two fishing grounds, a noi-thern fisliing gi'ound 

 and an equatorial ground, and thus the latitu- 

 dinal first moment of effort gives a better 

 representation of the balance between the two 

 grounds, rather than the central tendency of 

 either. 



Periodicities and Trends 



Periodicities and trends in spatial statistics 

 can be evaluated from time series plots of 

 these statistics. Figures 8, 9, and 10 show 

 spatial statistics for each month and year for 

 effort, catch, and CPUE. They also show the 

 values for the peak months. The figures illus- 

 trate the presence of annual periodicities as 



well as longer term trends. The annual peri- 

 odicities are summarized in table 3, which 

 gives the frequency by months of maximum and 

 minimum values of each spatial statistic for 

 effort, catch, and CPUE. 



The long-term trends of the spatial statistics 

 are indexed by the February values. Use of 

 February values is justified by the relatively 

 high within-year correlation among the points 

 for each statistic in any year. The trends are 

 interpreted from the slopes of least-square 

 regression lines fitted to the data (figs. 11-13). 

 A discussion of the annual periodicities and 

 longer term trends for effort, catch, and CPUE 

 follows. 



Figure 8. --Spatial statistics for effort ig'^g-GI on the North Pacific albacore 

 longline grounds. The heavy line represents the peak fishing period from 

 December to March. 



10 



