IRVINE ET AL: AERIAL SURVEYS FOR MANATEES AND DOLPHINS 



ences in reported results. During our surveys we 

 often encountered several herds within a few 

 kilometers of each other, after not seeing dol- 

 phins for distances of 20 km or more. Although 

 such assemblages may have been dispersed sub- 

 groups of a larger herd, they did not meet our 

 arbitrary criteria for defining a "herd." Our 

 spatial definition of herd may be unsatisfactory 

 if bottlenose dolphins, like some other cetaceans, 

 maintain acoustic contact over many kilometers 

 (Payne and Webb 1971). Acoustic contact among 

 free ranging groups of T. truncatus, however, 

 has not been demonstrated, and we know of no 

 more appropriate basis for defining herds from 

 aerial sightings. 



The proportion of dolphin calves noted during 

 our surveys (5.3%) is low when compared with 

 other reports. Leatherwood (1979) observed 8.1- 

 10.1% calves during aerial surveys in eastern 

 Florida in August, while Irvine et al. (footnote 8) 

 reported a maximum of 11% from May to July 

 during surface surveys near Sarasota, Fla. 

 Shane and Schmidly (footnote 7) noted that 

 calves constituted 7.6% of all dolphin sightings 

 during surface surveys near Port Aransas, Tex., 

 and Barham et al. (1980) sighted 9.3% calves 

 from the air in the same area. Leatherwood 14 

 observed 7.7% calves in 1974 and 7.9% calves in 

 1975 near the mouth of the Mississippi River. 

 Our calf counts may be lower because we only 

 counted very small animals; calves may grow to 

 2 m long within the first year (Leatherwood foot- 

 note 14) and therefore large calves may not have 

 been distinguished as such. 



SUMMARY AND CONCLUSIONS 



Most of the manatees (58.5%) were located in 

 the Everglades National Park (Monroe County) 

 and Ten Thousand Islands (Collier County) 

 areas, and most (80.1%) were in brackish water. 

 Because these areas are relatively undisturbed 

 by human development, they have great value as 

 locations to protect and study the endangered 

 manatee. 



Dolphins were well dispersed in the survey 

 area. Fifty -one percent were sighted in the Gulf 



"Leatherwood, S. 1977. Some preliminary impressions 

 on the numbers and social behavior of free swimming bottle- 

 nosed dolphin calves {Tursiops truncatus) in the northern Gulf 

 of Mexico. In S. H. Ridgway and K. W. Benirshke (editors), 

 Breeding dolphins: Present status, suggestions for the future, 

 p. 143-167. National Technical Information Service PB 273 

 673. 



of Mexico, 49% were in brackish water, and none 

 were located in freshwater. 



Seasonal movement patterns and reproductive 

 trends based on calf sightings of both dolphins 

 and manatees are unclear. While the survey re- 

 sults are valuable as indicators of relative abun- 

 dance, they are not useful to estimate total abun- 

 dance because the percentage of animals not 

 observed is unknown. 



ACKNOWLEDGMENTS 



This research was supported by the Bureau of 

 Land Management under contract to the Nation- 

 al Fish and Wildlife Laboratory (MOU-AA551- 

 MU9-19). Jean Duke and C. R. Smith partici- 

 pated in several survey flights. We thank James 

 Kushland and Sonny Bass for assistance with 

 flights over the Everglades National Park; Tom 

 Fritts, Steve Leatherwood, James Powell, Tom 

 O'Shea, Galen Rathbun, and Susan Shane made 

 constructive comments on earlier versions of the 

 manuscript. Esta Belcher prepared the illustra- 

 tions and Luanne Whitehead typed the manu- 

 script. 



LITERATURE CITED 



Barham, E. G., J. C. Sweeney, S. Leatherwood, R. K. Beggs, 

 and C. L. Barham. 



1980. Aerial census of bottlenose dolphin, Tursiops 

 truncatus, in a region of the Texas coast. Fish. Bull., 

 U.S. 77:585-595. 



Hartman, D. S. 



1979. Ecology and behavior of the manatee Trichechus 

 manatus in Florida. Am. Soc. Mammal. Spec. Publ. 5, 

 153 p. 

 Helwig, J. T., and K. A. Council (editors). 



1979. SAS users guide. SAS Inst. Inc., Raleigh, N.C. 

 Irvine, A. B., and H. W. Campbell. 



1978. Aerial census of the West Indian manatee, Tri- 

 checus manatus, in the southeastern United States. J. 

 Mammal. 59:613-617. 



Irvine, A. B., M. D. Scott, R. S. Wells, and J. H. Kaufmann. 



1981 . Movements and activities of the Atlantic bottlenose 

 dolphin, Tursiops truncatus, near Sarasota, Florida. 

 Fish. Bull., U.S. 79:671-688. 



Layne, J. N. 



1965. Observations on marine mammals in Florida 

 waters. Bull. Fla. State Mus., Biol. Sci. 9:131-181. 

 Leatherwood, S. 



1979. Aerial survey of the bottlenosed dolphin, Tursiops 

 truncatus, and the West Indian manatee, Trichechus 

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 Fish. Bull., U.S. 77:47-59. 



Leatherwood, S., J. R. Gilbert, and D. G. Chapman. 



1978. An evaluation of some techniques for aerial cen- 

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