FISHERY BULLETIN; VOL. 69. NO. 1 



Table 1. — Summary of average growth rate estimated 

 from the progression of modes or means (see Figs. 3 

 and 4). 



Year 

 class 



Recruitmenf 

 month 



Number 

 monHis 

 followed 



Growth rotes 



Modes 



(Fig- 3 for 



1965 and 1966 



year classes) 



Modes 

 (Fig. 4) 



size-fre(iuency histograms of all stations com- 

 bined (Fig. 3) illustrate the increasing modal 

 lengths between December and June for the 1965 

 and 1966 year classes. Recruitment of small 

 E. pacifica is also obvious during the spring of 

 1966 and 1967 and also shows a shift in modes 

 with time. The 1963 and 1964 year classes (not 

 shown here) showed similar trends. 



A modified histogram plot (Fig. 4) was used 

 to show the data for all 4 years and all 4 stations 

 together. The advantage of this method is that 

 one can follow the main modes of different sizes 

 throughout the 4-year period. A disadvantage 

 is that these plots are distorted by the arbitrary 

 constraints that (1) at least 50 individuals per 

 103 m3 of water within one size-group had to 

 be present for plotting and (2) concentrations 

 above 5000/103 m3 were plotted only as 5000/ 

 103 m3. All of the years represented in Figure 

 4 show some similarity. The main recruitment 

 pulses are in the fall and summer, and the max- 

 imum size attained is approximately 22-24 mm 

 length. After about 1 year, late in the second 

 summer or fall, these large individuals dis- 

 appeared from our collections. Interestingly, 

 many of the modes that were composed of small 

 euphausiids during the spring and early summer 

 disappeared or were undiscernible by the fall. 

 Either these individuals were subjected to high- 

 er mortality than the fall recruits or were trans- 

 ported out of the area. Apparently they made 

 no major contribution to the local adult popu- 

 lation. 



Average lengths of size modes were also 

 calculated for each collection using the com- 

 puter techniques described by Hasselblad 

 (1966). The means were generally close to 



the values for the modal lengths of various col- 

 lections shown in Figures 3 and 4 and, therefore, 

 are not illustrated here but are given in Table 1. 



Our estimates of the growth of E. pacifica 

 by all these methods are summarized in Table 1. 

 As expected, estimates are similar for the same 

 year classes. Growth varied from 1.6 to 2.9 mm 

 per month among year classes, averaging about 

 2.0 mm per month. Growth rates were fastest 

 for young stages. Year-classes 1963 and 1964 

 had slower average rates (1.6 and 2.0 mm/ 

 month) and were calculated over a longer period. 

 Year-classes 1966 and 1967, on the other hand, 

 were represented for the shortest periods of time 

 and had the fastest average rates (2.9 and 2.6 

 mm month) . This deceleration of growth at the 

 larger sizes is also apparent in Figure 3 where 

 the growth rate from January to March 1966 

 was about 3.2 mm/month, while from March 

 to June it was about 2.0 mm/month. 



Our estimates are biased in several ways. 

 They favored the recruitment pulses of the fall 

 because the smaller modes of young that ap- 

 peared earlier (June through September) did 

 not comprise a good series of modal sequences. 

 Moreover, the modes and means of the smaller 

 sizes of E. pacifica are probably slightly over- 

 estimated since catch curves (Fig. 2) indicate 

 escapement from our nets of individuals below 6 

 mm. This may cause an underestimation of 

 growth rates. 



DISCUSSION 



Generalized growth curves of E. pacifica for 

 three regions of the North Pacific are con- 

 trasted in Figure 5. On the basis of bimodal 

 size-frequency distributions of winter and 

 spring samples, Ponomai-eva (1963) concluded 

 that E. pacifica lives for a period of 2 years. 

 She found predominantly 8 and 14-15 mm indi- 

 viduals in the winter and 12-13 mm (her 1- 

 yearolds) and 19 mm (2-year olds) in the spring. 

 Off Oregon not only were 12-13 mm individuals 

 rare or absent in sirring samples, but also 13-14 

 mm individuals, the size that Ponomareva would 

 expect to find in the summer and fall, were ab- 

 sent. Moreover, our data, unlike Ponomareva's, 

 show no large seasonal fluctuations of growth 

 with retarded growth of the 13-14 mm sizes 



82 



