CHENG and SHULENBERGER: DISTRIBUTION AND ABUNDANCE OF HALOBATES 



behavior (Cheng 1973c; Cheng and Enright 1973). 

 Actual maximum densities are probably consider- 

 ably higher. 



Cooccurrence 



The incidence of cooccurrence of two or more 

 Halobates spp. in our samples was low. Out of 498 

 positive samples, only 33 contained two species 

 (Table 4); 1 sample had three species. This con- 

 firms the impression given by distributional maps 

 (Figures 2-7) that the various species seldom occur 

 together. Separation of distribution holds, even for 

 species with overlapping ranges (H. micans andH. 

 sobrinus). In those samples in which these two 

 species cooccur, there is no correlation of abun- 

 dance (Figure 9). This indicates that local varia- 

 tions in abundance of these two species are not 

 merely responses to vectorial ( i.e., physical) forces. 

 If vectorial forcing were the case, then the two 

 species should be abundant in the same samples 

 and be positively correlated when they cooccur. 



The mean number of individuals per positive 

 tow (pooled for all cruises and by cruise) is much 

 higher in//, sobrinus than in//, micans, although 

 the percentage of positive tows is similar for both 

 species (Table 5). However, the highest frequency 

 of capture (i.e., percentage of positive tows) for//. 

 sobrinus occurred in the same series as the lowest 

 frequency for H. micans (Figure 10). This may 

 represent some temporal partitioning of re- 

 sources, but is undoubtedly partly a function of the 

 species' nonoverlapping centers of high abundance 



Table 4. — Cooccurrences of Halobates spp. in 498 positive 

 EASTROPAC samples (at least one specimen of each species per 

 sample). 



40 r- 



Species 



H. micans H. sobrinus H. sericeus H. splendens 



H. micans 

 H. sobrinus 

 H. senceus 

 H. splendens 



28 



<0 



10 I* 







• • 





» • 



1 



J 



10 20 30 



No. /-/. mi cons 



Figure 9. — Abundance ofHalobates micans andH. sobrinus in 

 samples in which the species cooccurred. Abundance of the two 

 species is not correlated (Olmstead and Turkeys' comer test for 

 associativity, P>0. 05). 



(Figure 5), differences in cruise tracks, and differ- 

 ences in times of the year (Figure 1). 



Temperature Effects 



Temperature of surface waters appears to be 

 important in the distribution of Halobates spp. 

 Figure 11 shows the number of individuals per 

 positive sample plotted against surface water 

 temperature for both March and August 1967. 

 Abundance was very low in samples taken at <24° 

 C and >28° C. The optimal temperature band ap- 

 pears to be quite narrow. The shape of these curves 

 is not an artifact of the number of samples ob- 

 tained at each temperature: the data have been 

 standardized to a per tow basis and there were 

 many tows taken at each temperature. 



Table 5. — Occurrence ofHalobates micans andH. sobrinus by EASTROPAC cruise series. 



589 



