STOCK DISCRIMINATION USING OTOLITH SHAPE-ANALYSIS

Otolith shape has long been known to be species specific, but recent r eports have pointed to its value as an indicator of stock identity. To test this hypothesis, all three pairs of otoliths were sampled from 2 349 Atlantic cod (Gadus morhua) collected on spawning grounds througho ut the northwest Atlantic. Otolith shape was determined with Fourier a nalysis and combined with measures of otolith area and perimeter. Ther e were highly significant differences in otolith shape among most of t he cod samples, but shape also differed among ages, sexes, and year-cl asses. The sagittal otoliths (largest pair) provided slightly better s tock discrimination than did the lapillar or asteriscal otoliths. The first discriminant function was highly correlated with both fish and o tolith growth rate, indicating that stock discrimination improved as t he difference in stock-specific growth rate increased. Reevaluation of published studies on other species indicated that growth rate contrib utes more variation to regional differences in otolith shape than does stock origin. Differences in otolith shape among ages, sexes, and yea r-classes were also attributable to growth rate differences. To the ex tent that growth rates vary more between than within stocks, otolith s hape analysis can provide an easily determined measure of stock identi ty.