FISHERY BULLETIN: VOL. 84, NO. 4 



hake undergo as they increase in size. Thysanoessa 

 spinifera appears to be more important to larger 

 hake whereas Euphausia pacifica is more important 

 to smaller individuals. Pink shrimp and glass shrimp, 

 Pasiphaea pacifica, were consumed almost exclu- 

 sively by fish >55 cm. Eulachon and pleuronectids 

 were also predominantly consumed by larger hake. 

 Cannibalism was also observed among larger 

 individuals. 



Diel Feeding Pattern 



A number of previous researchers have postulated 

 that species in the genus Merluccius exhibit a diel 

 feeding pattern (Outram and Haegele 1972; Bow- 

 man and Bowman 1980). Alton and Nelson (1970) 

 as well as Livingston (1983) described Pacific hake 

 as nocturnal predators that migrate vertically to 

 feed near the surface during hours of darkness and 

 dive to deeper water during daylight hours. Brin- 

 ton (1967) and Alton and Blackburn (1972) showed 

 that this same vertical migration pattern exists for 

 the two species of euphausiids found in this study. 

 If the Pacific hake follow the euphausiids on their 

 vertical diel migration, the expectation is that the 

 relative proportion of both species of euphausiids 

 in the diet should not vary significantly by time of 

 day. As reported above, our findings conflict with 

 this expectation. 



To further examine this apparent deviation, we 

 considered potentially confounding factors; such as 

 differences in the distributions of the euphausiid 

 species and various size classes of Pacific hake. Brin- 

 ton (1962) reported that T. spinifera is a neritic 

 species and E. pacifica is a more oceanic species. 

 Analysis of length-frequency data from the cruise 

 during which this study was conducted shows that 

 Pacific hake of different size classes segregate by 

 depth. Pacific hake <40 cm in length made up 37% 

 of the catch in <100 m of water, but these same size 

 classes comprised 62% of the catch taken in MOO 

 m of water. Hence, the smaller individuals were 

 found in greater abundance in the habitat associated 

 with E. pacifica. 



This phenomenon of smaller fish occurring in 

 deeper water and consequently consuming greater 

 quantities of E. pacifica explains the apparent dif- 

 ference in importance of the two species of euphau- 

 siids by time of day (Fig. 2a). Only 7% of the non- 

 empty stomachs taken before 1200 h were from fish 

 <40 cm in length whereas of the fish sampled after 

 1600 h, 34% were <40 cm in length. Thus, we regard 

 the observed differences in consumption of T. spini- 

 fera and increasing importance of E. pacifica by 



time of day as spurious, confounded by the differ- 

 ences in the diets and distributions of various size 

 classes of Pacific hake. 



This study coincided with the strong presence of 

 El Nino in 1983 which may have altered the nor- 

 mal migration pattern of Pacific hake and conse- 

 quently the residence time estimates, and may also 

 have affected the abundance of the prey base. 

 Hence, there may be some error in the consump- 

 tion estimates presented herein. Miller et al. (1984) 

 noted a decline in the relative abundance of T. 

 spinifera off the Oregon coast during 1983 in com- 

 parison with other years. Thus, feeding to satiation 

 during evening hours may have been impossible; 

 consequently, feeding occurred whenever euphau- 

 siids were encountered. Additional circumstantial 

 evidence of aberrant feeding behavior of Pacific 

 hake in 1983 is their severely depressed growth 

 (Francis and Hollowed 1985). Food resources may 

 only have been sufficient for maintenance metabo- 

 lism with little energy remaining for growth. These 

 observations may explain why the diel feeding pat- 

 tern observed was weak. 



Trophic Interaction 



The seasonal migration pattern and consequent 

 latitudinal stratification of Pacific hake stocks by 

 size class makes it difficult to compare food habit 

 studies conducted at different times of the year and 

 at different locations on the Pacific coast. Nonethe- 

 less, examining only the role of pink shrimp in the 

 diet, we find first mention of Pacific hake preying 

 on this species by Gotshall (1969a, b). Analyzing 

 Pacific hake stomachs collected off California be- 

 tween 1966 and 1969, Gotshall found high incidences 

 (54% frequency of occurrence) of pink shrimp dur- 

 ing late summer and early fall, particularly in Pacific 

 hake collected over shrimp beds. The study was an 

 attempt to use Pacific hake as biological samplers 

 to estimate pink shrimp abundance, focusing sam- 

 pling effort on known pink shrimp beds, and, as 

 such, the sampling design was quite different from 

 other studies. 



Outram and Haegele (1972) reported that 3% of 

 the Pacific hake stomachs collected off the coast of 

 British Columbia contained pink shrimp. Pink 

 shrimp were found in 5.7% of the Pacific hake 

 stomachs collected during the summers of 1965 and 

 1966 of f Washington and Oregon (Alton and Nelson 

 1970). Livingston and Alton (1982) found that pan- 

 dalid shrimp constituted 0.3% by weight of the con- 

 tents of the 1,430 stomachs of Pacific hake taken 

 off the coasts of Washington and Oregon during the 



954 



