Duration of unassisted swimming activity for spotted dolphin calves: implications for mother-calf separation during tuna purse-seine sets

Fishery Bulletin, Jan, 2006 by Elizabeth F. Edwards

Abstract--Size-related differences in power production and swim speed duration may contribute to the observed deficit of nursing calves in relation to lactating females killed in sets by tuna purse-seiners in the eastern tropical Pacific Ocean (ETP). Power production and swim-speed duration were estimated for northeastern spotted dolphins (Stenella attenuata), the species (neonate through adult) most often captured by the fishery. Power required by neonates to swim unassisted was 3.6 times that required of an adult to swim the same speed. Estimated unassisted burst speed for neonates is only about 3 m/s compared to about 6 m/s for adults. Estimated long-term sustainable speed is about 1 m/s for neonates compared to about 2.5 m/s for adults. Weight-specific power requirements decrease as dolphin calves increase in size, but power estimates for 2-year-old spotted dolphin calves are still about 40% higher than power estimates for adults, to maintain the same speed. These estimated differences between calves and adults are conservative because the calculations do not include accommodation for reduced aerobic capacity in dolphin calves compared to adults. Discrepancies in power production are probably ameliorated under normal circumstances by calves drafting next to their mothers, and by employing burst-coast or leap-burst-coast swimming, but the relatively high speeds associated with evasion behaviors during and after tuna sets likely diminish use of these energy-saving strategies by calves.

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Dolphin calves draft in echelon position next to their mothers for the first few weeks after birth and continue to return to drafting in echelon position frequently throughout at least their first year (Edwards (1)). "Echelon position' is the physical positioning of the calf within a few centimeters of the mother, near her mid-section, with fin motions reduced or absent (e.g., Norris and Prescott, 1961). This position takes advantage of the mother's flow field, reducing or effectively eliminating the energy cost to the calf of moving forward through the water (Weihs, 2004). Because drafting appears to be ubiquitous among dolphin calves, particularly during the neonate stage, it appears likely that drafting is an essential factor in maintaining physical association between calves and their mothers, especially when the calves are small.

In the eastern tropical Pacific Ocean (ETP) a situation occurs in which it becomes important to consider the consequences for calves of losing their drafting association with their mother. In this area, a tuna purse-seine fishery targets schools of large yellowfin tuna that associate closely with schools of dolphins, primarily the spotted dolphin (Stenella attenuata) (NRC, 1992). The associated schools of tunas and dolphins are located by a helicopter sent out from the purse-seine vessel and are subsequently captured through the actions of several high-powered speedboats, which are released from the purse-seiner to overtake and herd the associated animals into the closing arc of the purse-seine (NRC, 1992). Examination of the dolphins found dead in the net has revealed that 75% to 95% of the lactating females killed in the sets are not killed with an accompanying calf (Archer et al., 2004). This observed calf deficit is a potentially important factor in the lack of recovery of ETP dolphin populations despite over a decade of very low fishery mortality (Wade et al. (2)), but little is known about why or how the separation of mothers and calves occurs (Archer et al., 2004).

The chase, encirclement, and release procedure of purse-seine fishing operations tends to be relatively prolonged--chases averaging about 30 minutes, capture and confinement about 90 minutes, and postrelease swimming at least 90 minutes (Myrick and Perkins, 1995; Chivers and Scott (3)). The set procedure also tends to induce relatively high, sustained swimming speeds in the dolphins. Swimming speeds of two spotted dolphins carrying velocity 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 (3)). Compared to adults, dolphin calves have smaller, less-coordinated muscles and lower aerobic capacity, especially at birth but persisting 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 maintain speed with adults while swimming alone.

The present study examines the possibility of energetics-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 swimming 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.