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tion. The differences are most clearly seen in the older 
ages late in the simulated time span (Tables 1-3). 
The January catches were corrected to lower values 
than those of the observed catch, but the values for 
December corrected catches were higher than those 
for observed catches in order to reflect more accu- 
rately these changes in the fishing mortality rate 
caused by the timing of the catch. 
In each scenario the recorrected catch matrix gave 
the lowest bias in estimated population size, whereas 
the observed catch resulted in the highest bias in 
two of the three scenarios (Table 4). The bias was 
largest for the observed catches that occurred in 
January and December. Use of observed January 
catches in the VPA overestimated the true popula- 
tion size, and use of observed December catches un- 
