Pesticides and Salmon

Pesticides may harm endangered salmon.

Several species of salmon on the West Coast have been listed as threatened under the Endangered Species Act. Pesticides are one of many threats to the survival of these iconic fish.

Fish kills from pesticides are, fortunately, rare. However, scientists have found that low levels of pesticides can affect salmon biology and ultimately affect the survival of the species.

Salmon have a very sensitive sense of smell. Relatively low concentrations of pesticides can interfere with their ability to smell, which may affect their behavior in a number of ways. Examples include altering the ability to feed and to avoid predators, disrupting schooling behavior, and inhibiting normal migration.

Salmon use smell to detect predators by sensing "danger" pheromones given off by other fish. When salmon sense this signal, they tend to stop moving and rest, making them less visible to a predator. A study done by scientists in Seattle found that juvenile salmon exposed to a pesticide at relatively low concentrations were less able to react to this signal. They continued to move around and feed rather than stop and hide.

Salmon are also thought to use smell to find the proper streams for spawning. Pesticides and other pollutants may interfere with this process.

Other effects on salmon from pesticides may include:

Reduced food sources: Pesticides can reduce the populations of salmon food sources (other aquatic life such as insects), thus affecting the overall health of salmon populations.

Immune system disruption: Some studies have found that exposure to relatively low concentrations of pesticides can disrupt the immune systems of salmon.

Threat to reproduction: Pesticides at low concentrations may act as mimics or blockers of sex hormones, disrupting normal sexual development.

References

Diminishing returns: Salmon decline and pesticides. Ewing, Richard D. Oregon Pesticide Education Network, 1999. www. pesticide.org/salpest.pdf.

Scholz, N.L., Truelove. N. K., et al. Diazinon disrupts antipredator and homing behaviors in chinook salmon (Oncorhynchus tshawytscha). Canadian Journal of Fisheries and Aquatic Sciences (2001): 57.

Pesticides and Streams

Pesticides have been found in local streams at levels high enough to harm salmon and other aquatic life. Many of the pesticides found are those commonly used by homeowners on lawns and gardens.

A study in King County in 1999 compared pesticides in streams with retail sales of pesticides. Twelve sites were sampled on 10 streams in urban watersheds during spring rainstorms. Sampling was done during storms because pesticide runoff is greatest during storms, so scientists are more likely to find pesticides at levels of ecological concern.

Scientists found 23 pesticides in the streams. Concentrations of five pesticides exceeded limits to protect aquatic life: the insecticides carbaryl, chlorpyrifos (Dursban), diazinon, malathion and lindane. The first four are organophosphate (OP) insecticides, which affect the nervous system.

At the time of the study, chlorpyrifos and diazinon were two of the most widely used garden insecticides. The US Environmental Protection Agency banned both for home use in 2003 because of concerns about its effects on human health, especially the nervous system.

Lindane is an organochlorine, an older class of pesticides that includes DDT. Organochlorines are long-lived in the environment.

Pesticides used on lawns and gardens have an impact on urban streams. The pesticides found in streams were compared with pesticides sold at 10 large home and garden stores such as Home Depot and Lowe’s. The four pesticides with the highest sales were found at all 12 stream sites: diazinon and the herbicides MCPP, 2,4-D and dichlobenil (Casoron). MCPP and 2,4-D are used in many weed-and-feed products. Residents also bought and applied the four OP pesticides that exceeded levels set to protect aquatic life.

Many pesticides found in urban streams had no retail sales. Almost half of the pesticides found were not sold in the home and garden stores. These pesticides are likely being applied to nonresidential areas such as rights-of-way, parks and recreational areas.

References

Pesticides detected in urban streams during rainstorms and relations to retail sales of pesticides in King County, Washington. http://wa.water.usgs.gov/pubs/fs/fs.097-99/ fs.097-99.pdf.