EDWARDS and KLEIBER: NONRANDOMNESS ON LINE TRANSECT ESTIMATES OF DOLPHINS 



mental topography, representing a 300 mile 

 gi'adual gradient from "best" to "worst" condi- 

 tions (Fig. 2a). In the complex, steep environ- 

 ment the precipitous slopes generate a distance 

 of only about 75 miles between ma.ximum and 

 minimum values for environmental quality, the 

 slopes being separated by a "desert" of unfavor- 

 able habitat about 150 miles wide (Fig. 2b). 



These two topographies were chosen to 

 bracket a range of reasonable possibilities for 

 patterns in environmental characteristics that 

 may cause nonrandom clustering of dolphin 

 schools. The factors of peak gi-adient and peak 

 spacing (number of peaks) are confounded here 

 because we tested only the two topogi'aphies, 

 simple:gentle and complex:steep. Gentle gi-adi- 

 ents are confounded with few peaks; steep gi'adi- 

 ents are confounded with many peaks. We did 

 not test the other two possibilities (simple:steep 

 and complex:gentle) because these are both 

 intermediate topogi'aphies that would have gen- 

 erated intermediate results. In the interest of 

 simplicity, we restrict this simulation study to 

 the two extreme cases. 



The rate at which the topogi'aphy moved (1 

 knot) was chosen to simulate movement of major 

 habitat features affecting dolphin school move- 

 ments. Because direct identification and 

 measurements of such features have yet to be 



made, the choice of rate was based on reported 

 speeds of major ocean currents in the eastern 

 tropical Pacific and apparent seasonal move- 

 ments of major concentrations of dolphin 

 schools. Reported current speeds include 0.1 to 

 0.3 knots for the core of the Pacific North Equa- 

 torial Current bordering the fishery area on the 

 north (Seckel 1975), 1.2 to 2.4 knots for the equa- 

 torial undercurrent underlying the fishery area 

 (Wyrtki 1966), and 1.2 to 2.4 knots for maximum 

 speed of the Equatorial Countercurrent surface 

 waters encompassing a majority of the fishing 

 area (Wyrtki 1966). School sightings data from 

 research ships indicate that major concentra- 

 tions of dolphin schools may move seasonally 

 between distant areas at approximately 0.3 

 knots (200 nmi/mo)^^ 



Our choice of 1 knot was based on the assump- 

 tion that the mechanism(s) responsible for ag- 

 gregating dolphin schools are most probably 

 related to distributions of prey and water mass 

 signatures indicating presence or absence of the 

 prey. Dolphins in the ETP consume small (10-50 

 cm) fish and squid (Perrin et al. 1973). This 

 mobile prey base will in turn be responding to 



■■^S. B. Reillv, Southwest Fisheries Center, National Mar- 

 ine Fisheries Service, NOAA, P.O. Box 271. La Jolla, CA 

 92038, pers. commun. December 1987. 



COMPLEX TOPOGRAPHY 



o 



^ ^" 



■V 



^ O 



topography (a) and the complex topography fb). 



863 



