Fish have in their inner ear crystalline structures formed by calcium carbonate, in the mineral form of aragonite, which together with the lateral line system are responsible for their orientation and balance in the water column. The shape and size of the otoliths vary a lot, being characteristic of each species. These structures show a regular deposition of increments (or rings) that are related to annual, seasonal and daily variations in fish growth.
The annual growth rings (also called anulli), present an alternation of opaque and translucent bands. The first bands coincide with periods of rapid fish growth, usually related to spring and summer. The latter are due to the lower rate of aragonite deposition during periods of slow growth, usually autumn and winter. Anulli are used to estimate the age of fish in fisheries biology. This method is very useful for the rational and sustainable management of fish stocks as it allows the determination of growth curves, mortality rates and recruitment rates.
This activity aims at learning conventional techniques of extraction, preparation and reading of otoliths for fish age estimation. Through the experimental activity, the students are made aware of the importance of estimating the age of fish stocks for the sustainable management of fishery resources, as well as for the influence of humans on the Ocean and the exploitation of its potentialities. This protocol is related to the Natural Science Curricular Areas of the 2nd and 3rd Cycles of Basic Education and Biology (12th year, Food Production and Sustainability) of Secondary Education. It is part of Essential Principle 6 "The Ocean and Humanity are strongly interconnected" on the scientific culture of the Ocean, promoted by the project “Conhecer o Oceano”.
Materials
- Sardine specimens (note: these may be overfished or animals whose organoleptic quality indicates that they are no longer fit for consumption)
- Ruler
- Knife
- Fine-tipped Forceps
- Gloves
- Distilled water
- Petri dish or watch glass
- Dissecting needle
- Black-bottomed container (e.g. black plastic lid)
- Clarifying solution (ethanol and glycerol, 1:1)
- Dissecting microscope
- Absorbent paper
Protocol
Part A:
- Choose a sardine specimen at random.
- Assign the selected specimen a record number. Record the total length of the selected individual in the observation record table.
- Remove the pair of sagittal otoliths. To perform the otolith extraction cut through the dorsal surface of the head in the direction of where the operculum fuses with the head, stopping slightly before. Next make a transverse cut from the dorsal part of the head. After removing the brain, remove the otoliths using fine-tipped forceps.
- Place the otoliths, initially, on the back of the hand (the otoliths adhere easily to the latex glove). After extracting the pair of otoliths place them in a container of distilled water. Remove any remaining tissue with the help of forceps and a dissection needle.
- Dry the otoliths on absorbent paper.
Part B (Age estimation)
- Place the otolith pair into a black-bottomed container.
- Immerse the otoliths in the clarifying solution, composed of ethanol and glycerol in a 1:1 ratio.
- Place the container under the dissecting microscope and observe. Choose a magnification that allows the entire structure of the otolith to be seen. Opaque areas appear light and translucent areas appear dark.
- Draw the external and internal views of one of the otoliths (visualization of the sulcus acusticus indicates that the internal side is being observed) on the result record sheet.
- Identify the different constituent parts of the otolith (anterior and posterior, dorsal and ventral, rostrum and anti-rostrum, excisura, core, primordium, rim and sulcus acusticus).
- Interpret and count the annual growth rings (annual growth area = opaque area + translucent area) and determine the type of rim (opaque or translucent).
- Take note of the record number of the specimen, the number of annual growth rings and the type of rim in the observation record table.
- January 1 st is the conventional date of birth for fish in the northern hemisphere. Considering the observed annual deposition pattern (opaque + translucent areas), assign this specimen of sardine an age group based on the number of growth areas counted, the type of rim observed (by convention read translucent bands, the last one not being counted if it is on the edge), the adopted date of birth and the date of capture.
Results
Discussion of results
The objective of this experiment was to teach the way to extract, prepare and understand the use of otoliths in estimating a fish’s age for the management of the fish stock in a sustainable manner.
Through the experimental activity, the students were aware of the importance of estimating the age of fish stocks for the sustainable management of fishery resources, as well as for the influence of humans on the Ocean and the exploitation of its potentialities.
Conclusion
During our experiment, we measured the fish, recorded its size and removed the otoliths with the help of scissors. The scientist that guided us through it,had prepared the otoliths to be observed in the microscope , due to the lack of time. We observed them and identified the different growth rings to be able to evaluate and discuss their age. We didn't have time to draw them because of the lack of time.
With the help of this experiment we learnt the importance of otoliths in dating fish to be able to preserve our planet through sustainable fishing.
Authors: Rita Almeida, Daniela Garne, Gustavo Valente, Leonor Costa, Finja Hannesson, Beatriz Mendonça, Laura Escolar, Maria Costa, Serena Gianuzzi, Manuel Nunez, Tiona Laima, Margarida Sá, Nele-Charlotte, Marta Moreira, Mayra Torres, Mariana Pereira, Liliana Rotolo, Nuno Pedrão, Eloanora Quaranta, Leonor Gilvaz and Ana Santos.