Analysis of the sexual maturity of fish

How It Works

By examining the development of fish reproductive organs (gonads), we can determine which individuals are likely to release eggs or sperm during the next spawning season — i.e., which ones will reproduce.

The individuals we assess contribute to the so-called spawning stock biomass (SSB), an essential reference point in stock assessments.

As fish mature, their gonads undergo changes in size, consistency, and color. By exposing the gonads, we can visually assess these characteristics, allowing us to determine the fish's sex and sexual stage. This method is ideal for monitoring fish stocks and estimating the proportion of fish in each age group that is mature enough for spawning.

Studying spawning patterns also enables us to identify spawning areas, determine the timing of spawning seasons, and estimate the size at maturity — critical information needed for effective stock management.

Histological Analyses of Fish Genitalia

While the visual method for assessing fish sexual maturity is quick and cost-effective, some stages of development can be challenging to distinguish with the naked eye. This can lead to inaccurate estimates of the reproductive portion of the stock.

To improve accuracy, we also use histological analyses, where we examine the fish's genitalia under a microscope. In this process, tissue samples are embedded, sectioned, stained, and analyzed to study the structure of the cells and tissues.

We then compare these histological results with our visual assessments to refine and improve our methods. To date, we have applied this approach to species such as cod, European plaice, herring, sprat, and witch.

The maturity stage of a herring is examined, and its gonads are carefully extracted for analysis.