The overlapping scales of the cisco or lake herring (Coregonus alba), a freshwater fish from North America, are magnified 35 times in this photograph. The scales grow throughout the fish's life, increasing in size with the fish. As each scale grows it forms a concentric pattern of ridges. The ridges are close together during winter when growth is slow, and more widely spaced in summer when growth is faster. The number of winter growth ring sets gives the fish's age, just as the growth rings in a tree trunk gives a tree's age.