FISHERY BULLETIN: VOL. 83, No. 4 



Table 2.— Number of days according to combinations of four species of dolphins that were encountered 



in the Central American Bight. 



'Northern (N) and southern (S) areas are partitioned to illustrate species differences by area; the dividing line is perpen- 

 dicular to the coast at lat. 16°N. 



tics is supported by physical environmental measure- 

 ments. The data indicated a species differential with 

 respect to waters with temperature >25°C and 

 salinities <34.5''/oo, and waters of <25°C and 

 >34.5%o. The former is primarily Tropical Surface 

 Water and the latter primarily Equatorial- 

 Subtropical Surface Water. Of 217 spotted and/or 

 spinner dolphin schools encountered during the 

 January-March cruises, and having associated 

 temperature and salinity measurements, 80.6% were 

 in this TVopical Surface Water, while only 19.4% were 

 in the Equatorial- Subtropical Surface Water. In com- 

 parison 53.7% of 229 similarly observed schools of 

 striped and/or common dolphins were in the Ti"opical 

 Surface Water and 46.3% were in the Equatorial- 

 Subtropical Water. The differential in percentages 

 by species pair reflects the more uniform "Inner 

 Tropical" Surface Water habitat of spotted and spin- 

 ner dolphins and the importance to striped and com- 

 mon dolphins of both the variable Tropical Water off 

 Central America and the variable waters along the 

 Equator, west of Peru, and southwest of Baja 

 California. 

 Because the school sighting data are represented 



by many combinations of temperature and salinity 

 from various areas, it is useful to summarize these 

 results in terms of an integrated measure of temper- 

 ature and salinity, i.e, sigma-^, a measure of water 

 density (see Sverdrup et al. 1942). Table 3 lists the 

 numbers and percentages of spotted and spinner and 

 of striped and common dolphins according to their 

 occurrence at different intervals of sigma-^. The 

 percentages of striped and common dolphin schools 

 were higher than that of spotted and spinner dolphin 

 schools for sigma-t ^22.5 (primarily Equatorial- 

 Subtropical Water). The opposite was true for sigma- 

 t <22.5 (primarily Tropical Water). The difference 

 in percentage distribution by species pair is signifi- 

 cant (P < 0.01, Kolmogorov-Smirnov test). 



Seasonal Features of Habitats 



It is not clear how dolphin populations are affected 

 by seasonal changes in the environment, although 

 the available data suggest what may happen. The far 

 offshore habitat of spotted and spinner dolphins, be- 

 tween long. 120°W and 140°W at about lat. 10°N, 

 is an important "porpoise-tuna" fishing area during 



Table 3. — Percent distribution of dolphin schools^ according to surface water density 



(sigma-f). 



'January-March, research cruises, 1976, 1977, 1979, 1980. 

 ^Includes mixed spinner + spotted dolphin schools. 



632 



