Salish Sea Marine Survival Project

24 WHAT CAN WE LEARN FROM OTOLITHS? WHAT ARE OTOLITHS? Otoliths are ear bones found in all teleost (bony) fish and play an important role in balance, orientation and hearing. They consist of three pairs (left and right sides) of small carbonate bodies called lapilli, asterisci and sagittae. In Pacific Salmon, the first two of these are very small but the sagittae (~5 mm) are larger and are the ones most often studied. Otoliths are actually crystals and formed by a precipitation of protein and calcium carbonate on the exposed surfaces. When examined under a microscope, they show a pattern that looks like the rings of trees. There are a number of concentric zones of differing sizes, and depending on the amount of organic material in each ring they may be opaque or transparent. As juvenile Pacific Salmon move from freshwater into estuarine and then marine environments, the otoliths display bands of different widths and particular visible "checks" which mark periods of change or stress. Ultimately, the otolith with its variable bands and check marks provides an invaluable record of a fish's age, and growth and residence in different habitats. As a fish grows, the width of the otolith also grows as a result of the precipitation noted above. The amount of precipitation depends on temperature and food availability, resulting in different widths of the bands. Wider bands if a fish grew a lot in a particular environment. These bands are actually deposited daily providing a daily record of a fish's growth and habitat conditions; however, to see the daily growth rings/bands requires high levels of amplification under a microscope (Figure 1 bottom, shows an example of daily growth rings in salmon). Thermal Marking Studies at Hatcheries The relationship between temperature and precipitation resulting in formation of daily growth rings has been utilized across the Pacific nations to identify the hatchery-of-origin of various salmon species. In Canada, the Salmon Enhance- ment Program (SEP) utilizes thermal otolith marks to mass- mark juvenile salmon by creating abrupt changes in water temperatures during incubation which results in a pattern of dark rings in the microstructure of the otoliths. Hatcheries use specific sequences of temperature changes to result in unique patterns for marking the otoliths of their fish. Thermal marking is useful for situations when hatchery salmon juveniles are too small for coded wire tag (CWT ) applications or if a release group is particularly large, making it impractical to mark all of them. Thermal marking of otoliths has been important to identify the hatchery origin of adults and for comparative hatchery studies, ultimately providing information on estimates of hatchery contributions in a fishery or fish returning to the rivers, as well as on high seas distribution and migratory characteristics of Pacific Salmon. Figure 1. Top: Example of a sockeye otolith with identification of each annulus (red dots) that demark the slower winter growth. Note that the number of annuli from the centre of the otolith (the focus) to an edge of the otolith provides the age of the fish. This sockeye spent 1 year in freshwater, and experienced 4 winters at sea, for a total age of 5 years. Bottom: photograph shows the daily growth rings visible under higher magnification. Images provided by Chrys Neville, Cam Freshwater, Jeff Till and Stephen Wischniowski, DFO Figure 2. Depiction of an adult Chinook otolith, showing the hatch and emergence checks, as well as the portion of the otolith formed in freshwater and the ocean life history components. Figure provided by Lance Campbell, WDFW.

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