Maturity staging is used to estimate the size of the spawning fraction of a fish stock and advice on fishing quotas.
In order to standardise guidelines for maturity determination among countries, and thus increase the accuracy of fish stock assessments, researchers from DTU Aqua gathered biologists from 15 different countries with the aim to make common guidelines for maturity staging of sprat and herring.
On board research vessels and in laboratories around the world, biologists determine the sex and maturity of fish by looking at the development of their ovaries and testicles. The purpose of this so-called maturity staging is to describe where the fish are in their reproductive cycle and thereby assess whether the sampled fish are sexually mature and ready to reproduce.
When sampling of a fish stock covers its distribution area, biologists can estimate the size of the spawning stock of the species in the area. These data are subsequently used to estimate the size of the next generation of fish as part of the stock assessment. This knowledge is used when advising on fishing quotas.
In order to make reliable assessments of the development of fish stocks, it is important that maturity staging is conducted in the same way in marine research institutes across borders. Therefore, on 20 - 23 June, 2011, senior research scientist Jonna Tomkiewicz and PhD-student Rikke Hagstrøm Bucholtz from the National Institute of Aquatic Resources (DTU Aqua) in Denmark gathered 40 biologists from 15 European countries at the workshop 'Maturity Staging of Herring and Sprat’ with the aim to agree on common guidelines for maturity staging of sprat and herring.
”The International Council for the Exploration of the Sea (ICES) took the initiative to the workshop in order to improve consistency in maturity staging of herring and sprat among countries. Standardised guidelines will improve the quality of the data used in assessment of these fish stocks,” says Jonna Tomkiewicz, who held the workshop at DTU Aqua with financial support from EU’s Data Collection Framework.
Four herrings which offhand look similar. To the left of each fish is its sex organ which has been removed from its abdomen. A glance at the organs reveals that the four herrings are a male (right) and three females in different maturity stages. The female second from left has a more developed ovary (maturing and would have participated in spawning) while the two other females are immature.
Photo: Cordula Schmitz.
Maturity staging is not always easy
”It can be difficult to determine the maturity stage of a fish. In particular, if the fish are relatively undeveloped. Therefore, biologists can disagree on the maturity of the same fish. Biologists from one institute will say that the fish is sexually mature and would have spawned, while biologists from another institute will say that it is immature,” explains Jonna Tomkiewicz.
To improve methods, DTU Aqua asked the 15 institutes participating in the workshop to sample and preserve ovaries and testicles from herring and sprat. Information about the samples included the visually determined maturity stage of each sample.
Jonna Tomkiewicz and her colleagues at DTU Aqua now made a histological analysis on each of the sampled tissues. This made it possible to make an accurate determination of the maturity stage. This knowledge enabled the 40 participants in the workshop to produce standardised maturity scales for herring and sprat with descriptions and pictures of the different maturity stages.
15 countries agree
”The agreement among biologists from 15 countries, who worked together to establish reliable criteria for the maturity stages, will help to improve the consistency in future maturity data across borders,” says Jonna Tomkiewicz, who taught the foreign biologists how to do maturity assessments on herring and sprat by histological analysis and microscopy.
”The participant were very engaged and the workshop was a good experience. I hope that common guidelines for maturity staging of sprat and herring will help increase data quality and thereby enhance stock assessments,” says Jonna Tomkiewicz.
FACT: Maturity staging form the basis of stock assessment
Biologists determine the maturity of a fish by assessing its developmental stage in relation to the reproductive cycle. This enables them to identify whether the fish is mature and ready to spawn, or still immature.
The maturity stage is determined by a glance at the testicles, if it is a male, or the ovaries or the spawn, if it is a female, or through microscopy of testes or ovarian tissue preparations.
Data on maturity and proportion of spawners in the stock are used in fish stock assessment to estimate how many fish are likely to spawn and produce offspring the following year, thereby indicating how many fish there will be in the future. This knowledge is used when giving advice on fishing quotas.
The 40 participants from 15 different countries participating in the workshop ‘Maturity Staging of Herring and Sprat’ that were held at DTU Aqua in Denmark on June 20-23, 2011. DTU Aqua has previously hosted similar workshops for maturity staging of cod and other gadoids.
Photo: Lis Winter, DTU Aqua