Forney et al.: Abundance of cetaceans in California waters: aerial surveys 



17 



Table 1 



Survey effort (in km) stratified by sea state and percent 

 cloud cover. 



Beaufort sea state 



Cloud cover and 1 2 



Total 



1991 



0-24 

 25-49 

 50-74 

 75-100 



Total 



1992 



0-24 

 25-49 

 50-74 

 75-100 



Total 



212 

 26 

 45 

 76 



359 



406 

 

 2 



78 



913 

 66 

 58 



129 



1,166 



933 



8 



43 



251 



486 1,235 



1,932 



96 



331 



980 



3,338 



1,349 

 141 

 192 

 758 



2,440 



1,346 



85 



241 



532 



4,403 

 273 

 676 



1716 



2,205 7,069 



1,220 



113 



47 



433 



1,813 



3,908 

 262 

 284 



1,519 



5,973 



Both years combined 



0-24 

 25-49 

 50-74 

 75-100 



Total 



618 

 26 



47 

 154 



845 



1,846 



74 



101 



380 



2,401 



3,280 2,566 8,311 



238 199 536 



523 288 960 



1,737 965 3,235 



5,778 4,018 13,042 



(each observer made a confidential record of best, high, 

 and low estimate into a personal field notebook). Any 

 additional schools sighted while the aircraft was di- 

 verted from the transect were recorded as 'off-effort' 

 sightings. Only sightings made during active searches 

 on predetermined transect lines Con-effort') were in- 

 cluded for abundance estimation. The secondary obser- 

 ver only reported sightings missed by the primary observer 

 team; these secondary sightings were used to estimate 

 the fraction of animals missed on the transect line. 



Analytical methods 



Stratification 



Because we were not able to complete both grids in all 

 regions of the coast, the study area was divided into 

 four a posteriori geographic areas to approximate uni- 

 form coverage within each stratum (Fig. 2). Environ- 

 mental conditions such as sea state and percent cloud 

 cover were recorded throughout the survey, as they have 

 been shown to influence cetacean sighting rates (Holt 

 and Cologne, 1987; Forney et al., 1991). However, be- 

 cause of the small number of sightings made during 

 each combination of environmental conditions, it was 

 not possible to evaluate their effect quantitatively. 



Because of the difficulty in identifying beaked whales 

 to species level during aerial surveys, only a combined 

 abundance estimate was obtained for this group. In 

 the preliminary analyses of the 1991 aerial survey data, 

 Forney and Barlow ( 1993) assigned other unidentified 

 species based on a 'nearest identified neighbor' ap- 

 proach. In the analyses presented here, unidentified 

 cetacean sightings were treated separately as either 

 'unidentified dolphin or porpoise,' 'unidentified small 

 whale,' or 'unidentified large whale,' because they rep- 

 resented only a small fraction of the total animals seen. 



The small number of sightings for each species 

 made it necessary to pool distributions of perpendicu- 

 lar sighting distances for line-transect calculations. 

 Forney and Barlow (1993) created preliminary spe- 

 cies groups based on considerations of school size, 

 body size and behavior, and pooled distributions for 

 groups that were not statistically different from one 

 another. The same procedure was used for this analy- 

 sis, resulting in the same three species/group-size 

 categories: 1 ) small cetacean groups with 1-10 animals; 

 2) small cetacean groups with more than 10 animals; 

 and 3) medium and large cetaceans (Table 2). 



Table 2 



Estimates of flO) and g(0), and number of sightings (n) 

 for the three species/group-size categories used in the 

 analysis. 



Small cetaceans 



Group size n /t0) g(0) 



1-10 

 > 10 



99 

 53 



4.70 

 2.85 



0.67 

 0.85 



Species 



Harbor porpoise, Phocoena phocoena 



Dall's porpoise, Phocoenoides dalli 



Pacific white-sided dolphin, Lagenorhynchus 



obliquidens 

 Risso's dolphin, Grampus griseus 

 Bottlenose dolphin, Tursiops truncatus 

 Common dolphins Delphinus delphis and D. capensis 

 Northern right whale dolphin, Lissodelphis borealis 



Medium and large cetaceans 



Group 



size 



n f{0) giO) 



1-22 57 2.49 0.95 



Species 



Killer whale, Orcinus orca 



Small beaked whales, Ziphius cavirostris and 



Mesoplodon spp. 

 Sperm whale, Physeter macrocephalus 

 Right whale, Eubalaena glacialis 

 Gray whale, Eschrichtius robustus 

 Minke whale, Balaenoptera acutorostrata 

 Blue whale, B. musculus 

 Fin whale, B. physalus 

 Humpback whale, Megaptera novaeangliae 



