Reading scales

Ages of individual fish are a critical piece of life history data used by fisheries managers. From a fisheries management perspective it is important to know how long a fish lives, how big a fish can grow, how many offspring a fish can have (i.e. fecundity) and how often it can reproduce, and these various parameters are collectively called a fish’s “life history.” One way for fish biologists to acquire age information is by collecting scale samples. Scales have growth rings, just like trees, which show the growth history of fish. When collecting scale samples from salmonids, it is important to choose scales that are the oldest and have not been regenerated. Scales can be collected from juvenile salmonids by using a knife to scrape between the dorsal fin and lateral line, where the oldest scales form. Scales on adult salmonids can often be acquired by removing individual scales with forceps. In the field, scales are placed on waterproof paper, labeled and stored in a small coin envelope.

In the laboratory, a scale can be “read” by compressing it between glass slides and viewing the slides under a microscope. Cycloid fish scales, like those of trout and salmon, add rings (‘circuli’) as they grow. The rate at which fish, and their scales, grow depends on how much food is available, water temperature and other stressors. In temperate climates where the winters are cold and there is less food available, very little growth occurs and the rings are closer together. During the summer months when food is more abundant, the growth rings are more widely spaced. Annuli, usually identified as a zone of crowded circuli, are formed each year and are counted to estimate the age of the fish. For anadromous fish such as salmon and steelhead, certain characteristics of their scales can show at what age they migrated to sea (as a juvenile), the length of time they spent at sea, whether they have spawned previously and their approximate age when they returned to the river. It is surprising how much valuable information can be obtained from such as small sample.