Abstract.- To estimate mortality 

 and abundance of walleye pollock 

 Theragra chalcogranuna larvae in 

 Shelikof Strait, Alaska, during spring 

 1981, a diffusion-advection model, 

 combined with growth and death of 

 fish larvae, was applied. Physical 

 parameters (diffusion coefficients 

 and advection rates) were derived 

 from the distributional variances and 

 centroids of fish larvae collected in 

 ichthyoplankton surveys. The diffu- 

 sion coefficient and the advection 

 rate in the along-strait direction were 

 65.2 km-/day and 2.7-4.2 km/day, 

 respectively, which compared favor- 

 ably with values obtained from 

 moored current-meter data. Simula- 

 tion revealed that the expected dis- 

 tribution of larvae was similar to that 

 observed from ichthyoplankton sam- 

 plings, and that around 20% of the 

 larvae drifted out of the survey area 

 in Shelikof Strait within the 1 -month 

 sampling period. The larval fraction 

 dispersed out of the survey area was 

 used to revise larval mortality and 

 abundance estimates. Revised mor- 

 tality (0.070/day) was close to that 

 (0.063/day) determined from examin- 

 ing larval patches. The simulation in 

 this paper resulted in an increase by 

 a factor of 1.5 in the estimated total 

 larval abundance compared with ear- 

 lier estimates and field observations. 



Oceanic Dispersion of Larval Fish 

 and Its Implication for Mortality 

 Estimates: Case Study of 

 Walleye Pollock Larvae in 

 Shelikof Strait, Alaska 



Suam Kim 



Alaska Fisheries Science Center. National Marine Fisheries Service, NOAA 

 7600 Sand Point Way NE, Seattle, Washington 981 15-0070 

 Present address: Korea Ocean Research and Development Institute 

 Polar Research Laboratory. An San, P O Box 29, Seoul 425-600, KOREA 



Bohyun Bang 



Division of Physical Oceanography and Environmental Engineering 

 Virginia Institute of Marine Science, College of William and Mary 

 Gloucester Point, Virginia 23062 



Manuscript accepted 24 January 1990. 

 Fishery Bulletin, U.S. 88:303-311. 



Most marine fish have a period of 

 planktonic existence during egg and 

 larval stages. Since early in this cen- 

 tury it has been believed that survival 

 during early life determines year- 

 class strength and recruitment vari- 

 ability to fisheries (Hjort 1914). Fur- 

 thermore, recruitment processes are 

 quite complex because the biological 

 and environmental factors which act 

 on eggs and lar-vae are closely related 

 (Wooster et al. 1983). Hence, the 

 relationship between organisms and 

 their environment is critical for un- 

 derstanding recruitment variability. 

 The observed patterns of egg and lar- 

 val distributions can be considered to 

 be the result of a combination of fun- 

 damental processes, including spawn- 

 ing time and location, advection, dif- 

 fusion, growth and mortality. These 

 parameters could be identified using 

 biological data. 



The early-life stages of walleye pol- 

 lock Theragra chalcogramma whose 

 biomass is the largest of a single spe- 

 cies in world fisheries (Sharp 1987), 

 have been the objects of considerable 

 research in recent years. In Shelikof 

 Strait (Fig. 1), about 90% of the eggs 

 were produced between 25 March 

 and 15 April 1981, and they have 



approximately a 2-week embryonic 

 period at 5°c"(Kim 1989). Spawning 

 produces a patch of planktonic eggs 

 and larvae that can be followed as 

 they develop and are advected in pre- 

 vailing currents toward the south- 

 west. In Shelikof Strait walleye pol- 

 lock eggs exist at depths below 150 

 m due to their high specific gravity. 

 Their transport rate from the spawn- 

 ing area is very small because of 

 weak circulation in deep water (Ken- 

 dall and Kim 1989). 



Eggs of late developmental stage 

 would move upward fast due to the 

 decreased specific gravity of old 

 eggs, and the eggs hatch mid-depth 

 in the water column (Kim 1987). Also 

 the specific gravity of newly hatched 

 larvae is continuously decreasing, so 

 that most larvae are found within the 

 upper 60 m of the surface (Kendall et 

 al. 1987). Kim and Kendall (1989) de- 

 scribed the distribution and transport 

 pattern of larvae in Shelikof Strait 

 during spring. Young larvae occupy 

 a relatively small area and form a 

 dense patch, whOe older lai'vae spread 

 over a broader area in Shelikof Strait, 

 showing the importance of diffusion 

 on the larval patch. These larval 

 patches have been identified for at 



303 



