The maturity status of a fish is one of the most important biological parameters in data collection. The degree of development of the reproductive organs allows the identification of that part of the stock able to reproduce, i.e. release eggs and sperms, during the next spawning season.
Those individuals contributing to the reproductive potential of the stocks are consequently included in the Spawning Stock Biomass (SSB) that is one of the reference points in stock assessment.
Furthermore, by studying the spawning patterns of a stock it is possible to delineate location of spawning areas, timing of spawning seasons and estimation of size at maturity, which are some of the critical information required to manage a stock.
During the maturation process the fish reproductive organs, i.e. gonads undergo different modifications in the gross morphology showing changes in size, vascularisation, consistency and colour.
The judgement of the reproductive status based on the gross anatomy of the gonad is therefore ideal for routine monitoring of fish stocks in order to estimate the maturity ogives, i.e. the proportion of mature fish per age class.
ICES is continuously organizing workshops on maturity for different species. The aim is to reach a standardized level of judgment important on a stock level especially when data from different institutes and countries are pooled together for stock assessment.
The macroscopical (visual) examination of reproductive organs is a low cost and quick method for assessing maturity, allowing the analysis of a large amount of samples.
However some of the gonadal features cannot always be discriminated by the naked eye during certain phases of the developmental process.
This may lead to misjudgement of the gonadal maturity status and consequently an incorrect estimation of the reproductive part of the stock.
In this cases histology, which is the study of the microscopic anatomy of cell and tissue, is of a great help. The modifications of the gross morphology mirror a series of developmental changes on a cellular level identifiable by the means of histological analyses.
It is performed by embedding the tissue in resin or paraffine, sectioning it and staining the sections, followed by examination under a light microscope.
At the Institute of Marine Research in Lysekil, there is a well furnished laboratory for histological analysis used for validating the routinely used macroscopical maturity scales. At present this work has been done for cod, plaice, herring and sprat and it is an ongoing project for witch flounder.
A useful result of a validation work is a manual including both macroscopic and microscopic pictures for each stage of the maturity scale. This is a valid tool to be used by the staff when in doubt while evaluating gonadal maturity status.
Meanwhile is a handbook for new technicians start to work with a new species. The Coastal and Marine laboratories are presently collecting materials for building Baltic Herring and Sprat manuals.
Study of maturity in a herring. Gonads (red) are removed.
Histological section of gonads seen in a microscope.