Liver Glycogen as a Sensitive Indicator of Food Limitation in Delta Smelt

Assessing habitat quality is a major goal of conservationists and restoration practitioners, but to associate habitat quality with biomarkers of vagile animals, the biomarkers must respond rapidly. Here, we identified a biomarker capable of rapidly detecting food limitation in the imperiled Delta Smelt (Hypomesus transpacificus), a pelagic fish endemic to the San Francisco Estuary (SFE). We conducted an experiment with fed and unfed treatments of hatchery-raised, sub-adult Delta Smelt that were sampled at 12 time points: 0, 1, 2, 3, 4, 5, 6, 7, 9, 11, 14, and 21 days. We then compared four biomarkers using Day 21 fish: RNA/DNA in liver, triglycerides in liver, glycogen in liver, and glycogen in muscle. Of the liver endpoints, glycogen had the largest difference between treatments at Day 21, so we compared it to muscle glycogen across all time points. Liver glycogen declined by 60% after 1 day of fasting and remained depressed in the fasting treatment across all subsequent time points. Muscle glycogen also responded rapidly, taking only 2 days to decline by 39% in the fasting treatment, but the difference was inconsistent across subsequent time points. When applied to hatchery-released Delta Smelt collected from the SFE, liver glycogen concentrations were less than half that of the fed hatchery fish, consistent with the hypothesis of food limitation in the wild, but also several other potential causes. This study highlights the utility of liver glycogen as an indicator of recent foraging success in Delta Smelt.

Automated summary

This study aimed to find a biomarker capable of rapidly detecting food limitation in Delta Smelt, and identified liver glycogen as a responsive biomarker. The experiment showed that liver glycogen and muscle glycogen both decreased rapidly in the fasting treatment, and liver glycogen concentrations in wild Delta Smelt were lower than those in hatchery fish.