FACTORS AFFECTING THE DISTRIBUTION, ABUNDANCE, AND 

 SURVIVAL OF PANDALUS JORDANI (DECAPODA, PANDALIDAE) 



LARVAE OFF THE OREGON COAST 



Peter C. Rothlisberg' and Charles B. Miller : 



ABSTRACT 



Abundance, distribution, and survival of larval pink shrimp, Pandalus jordani, differed between 1971 and 

 1972. Consistent southwest winds in the February- March spawning season of 1972 kept surface flow onshore 

 and larvae closer to the coast than did the mixed winds of 1971. The early season of 1972 was warmer than 

 that of 197 1 , and development was faster: Zoea V were prevalent at the end of April 1972, compared with me- 

 dian advancement to Zoea HI by early May in 1 97 1 . Corresponding to the lesser dispersal and faster develop- 

 ment of 1972, survival was substantially better than in 1971. 



Overall larval survival at settlement time appears from analysis of long-term fishery data and upwelling in- 

 dices to have some dependence upon the strength of June to August upwelling. Extrapolation from 

 laboratory studies suggests that is because survival is enhanced by the temperatures consistently 12°C and 

 below maintained by strong upwelling. Timing of spawning and development interacts with timing of the flow 

 regime. Summer upwelling generally keeps the habitat suitably cold for optimal development and survival 

 and returns larvae to seaward for settlement roughly at the beds from which they were spawned. 



Hjort (1914, 1926) was the first to suggest the impor- 

 tance of larval mortality in establishing year class 

 strength of marine fish. This concept has been useful 

 generally, and, in particular, larval mortality most ful- 

 ly explains fluctuations of stocks in species both 

 short-lived and fecund. However, larval mortality per 

 se is only one component of total mortality. Factors 

 affecting parental stock size, fecundity, spawning 

 and hatching rates, larval dispersal, metamorphosis, 

 and postlarval and prerecruitment mortality will also 

 generate year-to-year variation in population size. In 

 many marine animals these life history phases are in 

 totally different habitats and have very different 

 durations. 



Given the complexity of the life cycle and the variety 

 of habitat features which can therefore act important- 

 ly, it is not surprising that sound explanations of year 

 class variations have begun to emerge only now. Long 

 time series of catch data and well-developed un- 

 derstanding of oceanographic processes are both re- 

 quired. Interesting recent examples include 

 Southward et al. (1975), Boudreault et al. (1977), 

 Driver (1978), and Dow (1978). Creation of indices of 

 coastal upwelling strength (Bakun 1973) hasgivenus 

 an important variable for study of factors influencing 



'Marine Science Center, Oregon State University, Newport, Oreg.; 

 present address: Division of Fisheries Research, CSIRO Marine 

 Laboratories, P.O. Box 120, Cleveland, Queensland 4163, 

 Australia. 



2 School of Oceanography, Oregon State University, Corvallis, 

 OR 97331. 



Manuscript accepted November 1982. 

 FISHERY BULLETIN: VOL. 81, NO. 3 



^5$ 



Aix 



year class strength in marine populations. Upwelling 

 acts in several ecologically significant ways, affecting 

 temperature, salinity, nutrient concentrations, and 

 current patterns. Enhancement of productivity in 

 nearshore regions is well documented (Steemann 

 Nielsen and Jensen 1957; Ryther and Menzel 1965; 

 Ryther 1969). However, not only enhancement of 

 productivity, but the specific changes in flow caused 

 by upwelling can have important effects. Success of 

 life stages is likely to be related to the time of onset of 

 seasonal upwelling, intensity and duration of upwell- 

 ing, and even to details of its intermittency. Effects 

 can be negative. Lasker (1978) found that upwelling 

 dispersed the food of larval northern anchovy, En- 

 graulis mordax, reducing food availability below 

 levels established in the laboratory (Lasker 1975) as 

 minimal. Coe (1956), Winnor (1966), Lough (1976), 

 and Nelson et al. ( 1 976) have all shown that upwelling 

 can both improve and reduce survival by transport of 

 larvae toward or away from favorable habitat. 



We report here an attempt to evaluate the impor- 

 tance of larval survival to the year class strength of 

 the pink shrimp, Pandalus jordani, and to determine 

 which aspects of ocean dynamics affect larval sur- 

 vival. First, larval distribution and abundance of P. 

 jordani were ascertained by field sampling. Second, 

 apparent survival in several spawning seasons was 

 compared with laboratory studies of the effects of 

 habitat factors (Rothlisberg 1979) and with the hy- 

 drographic regime during sampling to estimate the 

 degree to which environmental factors determined 



455 



1983. 



