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



bution of each prey type was essentially the same, 

 except that invertebrate eggs contributed slightly 

 more (in all cases <0.1%) to the total volume when 

 the spheroidal shape was assumed. After the volume 

 of each individual prey was calculated, a median 

 volume for each prey type was determined. Median 

 values were used because they are free from implicit 

 assumptions regarding the normality of these data. 

 Each median volume was then multiplied by the raw 

 data from the numerical percentage composition 

 analysis to produce corresponding volumetric per- 

 cent composition data. Data were examined by size 

 group and year. 



The three analyses (%N, %F0, and %VOL) were 

 coalesced to yield a more comprehensive assessment 

 of prey importance, the index of relative importance 

 (IRI = (%N + %VOL) X %FO)(Pinkasetal. 1971). 

 For this combined analysis copepods were classified 

 by size. Copepods <1 mm in length (TL) included 

 Corycaeus anglicus, Oithona similis, Oncaea sp., 

 Paracalanus parvus, and copepodites. Broken cope- 

 pods of an indeterminable size were classified as 

 unidentified small copepods. Copepods 1-2 mm long 

 included Pseudocalanus sp., Clausocalanus spp., 

 Ctenocalanus vanus, Aedideus sp., Scolecithricella 

 minor, and Acartia spp. Calanus spp. and Metri- 

 dia lucens were the only copepods >2 mm that were 

 ingested. 



RESULTS 



Composition of the Diet of 

 Small Larvae (<12.5 mm SL) 



During 1980, copepod nauplii comprised over 80% 

 of the prey items ingested by number, although 

 copepods <1 mm long contributed slightly more to 

 the diet in terms of volume (Table 1, Fig. 2). The 

 index of relative importance, IRI, (Pinkas et al. 

 1971) indicated that nauplii were more important 

 than copepods (Table 1, Fig. 3). Many of the cope- 

 pods <1 mm that were ingested by small larvae were 

 in pieces and impossible to identify. Of those that 

 could be identified, Oithona similis was the domi- 

 nant species, followed by Paracalanus parvus (Table 

 2). The only noncopepod prey of much consequence 

 was pteropods (Figs. 2, 3), locally comprising as 

 much as 45.7% of the diet, by number. 



In 1983, copepod nauplii were again the dominant 

 prey in terms of number, but copepods represented 

 more than twice as much in volume as they did in 

 1980 (Table 1, Fig. 2). While both were significant 

 components of the diet, copepod nauplii had a slight- 

 ly larger IRI value than copepods <1 mm (Fig. 3). 



Although the proportion of 0. similis in the diet was 

 similar in both years, P. parvus contributed more 

 to the diet during 1983 (Table 2). A comparison of 

 the sizes of copepods ingested during 1980 and 1983 

 (Table 3) showed that more small copepods were 

 eaten in 1983, both in terms of the number of cope- 

 pods ingested (x^ = 8.46, 1 df, P < 0.005) and 

 volume (x^ = 755.32, 1 df , P < 0.001). 



Composition of the Diet of 

 Medium-Sized Larvae (12.6-20.5 mm SL) 



While copepods ranked first in all analyses in 

 1980, other major prey items varied with method 

 of analysis (Table 1, Fig. 2). From the IRI it is clear 

 that copepods <1 mm were the most important prey, 

 with 0. similis the dominant species by number 

 (Table 2), and P. parvus dominant by volume. In 

 addition, sizable numbers of 1-2 mm copepods, 

 especially Acartia longiremis, were ingested at 

 several stations. Copepod nauplii ranked second in 

 IRI value. Of noncopepod prey items pteropods, 

 euphausid larvae, and appendicularians were all of 

 consequence in the diet. 



In 1983, while copepods <1 mm were again the 

 principal prey, they comprised a greater portion of 

 the diet than in 1980, with P. parvus dominating 

 both in terms of number (Table 2) and volume. Cope- 

 pod nauplii ranked second by number, but accounted 

 for little prey volume, while euphausid larvae, the 

 only noncopepod prey to contribute substantially to 

 the diet, ranked higher than nauplii both in terms 

 of volume and IRI. Of the copepod species ingested, 

 the most striking difference between the years was 

 the increased contribution of P. parvus in 1983 

 (Table 2). 



Copepods >1 mm contributed more to the diet in 

 1980 than 1983 while small copepods comprised a 

 greater portion of the diet during 1983 (Table 3), 

 both in terms of number (x^ = 130.25, 1 df , P < 

 0.001) and volume (x^ = 9906.86, 1 df , P < 0.001). 



Composition of the Diet of 

 Large Larvae (20.6-28.5 mm SL) 



While ranking second to small copepods by num- 

 ber and second to large copepods by volume, appen- 

 dicularians comprised more than 30% of the diet by 

 number and volume during 1980, and ranked highest 

 in IRI value, slightly ahead of copepods <1 mm 

 (Table 1, Fig. 3). In contrast, euphausid larvae and 

 amphipods were the most important noncopepod 

 prey items in the diet during 1983, although each 

 contributed <15% to the diet. In 1983 the diet 



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