BRODEUR and PEARCY: FOOD HABITS AND DIETARY OVERLAP OF SOME SEBASTES 



S. pinniger and S. flavidus at this time. An 

 inshore-offshore peak in the biomass of midwater 

 collections occurred on the edge of the continental 

 slope off the central Oregon coast (lat. 44° 39' N), a 

 zone where oceanic macrozooplankton and micro- 

 nekton may be concentrated by advection (Pearcy 

 1976). This is the region where pelagic-feeding 

 rockfishes are often concentrated (Gabriel and 

 Tyler 1980). 



The majority of the prey species found in the 

 stomachs of the rockfish species examined are 

 pelagic species that undertake extensive diel 

 vertical migrations and are important compo- 

 nents of the biological sound scattering layer in 

 the Northeast Pacific (Pearcy and Laurs 1966; 

 Brinton 1967; Pearcy and Mesecar 1971; Pearcy 

 1972; Alton and Blackburn 1972). In this study, 

 both of the euphausiid species of interest, E. 

 pacifica and T. spinifera, have been found to have 

 substantially different daytime and nighttime 

 vertical distributions. According to Alton and 

 Blackburn (1972), catch rates of T. spinifera 

 off the coast of Washington were the highest near 

 the bottom during the early evening hours (1800- 

 2000 h) and at the surface a few hours later (2100- 

 2300 h). 



The diurnal downward migration of these orga- 

 nisms over the continental shelf may result in a 

 substantial biomass in close proximity to near- 

 bottom predators, such as rockfishes, which feed 

 on pelagic prey during the day. Deeper migration 

 to daytime depths typical of their more open ocean 

 conspecifics is restricted by the shelf, especially 

 in shoaler areas such as Heceta Bank. Isaacs and 

 Schwartzlose (1965) found dense populations of 

 predators, including many rockfishes, on shallow 

 banks off California; these predators presumably 

 take advantage of net inshore transport by cur- 

 rents of oceanic organisms over the bank. Pereyra 

 et al. (1969) reported high incidences of predation 

 on mesopelagic organisms by aggregations of S. 

 flavidus residing on the shelf edge near a deep 

 canyon. Vertically migrating mesopelagic orga- 

 nisms may also constitute an important food 

 source for many species of slope fishes (Sedberry 

 and Musick 1978). 



Diel vertical distribution patterns of offshore 

 rockfishes are not well documented. Based on 

 acoustic observations, Westrheim (1970) con- 

 cluded that schools of Pacific ocean perch move off 

 bottom at night. Pereyra et al. (1969) and Love 

 (1981) caught rockfishes that were apparently 

 feeding well off the bottom at night. Lorz et al. 

 (1983) concluded that S. flavidus off Washington 



fed on euphausiids during night or early morning 

 hours, when these euphausiids would be expected 

 to be in surface waters. Similar migrations were 

 seen on Heceta Bank during this study. Figure 10 

 shows an acoustic 33 kHz transect taken across 

 Heceta Bank during the late morning (about 1023- 

 1050 h PST). Many large "spikes" offish aggrega- 

 tions were apparent extending over 100 m above 

 bottom. Some of these were probably caused by 

 rockfish ascending in the water column to feed. 

 Figure 11 is a 33 kHz echogram on Heceta Bank 

 made around 1800 h PST The "haystacks" shown 

 are characteristic of tight aggregations of S. 

 pinniger just above bottom (Barss 6 ) and may 

 represent feeding aggregations. Also visible in 

 this echogram is more diffuse scattering in the 

 water column (20 m off bottom) probably caused 

 by euphausiids. The tow made concurrently with 

 this trace did yield a large catch of rockfish (97% 

 S. pinniger), most of which had stomachs full of 

 fresh euphausiids. This stratification of large 

 sound scatterers below diffuse midwater scatter- 

 ing prey was often observed during the acoustic 

 surveys. Atlantic cod appear to interact with 

 pelagic prey in a similar fashion (Brunei 1965; 

 Pearcy et al. 1979; Falk-Peterson and Hopkins 

 1981). 



The two primary species examined in detail in 

 this study appear to forage mainly during the 

 midday and evening dusk periods, although sam- 

 pling was limited during nighttime. The similar 

 diel patterns of feeding intensity suggest that 

 temporal partitioning of feeding time is not occur- 

 ring between S. pinniger and S. flavidus. The 

 differing utilization patterns of euphausiids and 

 fishes seen for the two species (Table 13) may be 

 related to the vertical positioning of the two 

 species in the water column. Sebastes flavidus 

 may feed high in the water column, prey upon 

 adult herring and pelagic juvenile fishes during 

 the daytime, and intercept euphausiids during 

 crepuscular periods, whereas S. pinniger may 

 stay nearer the bottom where they may feed al- 

 most exclusively on increased daytime aggrega- 

 tions of euphausiids. 



The occurrence of a high percentage of empty 

 stomachs and generally low feeding intensity 

 indices in S. alutus, which were caught mainly 

 in late afternoon in our study, suggests that this 

 species is more nocturnal in its feeding patterns, 

 assuming that this species has similar regurgita- 



6 W. Barss, fishery biologist, Oregon Department of Fish and 

 Wildlife, Newport, OR 97365, pers. commun. December 1980. 



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