MONTHLY AND ANNUAL VARIATION OF CATCH RATE SHOWN IN THE “SELF-NAVIGATION FISHING VESSEL TYPE” MOTHERSHIP TUNA FISHERIES

The author has studied on monthly variation and annual variation of catch rate which have been shown by member fishing vessels belonging to the so-called “self-navigation fishing vessel type” mothership tuna operating in the mid-Pacific Ocean and has obtained the follow-ing result : 1) There have been cases in which the catch rate of yellowfin tuna, albacore and strip-ed marlin, by mothership unit, had continued degradation as the 10-day periods of months had avanced from the start, but there have also been such other cases to approximately the same degree in which the catch rate had once advanced and then degradated, had once degradated and then advanced, had kept leveling or had continued degradation (Fig. 2). 2) In the year-round variation of average catch rate of various years, the rate of yellowfin tuna had been high twice in May-to-August and November-to-January without exception while the rate of albacore had advanced in May through July and degradated in August through January, so that we may here possibly conceive a periodical nature (Fig. 3). 3) In both cases of yellowfin tuna and albacore, the year-round variation of average catch rate of mothership-belonging fishing vessels have almost coincided with the overall year round variation of average catch rate of land-based fishing vessels of the same fishing area and the neighboring fishing areas (Fig. 4). 4) The catch rate of yellowfin tuna has shown an every-other-year variation in which it was high in even years and low in odd years while the catch rate of albacore advanced in 1957 through 1958, degradated from 1958 through 1960 and then advanced again from 1960 through 1961, and so far a tendency of gradual decrease of catch rate has not been perceived (Fig. 5) for both species. 5) The year-round variation of catch rate of mothership-belonging fishing vessels has almost coincided with the catch rate of land-based fishing vessels which made operations in the same fishing area and in the neighboring fishing areas (Fig. 5). 6) These results would show that the 10-day variation and annual variation of catch rate of mothership-belonging fishing vessels has been caused greatly by the seasonal variation and annual variation of the fish stocks existent in the fishing grounds.