A contrary, yet non-generalizable study that found mixed evidence, including individual cases of a high sperm concentration, was published in March 2011.

Even in moving water, sperm concentrations are higher and fertilisation is more likely to occur.

We agree with Anna Brake and Walter Krause that our study does not provide evidence for a continuing decrease in mean sperm concentration.

We chose linear regression for describing the overall changes in mean sperm concentrations, but other statistical models have also been tested.

Regardless of which statistical model was applied, however, the main conclusion was the same - namely, almost a 50% reduction in sperm concentration from 1940 to 1990.

We agree that such changes could theoretically cause some variation in determination of sperm concentration.

The two highest dose groups of male rats had a significant reduction in sperm concentrations, although concentrations were still within the historical range for that rat strain.

Also, sperm concentration markedly increases.

With some techniques, sperm concentration and motility measurements are at least as reliable as current manual methods.

In the urchin, fertilisation rates were reduced at a pH below 7.3, but only at low sperm concentrations.