ing the possible role of net entanglement on the mor- 

 tality of pinnipeds at sea. 



Materials and Methods 



A female California sea lion, Zalophus califor- 

 nianus, was used in this work. The animal was kept 

 in large seawater holding tanks at Scripps Institu- 

 tion of Oceanography. Its weight (45 ± 0.5 kg) re- 

 mained constant throughout the course of this study, 

 conducted during April 1983. 



lb measure drag, the sea lion was trained to bite 

 onto a neoprene mouthpiece and be towed through 

 the water behind a moving cart (Fig. 1). The cart, 

 powered by a variable speed electric motor, travel- 

 led around a circular "ring" tank which had a depth 

 of 3.5 m and inner and outer diameters of 14.5 and 

 21 m, respectively. A line was connected to the 

 mouthpiece and the other end secured to a load cell 

 (Western Scale Co.) which produced a voltage out- 

 put proportional to the amount of tension on the lina 

 The tow line extended down from the load cell, 

 through a streamlined strut and around a teflon 

 pulley attached to the end of the strut (Fig. 1). The 

 pulley, enclosed by a streamlined fiberglass housing, 

 was set at a depth of 1 m (>3 body diameters) to 

 eliminate surface wave effects on drag (Hoerner 

 1959). 



Drag was measured by continuously recording the 

 signal output from the load cell during each towing 

 session. The signal was amplified and recorded on 

 a Brush^ 220 strip chart recorder (Gould In- 

 struments). At the end of each session, the load cell 

 was calibrated using a hand-held dynamometer. A 

 tachometer, attached to one of the outer cart wheels, 

 was used to determine cart velocity. This was 

 simultaneously recorded on the strip chart. The sea 

 lion's velocity, while it was being towed down the mid- 

 dle of the tank, was computed using the speed of the 

 outer wheel and the tank's circumference After each 

 experiment the data were smoothed by eye and drag 

 and velocity determined. Only steady traces which 

 varied less than ±3% were analyzed. Drag was then 

 converted to newtons by multiplying the kilogram 

 force reading of the load cell by the acceleration of 

 gravity. 



Once the sea lion's drag without a net was 

 measured, the animal was trained to place its head 

 through an opening cut in the mesh of a 1/8-in (3.2 

 mm) nylon twine trawl net. The opening was near 

 the center of the net which measured 1.4 m x 5 m, 



with a stretched mesh size of 19 cm. The net had 

 a dry weight of 580 g. After several trials, the sea 

 lion became accustomed to the procedure and would 

 allow itself to be towed with the net trailing from 

 its neck. The net was removed after each session. 



Results 



Drag on the sea lion, both with and without the 

 net, increased with velocity (Fig. 2). This rise, 

 however, was significantly greater when the animal 

 was entangled, with the difference between the two 

 curves increasing throughout the range of speeds. 

 At the highest velocity of 3.5 m/s, the entangled drag 

 was 111 N greater than that of the free animal (Tkble 

 1). Therefore, to maintain a cruising speed of 2.0 m/s 

 an animal of this size, entangled in a net with similar 

 hydrodynamic characteristics, would experience the 

 equivalent drag of a free animal swimming at speeds 

 above 4 m/s. 



Power that the sea lion must expend for swimming 

 can also be calculated from these measurements. 

 Since drag is a force, power output (in watts) is a 

 product of drag times swimming velocity (Webb 

 1975): Pq = drag x velocity. Tkble 1 shows the 

 results of such calculations and the effect of the net 

 on the sea lion's required output. 



Power output is a measure of the mean rate of 

 energy expended by the swimming muscles at a 

 given velocity (Webb 1975). It does not, however, 

 reveal the total energetic requirements of the sea 



150t 



CO 



§100 



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o 50 



< 

 tr 

 o 



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WITH NET / 



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^^-^» WITHOUT 



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^Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



12 3 4 



VELOCITY (M/S) 



Figure 2.— Drag of a 45 kg sea lion with and withoug a net trail- 

 ing from its neck. In both cases drag increased geometrically with 

 speed. The regression equation with the net was 17.19 vel.'^^, SEE 

 (standard error of estimate) = 0.052. The equation for drag without 

 the net was 2.93 vel.^•"^ SEE = 0.118. 



693 



