126 



Fishery Bulletin 104(1) 



and Scott^). The set procedure also tends to induce rela- 

 tively high, sustained swimming speeds in the dolphins. 

 Swimming speeds of two spotted dolphins carrying ve- 

 locity tags during and after tuna purse-seine sets in 

 the ETP averaged 1.7-2.8 m/s during chase and 2.6-3.1 

 m/s after release, compared to undisturbed speeds of 

 1.2-1.9 m/s (Chivers and Scott^). Compared to adults, 

 dolphin calves have smaller, less-coordinated muscles 

 and lower aerobic capacity, especially at birth but per- 

 sisting through the first year or more (Dolar et al., 1999; 

 Dearoff et al., 2000; Noren et al., 2001; Noren et al., 

 2002; Eitner et al, 2003; Noren et al., 2004). Therefore, 

 it appears possible that the observed calf deficit in the 

 kill may result at least in part from energetics-related 

 separation of calves from mothers during the chase or 

 after release, as well as the inability of calves to main- 

 tain speed with adults while swimming alone. 



The present study examines the possibility of ener- 

 getics-related separation, by estimating the length of 

 time (duration) during which spotted dolphin calves of 

 different sizes ranging from neonate through two years 

 of age can swim unassisted at various velocities. These 

 velocity-durations are then compared to adult swim- 

 ming capacities, in order to determine whether calves 

 swimming without assistance may experience energy- 

 based difficulty keeping up with adult dolphins during 

 evasion from tuna purse-seine sets. 



Materials and methods 



Duration limits for unassisted swimming by spotted 

 dolphin calves were calculated by combining estimates 

 of mass-specific energy cost of steady, submerged, unas- 

 sisted swimming by neonate through adult spotted 

 dolphins, with reported swimming speed durations of 

 adult dolphins of various species (Table 1). Swimming 

 duration for adults was used because this information 

 was not available for dolphin calves. The data were com- 

 bined based on the assumption that the duration a given 

 unit of dolphin muscle can sustain a given mass-spe- 

 cific energy cost is the same for both adults and calves 

 within any species of dolphin. It is much more likely that 

 swimming capacity of calves within any dolphin species 

 is significantly lower than that of adults and that dif- 

 ferences are particularly pronounced at birth {Dolar et 

 al., 1999; Dearoff et al., 2000; Eitner et al., 2003; Noren 

 et al., 2001, 2002, 2004), but the exact differences are 

 unknown. The swimming duration estimates presented 

 in this study therefore represent maximum durations. 

 It is also possible that species differ in power produc- 

 tion capacities, based on observations of blood chemistry 

 differences between species (Ridgway and Johnston, 

 1965), but it is not unreasonable to assume that relative 



differences between species are maintained throughout 

 the size ranges of individuals within a species, so that 

 younger dolphins are probably less adept than adults 

 in any particular species. Thus, although it is not yet 

 possible to quantify differences between calf and adult 

 swimming-duration capacities in Stenella attenuata in 

 nature, they are likely greater than estimated in the 

 present study, so that problems apparent from this study 

 are likely greater during actual tuna purse-seine sets. 

 The energy cost of a steady, submerged, unassisted 

 swimming rate was estimated in this study for veloci- 

 ties ranging from 1.5 to 6.0 m/s in order to encompass 

 speeds from slow undisturbed swimming to the maxi- 

 mum likely to occur during attempted evasion from 

 tuna purse-seine sets. Average velocities observed dur- 

 ing tracking of dolphins before and after experimental 

 sets varied from about 1.5 m/s m to about 3 m/s (Chiv- 

 ers and Scott') but short term speeds attained during 

 evasion maneuvers were likely to have exceeded these 

 averages (Table 1). Total body energy costs were esti- 

 mated first and then converted to mass-specific costs by 

 dividing total estimated energy costs by the estimated 

 total muscle weight of each size of modeled dolphin. 

 Muscle-mass-specific measures are more appropriate 

 than simply dividing by total body weight for compari- 

 sons between sizes because muscle fraction of body 

 weight increases with body weight in Stenella attenuata 

 in the ETP, from 35-40% in neonates to 55-60% in 

 adults (Edwards, 1993). 



Data sources 



Total body energy costs were estimated for eight mod- 

 eled Stenella attenuata ranging in size (age) from new- 

 born through adult. Sizes were selected to emphasize 

 changes during the early months (Table 2). Size-at-age 

 to two years old was estimated from Hohn and Ham- 

 mond (1985). Size of an adult reproductive female was 

 estimated from Perrin and Reilly (1984). Total body wet 

 weight, wetted surface area of body, fins, and flukes, 

 maximum body diameter, and fraction of total body 

 weight composed of muscle were estimated for each size 

 of modeled dolphin by using regression equations devel- 

 oped from morphological measurements of ETP dolphins 

 (Edwards 1993, Edwards"*). 



The morphological measurements were taken from 

 35 spotted dolphins ranging in size from 0.71 to 2.1 m 

 total length (tip of rostrum to fluke notch). This size 

 range encompasses all life-history stages (near-term 

 fetus through mature adult) of the spotted dolphins 

 found in the eastern tropical Pacific Ocean. Spotted 

 dolphins are about 0.8 m total length at birth (Hohn 

 and Hammond, 1985). Specimens included 22 females 

 and 13 males (Edwards, 1993). Male specimens included 



■' Chivers, S., and M. Scott. 2002. Tagging and tracking of 

 Stenella specie.s during the 2001 Chase and Encirclement 

 Stress Studies cruise. National Oceanographic and Atmo- 

 spheric Administration Administrative Report LJ-02-33. 

 23 p. Southwest Fisheries Science Center, 8604 La JoUa 

 Shores Drive, La Jolla, CA. 92037. 



^ Edwards, E. 2002. Energetics consequences of chase by 

 tuna purse-seiners for spotted dolphins (Stenella attenuata) 

 in the eastern tropical Pacific Ocean. National Oceano- 

 graphic and Atmospheric Administration Administrative 

 Report LJ-02-29. 32 p. Southwest Fisheries Science Center, 

 8604 La Jolla Shores Drive, La Jolla, CA 92037. 



