Otolith patterns in juvenile plaice of the Baltic Sea

About this website

Welcome! My name is Lise Coquilleau, I am a master student enrolled in the IMBRSea Programme. As part of my studies, I did an 8-week internship with one of their partners: the Thünen Baltic Sea Fisheries Institute, in Germany.

I have always been curious about fisheries management, and the originally planned internship was going to be a smörgåsbord of opportunities to explore various aspects of fisheries research, by interacting with different scientists around the institute. Unfortunely, due to the COVID-19 crisis, employees were instructed to work from home as much as possible, so I focused on a single topic that could be done remotely instead: exploring the otolith patterns of juvenile European plaice from the Baltic Sea.

However, I managed to move to Germany right before travel restrictions were implemented, a month prior to the start of my internshsip, so I did get the chance to conduct a little bit of fieldwork. Read on to find out more!

The Thünen Institute for Baltic Sea Fisheries

  • The Thünen institute is a German federal research institute for rural areas, forestry, and fisheries. It is divided in 14 specialized structures, one of them being the Baltic Sea Fisheries Institute in Rostock. You can find a list of their ongoing projects here.

  • Not only do they conduct research and monitoring, they also guide decision-makers and give advice to stakeholders. They are increasingly collaborating with international organizations such as ICES (International Council for the Exploration of the Sea).

  • One of their main objectives is to assess the stocks of commercially important species, so that they can be harvested sustainably.

European Plaice (Pleuronectes platessa)

  • A benthic flatfish wide-spread in Europe: from the Mediterranean Sea, all the way to the Barents Sea.

  • One of the most important commercial flatfish species in the EU: trawls harvest ~100 000 tons/year.

  • Plaice spawn in winter/spring. Larvae are bilaterally symmetrical and pelagic. As they grow, they move to coastal nurseries (shallow subtidal sand or mud flats) and metamorphosize: eyes migrate to the right side of the head and the body flattens.

  • Most age 0 juveniles then migrate to deeper waters in winter, but some come back to the nursery as age 1 fish, in early spring. Based on my supervisor Dr. Uwe Krumme's year-round survey, some also seem to stay all throughout winter.

Plaice in Bergen, Norway.
Watch this 30-sec clip from NOAA to learn the basics of otoliths.

What are otoliths?

Otolith can directly be translated as "ear-stone". They are made from a matrix of protein and calcium carbonate.

They grow continously, without being reabsorbed during periods of low energy reserves, unlike bones or scales. The composition and opacity of the material being deposited varies depending on both external and internal factors, including seasonal changes in water temperature and salinity, body condition...

This makes otoliths extraordinary time capsules, a precious tool for studying different variables over a fish's entire lifespan! By looking at their isotopic composition and ring patterns, we can infer diet, migratory movements, growth rate, timing of major life-history events, and age.

Context and Objectives

Determining age by studying calcified structures is a discipline called sclerochronology, and it is extremely valuable since stock assessment models depend on age-structured data.

In theory, you can count annual rings on otoliths just like you can count tree rings to age them. But it's often more complicated than that, and errors can have big consequences on how well a fish population is managed. For example, rockfish used to be greatly under-aged, leading scientists to believe that they were fast-growing and could sustain much more intensive fisheries than in reality, which caused their collapse in Canadian waters.

The Baltic Sea, with its fluctuating salinity levels, can confuse things even further by creating false rings and "noise" in the main underlying pattern. There are still many aspects we don't understand about reading otoliths from Baltic Sea plaice, especially patterns deposited in early life -- in fact, one of the studies that inspired this project is an unpublished paper aiming to confirm the position of the first true annual ring.

The main goals of this professional practice were to: 1) Standardize an approach to taking measurement on juvenile plaice otoliths. 2) Create categories to differentiate otoliths based on general patterns. 3) Guide future research to validate age determination.

Collecting juvenile plaice

Although I worked from home at my desk most of the time, I was able to assist with collection of juvenile plaice twice. Sampling is done at sunset, once a month, around when the moon is full. Plaice are nocturnal and visual predators, so the full moon is when they are most active - and catchable!

After we collect the fish, they are sent to the lab where a technician measures and weighs them, then surgically extracts the otoliths and photographs them.

That's when the bulk of my work begins...

I used a small push net to skim the top layer of sand in water <1m depth. This oversized suit kept me nice and warm while I was waist-deep in 10°C water for over an hour.
The study area is on sand flats along the coast of the South Baltic Sea.
A sample of juvenile plaice I caught in June: newly settled age 0 fish are mixed with age 1 fish during this period. It's easy to tell them apart by size, but how do their otoliths differ?

Analyzing otolith patterns

Professional Practice

What's next?

I barely grazed the tip of the iceberg during my internship, but luckily there are exciting horizons ahead! This summer, an undergraduate student will be continuing the data collecting process I initiated, to increase our sample size, and next academic year another student will be conducting their Bachelor's thesis on similar questions but with flounder. Here are some of the next steps to be taken, taking into consideration what we learned from my experience:

Break (and bake) method

  • Breaking the otolith at its core reveals the inner ring pattern, unobscured by accumulation of new material.

  • The broken otolith is sometimes burned or baked to enhance pattern visibility.

  • This is commonly used with otoliths of adult fish, but it could also be useful for these juvenile fish.

"Backwards" approach

  • Many of the interpretation issues I had were related to small, ambiguous formations. However, these patterns may not even be visible in age 1+ plaice.

  • Instead of starting the analysis with the smallest available otoliths, we could look at age 2 otoliths (as pictured below) and find which patterns from juvenile otoliths stay noticeable in adults.

Mark-recapture study

Starting fall 2020, a new study with flounder and plaice will be launched:

  • Fish are bathed in a chemical called tetracycline, tagged, then released in a brackish pond.

  • Fish are recaptured after a lapse of time & their otoliths are extracted.

  • A thin ring, fluorescent under UV light, is a mark from when the fish was bathed in tetracycline.

  • This method allows us to study ring formation during a known amount of time.

A few closing thoughts on what I learned

  • It was my first time conducting this kind of research: exploratory, with subjective elements... There weren't always clear-cut right or wrong answers, and the freedom given to me was overwhelming at times, but that made it a great opportunity to build confidence.

  • Science is a process, make sure to communicate throughout it all - the more ideas bounce back and forth between different people, the more polished they become. I often felt like I should wait until I had something more "finished" to show my supervisors, but the reality is, I always felt better and more clear-headed after having a quick check-in meeting over Skype.

  • For every answer research provides, at least 10 new questions appear - and that's as it should be!

References and further reading

Beamish, R. J., E. B. Brothers, J. M. Casselman, R. I. C. C. Francis, H. Mosegaard, J. Panfili, E. D. Prince, R. E. Thresher, C. A. Wilson, and P. J. Wright. 1987. Glossary for Otolith Studies. 527–530.

Campana, S. E. 2001. Accuracy, precision and quality control in age determination, including a review of the use and abuse of age validation methods. Journal of Fish Biology, 59(2):197–242, https://doi.org/10.1006/jfbi.2001.1668.

Freyhof, J. "Pleuronectes platessa". IUCN Red List of Threatened Species. 2014: e.T135690A50018800. doi:10.2305/IUCN.UK.2014-1.RLTS.T135690A50018800.en

Matta, M. E., and D. K. Kimura. 2012. Age determination manual of the Alaska Fisheries Science Center Age and Growth Program. NOAA. Professional Paper NMFS 13, NMFS 13(February):97,.

Rijnsdorp, A. D., P. I. van Leeuwen, and T. A. M. Visser. 1990. On the validity and precision of back-calculation of growth from otoliths of the plaice, Pleuronectes platessa L. Fisheries Research, 9(2):97–117, https://doi.org/10.1016/0165-7836(90)90058-4.

Van Der Sleen, P., C. Stransky, J. R. Morrongiello, H. Haslob, M. Peharda, and B. A. Black. 2018. Otolith increments in European plaice (Pleuronectes platessa) reveal temperature and density-dependent effects on growth. ICES Journal of Marine Science, 75(5):1655–1663, https://doi.org/10.1093/icesjms/fsy011.

Wright, P., J. Panfili, B. Morales-Nin, and A. J. Geffen. 2001. A- Otoliths; Pp. 464. In Manual of fish sclerochronology, Ifremer-IR. J. Panfili,, H. De Pontual,, H. Troadec, and P. J. Wright, eds, Brest, France.


A big thank you to my supervisors Dr. Uwe Krumme and Dr. Kate McQueen for their trust, patience, optimism and encouragement.

Thank you also to my friends & family, who remotely kept me (somewhat) sane while I lived alone in an unfamilar country during a pandemic!