FISHERY BULLETIN: VOL. 83, NO. 4 



170° 

 30° 



Hawaiian Islands 



Christmas Island * 



o ^o-^o' 



10' - 



Marqussas Islands 



Figure 5.— Distribution of striped and common dolphin schools seen during the January-March research cruises. Larger circles indicate 



schools >300 animals. 



distance searched during 1 d). Data for days during 

 which the Beaufort wind force was equal to 4 (11-16 

 kn) for more than 50% of the time, or averaged more 

 than 4, were not used. Only schools sighted at 

 distances not more than 3 perpendicular nmi from 

 the ship's track and not more than 5 radial nmi dis- 

 tant were considered, to reduce the effects due to 

 distance on sightability. The latter criterion was not 

 applied to the 1976 cruises, because the radial 

 distances then were frequently overestimated. An 

 areal smoothing procedure that consisted of 

 calculating the average number of schools per 100 

 mi searched within sequential, overlapping 5° 

 squares was employed. Sequential squares were off- 

 set 2.5° in latitude and longitude, so that each was 

 wholly overlapped by 25% of the area of each of four 

 adjacent squares. A day's sighting rate was assign- 

 ed to a particular square if more than 50% of the 

 search effort occurred therein, and the value was 

 plotted at the center point of that square If the 

 search effort fell equally in two squares, the day's 

 results were assigned to both squares. These moving, 

 areal means of sightings per 100 nmi searched were 

 plotted and contoured. Contouring (and interpola- 



tion) constituted a second level of areal smoothing. 



The maps so generated describe the distribution 

 of relative abundance of species schools as surveyed 

 during January to March. These relative abundance 

 data were combined for spotted and spinner dolphins 

 and for striped and common dolphins, both because 

 these species pairs had similar distributions and 

 because pooling gave desirable sample sizes. 



The combined spotted and spinner dolphin map 

 (Fig. 6) shows some patterns already noted from the 

 school distribution. These schools appeared relatively 

 more abundant off southern Mexico (mostly spotted 

 and eastern spinner dolphins) and again along an 

 east-west band just north of the Equator, especially 

 west of long. 105°W. Another band of greater abun- 

 dance occurred south of the Equator. Spotted and 

 spinner dolphins appeared less abundant just west 

 of Costa Rica, off the coast of northern South 

 America, and along the Equator. A weak lobe of 

 higher relative abundance extended west of long. 

 120°W broadly about lat. 10°N. 



Striped and common dolphins show a relative 

 abundance pattern in which areas of higher density 

 tend to be complementary to that of spotted and 



628 



