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Fishery Bulletin 103(4) 



1999). In May 1998, the catch rate for pelagic-young- 

 of-the-year rockfish was the lowest in the history of 

 tri-annual rockfish surveys (Lynn et al., 1998). It is, 

 therefore, reasonable to assume that sea lions were 

 probably nutritionally stressed by the lack of prey and 

 change in prey species and found a hooked salmon an 

 attractive and easy meal. 



Mean numbers of California sea lions recorded dur- 

 ing the northward migration in summer and autumn 

 of 1998 were approximately 2000 individuals greater 

 than in the summer and autumn of 1997 and 1999, 

 most likely in response to poor foraging conditions in 

 southern California resulting from ENSO conditions 

 (Weise, 2000). During the 1983 and 1992 ENSO events, 

 numbers of sea lions increased along the central Cali- 

 fornia coast owing to the enhancement of the normal 

 northward migration of sea lions resulting from poor 

 food availability in the Southern California Bight (Syde- 

 man and Allen, 1999). During the 1983-84 ENSO, 

 older juvenile sea lions migrated in greater than usual 

 numbers from southern to central California (Trillmich 

 et al., 1991). Greater numbers of female sea lions were 

 counted on Ano Nuevo Island in summer and fall 1998, 

 presumably in response to poor foraging conditions in 

 southern California (Morris, unpubl. data s ). Increases 

 in numbers of sea lions in Monterey Bay during 1998 

 were most likely due to increases in numbers of juve- 

 niles and adult females that moved northward because 

 of the lack of schooling prey species in southern Cali- 

 fornia resulting from the ENSO. 



Presumably as a result of ENSO conditions, total 

 landings of salmon and the catch per unit of effort in 

 commercial and recreational fisheries were significantly 

 less in 1998 than in 1997. During our sampling effort 

 in 1998, approximately 2000 fewer fish were landed 

 in commercial and recreational fisheries than in 1997, 

 although approximately double the percentages of fish- 

 eries (total salmon landings) were sampled dockside. 

 Numbers of salmon landed in Monterey Bay in 1998 

 decreased by 59.6% in the commercial fishery and 49.4% 

 in the recreational fishery (PFMC 4 ). In California dur- 

 ing 1998, numbers of salmon landed in the commercial 

 fishery were 55.7% less than in 1997, and 46.7% less in 

 the recreational fishery. In 1998, CPUE of the commer- 

 cial fishery declined proportionally more than in other 

 fisheries, which corresponded to proportionally greater 

 percentages of fish taken by sea lions. In Monterey Bay, 

 numbers of angler trips in 1998 declined by 38.6% in the 

 commercial fishery, and 39.9% in the recreational fish- 

 ery (PFMC 4 ). Therefore, there were fewer boats actively 

 fishing, fewer fish being landed, and greater numbers of 

 sea lions in the area, under these conditions, when a fish 

 was hooked, it was more likely to be depredated. 



Conversely, in 1999 the depredation levels in the com- 

 mercial and recreational salmon fisheries in Monterey 



K Morris, P. A. 1999. Abstract. 13 th Biennial conference 

 on the biology of marine mammals; Maui, HI, 131 p. The 

 Society for Marine Mammology. http://www.marinemam- 

 mology.org/ 



Bay were significantly less as a result of cool and highly 

 productive La Nina oceanographic conditions. Follow- 

 ing one of the strongest ENSO events on record during 

 1997-98, there was a dramatic transition to highly pro- 

 ductive cool-water La Nina conditions and anomalous, 

 upwelling-favorable, wind forcing along the West Coast 

 (Schwing et al., 2000). Upwelling anomalies off the 

 central California coast during 1999 were the greatest 

 in the 54-year record of the upwelling index (Schwing 

 et al., 2000). Record harvest levels of Pacific sardines 

 (CalCOFI, 2000) and greater frequency of occurrence 

 of sardine in the diet of sea lions in central California 

 during the 1999 La Nina (Weise, 2000) indicated that 

 ample prey fishes were available for foraging California 

 sea lions; therefore, depredation pressure on the salmon 

 fisheries was reduced. 



Monterey Bay was selected for the present study be- 

 cause it experienced the greatest levels of depredation 

 during the 1995 commercial and recreational fisher- 

 ies season (Beeson and Hanan 1 ). Although Monterey 

 Bay experienced increased levels of pinniped predation 

 in recreational fisheries in 1997 and commercial and 

 recreational fisheries in 1998, these levels were prob- 

 ably not representative of the whole California coast 

 but were more likely the worst-case scenario. Pinniped 

 depredation may be increasing in other areas along the 

 California coast as the sea lion population increases, 

 but probably not to the degree that was observed in 

 Monterey Bay. Pinniped predation of hooked fish in 

 salmon fisheries is probably spatially and temporally 

 variable. Whereas this variability complicates evaluat- 

 ing pinniped impacts on fisheries, it is important for 

 fishery managers to take this variability into account. 



Estimated levels of depredation reported for the com- 

 mercial and recreational salmon fisheries in Monterey 

 Bay may be affected by many assumptions. Lack of 

 direct validation for information received during dock- 

 side surveys had unknown impacts on estimates of 

 predation levels, but concurrent onboard sampling ap- 

 peared to alleviate this bias. Commercial and private 

 skiff salmon boats bypass the sampling docks when 

 no fish are landed or they dock in a harbor slip. Boats 

 that bypass sampling docks may have no fish because 

 of predation by sea lions, and not sampling these boats 

 would result in underestimates of predation levels, but 

 the magnitude of this decrease was difficult to evaluate. 

 Surveys of fishermen were limited by crew cooperation 

 and therefore, not all fishing styles and locations were 

 sampled. The lack of some data would have an impact 

 on predation levels. Surveys of fishermen also were 

 limited to boats fishing for one day because boats fish- 

 ing for multiple days often fished outside the study area 

 during the course of a trip; however, boats fishing for 

 multiple days were surveyed at dockside so that any 

 biases of onboard samples would have been detected in 

 comparisons of dockside and onboard predation levels. 



Depredation of salmon by California sea lions in Mon- 

 terey Bay could negatively impact salmon populations 

 along the Central California coast. Pinniped depre- 

 dation of hooked salmon from the California Central 



