Kramer: Growth and mortality rates of juvenile Paralichthys californicus 



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LagOOn (N mssion Bay 42,067, SE 8543; -/Va^ H edionda Lagoon 



14,432, SE 3312), but there was a second large peak 

 in Mission Bay for individuals at about 90 days (N 

 43,697, SE 7850) (Fig. 8). 



The class composed of transforming larvae (age 30 

 days) was the most abundant age class on the open 

 coast in 1988 (N 191,553, SE 17,339) (Fig. 8). Abun- 

 dance rapidly decreased with age on the open coast, 

 with essentially no halibut 70-180 days of age present 

 on the open coast (Fig. 8). The decline in abundance 

 of halibut on the open coast corresponded to an increase 

 in the bays (Fig. 8). 



Mortality 



Total age-specific abundance was determined by com- 

 bining data from the bay and open coast habitats (Fig. 

 9). In the survey area, the total loss of juvenile halibut 

 ages 30-115 days was estimated at 183,250 (95% CL 

 of 148,800 and 210,350) (Fig. 9). 



Instantaneous mortality rates (z (t) ) were calculated 

 by age-class using abundance-at-age, with age obtained 

 from the linear regression of In-transformed abundance 

 on In (age) (Table 3, Fig. 9), and the duration of each 

 age-class calculated from the age-at-size relationship 

 (Table 4) (Lo 1985). Instantaneous mortality rates 

 (z (t) ) were highest (0.044) for the youngest juveniles, 

 and decreased with increasing age but became constant 

 (x 0.0124, SD 0.001) for juveniles 70 days of age and 

 older (Table 4). 



I also calculated habitat-specific instantaneous mor- 

 tality rates (z (t) ) for juveniles < 70 days of age that 

 were taken only on the open coast, and for those 



88-115 days of age taken only in the bays (i.e., im- 

 migration completed (Table 5). 



The apparent mortality in bays was much higher than 

 that predicted from the combined bay and coast data, 

 ranging from 0.043 to 0.037 for the bay model and from 

 0.011 to 0.014 for the total mortality model (Tables 4, 

 5). The age-specific mortality of halibut from the bays 

 declined with increasing age, and was not constant as 

 predicted by the total mortality model (Tables 4, 5). 



To test for differences between the age-specific in- 

 stantaneous mortality rates (z (t) ) of the total popula- 

 tion, and of the open coast and bays, I used ANCOVA 

 on the age-specific mortality coefficient, Beta. Beta is 

 related to the instantaneous mortality rate (z (t) ) by the 

 equation: z (t) = Beta/t (Lo 1985). The Beta coefficients 

 for the total population (1.94, SE 0.22) and the open 

 coast juveniles (ages <70 days, Beta 3.58, SE 1.10) 

 were significantly different (P<0.01) (Table 6). The dif- 

 ference in the Beta coefficient between juvenile halibut 

 on the open coast and the total halibut abundance-at- 

 age is probably due to movement of halibut from the 

 coast to the bays. Nearly half of the decline in abun- 

 dance of juveniles along the coast could be caused by 

 their movement into the bays (1.94/3.58 = 0.54). The 

 Beta coefficients for the total population and the bay 

 juveniles (ages 94-115 days) also differed significant- 

 ly (P< 0.05), with a Beta of 0.69 (SE 0.77) for the total 

 population, and 2.96 (SE 0.65) for the juveniles from 

 the bays (Table 6). Mortality rates in the bays appear 

 to be underestimated by the abundance-at-age model 

 for the total halibut population. 



