ROTHLISBERG and MILLER: LARVAL PANDALl S JORDAN! OFF OREGON 



100.0 



> 100 



> 



a: 



Z) 



to 



u 



9f: 10 



CL 



01 



i \ 



■•1971 

 ■0 1972 



VI VII VIII IX X 



LARVAL STAGE 



■+■ 



-t- 



XI XII XIII JUV 



FIGURE 12.— Larval survival (percent) based on total numbers at 

 each stage caught in the plankton sampling along NH line. 



tively. Compared with the long-term variation in the 

 commercial estimate of egg number, 1971 and 1972 

 were both close to average (mean 1961-80 = 1.93). In 

 1971, only 26.9% survived to Zoea JJ and <1% sur- 

 vived past Zoea VII. Only two Stage I juveniles 

 (Rothlisberg 1980) were found in 1971, representing 

 0.17% of the hatch. In 1972 over 64% survived 



through Zoea III, with numbers and percentages de- 

 creasing gradually with age. Five Stage I juveniles 

 were caught, 1.06% of the hatch. Percent surviving to 

 the juvenile phase in 1 97 2 was an order of magnitude 

 higher than in 1971. This is not, of course, simply a 

 comparison of two versus five survivors. There was a 

 consistent difference between the years throughout 

 the developmental sequence. Some survival es- 

 timates from the decreasing abundance at successive 

 stages are shown at the bottom of Table 2. Such es- 

 timates depend on the assumption that reproduction 

 is relatively synchronous (Mullin and Brooks 1970; 

 Fager 1973). In both years the main pulse of hatching 

 of P. jordani occurred within a relatively limited 

 period, essentially synchronously. 



Some of the variability of percent survival at in- 

 dividual stages (Fig. 12) can be due to the relation- 

 ship between sampling frequency and molting 

 frequency (mean intercruise period was 14.5 d, while 

 mean intermolt period is 6.8 d, a ratio of 2.1:1). 

 Stages can effectively be passed by the bulk of the 

 population between two samplings. A simulation 

 model of survival through stages of development and 

 time was developed to illustrate this effect. The mod- 

 el imposed an intermolt period of 6 d over the entire 



TABLE 2.— Estimates of larval survival (<?"") from fishery data supplied by Fish Commis- 

 sion of Oregon (J. Robinson, see text footnote 5) and from field estimates of larval abun- 

 dance i = In (N t /N )/t where t = 1.5 yr, the interval from hatching to recruitment to the 

 commercial gear, (: uv = calculation for first year instantaneous mortality after recruit- 

 ment to the bottom, from stomach analysis of hake, Merluccius productus, by Gotshall 

 (1969, 1972); i^d = instantaneous larval mortality as a proportion of the total instan- 

 taneous mortality (i) with instantaneous juvenile mortality ((: uv ) held constant, see text. t= 

 1.274 yr (100 d), estimated time of larval period in plankton; t = 0.364, period between 

 calculations of N and JV, in 1971; f = 0.263, period between calculation of N and TV, 

 in 1972. 



467 



