Science 10, Advanced Level, Core Unit 3: Organisms and Their Internal Environment
Duration: 90 minutes
Key Terms: annulus, circulus, cleithrum, focus, otoliths
Students will collect and mount fish scales and use them to determine the age of fishes.
- Students will describe the structure of a typical fish scale and understand how the age of a fish can be determined by studying its scales.
- Through discussion, students will understand environmental factors that affect the growth of fishes.
Investigating the age and life span of fishes is especially difficult, if not impossible, from only brief observations in the wild. Fortunately, individual fishes keep a permanent record of their life history in some of the hard tissues of their bodies. As a fish grows, its scales must grow as well in order to keep its body covered. If a scale is lost or removed, a new one will replace it. The process is similar to the way a tree grows, but while trees add one ring per year, a fish scale may gain many rings (two, three or up to 20) in a single year.
The basis of most aging techniques in fishes is the annulus or year mark. In Ontario, where there are distinct seasonal changes, clear annular zones form during the colder months of winter. The focus is the first part of the scale to develop. A ridge called the circulus is laid down around the focus (appears as a dark ring) as the fish continues to grow. Several circuli are added to the scale each year, thus increasing the scale's size.
When conditions are good, as in late spring and summer, warmer water temperatures and more available light stimulate an increase in the metabolism of the fishes and circuli are formed further apart. When conditions are harder, as in winter, growth slows down and circuli, if formed, are much closer together and sometimes appear broken or fragmented. The rings create a zone called the annulus, which indicates the termination of that year's growth. The age of the fishes is determined by counting the number of annuli.
By determining the age of individual fishes, scientists can begin to gain some insight into the population as a whole. The rate at which fishes grow in a given lake or stream can be determined and compared to expected or normal growth rates. Since the onset of sexual maturity in fishes tends to slow the growth rate down, scientists can estimate the age at which a species reaches sexual maturity (which can be verified by experts during the spawning period). This can help in the development of fishing regulations to ensure that sufficient numbers of fishes can reproduce at least once before being caught by anglers or a commercial fishery. Fish populations stressed by humans tend to show typical age distributions. A fish population suffering from overfishing or excessive natural predation will lack older fish, and younger fish will predominate. What older fish that occur will tend to be larger for their age than in an unstressed population. If the situation continues, then young fish will decline as well since the reproducing fish are gone. A fish population of predominantly older fish may be underfished, or the spawning or nursery habitat may be lacking. Each species of fish has its own unique scales. Thus, fish scales can be useful as evidence when prosecuting someone with fishing out of season, or when trying to identify what kind of fish another fish might be eating. Some fishes can be aged more reliably by other body parts. Annual growth rings are laid down on vertebrae, otoliths (ear-stones), pectoral fin rays, dorsal fin spines (e.g., walleye) and cleithrum (cheek) bones (e.g., for northern pike).
Overhead transparency of Teacher Resource Sheet for each group; fish with scales intact (see note below); small knife; beaker or jar; few drops of dish detergent; paper towels; microscope slides; masking tape; 35 mm slide mount and screen; a dissecting or 40 to 50X microscope (optional; see Note below.)
Note: Scale samples can be taken from a live fish and the fish can be released without harm; scales will grow back. A dead specimen may be obtained from an angler or purchased from a fish market. The aging process is particularly clear on lake whitefish, which may be purchased with scales intact.
If microscopes are not available, a scale may be mounted in a 35 mm slide mount and projected on a screen. Do-it-yourself slide mounts are available at most photographic supply stores. Even holding up the mounted scale to a well-lit window can provide sufficient light to examine large scales (e.g., carp or lake whitefish). Scales can also be read on a microfiche reader.
- Brainstorm the resources and/or techniques that could be used to determine the age of a human. Discuss the reasons for the unreliability of age/height tables as a method for determining age.
- Explain that some living organisms keep a record of their age in some of their body structures (i.e., tree rings in a cross-section of a trunk, rings of cementum in the cross-sections of some mammal teeth and annular zones in some hard body parts of fishes). These structures change their growth patterns as a result of annual changes in the environment of the organism.
- Have students list the annual environmental fluctuations that could cause changes in the growth pattern of a fish. Examples are warmer water temperatures in summer, less light in winter, more biological activity leading to more available food in summer, etc. Upon completing the list, have students hypothesize which season a fish would grow the most.
- Prepare an overhead transparency of the scale on the Teacher Resource Sheet and have students note the position of the focus of the scale, the circuli and the annuli. Using the enlarged section of a scale, emphasize that the annulus is a result of slow growth in the winter. It should be readily apparent that the illustrated scale represents a fish that is eight and a half years old.
- Divide the class into groups with one fish per group. Have each group observe the organism and list the external hard structures that might reveal its age.
- Have students scrape away a few scales from the appropriate area on the fish using small knives. See diagram on previous page.
- Instruct some students from each group to soak the scales in a small jar or beaker of warm water to which a few drops of dish detergent have been added. By stirring the water vigorously, they can remove much of the mucus and other materials from the scales. Meanwhile, have the remaining students in the group measure the length of the fish from its nose to the fork in its tail.
- Have students remove the scales from the water solution and blot them dry on a piece of paper towelling. Show students how to mount a scale for observation by placing it between two microscope slides and wrapping masking tape around each end of the slides to flatten it and keep it in position.
- Have students observe the scale they prepared and mounted on a slide through a microscope.
- Have students sketch the scale, label the visible structures and determine the age of the fish by counting the annuli.
Note: Occasionally a false annulus will form due to lack of food, high temperatures or re- duced oxygen levels. It can usually be identified because the circuli rapidly become closely packed. In a true annulus, the circuli usually close gradually and seem to "cut over" one another. Regenerated scales usually have a very large focus and should not be aged.
- If fish scales came from the same species, have students exchange age and length data and prepare an age/length graph for the selected species. A good graph will require a minimum of 20 fish.
- Have students brainstorm how fishery biologists might use the age data collected from fishes in a given area to assist in the management of the fishery.
Obtain another fish scale and assess students' ability to determine the age of the fish and label the following parts: focus, circulus and annulus. Have them describe one way a fishery biologist might use this type of information in fisheries management.
- Compare annual growth rings on a tree disc with growth rings from a fish scale. Have students find out how scientists determine the ages of other living things.
- Obtain fish scales from several different fish species and have students distinguish among them. In what ways were the scales different? How were they the same?
- Contact your nearest MNR district office and find out if the fish & wildlife section, fisheries assessment or research units have any aged fish scales mounted on slides from old lake surveys that they would be willing to donate to your class. Have students determine the age of the scales and compare their results. These scale samples may not be available everywhere in the province.
Nielsen, L. A., and D.L. Johnson. Fisheries Techniques. Blacksburg, Virginia: American Fisheries Society, 1985.