Shimada and Kimura: Seasonal movements of Gadus macrocephalus 



803 



s 



=> o 

 z o 



40 60 80 



SIZE AT RELEASE (CM) 



100 



120 



a. asm 



20 



100 



120 



SIZE AT RECOVER i 



Figure 2 



Length-frequency histograms of Pacific cod at time of release and recovery, 

 1982-92. 



5) and aggregated by recovery season (winter= Jan- 

 Mar, spring=Apr-Jun, summer=Jul-Sept, and 

 fall=Oct-Dec; this definition of seasons is used 

 throughout the paper). For these plots each 2° x 5° 

 block is associated with just one arrow. 



The modeling of the tagged population was per- 

 formed similarly to that done by Hilborn ( 1990) and 

 by Heifetz and Fujioka ( 1991 ). However, rather than 

 describing movement through time as a Markov pro- 

 cess, we only estimated seasonal distribution, a much 

 easier task. Our model assumed that the quarterly 

 natural mortality rate (M) was the same in all sea- 

 sons and areas, and that the instantaneous fishing 

 mortality rate (F) varied by season but not by area. 

 Years were assumed to be homogeneous (i.e. we as- 

 sumed no year effects). Modeling the tagged popula- 

 tion allowed the estimation of seasonal distribution 

 by taking into account the actual time and area of 

 tagging and recovery. 



The model assumed that time (i) is divided into 

 seasons (four per year over all years, so that the sub- 

 script i runs from 1 to 4 x nyr, where nyr=10, the 

 number of years being modeled) and that there are 

 three areas (j) being modeled. 



Table 1 



Tabulations of tag releases and tag recoveries (top) 

 by areas described in Figure 3 (Area l=the eastern 

 Bering Sea shelf, Area 2=the outer eastern Bering 

 Sea shelf and upper slope. Area 3= the main spawn- 

 ing area) for Pacific cod, Gadus macrocephalus, in 

 the eastern Bering Sea. Percentage release and re- 

 coveries by fishing gear (bottom I. 



Release 



Recovery 



Area 



Number 



Percent 



Number Percent 



1 

 2 

 3 

 Other 



Total 



4,967 

 1,303 

 3,048 

 3,078 



12,396 



Release 



40.1 

 10.5 

 24.6 

 24.8 



91 

 107 

 158 



17 



373 



24.4 



28.7 



42.4 



4.5 



Recovery 



Gear 



Percent 



Gear 



Percent 



Trawl 



Hook and line 



Pot 



Unknown 



97.2 

 0.6 

 2.2 

 0.0 



trawl 

 longline 

 pot 

 unknown 



62 



24 



2 



12 



