Otolith Analysis in Sportfish Population Studies
 How Ear Bones Influence Future Fishing Regualtions
Georgia's saltwater recreational fishing regulations are developed to ensure fishing opportunities for current and future generations of anglers. These guidelines, such as size restrictions and creel limits, are based on estimates of current fishing conditions and careful predictions about the future. To make these estimates, fisheries scientists calculate growth, mortality and reproduction rates, and monitor changes in size and age compositions of fish populations. One of the most reliable methods for determining the age of a fish is to examine the otolith, or ear bone.
Otoliths are calcified structures located in the inner ear just behind the brain that assist fish with balance and hearing. While there are three pairs of otoliths, only the largest pair, usually the saggital otoliths, is used for age determination. In temperate waters, such as those occurring along the Georgia coast, seasonal growth periods appear on otoliths asalternating opaque and translucent bands. This pattern looks much like the annual growth rings present in the trunks of trees. Just as in trees, we are able to count the rings in these structures and determine the age of each fish. Similar seasonal bands can also be found in other hard parts such as scales, fin rays, spines, and vertebrae.
Generally, fish grow faster during the warm summer months and slower during the winter months when water temperatures are colder. A year of growth is comprised of one winter zone and one summer zone. Under reflected light, the winter zones appear as narrow white bands and are counted to determine the age of the fish in years.
In order to estimate the age distribution of fish being harvested by recreational fishermen, researchers must first gather samples which represent the existing population. While some fish are collected during state-conducted research surveys, the primary source of size and age samples processed by the Coastal Resources Division (CRD) is the Marine Sportfish Carcass Recovery Program. As part of this program, freezers have been located near fish cleaning stations at six sites along the Georgia coast. Bags and information cards are supplied at each of these sites, and anglers are encouraged to donate their fish carcasses after they have been filleted. Freezers are checked on a regular basis and the carcasses are brought back to CRD for processing.
Each donated carcass is assigned a unique identification number, which links biological information collected from the specimen with fishing trip information supplied by the angler. A length is taken for each fish, and when available the internal organs are examined to determine the sex and maturity of the individual. The otoliths are then removed from the fish, cleaned, and taken to the lab for age determination. Some species such as southern flounder (Paralichthys lethostigma) and Atlantic spadefish (Chaetodipterus faber) have relatively thin otoliths that can be read without much further preparation. For species with larger otoliths like red drum (Sciaenops ocellatus) and spotted seatrout (Cynoscion nebulosus), a narrow cross-section must be cut out of the otolith and mounted on a microscope slide before the fish can be aged.
Samples are first viewed under a microscope and then projected onto a computer screen. Using an image analysis program, each winter growth band is marked and a series of measurements are recorded. In order to ensure consistent results, otoliths are examined independently by two readers. If the readers agree then the determined age is accepted.
Fisheries scientists then examine the relationships between ages and lengths. From these data, they can estimate the number of existing fish and the percentage of individuals that make up each age group within the population. These results can be compared to past studies to examine changes in fish populations. With this information, predictions can be made about the future of Georgia's marine recreational fisheries and management guidelines can be proposed.
Reprinted from the Spring 2002 Georgia Sound Newsletter. Article by Celia Cackowski.
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