50 



Fishery Bulletin 89(1 



1991 



unprecedented in the history of the fishery. The in- 

 creased size of age-1 shrimp since 1979 has made them 

 more vulnerable to the gear and should have increased 

 the harvest rate on age-1 shrimp relative to 1966-78. 

 Fishing effort in the study area in 1987 and 1988 

 reached the two highest totals ever recorded, with 1989 

 likely to be as high or higher. The only other year in 

 which effort approached levels of 1987-88 was in 1980 

 (Fig. 12). Postulating some improvement in yessel and 

 gear efficiency in the years since 1980, the strong land- 

 ings of 1987-89 must have been partly a result of 

 record levels of effective fishing effort. In combination, 

 the strong landings, high effort, and dominance of age 

 1 shrimp in the catch for 1987 and 1988 argue strong- 

 ly for increased exploitation rates in those years. The 

 large landings in 1989 are probably caused by the same 

 factors. 



This raises the question of what impact the increas- 

 ing harvest of age-1 shrimp may have on the spawn- 

 ing population and subsequent recruitments. A 

 spawner-recruit relationship has not been demon- 

 strated for pink shrimp (Gotshall 1972). However, the 

 Pacific Fishery Management Council (1981) identified 

 some potential indicators of over-harvest of shrimp 

 stocks. These included increases in the percentage of 

 age-1 shrimp in the catch and in the percentage of 

 primary females. In the past, reductions in age-2 and 

 older shrimp were balanced by accelerated sex change 

 in age-1 shrimp, and possibly by increased fecundity 

 at age due to density-dependent growth. Levels of 

 primary females have reached nearly 50% in some 

 years. In such years, pink shrimp are virtually a single- 

 age spawning stock. At some level of exploitation, ac- 

 celerated sex change and density-dependent growth 

 will not prevent declines in larval release and subse- 

 quent recruitment. The strong year-classes passing 

 through the fishery since 1986 indicate that we've prob- 

 ably not reached that level of exploitation as yet. 



Acknowledgments 



This project was financed in part with Federal Inter- 

 jurisdictional Fisheries Act funds through the U.S. Na- 

 tional Marine Fisheries Service. Numerous individuals 

 provided assistance in the completion of this project. 

 P. Collier of the California Department of Fish and 

 Game and M. Gross of the Washington Department of 

 Fisheries provided unpublished data used in our anal- 

 ysis. J. Robinson, J. Golden, R. Starr, and M. Saelens 

 provided historical background information on the 

 shrimp fishery. The draft manuscript was reviewed by 

 R. Starr and also by D. Hankin of Humboldt State 

 University. Assistance on statistical questions was pro- 

 vided by P. Lawson. 



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