Condition refers to whether, and the extent to which, these organisms contain chemical or biological contaminants that could affect the health of people who eat them.

Several factors affect the condition of fish and shellfish, including chemical and biological contamination, aquaculture, and overharvesting.
 

• Chemical contaminants of greatest concern tend to be those that are persistent, bioaccumulative, and toxic (called PBTs). PBTs that are common in fresh and coastal waters include:
   
  • Mercury. This highly toxic metal is present in waters all over the globe as a result of long-range transport and deposition of airborne mercury as well as direct inputs to water. It accumulates in the food chain and becomes concentrated at dangerous levels in larger and longer-lived predators such as bass, tuna, swordfish, and some sharks.
    
    Mercury can be methylated by certain bacteria in bottom sediments to form methylmercury, which is more toxic and bioavailable than other forms of mercury. High levels of methylmercury can cause reproductive and other effects in wildlife.¹ In humans, exposure to elevated levels is primarily associated with developmental and neurological effects.²
     
  • Polychlorinated biphenyls (PCBs) and the pesticide DDT. Though no longer made in the United States, these chemicals persist in watersheds and near-shore sediments and can continue to contaminate fish and shellfish.
    
    These chemicals are also circulated globally as a result of use in other parts of the world. They have been linked to health effects such as cancer, nervous system damage, reproductive disorders, and disruption of the immune system in both humans and wildlife.
     
• Biological contamination can affect the condition of fish and shellfish—especially shellfish. Eating shellfish contaminated with pathogens from human and animal fecal waste can cause gastrointestinal illness and even death in people with compromised immune systems. Sources of fecal contamination include urban runoff, wildlife, wastewater treatment plants, agricultural runoff, and boating and marinas.
  
  Marine biotoxins produced by certain types of algae can also contaminate fish and shellfish. These toxins can cause massive fish kills and can affect seabirds, marine mammals, and humans. Consuming seafood contaminated with biotoxins can cause gastrointestinal illness, amnesia, memory loss, paralysis, and even death.
   
• Aquaculture (“fish farming”) can affect the levels of certain contaminants in consumable fish and shellfish. Dense colonies can increase stress and disease transmission, in some cases requiring the use of antibiotics.³ Several studies have found higher levels of certain contaminants in farmed fish compared with wild fish.⁴
   
• Overharvesting can affect the condition of fish and shellfish by disrupting the food web. Because of depleted food sources, predators can become more susceptible to disease. If people handle diseased fish, they can become infected, a problem especially for people with compromised immune systems.

Concern about fish and shellfish safety is higher for subsistence fishers, children, pregnant and nursing women (because of possible effects on the fetus or infant), and other groups who may be more vulnerable (such as the elderly or immunosuppressed individuals).

Two ROE indicators are available to describe the condition of consumable fish and shellfish. Coastal Fish Tissue examines the levels of contaminants in fish from coastal waters, while Lake Fish Tissue does the same for fish collected from lakes. Both are based on national surveys coordinated by EPA. There currently are no ROE indicators to describe the effects of fish and shellfish condition on human health.

On a local scale, information about the safety of fish and shellfish can come from consumption advisories issued by states and tribes. However, states and tribes vary in how they conduct monitoring, how they decide to place waters under advisory, and what advice they provide when contamination is found.

Further, trends in the number of advisories over time may reflect changes in the frequency and intensity of monitoring.⁵ Thus, advisories cannot provide a consistent national ROE indicator of trends in the condition of consumable fish and shellfish.