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Fishery Bulletin 104(1) 



1957; Wells, 1991). Other differences persist for weeks 

 or months, including undeveloped musculature (Ei- 

 tner et al., 2003) and uncoordinated swimming and 

 respiratory movements (Tavolga and Essapian, 1957; 

 Taylor and Saayman, 1972; Reiss, 1984; Cockroft and 

 Ross, 1990; Peddemors, 1990; Herzing, 1997; Mann 

 and Smuts. 1999). Aerobic capacity is also likely to 

 be reduced throughout the first few months, and not 

 likely to reach adult levels until two to three years of 

 age (Dolar et al., 1999; Dearoff et al., 2000; Noren et 

 al., 2001, 2002, 2004). The results presented in this 

 study also do not include the added costs of increased 

 drag due to swimming near the surface, and repeatedly 

 piercing it. which will occur much more often during 

 evasion of tuna sets than during normal swimming. 

 These other effects increase the likelihood that dolphin 

 calves, particularly the younger individuals, will have 

 problems coping with the swimming conditions induced 

 during tuna purse-seine sets. 



Difficulties with unassisted swimming for dolphin 

 calves may be ameliorated to some extent by employing 

 drafting (Weihs, 2004), burst and coast (Au and Weihs, 

 1980), or leap-burst-coast swimming behaviors (or a 

 combination of these behaviors) (Weihs, 2002). Theo- 

 retically, these strategies could significantly reduce the 

 cost to calves of moving through the water. However, it 

 is not clear that any of these energy-saving strategies 

 will be consistently attainable by spotted dolphin calves 

 during herd movements associated with evading or es- 

 caping sets by tuna purse-seiners in the ETP. 



Drafting can only be sustained through respiratory 

 leaps if mother and calf leave and reenter the water 

 with equal speed and efficiency (Weihs, 2004). Because 

 evasion of tuna purse-seine sets involves sustained 

 high-speed swimming characterized by repeated full- 

 body respiratory leaps from the water (Au and Wiehs. 

 1980), calves are likely to tire and lose coordination 

 more quickly than adults, and the smallest calves will 

 be the first to experience problems. These factors may 

 have contributed to the observed failure of a neonate 

 dolphin calf in the ETP to successfully maintain a 

 drafting relationship with its assumed mother during 

 a respiratory leap while attempting to evade a vessel 

 (Weihs, 2004). 



Once the drafting relationship is disrupted, the calf 

 appears likely to fall behind because of its physical 

 limitations, unless its mother alters her speed so that 

 the calf can reestablish the drafting relationship. How- 

 ever, review of dolphin mother-calf behavior indicates 

 that the mother is not likely to voluntarily leave other 

 adults during attempted evasion of tuna purse-seine 

 sets in the ETP (EdwardsM. Thus, the faster or longer 

 the chase or postrelease escape period (or combination 

 of all three factors), and the younger the calf, the more 

 likely it appears that the calf will become separated 

 from its mother and be left behind during periods of 

 fast swimming by the adults. Although fast swimming 

 for a few minutes may not pose a great problem for 

 many calves, longer periods appear increasingly likely 

 to lead to significant calf loss in purse-seine operations. 



The duration of the high-speed period is at least as 

 important as the speed maintained during the period, 

 given the power-duration relationship. If fast swim- 

 ming is concluded quickly, it is more likely that calves 

 could achieve the required power during the short time 

 required. As fast-swimming persists, power capacity 

 decreases rapidly (Fig. 1), so that more and more calves 

 are likely to be lost because they have exceeded their 

 ability to keep up with the adults in their school. 



Even under normal circumstances, burst and coast, 

 or burst-leap-coast, swimming patterns are not likely to 

 provide sustained benefits to free-swimming calves until 

 they approach adult size, coordination, and swimming 

 capacity, because the calves will still tire more quickly 

 than their associated adults. In the case of tuna purse- 

 seine set evasion, sustained use of these swimming 

 patterns may not be employed even by adults because 

 these behaviors become less efficient as swim speed 

 increases (Weihs, 2002). Video studies of swimming 

 behaviors of adult Tursiops have shown that burst and 

 coast periods decrease with increasing drag and cease 

 altogether if the drag is large (Skrovan et al., 1999). In 

 a study of dolphin gaits, an adult Tursiops experiencing 

 increased drag due to an instrument pack did not use 

 burst and coast propulsion during horizontal swimming 

 speeds and depths at which four other adult Tursiops 

 (without instrument packs) did employ short periods 

 of burst and coast swimming. Even animals swim- 

 ming without instruments at 1.5-3.7 m/s incorporated 

 only short periods of burst and coast, and glide periods 

 rarely exceeded 2 seconds. At the speeds characteristic 

 of tuna purse-seine sets, dolphins tend to swim steadily 

 or employ burst-leap-coast swimming behaviors (Weihs, 

 2002, Au and Weihs, 1980). With their smaller power 

 production capacities, calves will need to shift into 

 leap-burst-coast mode at slower speeds than will adults 

 (Weihs, 2002), and again will tire more quickly. 



A potential shortcoming of the results presented in 

 the present study is that the model produces absolute 

 values from single calculations, without any associated 

 statistics or sensitivity analyses. It is more likely that 

 a range of values for morphological, physiological and 

 behavioral characteristics occurs within real spotted 

 dolphin populations in the ETP, so that a range of re- 

 sponses is much more likely than a single response for 

 a given size, swimming speed, and duration of chase. 

 Lack of statistical and sensitivity analyses can be a 

 problem where responses tend to be subtle and there- 

 fore difficult to discern. However, the model results 

 presented in this study with respect to estimated size- 

 related differences in power production capacities of 

 spotted dolphins in the ETP are far from subtle, and 

 model results are reasonably similar to energy mea- 

 sures derived from observations on real dolphins indi- 

 cating that these model results are not unrealistic. In 

 general, results presented here imply that older calves 

 may be able to swim unassisted as fast and for as long 

 as their associated adults during or after most sets (or 

 during and after most sets) by tuna purse-seine vessels 

 in the ETP. The markedly smaller energy production 



