Aging and Chemical Analysis

The St. Andrews Biological Station (SABS) staff conduct ageing research for stock evaluations on a variety of marine finfish to provide scientific advice for fisheries management. Accurate ageing of fish is essential for effective evaluation of stock status. Age-based fisheries assessment models use the history of fish populations, including the number of fish in each year class, to help predict what will happen to the stocks in the future.

Since 1946, SABS has examined the otoliths, or earbones, of various fish species to determine their age. Otoliths are bones found in the head of a fish that lay down growth rings, much like those found in the cross section of a tree trunk. Using a microscope, staff examine the otoliths to assess growth patterns, spawning events and the age of the fish. This information is used with other biological information, such as length, weight, and maturity of samples collected from the Scotian Shelf, Bay of Fundy, Georges Bank, and Gulf of Maine. Information from samples collected during DFO’s annual ecosystem research surveys is augmented with that collected from commercial fishing vessels or from landing ports. This information is then compiled to provide an approximation of the size and strength of a year class within a certain fish stock over time.

Fish ageing research at SABS began with haddock and then expanded to include most commercial groundfish and pelagic stocks. Currently, three groundfish species from the Gulf of Maine and the Bay of Fundy are aged: cod, haddock, and pollock. Herring, a pelagic species caught in weirs and by seine in the Bay of Fundy, is also aged. SABS is also researching methods to accurately age Atlantic bluefin tuna, a large pelagic species that is highly migratory in the Atlantic Ocean.

Ageing techniques have evolved over the years. From the 1940s to 1985, otoliths were prepared by manually breaking them along a particular groove. In 1985, a new procedure with greater precision was introduced. The otoliths are embedded in a black polyester resin block (at left) and cut through the centre using a precision saw. Once cross sections are cut, they can be mounted on plexiglas slides for reading under a microscope.

Readers count how many distinct growth rings, or annuli, can be seen. Although this sounds simple, it is often difficult to distinguish early or compact annuli. Annuli develop when the growth rate of a fish changes. This can be seasonal, related to food availability, an increase or decrease in the level of activity, changes in water temperature, and/or spawning periods. These factors change the composition of the material from which the otolith is formed. For example, months associated with higher feeding rates and faster growth result in more calcium carbonate deposition which makes the otolith appear opaque when viewed with reflected light. Conversely, in the winter months when fish grow less slowly, less calcium carbonate is deposited resulting in more transparent rings. An experienced reader can tell the difference between the slower growth in winter months and the faster growth in summer months. It is more difficult to determine age when seasonal environmental changes are less intense (e.g., for tropical species).

Groundfish otoliths are relatively large and indicate growth patterns better than other bony structures. Fish species lacking ear bones of sufficient size and clarity (e.g., Atlantic salmon) require alternative ageing methods such as counting annuli on vertebrae, scales, or other bony structures.

For Atlantic bluefin tuna, initial ageing techniques of their vertebrae have found that double rings are formed during one year, making it difficult to determine where one annuli ends and a new one begins. Although bluefin tuna otoliths are smaller and more difficult to read than those of groundfish, they are still the predominant means of ageing. Therefore, SABS researchers collect otoliths from the bluefin tuna commercial fishery. In addition, studies on the chemical composition of the otoliths have provided insight into bluefin tuna migration habits.

Careful ageing and quality control is of utmost importance for determining stock status advice. Quality control is aided by comparing results of different “readers” and involves collaboration with colleagues in the United States for fish stocks that live in both US and Canadian waters. Two readers compare their results on the same otoliths to ensure they are obtaining the same results.

Another tool used to validate the readers’ ageing technique is comparison with tagging data. Fish caught from various locations across the region are identified with small plastic tags and then released. Tags are returned upon recapture in the fishery along with information on the time and location of capture, as well as size, length and sometimes spawning condition. Comparing otolith age estimates with these data helps ensure more accurate ageing estimates.

Although most otoliths are aged using manual observations from the microscope, there is an alternate method using camera image analysis. A camera is mounted on top of a microscope and transfers the image to a computer screen. The reader can assess this image and store it on a database. This alleviates the need for physical storage of the otoliths. This approach also allows the reader to mark where they see the annuli, making training for new readers more straightforward, and reassessing samples more efficient. Despite these positive features, microscope analysis remains the preferred ageing technique, since camera images are captured in two-dimensions only, whereas microscope analyses permit three-dimensional observations. However, the Station has established a comprehensive otolith image library for training, workshop and testing purposes.

Equipment: SABS has recently purchased a new state-of-the-art precision saw - Discotom-6 for sectioning otoliths

Herring otoliths: showing the location in the fish (top), extraction (centre), and otoliths of a 9-year old herring (bottom)