GROVER and OLLA: EFFECTS OF EL NINO ON SABLEFISH FOOD HABITS 



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liJ 2000 

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 en 



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500 



200 



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Pteropods 



Copepod 

 Naupili 



<lmm I l-2mm I >2mm 

 COPEPOOS 



Amphipods 



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Euphausid 

 Lorvoe 



Appendiculorians 



PREY TYPE 



Figure 3.— Indices of relative importance (IRI values) of dominant prey items in the 

 diet of larval sablefish, by size class and year. Only IRI values >100 are included. 

 In each case the sum of IRI values <100 accounted for <1% of the sum of all IRI values. 



our dietary observations which showed a marked 

 decrease relative to 1980. Plankton data regarding 

 the relative abundance of appendicularians and am- 

 phipods are less correlative with diet. 



Preliminary analyses of collections made off Ore- 

 gon in the spring and summer of 1983 (Miller et al. 

 1985) indicated that zooplankton density was re- 

 duced, possibly as low as 30% of that found in a non- 

 El Nino year. Indirectly lending support for this was 

 data from satellite imagery which monitored phyto- 

 plankton pigment images and which indicated that 

 primary productivity was substantially reduced dur- 

 ing 1983 (Fiedler 1984). 



Fulton and LeBrasseur (1985) suggested that 

 while interannual fluctuations in zooplankton bio- 

 mass affect planktivorous fish living in the open 

 ocean off the west coast of North America, major 

 shifts in the particle size of zooplankton may have 

 a greater effect. In particular, the extreme north- 

 ward shifting of the subarctic Pacific boundary, 

 which occurred during the 1957-58 El Nino event, 

 resulted in the replacement of large copepods with 

 small copepods off North America from lat. 40 °N 

 to 52 °N. They hypothesized that the absence of 

 large copepods and decreased biomass in these 



waters during a warm El Nino year would result in 

 reduced growth and perhaps reduced survival of 

 juvenile salmonids. 



Ontogenetic changes in the diet of sablefish lar- 

 vae included the diminution of small prey such as 

 copepod nauplii and pteropods, and the increasing 

 contribution of larger prey such as amphipods, 

 euphausid larvae, and appendicularians as larvae 

 grew. These observations parallel earlier findings 

 on prey size-selection of larval sablefish (Grover and 

 011a 1986) and agree with trends seen in many other 

 marine fish (Hunter 1981). All size classes of larvae 

 showed some flexibility in the prey they ingested, 

 from year to year as well as from station to station, 

 with large larvae ingesting the widest range of prey 

 items. The expansive range of prey ingested by large 

 larvae may have enabled them to ingest large num- 

 bers of small copepods during 1983, when larger 

 prey of high caloric value may not have been readi- 

 ly available. 



From all indications, 1983 was a year of reduced 

 planktonic productivity off Oregon and Washington 

 (Fiedler 1984; Miller et al. 1985). It was also a year 

 when P. parvus, a small copepod, was dominant both 

 in the plankton (Miller et al. 1985) and in the diet 



77 



