482 



Abstract.— Using improved indices of 

 age-1 recruitment and parent stock. I 

 investigated the existence of a stock- 

 recruitment relationship for ocean 

 shrimp, Pandalusjordani. I also exam- 

 ined the effects of large catches of egg- 

 bearing shrimp in April on subsequent 

 recruitment. After known environmen- 

 tal influences were accounted for, re- 

 gression analysis revealed evidence of 

 a quasilinear relationship between re- 

 cruitment and either spawning stock or 

 egg production. The stock-recruitment 

 relationship showed that average re- 

 cruitment increases if higher spawner 

 abundance is maintained in years of 

 low abundance. The recruitment-stock 

 relationship, however, showed that low 

 abundance of spawning fish was mostly 

 a result of environmentally driven re- 

 cruitment failures, rather than just 

 fishery impacts. Fishing did contribute 

 to the low numbers of spawning fish in 

 1993, and catches of egg-bearing 

 shrimp in April 1989 may have also de- 

 pressed the 1990 year class. The strong 

 recruitment-stock relationship for 

 ocean shrimp suggests that an increase 

 in escapement of all ages of shrimp in 

 response to a year-class failure may do 

 more to bolster recruitment than tra- 

 ditional strategies aimed at px-otecting 

 age-1 shrimp. 



A new method for indexing spawning 

 stock and recruitment in ocean shrimp, 

 Pandalus jordani, and preliminary evidence 

 for a stock-recruitment relationship 



Robert W. Hannah 



Oregon Department of Fish and Wildlife 



Marine Program 



2040 S E. Marine Science Drive 



Newport, Oregon 97365 



E-mail address bobhannatimhrnscorstedu 



Manuscript accepted llAugust 1998. 

 Fish. Bull. 97:482-494 (1999). 



Recruitment of the ocean shrimp, 

 Pandalus jordani , to the fishery in 

 waters off Oregon has been shown 

 to be influenced strongly by ocean 

 conditions during early larval 

 stages (Hannah, 1993). Specifically, 

 sea level height (SLH) during the 

 month of April, shortly after the 

 bulk of larval release, is strongly 

 negatively correlated with recruit- 

 ment of age-1 shrimp to the fishery 

 the following year ( Hannah, 1993 ). 

 Although the mechanism linking 

 recruitment to April SLH remains 

 unclear, April SLH probably inte- 

 grates the strength and timing of 

 the spring transition in coastal cur- 

 rents (Huyer et al., 1979). Ocean 

 shrimp larvae inhabit the near- 

 surface waters shortly after release, 

 occupying progressively deeper 

 strata as they develop (Rothlisberg, 

 1975; Rothlisberg and Miller, 1983). 

 The nature of the spring transition 

 probably influences the alongshore 

 transport of early larvae, as well as 

 the nearshore temperature regime, 

 either of which could strongly influ- 

 ence shrimp larval survival (Rothlis- 

 berg, 1975; Rothlisberg and Miller, 

 1983; Hannah, 1993; McConnaughey 

 etal., 1994). 



Attempts to relate ocean shrimp 

 recruitment to the size of the par- 

 ent spawning stock, even allowing 

 for the influence of April SLH, have 

 been unsuccessful (Abramson and 



Tomlinson, 1972; Gotshall, 1972; 

 Geibel and Heimann, 1976; Hannah, 

 1993 ). Efforts to date have, to a large 

 degree, relied on assumptions of 

 constant catchability and natural 

 mortality. Recently, both natural 

 mortality rates and the catchability 

 coefficient have been shown to be 

 quite variable for ocean shrimp 

 (Hannah, 1995). I have suggested 

 previously that variation in catch- 

 ability and natural mortality may 

 have created large enough errors in 

 prior indices of shrimp stock and 

 recruitment as to obscure an under- 

 lying relationship between these 

 two variables (Hannah, 1995). In 

 the present study, new indices of 

 ocean shrimp recruitment and 

 spawning stock were developed 

 which do not rely, or rely only very 

 minimally, on assumptions of con- 

 stant catchability and natural mor- 

 tality. The main objective of this 

 study was to re-examine the rela- 

 tionship between shrimp recruit- 

 ment and both shrimp spawning 

 stock and April SLH in the year of 

 larval release, using these improved 

 indices. 



Another problem with earlier 

 studies of ocean shrimp stock size 

 and recruitment is that they have 

 relied on measures of parent stock 

 or spawning biomass, rather than 

 on more direct measures of repro- 

 ductive output, such as estimates of 



