FISHERY BULLETIN: VOL. 87. NO. 1 



animals. Young fur seals are most often found in 

 trawl net fragments having a stretched mesh size 

 of 20 cm or more, with 23 cm mesh observed most 

 frequently (Scordino 1985). The diameter of a 23 cm 

 mesh is 14.6 cm, almost exactly the average head 

 diameter of the captive 17 mo old northern fur seals 

 (Table 1). Similarly, a 20 cm mesh net has a circular 

 diameter of 12.7 cm. Although this is slightly smaller 

 than the head diameter of captive 4 mo old fur seals, 

 it may well pose an entanglement threat for smaller 

 animals. 



At 17 months of age, the captive northern fur 

 seals had average shoulder diameters of 23 cm. A 

 23 cm mesh net would therefore lodge tightly 

 around the neck region but would not slip further 

 down the body. Based on these dimensions, a net 

 would have to have a stretched mesh size of 73 cm 

 or more before a fur seal of this age could pass 

 through a single mesh opening. 



Scordino (1985) has shown that most webbing 

 found on young seals weighs less than 150 g. He sug- 

 gested that the high incidence of small debris en- 

 tanglement may be due to the seals "playing" with 

 small pieces of debris, as they do with kelp. This sug- 

 gestion is supported by our observations of the fur 

 seals' investigative nature when presented with net 

 fragments. Prior to their first entanglement, all 

 animals showed an immediate interest when they 

 encountered a floating net and played with it almost 

 continuously until they became entangled. While it 

 is difficult to draw conclusions about the behavior 

 of northern fur seals at sea from studies of a small 

 number of captive animals, our observations none- 

 theless suggest that young fur seals are naturally 

 inquisitive. Interestingly, however, these captive 

 animals appeared indifferent to other floating ob- 

 jects (plastic bats, frisbees) that were occasionally 

 placed in their tank. 



Scordino's (1985) observations may also reflect a 

 high incidence of at-sea mortality caused by en- 

 tanglement in fragments larger than 150 g. Starva- 

 tion, resulting from an increased energy demand 

 during swimming, may be one consequence of en- 

 tanglement in larger fragments. Previous studies 

 have shown that entangled animals experience 

 greater drag during swimming and that this drag 

 increases exponentially with swim velocity and with 

 greater net size (Feldkamp 1985). Because swim- 

 ming energy requirements increase in relationship 

 to drag, it was expected that metabolic increases 

 would parallel increases in drag. Results from the 

 swimming experiments support these predictions 

 (Fig. 1; Table 2). At slow speeds, and with small (61 

 g and 100 g) nets, metabolism did not differ signif- 



icantly from that measured at zero flow. At the 

 higher speed of 1.1 m/s, metabolism was signifi- 

 cantly elevated by both the 100 g and 200 g nets. 

 With a 200 g net, animals visibly struggled against 

 the 1.1 m/s flow. On several occasions, the experi- 

 ment had to be stopped because of the fear of in- 

 jury to the animal. 



Metabolic rates at zero flow speeds and at 0.75 

 m/s were also higher during the winter experiments. 

 This may be accounted for by differences in water 

 temperature and body size. Miller (1978) has shown 

 that the metabolic rate of northern fur seals in- 

 creases linearly with decreasing water temperature. 

 In 15°C water, animals in Miller's study had a meta- 

 bolic rate of about 6.8 W/kg, close to the 6.95 W/kg 

 (Table 2; no net, zero flow) measured for animals 

 in this study. Under similar conditions during the 

 spring, when water temperature had increased by 

 1.7°C, our measurements showed a 25% reduction 

 in metabolic rate (Table 2). 



This reduction in metabolic rate during the spring 

 experiments was also observed for swimming and 

 entangled animals. At 0.75 m/s in the winter, a 100 

 g net resulted in an average metabolic rate of 8.4 

 W/kg (Table 2). Under similar conditions during the 

 spring, metabolism had dropped by 18%. Although 

 the reasons for these metabolic changes are diffi- 

 cult to interpret, given the small sample size and 

 changes in body mass, they do suggest, as did Miller 

 (1978), that water temperature is an important fac- 

 tor influencing the energetic demands of swimming 

 juvenile northern fur seals. 



Metabolic rate measurements suggest that if juve- 

 nile northern fur seals become entangled in nets of 

 200 g or more, they will experience considerable dif- 

 ficulties in swimming and likely suffer a greater 

 mortality than unentangled animals. Although our 

 measurements were conducted over relatively slow 

 swimming speeds, they do provide a basis of esti- 

 mating the impact of entanglement on the energetic 

 requirements of animals at sea. If animals with 200 

 g net fragments maintained an average speed of 1.1 

 m/s over the course of a day, they would need to 

 consume 284 kcal of fish/kg body mass to maintain 

 body weight. Using data on the caloric density of 

 pollock (1.4 kcal/g) and on fur seal assimilation effi- 

 ciencies (Miller 1978), this energetic requirement 

 equals roughly 5 kg of pollock per day, compared 

 with 1.9 kg for an unentangled animal. While it is 

 likely that entangled fur seals would not swim con- 

 stantly at sea, they may have to reach swim speeds 

 higher than 1.1 m/s in order to catch prey, thereby 

 increasing their metabolic expenditures. Moreover, 

 water temperature of the Bering Sea is consider- 



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