Thirty years of using the postovulatory follicles method: Overview, problems and alternatives

The postovulatory follicles’ method or POFs method was introduced by Hunter and Goldberg in the start of the 80s and still remains the most popular method for estimating population spawning frequency in wild populations of multiple spawners. During these last thirty years the method has been applied to more than 50 fish stocks almost all around the world's oceans. Even though the method was initially designed for the stock of northern anchovy, Engraulis mordax, it has now been applied not only to other multiple spawning clupeoids (ca. 40% of total applications) but also to a large number of other fish taxa. Despite its popularity the method can be quite inaccurate when its criteria are applied to other species and populations without prior validation. Four important sources of bias in the application of the method were identified: bias related to POF staging, bias in POF ageing, sampling bias and bias in the estimation of spawning fraction. Apart from all these potential sources of bias the method is quite costly and labor-intensive because it needs large number of adult samples, much histology and many work-hours from experienced personnel. In that respect, the development of alternative methodology for estimating spawning frequency seems worthwhile.