Age determination of Lingcod

In 1977, Dr. Dick Beamish and Doris Chilton of the Pacific Biological Station published an article showing that cross sections of the 4th to 8th fin rays from the second dorsal fin provided a method for estimating the age of lingcod1. This method has since been validated by a mark-recapture study in which lingcod received an injection of oxytetracycline (OTC). Other methods of aging, such as those using scales and otoliths, were found to underestimate ages for older fish.

photo: cross-section of a lingcod fin ray from a five year old fish (photo: Fish Aging Unit)Cross-section of lingcod fin ray from a five year old fish (Photo: Fish Aging Unit)

Ages are determined from fins in much the same manner as for other aging structures: sections of varying thickness are examined under a microscope, and the annuli, or rings, that are formed for each year of growth are counted and used to estimate the age. The cross sections must be made at right angles to the length of the fin ray, and it is therefore important that fins be dried flat, with the cut surface at right angles to the fin rays. In addition, the distance that the section is cut from the fin-ray base is important, and for this reason, all fins should be collected with the base intact.

drawing: 2nd dosal fin that includes the fin-ray base (M. Surry)Samplers collect a section of the 2nd dorsal fin (rays 4-8) that includes the fin-ray base. (Drawing: M. Surry)

One problem associated with using fin rays to age older fish, is that the fin-ray center may be resorbed, resulting in the loss of the first two annuli. It is therefore necessary to determine an average width for the first two annuli by examining the fins from juvenile fish. This measurement can then be used to estimate the position of the third annulus on older fish.