EARLY POSTNATAL GROWTH OF 



THE SPOTTED DOLPHIN, STENELLA ATTENUATA, IN 



THE OFFSORE EASTERN TROPICAL PACIFIC 



Aleta a. Hohn' and p. S. Hammond^ 



ABSTRACT 



Estimates of length at birth and early postnatal growth are made for the northern and southern popula- 

 tions of the offshore spotted dolphin in the offshore eastern tropical Pacific Length at birth is estimated 

 to be 85.4 cm for the northern population and 83.2 cm for the southern population. Analyses of series 

 of monthly distributions of length revealed two cohorts born each year in the northern population, at 

 least in the northern inshore part of its geographic range, but only one cohort born each year in the southern 

 population. Growth curves fitted to the means of the monthly distributions of length gave estimates of 

 length at 1 year of 126.2 and 132.6 cm and length at 2 years of 154.3 and 154.9 cm for the two cohorts 

 in the northern population, and length at 1 year of 127.9 cm for the southern population. A growth curve 

 fitted to lengths and ages (in dental growth layer groups) from the northern population gave estimates 

 of lengths at 1 and 2 years of 123.0 and 143.0 cm, respectively. 



The average growth rate of individual animals in a 

 population is an important characteristic because of 

 its correlation with other population parameters. In 

 fisheries biology, two commonly employed techniques 

 used to estimate growth rates are the aging of a sam- 

 ple of fish of known length and the following of a 

 series of length distributions through time These 

 techniques allow the relationship between length and 

 age (or relative age) to be applied to a much larger 

 sample of fish, provided that the aged sample is a 

 representative ona 



For most species of fish, length-age relationships 

 may be appropriate for the entire life of the animals, 

 or at least for the period of interest to a commer- 

 cial fishery. In marine mammals, however, length 

 changes little, if at all, after attainment of physical 

 maturity. Growth rates may change markedly even 

 while the animal is maturing, being high for an ini- 

 tial period after birth and then declining quite rapid- 

 ly. In delphinids, the growth rate has been found to 

 be high in the first year, with animals typically in- 

 creasing by 50-70% of their birth length (Sergeant 

 1962; Kasuya et al. 1974; Kasuya 1976; Miyazaki 

 1977; Hohn 1980; Perrin and Henderson 1984), but 

 then declined rapidly in the second year. During this 



'Southwest Fisheries Center La Jolla Laboratory, National 

 Marine Fisheries Service, J^OAA, P.O. Box 271, La Jolla, CA 92038. 



^Inter-American Tropical Tlina Commission, Scripps Institution 

 of Oceanography, La Jolla, CA 92098; present address: Sea Mam- 

 mal Research Unit, c/o British Antartic Survey, Madingley Road, 

 Cambridge CBS OET, United Kingdom. 



period, growth rates are high relative to the varia- 

 bility in age-at-length so that length distributions are 

 distinguishable as separate age groups. Consequent- 

 ly, length-age relationships for these animals are 

 most useful from birth until about 2 yr. 



In this paper, we have used both the technique of 

 following a series of length distributions from month 

 to month and the technique of aging a sample of 

 dolphins of known length to estimate the rate of 

 growth in the spotted dolphin, Stenella attenuata, 

 in the offshore eastern tropical Pacific (hereafter 

 referred to as the offshore spotted dolphin). In 

 neither of these two techniques did we have an ab- 

 solute measure of age. Consequently, we have esti- 

 mated length at birth independently and used this 

 to fix time at birth. Growth curves were fitted to the 

 length data by relative age and then length at birth 

 was substituted in order to predict length-at- 

 age 



THE SAMPLE 



The field data and specimens used in the follow- 

 ing analyses were collected by National Marine 

 Fisheries Service (NMFS) and Inter-American 

 Tropical lUna Commission (lATTC) scientific techni- 

 cians aboard commercial tuna purse seiners from 

 1968 to 1982. Procedures for collecting sample data 

 and specimens have been described by Perrin et al. 

 (1976). In all the following analyses, the data have 

 been stratified into northern and southern popula- 



Manuscript accepted December 1984. 

 FISHERY BULLETIN: VOL. 83. NO. 4, 1985. 



553 



