt’, and adds that it is essential that we provide refuges at temperatures below 20°C for the freshwater life stage of the longfin smelt if they are to survive this and future droughts that may grip California.

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