Anderson: Age. growth, and mortality of Pandalus boreahs Kroyer 



545 



ing. Seasonal trawl sampling and larval surveys at 

 Chiniak Bay, Alaska, showed juvenile young-of-the- 

 year reaching an average size of 7 mm CL by Septem- 

 ber (~153 days after hatching) for three consecutive 

 years (E. Munk, Kodiak Lab., Alaska Fish. Sci. Cent., 

 pers. commun., 10 June 1987). Since no other modes 

 were found between this juvenile group and the larger- 

 age 1.4 mode in survey sampling, it was assumed the 

 age designation of these groups was correct. Skuladot- 

 tir (1981) found difficulty in assigning ages to enter- 

 ing size modes (average size 10-12 mm CL), probably 

 because the stretched mesh gradually increased from 

 32 to 38 mm over five years of sampling (Hallgrfmsson 

 and Skuladottir 1981). Frechette and LaBonte (1981) 

 also noted a similar problem with a mesh size of 38 mm. 

 The mesh size in this study was 32 mm with a 32 mm 

 codend liner, which may have helped in the retention 

 of young shrimp. 



Abundance and variability of year-classes 



The estimated biomass of P. borealis in Pavlof Bay 

 peaked in 1977 and declined substantially in later years 

 (Table 1). In response to reduced biomass, the fishery 

 was closed by the ADF&G in 1979; the population 

 subsequently stabilized at a lower level after 1978. The 

 decline in abundance of the 1971 year-class (Table 2) 

 reflected that of the total population (Table 1). The 

 average number of P. borealis caught per survey tow 

 declined steadily from approximately 199,000 in 1976 

 to 10,000 in 1981 (Table 3). 



Dominant year-classes persisting in the population 

 result from favorable conditions during their embry- 

 onic, larval, or juvenile stages. The relationship of 



shrimp landings to water temperatures prevailing dur- 

 ing the spawning period 2 years earlier has been 

 demonstrated in Gulf of Maine stocks (Dow 1966). Eggs 

 incubated at 3 and 6°C resulted in larger newly hatched 

 P. borealis larvae (1.41-1. 49mm CL) than those incu- 

 bated at 9°C (1.09mm CL) (Nunes 1984). If larger 

 newly hatched larvae have a survival advantage, then 

 colder incubation temperatures (6°C or lower) might 

 enhance year-class strength. Warm temperatures may 

 lead to lower survival of eggs and larvae of P. borealis 

 due to increased egg parasitism and reduced food con- 

 version efficiency (Paul and Nunes 1983). Minimum 

 seawater temperatures in the study area normally 

 range from 3 to 6°C (McLain et al 1979). Royer (1989) 

 examined temperatures across the Gulf of Alaska and 

 found very low-frequency fluctuations of ±2.0°C oc- 

 cur in the upper 250 m of the water column north of 

 55 °N in the Gulf of Alaska (including Pavlof Bay). A 

 cold temperature anomaly of more than 1.2°C was 

 reported for the 1971-78 period in this region. These 



