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"Fish scales give us a huge amount of information. It's a magic tool for studying a life cycle as a whole," says Clarisse Boulenger, researcher at the Ecology and Ecosystem Health joint research unit. INRA researchers are reading the life stories of individual fish from their scales. "Age, growth, reproduction, species … We can even tell how much time a fish has spent in the river and the sea. We can get even more precise data from another small fish part, the otolith, which is made of calcium carbonate. It can tell us the times they have spent in sea and river, and where they were born, because every river is unique in its chemical composition." Also from otoliths, scientists can tell how long the fish has spent in the brackish water of the estuary. This gives an idea of the time it takes the migrating fish to adapt and change so as to survive in the sea. The information is obtained individual by individual, not at population level. DNA from the scale is used to characterise the individual genetically (e.g. sex and parentage) and to identify different populations.

From river to sea and back

It's a magical tool

Fish grow at different rates according to the season; more slowly in winter, faster in spring and autumn. This can be tracked by examining the spaces between the ridges on a scale, which are wider or narrower depending on the season. By combining information provided by the scales with catch and field data, the INRA scientists, who have been collecting fish scales since the 1970s, have found clearly noticeable changes over time, particularly in Atlantic salmon. The species has become smaller (all ages combined), and breeding adults that have spent more than one winter at sea are increasingly rare. So the fish scales are providing information not only about the fishes' life histories but also about the impact of human activity on their life patterns.

Excellent bioindicators

Where a population is in a bad state, so is its environment

We know that mankind is profoundly changing the environment with dam building, climate change, intensive farming and other factors. Even fishes' life patterns are being disrupted. "In January 2018, IBISA, the French oversight body for infrastructures in biology, health and agricultural science, approved our collection of 200,000 fish scales and other items as an official Biological Resource Centre (BRC). It is part of the RARe infrastructure, which comprises five pillars, including the environment pillar that our BRC belongs to," says Frédéric Marchand, joint director of the U3E ecology and aquatic ecotoxicology experimental unit. Fish scale studies are providing a better understand of the effects of climate change on trout and salmon, INRA's target species, which are excellent bioindicators. "We talk in terms of bioindicators because these fish have complicated life cycles, moving between different environments. If the population is in a bad state, it means that the environment the fish grew up in is also in a bad state. They are highly sensitive to temperature and nutrition. Studying fish scales, we see very clearly that their environmental conditions are deteriorating more and more," explains Clarisse Boulenger.

In danger of extinction

Studying fish scales helps towards taking care of these endangered migratory species. Salmon is on the IUCN's Red List as a vulnerable species (along with the panda); the eel is critically endangered (like the orang-utan). Thanks to the various data collected, suitable management tools have been introduced for most of the migratory species. Cogepomi (Comité de gestion des poissons migrateurs), the official migratory fish management body through which the Regional prefect lays down the rules for fisheries, makes some of the decisions concerning migratory species including Atlantic salmon, Allis shad and trout. Cogepomi validates the migratory fish management plan (Plagepomi) drawn up by INRA scientists, which is helping to change attitudes and promote proper care of fish and the environment.