#SummerScienceFriday | There are FISH in the LA River?

Every Memorial Day, two stretches of the LA River open to the public for recreational uses. The Elysian Valley is in northeast LA and the Sepulveda Basin is in the San Fernando Valley. Recreational uses include walking, kayaking, and even fishing! Despite the river being a concrete channel, it is a hub for wildlife, especially for fish species.

“You can definitely see amazing wildlife out there, from fish breeching to an osprey flying up above…”

– Fernando Gomez, chief ranger for the Mountains and Recreation and Conservation Authority

Fish and the Food Chain

Bioaccumulation is the process in which chemicals become concentrated at levels that are much higher in living cells than in open water. As these chemicals move up through the food chain, they become more concentrated in the organisms that consume them, a process known as biomagnification (2).

This concept explains why fish consumption could potentially be harmful to humans, considering we are top of the food chain predators. This is especially true for persistent chemicals that don’t break down over time such as PCBs, DDT, and mercury (3). PCBs used to be found in many electrical equipment, plastics, capacitors, and thermal insulation. Although PCB’s were banned in 1979 they can still be released through the disposal of these products (4). DDT was one of the first modern synthetic insecticides, but due to its wide-spread use, many insect pest species became resistant. It is known to be persistent in the environment, can accumulate in fatty tissues, and can travel long distances in the atmosphere. It is no longer used in the United States, but its heavy historical use and persistence has allowed residues to remain a problem (5).

Unlike PCBs and DDT, mercury is a naturally occurring element found in the Earth’s crust and can be released naturally by volcanoes and forest fires. Although it is naturally occurring, mercury could perhaps be the most dangerous bioaccumulated chemical due to anthropogenic emissions. The largest sources of mercury in the United States come from coal and other fossil fuels, which account for 44% of all anthropogenic mercury emissions (6). When coal is burned, Mercury is released into the atmosphere, deposited into water, and transformed into methylmercury by mircoorganisms. Methylmercury is a neurotoxin that negatively affects brain development in both children and adults (7). Once methylmercury is in the water, it can be accumulated in fish as they filter water through their gills. Since fish are a part of many of our diets and biomagnification occurs up the food chain, it is necessary that we monitor these chemicals in the fish we eat.

Are fish in the LA River Watershed safe to eat?

The Council for Watershed Health manages the Los Angeles River Watershed Monitoring Program (LARWMP) to monitor the health of the LA River Watershed. In order to assess human health risk, one of the components of this program is testing if target fish species are safe to eat. In 2018, common carp, bluegill, redear sunfish, and largemouth bass were sampled in Echo Park. These fish were tested for mercury, selenium, PCBs, and DDTs.

Common carp and redear sunfish were considered safe to eat for all chemicals and had an OEHHA recommended serving of three 8-oz servings a week. Bluegill exceeded thresholds for PCBs and had an OEHHA recommended serving size of two 8 oz servings a week. Largemouth bass had a recommended serving of one 8-oz serving a week for mercury and PCBs. These findings can probably be contributed to size and diet, with largemouth bass consuming larger fish. To see data from past LARWMP reports click here.

Fishing in the LA River: Future Steps

Even though regulations have been put in place to reduce mercury emissions, mercury concentrations in fish are actually rising. According to researchers, climate change and humans may be to blame. Overfishing has caused some fish, like cod, to change their diet and eat larger, more mercury concentrated fish, while warmer waters increase the need for fish, like tuna, to expend more energy to move through water (8). This is why it is important to get involved with conservation efforts, as well as to be aware of what types of fish you eat. Here are ways to do this:

Trout Unlimited is a conservation group that aims to protect critical habitat, reconnect degraded waterways, and restore population to coldwater fisheries by bringing together all stakeholders. To take part in their research and conservation programs click here.
Seafood Watch is an app by Monterey Bay Aquarium to provide users with recommendations to make sustainable fish choices. Learn more about it here.