612 



Fishery Bulletin 91|4). 1993 



pre-exploitation size (circa 1973) in 1979 (Smith 1 ). Since 

 1976, both stocks of spotted dolphin are believed to 

 have declined significantly in abundance (Anganuzzi 

 etal., 1992). 



As potential biological indices, we estimated ASM 

 and associated reproductive parameters for two stocks 

 of the spotted dolphin. Although for long-lived mam- 

 mals, like the spotted dolphin, population growth rates 

 are relatively insensitive to changes in ASM and re- 

 productive rates (Eberhardt and Siniff, 1977; Fowler, 

 1981; Reilly and Barlow, 1986), these parameters have 

 been correlated with changes in population abundance 

 for other marine mammal species (Fowler, 1987). For 

 instance, a decrease in ASM was correlated with a 

 decline in population abundance for baleen whale stocks 

 (Lockyer, 1984). Similarly, a decrease in ASM for 

 crabeater seals was correlated with an increase in the 

 per capita availability of food resources resulting from 

 the exploitation and subsequent reduction in size of 

 baleen whale stocks (Bengtson and Laws, 1985). Like- 

 wise, ASM for the striped dolphin of the western Pa- 

 cific declined in response to exploitation and reduced 

 population abundance (Kasuya, 1985). Based on the 

 predictions of population responses to changes in den- 

 sity (i.e., increases in ASM and decreases in pregnancy 

 rates as population abundance increases) presented 

 by Eberhardt (1977) and Eberhardt and Siniff (1977), 

 we predicted a decline in the ASM for both the north- 

 ern offshore and southern offshore stocks of spotted 

 dolphin after 1974 as well as a lower ASM for the 

 more heavily exploited northern offshore stock. Simi- 

 larly, a higher fraction of sexually mature females preg- 

 nant and those simultaneously pregnant and lactating 

 should be observed for the northern offshore stock. 



Methods 



Since 1968, when the National Marine Fisheries Ser- 

 vice (NMFS) first placed observers aboard U.S. vessels 

 to observe fishery activities, life history data has been 

 collected from all cetacean species incidentally killed 

 in the ETP yellowfin tuna purse-seine fishery. Begin- 

 ning in 1974, life history data collection procedures 

 were standardized, and the original sampling scheme 

 that selectively collected large, female specimens was 

 replaced by a random sampling scheme that selected 

 the first available dead dolphins brought aboard (Perrin 

 and Oliver, 1982). In 1979, the Inter-American Tropi- 

 cal Tuna Commission (IATTC) joined the NMFS in 



'Smith, T. D. (ed.). 1979. Report of the status of porpoise stocks 

 workshop (August 27-31, 1979, La Jolla, California). U.S. Dep. 

 Commer., NOAA, Natl. Mar. Fish. Serv., Southwest Fish. Sci. Cent., 

 P.O. Box 271, La Jolla, CA 92038. Admin. Rep. LJ-79-41, 120 p. 



placing observers aboard U.S. vessels and collecting 

 life history data. 



The offshore stocks of spotted dolphin are distin- 

 guished geographically as being north or south of 1 

 south latitude (Perrin et al., 1985). The specimens used 

 in our study were collected from these areas, but the 

 sample of northern offshore stock animals was addi- 

 tionally restricted to those animals collected west of 

 120° west latitude (Fig. 1). The northern offshore stock 

 was sub-sampled because exploitation has not been 

 spatially uniform. The yellowfin tuna purse-seine 

 fishery began primarily as a coastal fishery in 1959 

 and gradually expanded farther offshore. The 

 Commission's Yellowfin tuna Regulatory Area (CYRA) 

 was established in 1968 by the IATTC and provides a 

 useful boundary that separates the inshore fishery area 

 from the westernmost region of the ETP yellowfin tuna 

 fishery (Peterson and Bayliff, 1985). The area west of 

 the CYRA was not fished until the late 1960s, and 

 then only for a few months a year. Exploitation in this 

 region has been significantly less than inside the CYRA 

 but much more than in the southern area which has 

 been fished only sporadically since the early 1970s 

 (Punsly, 1983). Although spotted dolphin move along 

 the 10°N latitude as well as from the south to the 

 west (Perrin et al., 1979a; Au and Perryman, 1985; 

 Reilly, 1990), owing to the large size of the area, inter- 

 change between areas is probably limited. A standard- 

 ized data collection scheme was established at about 

 the same time as the fishery expanded into the west- 

 ern and southern regions of the ETP, and therefore we 

 anticipated that these two distinct geographic regions 

 would provide the best opportunity for testing poten- 

 tial biological indices. 



Female spotted dolphins with complete life history 

 data (i.e., geographic position, total body length, teeth 

 collected, and both ovaries examined) collected between 

 1974 and 1988 were selected for this study. Few speci- 

 mens have been collected from the southern offshore 

 stock, and therefore all available specimens were pre- 

 pared. Specimens from the northern offshore stock were 

 selected randomly by year and the sample sizes dis- 

 tributed as evenly as possible between years so that 

 annual estimates of ASM could be calculated. A maxi- 

 mum of 50 specimens per year was the target for se- 

 lecting specimens from the northern offshore stock 

 sample because Hohn (1989) showed that a sample 

 size of 50 accurately estimated ASM and reduced the 

 variance from an estimate based on a sample size of 

 25. Thin sections of the teeth were prepared and aged 

 by one of us (ACM) with techniques described in Myrick 

 et al. (1983). Prior to estimating age, all specimens 

 were numerically coded in random order to disguise 

 the specimen number so that no inference could be 

 made about the stock or collection year for any speci- 



