A glance at the sex organs in the body cavity of a fish reveals both its sex and maturity. 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 June 20-23, 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.
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.
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