ABRAMSON and TOMLINSON: APPLICATION OF YIELD MODELS 



have been in equilibrium during the period 

 studied since the level of effort fluctuated from 

 year to year. 



The actual catch exceeded the estimated max- 

 imum equilibrium yield (2.46 million pounds) 



1962 



YEAR 



1968 69 1970 



during the period 1954 through 1969 only three 

 times (Table 1): 1962, 1968, and 1969. Effort 

 has always been substantially below the esti- 

 mated level which would produce the maximum 

 sustainable yield, A literal interpretation of 

 these results would indicate the population has 

 been underexploited until recently. 



It is a problem in actual management situa- 

 tions to deduce how well a model such as this 

 represents a population. In years when the ob- 

 served catch-per-unit effort deviates substan- 

 tially from the corresponding expected value, it 

 cannot be determined whether deviations are due 

 to an actual departure from the expected popu- 

 lation size or due to a temporary change in the 

 catchability. In the management strategy 

 which we will discuss later, we are assuming the 

 population size is being predicted correctly by 

 the model and we are essentially ignoring devi- 

 ations between the observed and expected catch 

 insofar as they may represent actual population 

 deviations. 



Figure 1. — Ocean shrimp catches predicted by GEN- 

 PROD and observed catches for the years 1954 through 

 1969. 



(/> 

 a 



z 



o 



Q. 



I 



3 



o 



I 



tu 



Q. 



X 



o 



2,000 4,000 6,000 8,000 



EFFORT— HOURS 



10,000 12,000 



Figure 2. — Fitted model (m = 2) for catch-per-hour 

 as a function of hours under equilibrium conditions. 



DYNAMIC POOL MODEL 



Catch data by age categories, both in weight 

 and numbers, were utilized to estimate mortal- 

 ity, growth, and recruitment parameters neces- 

 sary in a dynamic pool model. 



AGE-STRUCTURED CATCH DATA 



Catches from the population were landed at 

 Eureka and Crescent City, Calif., and Brookings, 

 Oreg. Landings data were obtained mainly 

 from the Pacific Marine Fisheries Commission 

 Data Series (1965-1969). Catches south of the 

 California-Oregon border were recorded in that 

 publication in tables for PMFC Area 96, but 

 those from north of the border were included 

 in, but did not comprise all of, the catch reported 

 from PMFC Area 88. Catches within PMFC 

 Area 88 south of the Rogue River were obtained 

 from the Oregon Fish Commission (Jack Rob- 

 inson, Oregon Fish Commission, personal com- 

 munication). Catches made in the more recent 

 years were obtained from the California Depart- 

 ment of Fish and Game Shellfish Program (Dan- 

 iel Gotshall and Walter Dahlstrom, California 



1025 



