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

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 three fetuses, six immature, and four mature individuals. Female specimens included three fetuses, six immature, one mature resting, one mature lactating, and 11 mature pregnant individuals. All specimens were killed during tuna fishing operations in the eastern tropical Pacific Ocean. Two of the specimens were collected in February 1980, one in July 1983, nine in July 1985, two in August 1985, seventeen in December 1985, and four were collected without a date noted (Edwards, 1993). All specimens were processed according to the same procedures prior to dissection. Immediately after the sets, the dolphin specimens were brought on board the vessel and frozen whole in the brine wells of the vessel. The specimens were transported Srozen to port and then transported frozen to the Southwest Fisheries Science Center. Specimens were kept frozen until thawed in fresh water (about 27[degrees]C) just prior to dissection. Not all measurements were made on all specimens; therefore sample sizes differ between the regression equations presented below.

Energetics model The energetics model used to estimate total body cost of swimming was taken from Edwards (1992, based on Magnuson, 1978), except that 1) new data were used to estimate dolphin body parameters and 2) the estimate of fin plus induced drag was replaced by the multiplier 3 (see below). The model used standard hydrodynamic equations and methods (Hoerner, 1965; Hertel, 1969; Webb, 1975) to estimate hydrodynamic drag on a fully submerged streamlined body of revolution moving steadily in turbulent flow. Body surface area was increased to specifically include the surface area of fins and flukes (Fish(5)), and drag estimates were increased to account for body and fin movements. Because energy to move forward (thrust energy) must exactly balance the drag experienced by a steadily swimming animal, estimating total drag energy is equivalent to estimating thrust energy, i.e., the energy cost to swim (Fish and Rohr, 1999).

Model formulation Total power ([P.sub.t], in watts) required to overcome drag during steady, submerged swimming (Hertel, 1969) by a modeled dolphin of a given total length (L, rostrum to fluke notch) was estimated as

[P.sub.t] =[P.sub.c]/([E.sub.m][E.sub.p]),

where [P.sub.c] = mechanical power (in watts) required to overcome hydrodynamic drag;

[E.sub.m]= muscle efficiency; and

[E.sub.p] = "propeller efficiency" (efficiency of propulsion by flukes).

[E.sub.m] was assumed to be 0.2 from studies of muscle efficiencies in terrestrial animals (e.g., Goldspink 1988), man (Alexander, 1983; quoting Dickinson, 1929) and dolphins (Fish, 1993, 1996). [E.sub.p] was assumed to be 0.85 based on studies by Fish (1998), Webb (1975), and Yates (1983). [P.sub.c] was estimated as a function of total hydrodynamic drag ([D.sub.t], in dynes) and velocity (V, in m/s) as