FISHERY BULLETIN: VOL. 80, NO. 3 



would have spawned. Growth from July through 

 September could have been high as the apparent 

 pattern suggests, or it might have slowed down 

 and/or ceased as these fish matured and spawned. 



Age Determination Using Scales 



Silver seatrout can be aged using scales, 

 although few fish had scales with either an annu- 

 lus or false annulus. Only 38 of 1,483 fish (2.6%) 

 had an annulus, and no fish had more than one. 

 Only 41 fish (2.8%) had a false annulus, and they 

 included 5 fish with an annulus. These percent- 

 ages overemphasize the frequency of annuli and 

 false annuli because the stratified sampling used 

 to select specimens for intensive processing also 

 selected all the large fish which would most 

 likely show these marks. 



Repeated examination suggests that age de- 

 termination of C. nothus is consistent. We had 

 90% agreement in a second reading of scales from 

 225 fish, which included 123 fish >150 mm SL 

 and all 38 fish first determined to have an annu- 

 lus. The second reading identified an annulus in 

 45 specimens, including 30 of the 38 fish (79% 

 agreement) first determined to have an annulus. 

 The eight fish for which an annulus was not con- 

 firmed were collected in May and June about 

 when the annulus forms; their scales had small 

 marginal increments after an indefinite clear 



1 



Apr n=1 



La 



May n=14 



^ <£>_ 



L\- 



T 1 



Jun n=13 



o 5 l 



2 L 



i 1 — 



Jul n=1 



Hi 



O 5 " 



LU 



CC 



5- 

 5- 



^r 



Aug n=1 



1 



Sep n=4 



-^ 



Oct n=1 



_^_ 



i 1 



Nov n=2 



/\ 



1^ 



-i r* 1 1 



10 30 



MARGINAL INCREMENT (mm*42) 



zone, and secondary radii and/or cutting over 

 were not distinct. The second reading was done 

 without knowing sizes or collection dates. This 

 would minimize agreement between readings. 



May-spawned C. nothus form an annulus from 

 April (or earlier) to June when they reach 130- 

 190 mm SL and 1 yr of age, but time of annulus 

 formation may vary between spawned groups 

 and is not clear for August- or September- 

 spawned fish. Marginal increments were small- 

 est from April to June and generally increased 

 thereafter (Fig. 8), suggesting the first annulus 

 forms from April (or earlier) to June. The small- 

 est fish with an annulus was 139 mm SL and 

 most exceeded 150 mm SL. The proportion with 

 an annulus increased with increasing size, per- 

 centages being 16% at 150-159 mm SL (n = 55), 

 24% at 160-169 mm SL (n = 54), 60% at 170-179 

 mm SL (n = 10), and 100% at 180 mm SL and 

 greater (n = 6). The proportion of the fish >150 

 mm SL with an annulus (Fig. 9) was significantly 

 higher in May and June, when most of these 

 large fish were May-spawned, than it was in Sep- 

 tember and November, when most were August- 

 or September-spawned. Fish with an annulus in 

 the period August-November all exceeded 170 

 mm SL and probably were survivors of the May 7 e 

 group; those without an annulus then were 150- 

 170 mm SL and probably August- or September- 

 spawned. 



Back-calculated lengths agree with length fre- 

 quencies. Lengths at age I back-calculated using 

 the Lee method (Lagler 1956) varied from 132 to 

 176 mm SL in comparison to 130-190 mm SL 

 based on length frequencies. The mean back- 

 calculated length was 156 mm SL with 95% con- 

 fidence limits of 153-159 mm. 



30 



> 

 o 



z 



LU 



o 



LU 



cc 



QC 10 



□ Without an annulus 

 With an annulus 





Ds 



a 9 



_Ea. 



ra 



0. 



cc 



Q- 

 < 



< 



2 



z 



3 



o 



< 



o 

 o 



> 



o 



z 



MONTH CAPTURED 



Figure 8.— Monthly marginal increments for C. nothus with 



one mark. 



Figure 9.— Histogram showing by month the number and per- 

 centage of C. nothus>l50 mm SL with and without an annulus. 



494 



