How and Why We Age Fish

June 16, 2023
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Why do we want to know the age of fish?

To manage fish populations, whether it be for conservation, recreation or brood stock, we need to determine the ages of fish. Important management decisions require timely and accurate fish aging data.

The age distribution of fish within a lake provides an indication how healthy stocks are within that lake. A broad range of ages is usually a good sign that the stock is doing okay. Maybe we don’t see very many (or any) younger fish, indicating a potential problem with recruitment; if we see no older fish, this might indicate unequal fishing pressure or overfishing. Over the long term, knowing the age distribution of fish within a system is a key component that managers can use, together with many other environmental factors, to monitor and maintain the health of lakes and other waterbodies around the province.

Which fish get aged?

The BC provincial aging lab currently ages rainbow trout, cutthroat trout, eastern brook char, steelhead, largemouth bass, bull trout and kokanee.

How can you tell the age of a fish?

Scales, otoliths (inner ear bones) and other bony parts of fish (fin rays, dorsal spines, or vertebrae) often contain a record of seasonal growth patterns. This is because fish grow faster in the summer than in winter. Patterns in the scales and otoliths reflect this change: summer zones are usually wide, and winter zones are narrow. One pair of such zones is called an annulus. Therefore, by counting the number of annuli, fish age in years can be determined.

1. Aging using a fish’s scale

Scales collected in the field are further sampled in the aging lab by picking four or five scales from each fish, and mounting them onto sticky pages into a “scale book.” Each page of the scale book is then pressed together with a thin piece of special plastic, called an acetate, using heat (approximately 100°C) and pressure (approximately 10,000 psi). 

The pressed acetates with scale impressions are then placed within a microfiche reader that allows the scale impressions to be enlarged for viewing and age analysis.

2. Aging using a fish’s otolith

Primarily, otoliths within the aging lab are aged whole. This is exactly what it sounds like. Otoliths are placed on glass slides, covered with glycerine, and left to sit for at least 24 hours. This allows the glycerine to clarify the annuli within the otolith. A microscope with a camera is then used to transmit a larger image of the otolith onto a monitor for viewing, age analysis, and capturing images of the otolith for reference archiving.

3. Aging using thermal marking

Thermal marking of otoliths allows fisheries biologists to mass mark hatchery-reared fish by taking advantage of the otolith’s ability to record abrupt changes in temperature. Eggs in the hatchery undergo short-term temperature manipulations (heating or chilling) that alter how the growth increments look within an otolith. The process produces an obvious and unique pattern, a biological “bar code,” that can be recognized at any life stage, and provides definitive stock identification for these marked fish – including the brood year, which can then provide the age of the fish. Recent Provincial work involving thermally marked otoliths on pre-release kokanee requires further processing steps within the lab. These otoliths need to be attached to glass slides using an adhesive, and then hand-polished using several variations of extremely fine sanding/polishing paper. The polishing proceeds slowly, as it’s very easy to over-polish and lose all marks within the otolith. Continuous checking using a compound microscope is necessary to look for the specific thermal pattern.

The BC Provincial Aging Lab, located at the Fraser Valley Trout Hatchery, opened on June 5th, 2017, with support from the Freshwater Fisheries Society of BC, the Province, and B.C. anglers.

Author: Morgan Davies, Freshwater Fisheries Society of BC