Rundio et al. 2022 — High mercury in trout, sculpin, and invertebrates, Big Sur coast, California
This paper measured total mercury (THg) in fish muscle and both THg and methylmercury (MeHg) in terrestrial and aquatic invertebrates from a small stream-riparian food web on the Big Sur coast of California, where marine fog is a known vector of ocean-derived MeHg to terrestrial ecosystems. The study found that mean THg in age 0 (young-of-year) trout was 400 ng/g dw, rising to 1200–1300 ng/g dw in age 1+ trout and sculpin, with a maximum individual THg exceeding 3500 ng/g dw. Terrestrial invertebrates had mean MeHg concentrations three to four times higher than aquatic invertebrates of the same trophic level, with some centipede and scorpion individuals exceeding 1000 ng/g dw MeHg — a finding the authors attribute to marine fog as a delivery pathway for ocean-derived MeHg into coastal terrestrial food webs.
Key numbers
Fish (muscle tissue, dry weight basis):
- Age 0 trout (n=10, 72–90 mm): mean THg 400 ng/g dw
- Age 1+ trout (n=10, 125–300 mm): mean THg 1200–1300 ng/g dw; maximum individual >3500 ng/g dw
- Coastrange sculpin (n=10, 86–128 mm): mean THg 1200–1300 ng/g dw (similar to age 1+ trout)
- Conversion from dry to wet weight assumed 25% moisture (dry:wet = 1:4), yielding approximate wet weight equivalents of ~100 ng/g ww for age 0 trout and ~300–325 ng/g ww for age 1+ trout and sculpin
Invertebrates (dry weight basis, Table 1 in source):
- Aquatic detritivore/herbivore THg range: 13–70 ng/g dw (stoneflies, caddisflies, mayflies)
- Aquatic predator insects (dobsonflies, dragonflies): mean THg 85–120 ng/g dw
- Terrestrial detritivores (isopods, diplopods): mean THg 200–500 ng/g dw; %MeHg ~50–80%
- Terrestrial predators (centipedes, scorpions): individual samples >1000 ng/g dw MeHg
Methods (brief)
Fish were archived muscle samples from incidental mortalities in electrofishing surveys (2005–2019). Invertebrates collected by hand search in summer 2020. THg analyzed by Milestone DMA-80 direct mercury analyzer, EPA Method 7473. MeHg analyzed by KOH/methanol extraction, derivatization with sodium tetraethylborate, GC-CVAFS, EPA Method 1630. Certified reference materials run daily; recoveries 96–114% for THg CRMs and 90% for DORM-4 MeHg. Samples for fish expressed as muscle THg, converted to wet weight using 25% dry weight assumption.
Implications
Certification: freshwater salmonid products from coastal California fog zones may carry substantially elevated MeHg, warranting geographic origin disclosure and lot-level testing. Age-related bioaccumulation is strong (3x increase from age 0 to age 1+ trout). Fog-belt coastal sourcing is a meaningful variance factor.
Courses: illustrates marine-fog MeHg pathway as an underappreciated contamination route distinct from aquatic sediment methylation.
App: trout from coastal California should carry a MeHg risk flag; geographic provenance is a key variable.