FISHERY BULLETIN: VOL. 70, NO. 3 



Department of Fish and Game, personal com- 

 munication). Virtually all catches were made 

 during single day trips. 



Landings were stratified into port-months, 

 with Eureka-Crescent City as "California" and 

 Brookings as "Oregon." Relative age frequency 

 and weight at age were determined from samples 

 of most port-month catches. Values used for 

 California strata not sampled were either the 

 average of preceding and following strata or the 

 nearest sampled strata of the same season. The 

 Oregon Fish Commission provided values for all 

 Oregon strata. 



Several methods of drawing samples from 

 within strata were used by California. For all 

 but very recent years, the methods were equiva- 

 lent to assuming a simple random sample of 

 shrimp from within strata. These sampled 

 shrimp were aged by carapace length measure- 

 ments, and the fraction falling into a specific age 

 group determined its relative frequency. In re- 

 cent years a simple random sample of boatloads 

 was assumed drawn, and the length composition 

 of a subsample from each boatload was weighted 

 by the estimated number of shrimp in the load. 

 Estimates by strata, done separately for Oregon 

 and California, were combined to obtain the val- 

 ues in Table 2. 



The average weight at age was determined 

 by two methods: (1) the aged shrimp were 

 placed into length frequency groups, a length- 

 weight key was used to convert length to weight, 

 and average weight for each age group was 

 calculated; (2) the aged shrimp were weighed 

 and an average weight computed directly for 

 each age group. The study of aged catch data 

 was performed for the 1955 through 1968 sea- 

 sons. All aged shrimp fell into age groups 0, 

 I, II, or III, but the group was rare and omitted 

 from the study. 



Catch by age category for 2,598 million shrimp 

 (22.88 million pounds) harvested during 1955 

 through 1968 are listed by month in Table 2. 

 During the first 7 of these years, the fishery was 

 active during 39 months and captured an esti- 

 mated 954 million shrimp, excluding age 0, yield- 

 ing a monthly average of 24.5 million. These 

 shrimp weighed about 8.25 million pounds, av- 

 eraging 212,000 lb. per month of fishing and 



0.0086 lb. per shrimp. The fishery was active 

 during 46 months of the second 7 years and 

 caught an estimated 1,644 million shrimp, ex- 

 cluding age 0, for a monthly average of 35.7 mil- 

 lion. These weighed about 14.63 million pounds, 

 averaging 318,000 lb. per month and 0.0089 lb. 

 per shrimp. The relative frequencies in num- 

 bers during the first 7 years were: 0.559 for 

 age I, 0.422 for age II, and 0.019 for age III. 

 During the second 7 years the frequencies were 

 0.495 for age I, 0.463 for age II, and 0.042 for 

 age III, The reliability of the age frequency val- 

 ues is uncertain due to the aging method. 



GROWTH CURVE 



A growth in weight curve was obtained em- 

 pirically by plotting average weights of shrimp 

 by month and age for all seasons 1955 through 

 1968 (Tables 2 and 3, Figure 3). Dahlstrom 

 (1970) and Gotshall (California Department of 



025 



CO 

 Q 



z 

 o 



0. 



X 



o 010 



I I I I I I I I I I I 



I I I I I I I I I I I I I I 



00025 



- -kIk! 



/ 



.;.rf^ 



Seasons 1955— 1959 



Seasons 1960-1968 



-Empirical Growth Curve 



I I I I I I I I I 



I I I I I I I I 



I I I I I I I I 



X < 2 



o z 



_L_L 



AGE I 



AGE H 



AGE M 



Figure 3. — Ocean shrimp growth in weight by month 

 from sampling commercial landings. Seasons included 

 are 1955 through 1968. 



1026 



