Update: Mercury Testing of Sport-caught Food Fishes within Humboldt Bay

Ross TaylorHumboldt Bay

In August of 2016, Humboldt Baykeeper contracted with Ross Taylor and Associates to assist in implementing a study to test the methyl-mercury levels in commonly sport-caught food fishes from Humboldt Bay. This current study is funded by the California EPA as a follow up to a statewide study conducted by the State Water Resources Control Board’s Surface Water Ambient Monitoring Program (SWAMP) in 2009 and 2010 which tested methyl-mercury levels in commonly caught coastal fishes. The SWAMP study tested nearly 3,500 samples collected from 46 fish species captured at 68 locations along the California coast. SWAMP documented high levels of methyl-mercury in several species commonly caught along the north coast, including a leopard shark from Humboldt Bay that had one of the highest concentrations of methyl-mercury of all the fish tested.

For the Humboldt Bay project, the primary criteria for selection of focal fish species included:

  • The species is commonly fished for in Humboldt Bay and kept as a food source.
  • The species resides within Humboldt Bay for a significant part of the year.
  • The species is relatively long-lived, thus more susceptible to bioaccumulation of toxins.
  • The species is already associated with methyl-mercury health advisories within coastal waters of California.
  • The species is an important indicator to statewide tracking of methyl-mercury levels due to its wide distribution and abundant populations.

The fish species meeting these criteria included leopard shark, bat ray, California halibut, lingcod, and shiner surfperch. Several species of clams were also included in the study at the request of the Wiyot Tribe due to their cultural importance to the Tribe. These included Martha Washington, little neck and horse neck clams. For the Humboldt Bay study, flesh samples from five fish or clams of each species will be collected.

To avoid cross contamination of flesh samples, the Humboldt Bay study closely followed collection, handling and processing methods developed by the U.S. EPA. From each fish, approximately an eight ounce, skinless filet was kept, wrapped and frozen until being sent to a laboratory for testing. Each sample included a detailed label which identified the species, length and weight of the fish, and capture date and location.

In August of 2016, Baykeeper enlisted the services of Captain Phil Glenn and his boat the Bluefin to assist in collecting fish for the study. On two half-day trips we caught five legal-sized California halibut, three bay rays and one leopard shark (Photos 1-3). Numerous undersized halibut and brown smooth-hound sharks were caught and released. Additional sampling in September of 2016 conducted by Ross Taylor and Associates resulted in samples from two more bat rays and a lingcod. Clam samples were collected in south bay with assistance of the Wiyot Tribe on March 30, 2017. The remaining fish samples will be collected in May and June of 2017. If time and budget allows, additional clams may be sampled from other areas of Humboldt Bay. A final part of Baykeeper’s study is to interview fishermen around Humboldt Bay regarding fishing habits, species kept and how frequently they and their families eat fish caught from the bay. These interviews will include various user groups on Humboldt Bay, including the Latino and Hmong communities, the Wiyot Tribe, dock fishermen, jetty fishermen and boat-based anglers.

Eleven samples from Humboldt Bay were analyzed in the fall of 2016: five California halibut, five bat ray and one leopard shark. Consistent with the SWAMP report, the leopard shark was extremely high in methyl-mercury at 1.192 parts per million (ppm) (Figure 1). The California Office of Environmental Health Hazard Assessment’s (OEHHA) no consumption advisory tissue level of 0.44 ppm provides an upper bound threshold for assessment of methyl-mercury in California sport fish. This value represents a relatively high concentration above which consumption is not safe for the most sensitive fish consumers (children and women of childbearing age) and was set to protect for developmental neurotoxicity. The 0.13 ppm threshold shown on Figure 1 was set as an overall “do not consume” advisory. The lowest line on Figure 1 equals 0.07 ppm and concentrations below this level are considered safe by both OEHHA and CA EPA for subsistence consumption (up to 12 meals per month). Ten samples from a CA Fish and Game 2005 study were added since these two species are commonly caught by our north coast anglers: five lingcod from Cape Mendocino and five Chinook salmon from just outside Humboldt Bay (Figure 1). Note as the size (weight) of the lingcod increases, so does the level of methyl-mercury.

Results from the SWAMP study that HASA members should be aware of were the elevated methyl-mercury levels of several rockfish species tested from Cape Mendocino and Shelter Cove. All samples taken from copper rockfish, China rockfish, gopher rockfish and cabezon had methyl-mercury levels exceeding 0.44 ppm, with several individuals >1.0 ppm. Additionally, copper rockfish and gopher rockfish from northern Humboldt County also had levels above 0.44 ppm. These species tend to be long-lived and are classified as benthic (bottom) feeders. In contrast, blue rockfish and black rockfish of comparable sizes, tested below 0.44 ppm, probably due to these species attaining similar sizes at a younger age and also tending to feed higher in the water column (pelagic feeding versus benthic). The full SWAMP report is available at: http://www.waterboards.ca.gov/water_issues/programs/swamp/docs/coast_study/bog2012may/coast2012report.pdf

Baykeeper’s final report for the Humboldt Bay mercury study will be completed by the fall of 2017.