The young halibut are also vulnerable to capture by the expanding fleets of 

 foreign bottom trawlers now Fishing in the northeastern Pacific. Since 1960, annual 

 surveys by the Halibut Commission have demonstrated that there has been a drastic 

 decline in the number of young halibut on some grounds (Figure 11). This develop- 

 ment poses a serious threat to the ultimate survival of the North American setline 

 halibut fishery. 



Age and Growth 



Halibut live to a moderately advanced age. The growth rate or increase in size 

 with age ranges widely from one section of the Pacific Coast to another, and it may 

 also vary from one period of time to another (Figure 12). 



Females grow faster than males and also live longer. The oldest female recorded 

 was about 42 years, and the oldest male 27 years. 



-X 





Figure 13. Otolith from a halibut in its ninth year. Dark rings are the less opaque winter zones. 



Rings or annuli are formed each year on 

 the bones or hard parts of fish as in a tree 

 due to the succession of the fast summer and 

 slower winter growing conditions. The oto- 

 lith, a calcareous or stone-like body in the 

 internal ear of the halibut (which probably 

 serves as a hydrostatic or balancing organ), 

 shows such annual rings (Figure 13). 



The Halibut Commission "reads" about 

 40,000 such otoliths each year to determine 

 the age composition of the stock and the 

 growth rate (Figure 14). This information 

 is essential for the scientific management of 

 the resource. 



Figure 14. Measuring annual zones 

 for growth studies. 



