Abstract.- The trawl fishery for 

 ocean pink shrimp (Pandalus jordani 

 Rathbun) has increased dramatical- 

 ly since the early 1970s. Catch and 

 effort statistics and catch sampling 

 data from 1968-88 were analyzed to 

 evaluate changes in the shrimp popu- 

 lation structure. Carapace length at 

 age one and two have increased sig- 

 nificantly since 1978, concurrent 

 with a reduction in fishery catch per 

 unit effort, strongly indicating den- 

 sity-dependent growth. The seasonal 

 pattern of growth provides further 

 evidence for density-dependent 

 growth. The number of age three 

 shrimp in the catch has declined 

 markedly since 1978, while age one 

 shrimp have increased from 30.6% of 

 the catch to 69.2%. The percentage 

 of age one shrimp maturing as fe- 

 males has increased to 30-50% in 

 some years, while the overall per- 

 centage of males shows no trend. 

 The changes in growth, and age and 

 sex composition of the catch are at- 

 tributed in part to the impact of the 

 trawl fishery, which is currently con- 

 tinuing to intensify. Density-depen- 

 dent growth, and the ability to accel- 

 erate the sex change process make 

 pink shrimp resistant to over-har- 

 vest. However, at some exploitation 

 level the reduction of the age 1 

 spawning stock should begin to re- 

 duce subsequent recruitment. Re- 

 cent strong year classes indicate that 

 the fishery probably has not reached 

 that level of exploitation. 



Fishery-induced Changes 



\n the Population Structure 



of Pink Shrimp Pandalus jordani 



Robert W. Hannah 



Stephen A. Jones 



Oregon Department of Fish and Wildlife, Marine Region 

 Marine Science Drive, Bldg. 3, Newport, Oregon 97365 



Manuscript accepted 24 August 1990. 

 Fishery Bulletin, U.S. 89:41-51 (1991). 



The Pacific trawl fishery for pink 

 shrimp Pandalus jordani Rathbun 

 has developed from a fishery with 

 landings of around 220 mt in the early 

 1960s to a fishery regularly landing 

 in excess of 18,000 mt. In six of the 

 thirteen years since 1975, combined 

 landings for the states of California, 

 Washington, and Oregon have ex- 

 ceeded 24,000 mt. Pink shrimp range 

 from San Diego, California to Un- 

 alaska, Alaska (Butler 1964); how- 

 ever, the majority of the catch is 

 taken between Cape Mendocino, 

 California and Destruction Island, 

 Washington. The development of the 

 fishery has been well summarized by 

 Dahlstrom (1970), Fox (1972), Zirges 

 and Robinson (1980), and others. 



Saelens and Zirges (1985) de- 

 scribed the 1984 fishery for pink 

 shrimp and suggested that there was 

 some evidence of changes in the 

 shrimp population structure that 

 were possibly the result of fishing. 

 They noted improved growth and 

 higher levels of age-1 shrimp in the 

 catch, relative to earlier years. Fish- 

 ing effort and shrimp catch have con- 

 tinued to increase since 1984. Given 

 the continued development of this 

 fishery, we felt that some fishery- 

 induced changes in the population 

 structure would be evident in a tho- 

 rough review of the fishery sampling 

 data. We examined 23 years of infor- 

 mation from the pink shrimp fishery 

 off the coasts of Oregon and northern 

 California to search for the classic 

 population responses to increased 

 fishing. Specifically, we looked for 



persistent shifts in age and sex com- 

 position, changes in the age of female 

 maturity, evidence of reduced stock 

 biomass, and improved shrimp 

 growth as a response to lower bio- 

 mass. As a final step, we attempted 

 to relate the observed changes to the 

 development of the fishery and alter- 

 natively to environmental factors. 



Methods 



We examined monthly sample data 

 from the landed catch of pink shrimp 

 for the years 1966-88. The data is 

 comprised of several samples from 

 each statistical area (Fig. 1) and 

 month of the fishing season. The 

 season currently runs from April 

 through October, but has been longer 

 in the past. Individual samples for 

 each area and month (area-month) 

 were combined for analysis of age 

 and sex composition. Sample sum- 

 maries provide individual carapace 

 lengths and average weight ex- 

 pressed as the number of whole 

 shrimp per pound. Shrimp are classi- 

 fied as male, female, or transitional 

 based upon close examination of the 

 inner ramus of the first pleopod 

 (Tegelberg and Smith 1957). Shrimp 

 age is determined by modes in the 

 combined length-frequency histogram 

 (Zirges et al. 1981). Nadirs in the 

 histograms define the range of cara- 

 pace lengths corresponding to each 

 age group, then ages are assigned to 

 individual shrimp. 



Zirges et al. (1982) concluded that 

 pink shrimp from statistical areas 



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