357 
Age, growth, and calving season 
of bottlenose dolphins, 
Tursiops truncatus, 
off coastal Texas 
Stephanie Fernandez 
Institute Tecnologico y de Estudios Superiores de Monterrey 
Campus Guaymas, Apdo. Postal 484, Guaymas, Sonora 85400, Mexico 
Aleta A. Hohn 
Office of Protected Resources 
National Marine Fisheries Service, NOAA 
1335 East-West Highway, Silver Spring, Maryland 20910 
Present address: National Marine Fisheries Service 
1 0 1 Pivers Island Road 
Beaufort, North Carolina 28516 
E-mail address (for A. A. Hohn, contact author), aleta.hohn@noaa.gov 
Life history studies of bottlenose 
dolphins, Tursiops truncatus , in 
North America began over a cen- 
tury ago with observations by True 
(1890) on dolphins captured in a 
directed fishery along the coast of 
Cape Hatteras, North Carolina. He 
detected an approximately equal 
sex ratio but variability in size 
among individuals in almost every 
catch. He noted tooth development 
(eruption ) and state of fissure of the 
umbilical cord in young dolphins, 
as well as reproductive state in fe- 
males (i.e. lactating or pregnant). 
Furthermore, he suggested that 
calving took place primarily in the 
spring. Since then, much of the 
knowledge of life history of bottle- 
nose dolphins in the western Atlan- 
tic has been obtained from dolphins 
captured incidentally during fish- 
ing operations or directly for research 
or display, and trom stranded ani- 
mals (Sergeant et al., 1973; Hohn, 
1980; Hersh, 1987; Barros and 
Odell, 1990; Mead and Potter, 
1990). More recent and extensive 
information on age, growth, and 
population biology has been ob- 
tained from a long-term study of 
free-ranging bottlenose dolphins in 
Sarasota Bay, Florida (e.g. Hohn et 
al., 1989; Scott et al., 1990; Wells 
and Scott, 1990; Read et al., 1993). 
Equivalent life history studies 
have not been conducted on bottle- 
nose dolphins from the north or 
western Gulf of Mexico, although 
this is the most common cetacean 
inhabiting the coastal waters of the 
northwestern Gulf (Barham et al., 
1980; Fritts et al., 1983; Leather- 
wood and Reeves, 1983; Mullin et 
al., 1990). It also is the most com- 
monly stranded species along the 
Gulf coast, particularly along the 
coast of Texas. Strandings of bottle- 
nose dolphins have been recorded 
routinely in Texas since 1974 ( Jones, 
1987; Schmidly and Shane 1 ). Since 
late 1980, the Texas Marine Mam- 
mal Stranding Network (TMMSN) 
has recovered over 800 carcasses. In 
the spring of 1990 and 1992, the 
TMMSN documented the highest 
frequency of bottlenose dolphin 
strandings since the program be- 
gan (Hansen 2 ). Unusually high 
mortalities among bottlenose dol- 
phins along the Texas coast have 
emphasized the need for baseline 
information on their life history to 
allow better interpretation of the 
impacts of these mortalities on the 
populations. The available source 
of data and samples to begin such 
studies is the strandings them- 
selves. This study of stranded speci- 
mens was conducted to estimate 1 ) 
age structure of stranded speci- 
mens, 2) growth from length-at-age 
data, 3) mean length at birth, 4) 
calving season, and, where pos- 
sible, 5) age and length at sexual 
and physical maturation. These 
data provided the material for a 
preliminary examination of the life 
history of bottlenose dolphins that 
inhabit the coastal waters of Texas. 
Materials and methods 
From January 1981 through De- 
cember 1990, 898 stranded bottle- 
nose dolphins (373 males, 292 fe- 
males, and 233 specimens of un- 
known sex) were recovered by mem- 
bers of TMMSN along the 715-km 
Texas coastline. Tooth sections were 
prepared for 205 specimens (Table 1), 
which comprised the entire sample 
of specimens from which teeth were 
collected. The teeth were fixed in 
10% buffered formalin and pro- 
cessed following the decalcification, 
sectioning, and staining protocol 
described by Hohn et al. ( 1989 ). Age 
was estimated by counting the 
number of GLG’s (growth layer 
groups, Perrin and Myrick, 1980) 
in dentine and cement without ref- 
erence to data on the sex and length 
1 Schmidly and Shane. 1978. A biologi- 
cal assessment of the cetacean fauna of 
the Texas coast. Rep. U.S. Mar. Mamm. 
Comm., contract report MMC-74/05, 38 p. 
2 Hansen, L. J. 1992. Stranding rate and 
trends. In L. J. Hansen (ed.), Report on 
investigation of 1990 Gulf of Mexico bottle- 
nose dolphin strandings, p. 15-20. Na- 
tional Marine Fisheries Service, South- 
east Fisheries Center, Contribution MIA- 
92-93-21. 
Manuscript accepted 26 June 1997. 
Fishery Bulletin 96:357-365 ( 1998). 
