THE EFFECTS OF NET ENTANGLEMENT ON 



THE DRAG AND POWER OUTPUT OF 



A CALIFORNIA SEA LION, 



ZALOPHUS CALIFORNIANUS 



Interactions between pinnipeds and fisheries can be 

 broadly divided into two categories: the role of pin- 

 nipeds on the mortality of commercially important 

 fish species and the effect of commercial fisheries 

 on the dynamics of pinniped populations. Although 

 the former has received considerable attention 

 (Hirose 1977; Fiscus 1979, 1980; Matkin and Fay 

 1980; DeMaster et al. 1982), the importance of the 

 latter has been addressed only recently (Shaugh- 

 nessy 1980; Fowler 1982; Scordino and Fisher 

 19831). 



Fishery interactions may affect pinniped stocks 

 through changes in prey abundance, incidental takes, 

 or entanglement in discarded fishing gear and plastic 

 packing bands. Scordino and Fisher (fn. 1) have 

 shown that the number of entangled northern fur 



seals, Callorhinus ursiniLS, on the Pribilof Islands, 

 AK, has recently increased, and now comprises 0.4% 

 of the harvested animals. Fowler (1982^) reviewed 

 existing data concerning the accumulation of plastic 

 litter on beaches of several Alaskan islands. Using 

 the number of net fragments found on shore as a 

 rough estimate of the size distribution of material 

 adrift at sea, he concluded that at least 60% are 

 larger than those measured on fur seals. Because 

 most nets found on these animals weigh <600 g, a 

 significant mortality undoubtedly occurs at sea from 

 entanglement in larger fragments. 



This paper evaluates the hydrodynamic effect of 

 net entanglement and documents the behavior of an 

 entangled animal. A California sea lion was trained 

 to allow itself to become entangled in a twine trawl 

 net fragment and the subsequent rise in drag was 

 measured. Increased energy consumption and swim- 

 ming power requirements associated with dragging 

 net fragments were calculated from these measure- 

 ments. The results provide an initial basis for assess- 



^Scordino, J., and R. Fisher. 1983. Investigations on fur seal 

 entanglement in net fragments, plastic bands and other debris in 

 1981 and 1982, St. Paul Island, Alaska. Background paper sub- 

 mitted to the 26th Annual Meeting of the Standing Scientific Com- 

 mittee, North Pacific Fur Seal Commission, 33 p. 



^Fowler, C. W. 1982. Entanglement as an explanation for the 

 decline in Northern fur seals of the Pribilof Islands. Background 

 paper submitted to the 25th Annual Meeting of the Standing Scien- 

 tific Committee, North Pacific Fur Seal Commission, 24 p. 





y 



C 



Figure 1.— Instruments and cart used in the drag experiments. The sea lion was towed passively underwater and the resultant force 



recorded. See text for further details. 



692 



FISHERY BULLETIN: VOL. 83, NO. 4, 1985. 



