One study found that chemicals in household wastewater make about 20% of male fish in the river have characteristics of females.
Ingredients in some items such as birth control pills, cosmetics and plastics even make some male fish lay eggs.
Some components of oral contraceptives also cause males to lay eggs.
Lead researcher Professor Charles Taylor from Essex University said: 'Some of these chemicals may have more impact on fish than we expected.'
"The use of genetically modified fish allows us to observe the reactions to these chemicals in the fish's body at the right time, such as, we see, oestrogens. in some plastics that affect heart valves'.
Tests show that about 20% of freshwater males, such as rutilut (a carp species) in 50 different areas all have female characteristics.
More than 200 types of chemicals from wastewater treatment plants have been identified that have the same effects as estrogen and drugs such as antidepressants that also alter the natural behavior of fish.
Silver rutilut fish.
According to Professor Tyler: 'Other studies also show that many other chemicals released through wastewater treatment systems can also affect fish species, including antidepressants, which reduce natural timidity. of some fish species - including how they react to predators'.
Professor Tyler also found that fry and offspring produced by the offspring of the affected fish may also be more sensitive to these chemicals during subsequent contacts.