484 



Fishery Bulletin 93 13). 1995 



data on age and gonadal condition for 35 male and 

 52 female specimens. Most of the dolphins examined 

 in this study were collected during purse-seine re- 

 search operations off the southern California coast 

 (Hui, 1973). 



In the late 1970's, Japan, Taiwan, and the Republic 

 of Korea initiated high-seas driftnet fisheries for fly- 

 ing squid, Ommastrephes bartrami, and large-mesh 

 fisheries for pelagic fish species including albacore 

 tuna, Thunnus alalunga. The majority of squid 

 driftnet fisheries operated from June to November 

 in the central North Pacific Ocean between lat. 34°- 

 46°N and long. 171°E-147°W, while large-mesh fish- 

 eries operated farther south and west between lat. 

 29°N-45°N and long. 147°E-150°W from February 

 to November. Delphinus delphis was the most fre- 

 quently killed cetacean in the large-mesh fisheries 

 and was also found regularly entangled in the squid 

 driftnet fisheries (Hobbs and Jones, 1993). 



Beginning in 1990, under international coopera- 

 tive agreements, biological samples were collected 

 from marine mammals killed in the Japanese and 

 Taiwanese squid and large-mesh fisheries and from 

 marine mammals killed in the Korean squid fishery. 

 Combining the 1990 and 1991 biological samples 

 provided an opportunity to estimate growth and re- 

 productive parameters for D. delphis from the off- 

 shore waters of the North Pacific Ocean. 



Materials and methods 



Specimen collection 



Scientific observers collected biological data from all 

 cetaceans caught in Japanese, Korean, and Taiwan- 

 ese driftnet operations. Soon after arrival on deck, 

 each cetacean was identified, sexed, measured (total 

 length to nearest 1.0 cm), photographed twice (left 

 lateral and ventral), and given an individual speci- 

 men number. 



When an animal was dissected, the left lower jaw 

 was tagged and frozen intact. For males, the right 

 testis and epididymis were collected whole, tagged, 

 and preserved in 10% formalin. Females were 

 checked for evidence of lactation by longitudinal in- 

 cision through the left mammary gland. The ovaries 

 and uteri for most females were collected intact. The 

 left ovary and entire reproductive tract was tagged 

 and preserved in 10% formalin. If the animal was 

 pregnant with a large fetus or was recently postpar- 

 tum, only the ovaries and a cross section of the left 

 uterine horn were collected. Fetuses were sexed, 

 weighed to the nearest 1.0 g, and measured to the 

 nearest 1.0 cm. 



Frozen and preserved samples were shipped to the 

 National Marine Mammal Laboratory in Seattle, 

 Washington, for analyses. 



Age determination 



Teeth were extracted from the center of the left lower 

 jaw for age determination. Each tooth was decalci- 

 fied and sectioned (24 um) longitudinally on a freez- 

 ing microtome. Tooth-section preparation and den- 

 tinal growth-layer group (GLG) counting procedure 

 followed guidelines developed by Myrick et al. ( 1983) 

 for Stenella spp. Six to eight stained sections from 

 the center of each tooth were mounted on a glass 

 slide and examined under a compound microscope 

 at 40x and lOOx magnification with transmitted light. 

 Dentinal GLG characteristics have been described 

 for Delphinus (Perrin and Myrick, 1981). Annual 

 deposition of dentinal GLG's has been established 

 for tetracycline-marked common dolphins (Gurevitch 

 et al., 1981). We followed these findings and assumed 

 one GLG represented one year of growth. 



Each tooth was read independently by two read- 

 ers. Ages were recorded to the nearest 0.5 layer ex- 

 cept for animals with less than one complete GLG. 

 In these cases, we estimated the thickness of the in- 

 complete layer to the nearest 0.2 GLG. Predeter- 

 mined limits on reader variability were established 

 following those used for Lissodelphis borealis in Ferrero 

 and Walker (1993). This procedure allowed for a 0.5- 

 layer difference between readings for estimated ages 

 up to 5 years (measured from the median reading), one 

 layer for estimates between 5 and 10 years, and one 

 additional layer for every 5-year interval thereafter. 

 Within these limits, we averaged the two readers' esti- 

 mates to obtain the final age. When readings differed 

 by more than these limits, the tooth was reread. 



Examination of reproductive organs 



Males Right testes with epididymides were weighed 

 to the nearest 0.01 g and measured to the nearest 

 0.1 cm. A 1-cm 3 block was removed from the center 

 of each testis; a similar section of epididymis was 

 removed at mid-length, and both were prepared for 

 histological analysis. Paraffin-embedded tissues were 

 sectioned at 6 um, stained with hematoxylin and 

 eosin, and mounted on glass slides. Testes and epi- 

 didymides were examined for evidence of spermato- 

 genesis by using a compound microscope at lOOx with 

 transmitted light. Males were considered mature if 

 sperm were present in testes tubules. 



Females Ovaries were weighed to the nearest 0.01 

 g. Maximum diameter of the left uterine horn was 



