Table 1.— The increase in drag and power output, and the estimated power 

 input of the entangled sea lion. Drag with and without the net was calculated 

 from the best fit regressions determined by the experiments. Power output 

 was calculated by multiplying drag and the appropriate velocity. Weight 

 specific power input is based on an efficiency (power out/power in) of 10%. 



lion. Animals are not 100% efficient in converting 

 metabolic energy to mechanical power needed for 

 locomotion (Hicker 1975). Studies of penguins and 

 fish which swim with their pectoral fins show that 

 efficiency (power output/power input) varies between 

 5 and 15% (Webb 1973; Hui 1983). Female north- 

 ern fur seals consume about 8 W/kg while at sea 

 (Costa and Gentry in press). Using this number, the 

 power output values estimated for the sea lion, and 

 an assumed cruising speed of 2.5 m/s, an efficiency 

 for fur seals of roughly 10% is obtained. Tkble 1 

 shows estimated energy requirements based on this 

 efficiency for the sea lion with and without the net. 

 Again it can be seen that to maintain a swimming 

 speed of 2.5 m/s the sea lion would need to increase 

 its metabolic expenditure by 50-fold, an impossibly 

 high figure (Bartholomew 1977). 



Discussion 



There is little doubt that for an animal of this size, 

 entanglement in a 600 g net will substantially in- 

 crease its chance of mortality at sea. Drag, and hence 

 the power required for swimming, is increased by 

 the presence of a net. As a result, these animals will 

 swim slower, at a greater energetic cost than free 

 animals. Drag of the net, which rises geometrically 

 with velocity, will prevent activities requiring high 

 speeds as would be the case if such animals engaged 

 in the pursuit of rapidly moving or evasive prey 

 species. Additionally, migration or travel to and from 

 the rookery will be energetically more costly. It is 

 likely, then, that once an animal becomes entangled 

 in net fragments of this size or larger, it enters a 

 state of negative energy balance 



The animal's size as well as the size of the net plays 

 an important role in the amount of drag experienced. 

 A larger net will present more surface area to the 



water. Since drag is dependent on surface area 

 (Vogel 1981), larger net fragments will result in 

 greater drag. Similarly, if two different-sized animals 

 are entangled in nets of the same dimensions, the 

 smaller will experience a larger relative increase in 

 its drag and power requirements. If animals of dif- 

 ferent age classes encounter net fragments with 

 equal probability, it is expected that the younger age 

 classes will suffer a proportionally higher mortality. 



Although starvation is undoubtedly the long-term 

 result of net entanglement, other factors may have 

 a more immediate effect. This was particularly evi- 

 dent during an observation of an actual entangle- 

 ment. In the initial training phase of the sea lion, 

 a net with a larger mesh size was used. At one point, 

 while the net was floating in the water, the sea lion 

 swam up from below and inserted its head through 

 one of the mesh openings. Upon sensing the net 

 around its neck it gave a strong backwards stroke, 

 trying to retract its head. The backward movement 

 brought some of the trailing net in front of it and 

 when the animal then swam forward and dived 

 underwater, another strand slipped onto its neck. 

 This caused a violent reaction with the sea lion 

 twisting and thrashing wildly. The twisting further 

 entangled the animal and tightened the net. Within 

 IV2 to 2 min the animal was completely entangled 

 with three or four loops of mesh tight around its 

 neck. 



The net was so tight that an observer on the side 

 of the tank was unable to pull it from the sea lion's 

 head, and it was necessary to drain the holding tank. 

 During this time, the animal swam around the tank 

 barking and often thrashing about while trying to 

 bite the net. This appeared to further tighten the 

 net which, when finally removed, was so tight that 

 a finger could not be slipped between the net and 

 the animal's neck. 



694 



