DARK: AGE AND GROWTH OF PACIFIC HAKE 



same trawls, but with 3.8-cm liners in the cod ends 

 much of the time. Liners were used to determine 

 the availability of the youngest age groups. 

 Research and commercial samples were never 

 taken in such a manner that length frequencies 

 could be compared to isolate the effects, if any, of 

 the 3.8-cm liner. But there was probably no sig- 

 nificant selection for fish length without the liner 

 since similar unlined trawls used in the Puget 

 Sound hake fishery apparently retain all fish of 35 

 cm or greater.^ Because very few hake off 

 Washington were as small as 35 cm, sampling gear 

 differences were not considered to be a significant 

 source of sampling error. Therefore research and 

 commercial samples taken in 1966-67 were com- 

 bined at times to increase sample sizes and to 

 improve temporal and areal sampling coverage. 



DETERMINATION OF 

 AGE COMPOSITION 



Aging Technique 



A prerequisite to any growth study is a method 

 for reliably determining the age of individual fish. 

 European investigators (Birtwistle and Lewis 

 1925; Hickling 1933; Bagenal 1954) found the 

 otolith to be the most useful structure in deter- 

 mining the age of the European hake, Merluccius 

 nierliiccius. Bigelow and Schroeder (1953) arrived 

 at the same conclusion while studying the silver 

 hake, M. bilinearis. Apparently Best (1963) was 

 the first to age Pacific hake. He found that from 

 the standpoint of availability otoliths were 

 superior to scales as most scales were absent on 

 trawl-caught specimens. 



Our collection of otoliths was standardized in an 

 effort to control sampling variation. Samplers at- 

 tempted to always collect the otolith from the 

 right side of the head to avoid any confounding 

 effects due to possible otolith asymmetry. If the 

 right otolith was damaged during the extraction 

 process, the left otolith was accepted as an alter- 

 nate (5-10% of all samples). Otoliths were 

 thoroughly cleaned and preserved in a solution of 

 10-30% ethyl alcohol. Occasionally otoliths with a 

 uniform chalky appearance were encountered and 

 were cleared by dipping them in a weak solution of 

 hydrochloric acid. This practice was followed with 



■'Larkins, H. A., H. H. Shippen, and K. D. Waldron. Features of 

 a northern Puget Sound hake population. Unpubl. manuscr. 

 Northwest Fish Cent., Natl. Mar. Fish. Serv., NOAA, Seattle, 

 Wash. 



care to prevent the dissolution of annuli at the 

 otolith edge. 



Otoliths were placed in a petri dish with the 

 bottom painted black, illuminated with a reflected 

 light, and read under a dissecting microscope at a 

 magnification of 6.6 x. Each otolith was read by 

 two readers and if the ages did not agree, as was 

 the case in 25-40% of the otoliths processed, the 

 otolith was examined by a third reader. The best 

 estimate of age was taken as the age agreed upon 

 by any two readers. When all three readers 

 disagreed (about 5% of the readings), the middle 

 reading was used. If one reader could not make a 

 determination and agreement could not be 

 reached by the other two, the otolith was con- 

 sidered unreadable. Generally there was a 3-5% 

 rejection rate. 



The majority of the hake otoliths were collected 

 during the summer (May-September). Assuming 

 that the past winter was represented by the last 

 (most recent) translucent zone, the age was taken 

 to be simply the total number of translucent zones 

 on the otolith. The few winter (February-March) 

 samples collected were composed mainly of fish 

 completing their first or second year of life. The 

 same aging criteria cited above were applied to 

 winter samples, except those otoliths without a 

 translucent zone were assigned to age "1" instead 

 of "0." This was done on the premise that the 

 translucent zone would have been deposited 

 shortly after the sample was taken, since young of 

 the year would not have been captured by the 

 sampling gear. 



Validity of Aging Technique 



Because the use of otoliths in aging Pacific hake 

 had not been completely evaluated, some attention 

 was devoted to determining the reliability of the 

 procedure. Graham (1929) gives three indirect 

 methods of evaluating the use of scales and 

 otoliths for age determination: 1) agreement with 

 Petersen's (1895) method; 2) seasonal changes in 

 scale or otolith margins; and 3) observation of a 

 strong year class over a period of years. 



The Petersen's (1895) method, which compares 

 the relative abundance of age groups as deter- 

 mined by length distribution with age groups as 

 determined by analysis of scales, otoliths, or other 

 structures, is generally only applicable to the first 

 three or four age groups. For Pacific hake, the 

 length distributions of the age groups overlap ex- 

 tensively after age 3, restricting the use of the 



339 



