ENERGETIC AND BEHAVIORAL EFFECTS OF NET ENTANGLEMENT ON 

 JUVENILE NORTHERN FUR SEALS, CALLORHINUS URSINUS 



Steven D. Feldkamp,' Daniel P. Costa, ^ and 

 Gregory K. DeKrey^ 



ABSTRACT 



The energetic costs and behavioral changes associated with net entanglement were studied in three cap- 

 tive juvenile male northern fur seals, Calhrhinus ursirms. Rates of energy expenditure were highly depen- 

 dent upon swim velocity and size of the net fragment. At a speed of 1 . 1 m/s, northern fur seals expended 

 a mean ( ± SD) of 6.5 ( + 0.7) W/kg before entanglement, 9.7 ( ± 3.8) W/kg when entangled in 100 g nets, 

 and 13.8 W/kg with 200 g nets. These results showed that a free-ranging animal entangled in a net frag- 

 ment of 200 g or larger will experience considerable difficulty swimming. 



The northern fur seals' average daily metabolic rates (ADMR) were measured with doubly labeled 

 water over 6 day periods before and during entanglement in 225 g net fragments. Concurrent behavioral 

 observations revealed a 75% reduction in time spent swimming and a 138% increase in time spent resting 

 due to entanglement. Nevertheless, the northern fur seals' mean ADMR rose from 8.0 (±0.4) W/kg to 

 9.3 (±1.9) W/kg. While this increase was primarily due to one animal's performance, it suggests that 

 entanglement may also elevate the costs of resting and grooming. 



At 17 months of age, the northern fur seals had averaged head diameters ( ± SD) of 14.7 ( ± 0.2) cm, 

 making them most susceptible to entanglement in nets with stretched mesh sizes of 23 cm or more. Obser- 

 vations showed that these juvenile fur seals were naturally inquisitive and rapidly became entangled 

 upon their first encounter with a floating net. Subsequent entanglements depended more upon each 

 animal's behavior than upon net fragment size. Captive animals were unable to free themselves from 

 the entangling fragments. 



Since the mid-1950's, the Pribilof Island population 

 of northern fur seals, Callorhinus ursinus, has 

 undergone several declines. The initial reduction in 

 population size can be attributed to a harvest of 

 adult females, conducted from 1957 through 1968 

 (York and Hartley 1981). However, from 1974 until 

 1981, the number of pups born each year continued 

 to decline (Fowler 1985; York and Kozloff 1987). As 

 a result, the present northern fur seal population 

 numbers 800,000 animals, down from an estimated 

 1.2 miUion in 1976. 



In the mid-1960's, the percentage of young male 

 northern fur seals found entangled in synthetic 

 trawl net fragments and other marine debris began 

 to rise, reaching a peak of about 0.7% in 1975 

 (Fowler 1987). Since 1976, the entanglement rate 

 has remained roughly stable at 0.4% of the subadult 

 male population. The northern fur seal population 

 declines, concurrent with the rising entanglement 

 rate, have led some authors to speculate that en- 



'Long Marine Laboratory, Institute of Marine Sciences, Univer- 

 sity of California, Santa Cruz, CA 95064; present address: P.O. 

 Box 524, Roseburg, OR 97470. 



'^Long Marine Laboratory, Institute of Marine Sciences, Univer- 

 sity of California, Santa Cruz, CA 95064. 



Manuscript accepted October 1988. 

 Fishery Bulletin. U.S. 87:85-94. 



tanglement may be one contributing factor (Fowler 

 1985, 1987). Using available data on entanglement 

 rates, net size distribution, and assumed mortality 

 rates, Fowler (1982) derived and demonstrated a 

 model that entanglement induced mortality could 

 account for the current population trends. Although 

 based on several unverified assumptions, it none- 

 theless points to the potential seriousness of net 

 entanglement. 



Several lines of indirect evidence suggest that en- 

 tanglement related mortality has its greatest impact 

 on younger age classes (less than 2-3 years old). 

 Since 1965, the at-sea survival rate of 0-2 yr old 

 northern fur seals has declined relative to the sur- 

 vival rate of nursing pups on land (Fowler 1985). 

 Prior to 1965, these parameters were positively 

 correlated. Furthermore, this decline in the ex- 

 pected survival rate is correlated with the increased 

 incidence of observed entanglements (Fowler 1985). 

 Working with captive animals, Yoshida and Baba 

 (1985) have also demonstrated that younger animals 

 entangle themselves more frequently than older 

 ones. The impact of entanglement would be more 

 severe on these smaller animals; because of their 

 size, smaller animals will suffer relatively higher 

 drag and greater power requirements during swim- 



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