Biological and physical processes in and around Astoria submarine Canyon, Oregon, USA

Journal of Marine Systems 50 (2004) 21 – 37 www.elsevier.com/locate/jmarsys

Biological and physical processes in and around Astoria submarine Canyon, Oregon, USA Keith L. Bosley a,*, J. William Lavelle b, Richard D. Brodeur c, W. Waldo Wakefield a, Robert L. Emmett c, Edward T. Baker b, Kara M. Rehmke d a

NOAA Fisheries, Northwest Fisheries Science Center, Fishery Resource Analysis and Monitoring Division, Hatfield Marine Science Center, 2032 S.E. OSU Drive, Newport, OR 97365, USA b NOAA Pacific Marine Environmental Laboratory, OAR 7600 Sand Point Way NE, Seattle, WA 98115-6349, USA c NOAA Fisheries, Northwest Fisheries Science Center, Fish Ecology Division, Hatfield Marine Science Center, 2030 S.E. OSU Drive, Newport, OR 97365, USA d Oregon State University, Hatfield Marine Science Center, 2030 S.E. OSU Drive, Newport, OR 97365, USA Received 13 February 2003; accepted 10 June 2003 Available online 21 July 2004

Abstract Astoria Canyon represents the westernmost portion of the Columbia River drainage system, with the head of the canyon beginning just 16 km west of the mouth of the Columbia River along the northern Oregon and southern Washington coasts. During the summer of 2001, physical, chemical, and biological measurements in the canyon were taken to better understand the hydrodynamic setting of, and the feeding relationships among, the pelagic and benthic communities. Results show that currents were strongly tidal, and transport, where measured, was primarily up and into the canyon below shelf depth as previous studies in the canyon have shown. Temperature time series suggests that the largest diurnal oscillations occurred at, or were trapped near, the bottom of the canyon. Within the upper canyon, subtidal temperature was correlated with upper-level shelf-edge currents, linking subtidal upwelling events in the canyon with near-surface subtidal along-shore flow. Invertebrates, such as shrimp, euphausiids, and squid, as well as mesopelagic fishes, dominated the Isaacs – Kidd midwater trawl catches along the canyon walls. Large trawl catches were comprised mainly of hake and rockfishes (shallow trawls) and macrourids, scorpaenids, stomiids, and zoarcids (bottom trawls). Gut-content analysis of rockfishes and lanternfishes revealed substantial use of midwater prey such as euphausiids and mesopelagic fishes. The d13C values of fishes and invertebrates reflected local primary production, as indicated by particulate organic matter (POM) d13C values from samples collected at various depths along the axis of the canyon, as well as across the canyon at several sites. The d15N values of fishes and invertebrates indicated lanternfishes, along with euphausiids, amphipods, shrimp and squid, may be important dietary components of higher-trophiclevel fishes in both the benthic and benthopelagic food webs. The d13C and d15N values of Sebastes species showed significant enrichment in the adults of species that are largely piscivorous relative to the values of adults of more omnivorous species. D 2004 Elsevier B.V. All rights reserved. Keywords: Submarine canyons; Astoria canyon; Currents; Zooplankton; Micronekton; Fish; Stable isotopes

* Corresponding author. Tel.: +1-541-867-0506; fax: +1-541-867-0505. E-mail address: [email protected] (K.L. Bosley). 0924-7963/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmarsys.2003.06.006

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1. Introduction Canyons represent just one type of abrupt topography in which enhanced productivity is often found. Factors that lead to enhanced productivity in canyons are the regional and local topography, the regional and local nutrient supply, vertical migration habits of species, and the ocean currents within and above the canyon. Genin (2004) has reviewed the biophysical mechanisms by which fishes and their prey resources become aggregated near abrupt topography. These include funneling and trapping of plankton (Koslow and Ota, 1981; Greene et al., 1988; Macquart-Moulin and Patriti, 1996) and counter-upwelling depth retention (Allen et al., 2001). The topographic complexity of canyons, the time-varying nature of flow and forcing, and the difficulty in sampling behaviorally dynamic biological populations in physically dynamic environments, however, make fully unraveling trophic pathways in these environments a challenge. Compared to neighboring shelf and slope regions, submarine canyons exhibit intensified flow and turbulence. Wind-driven, pressure-driven, and tidal flows dominate motion (e.g., Hickey et al., 1986; Hunkins, 1988; Noble and Butman, 1989; Lafuente et al., 1999). Focusing of flow by canyon topography makes canyons a major site in cross-slope/cross-shelf exchange (e.g., Hickey, 1997). Freeland and Denman (1982), noting the persistent pool of nutrients on the shelf at the head of the Juan de Fuca Canyon in the Northeast Pacific Ocean, recognized that canyons may serve as conduits of deep nutrients to shelf water (see also Sobarzo et al., 2001). Internal tide focusing and breaking in canyons (Gardner, 1989) can lead to intensified mixing near canyon bottoms (Lueck and Osborn, 1985; Carter and Gregg, 2002; Kunze et al., 2002; Petruncio et al., 1998). Increased turbulence in the bottom f 100 m leads to resuspension, a slowing of the deposition of particulate matter descending from the euphotic zone, and to higher levels of particulates and nutrients in the water column within the canyon. Density-driven, down-canyon flows are also not uncommon and they can temporarily change habitats along a canyon floor by scouring the seafloor, by increasing turbidity, and through sediment deposition (Kampf and Fohrmann, 2000).

Astoria Canyon, along the west coast of North America and just beyond the mouth of the Columbia River, is considered to be a steep and narrow canyon (Hickey, 1997). Flow is marked by spatial variability at length scales of a few kilometers or less; diurnal and semi-diurnal tidal motions account for much of the variance (Hickey, 1997). This results in suspended matter fields that are marked by considerable temporal variability (Plank et al., 1974). During times of maximum upwelling (southward along-shore near surface flow), flow across the entire width of the canyon is landward, but prior to and after maximum upwelling, canyon flow is marked by strong lateral shear (Hickey, 1997). A cyclonic vortex at the head of canyons is typical of flow during such times (Allen, 1996; Hickey, 1997; She and Klink, 2000) and, in the case of Astoria Canyon, it extends as much as 50 m above canyon rim depth. Water properties are domed upward at the head of some canyons (Hickey, 1995) in response to upwelling. In Astoria Canyon, shelf flows crossing the canyon in the near surface (0 –100 m) are not deflected (Hickey, 1997). Stable-isotope ratios present a useful tool for studying the processes, connections, and energy flow within aquatic ecosystems (see reviews by Fry and Sherr, 1984; Owens, 1987; Peterson and Fry, 1987). Two elements, carbon and nitrogen, are particularly useful as natural tracers of both the flow of organic matter and food-web structures in marine ecosystems. It is possible to obtain time-integrated descriptions of diets, sources of prey, and the trophic status of a fish by quantifying the ratios of 15N/14N and 13C/12C. The carbon sources in a geographic area can be distinguished isotopically because the relative amount of 13 C in the tissues of primary producers varies depending on the mechanism of carbon fixation and often varies substantially between habitats that are dominated by different types of primary producers (Fry and Sherr, 1984). The trophic level of a secondary consumer can be inferred because heavier isotopes, especially 15N, are retained in the tissues of a consumer in increasing amounts relative to the amount of heavier isotopes found in its diet, a process referred to as fractionation (DeNiro and Epstein, 1981). A onetrophic-level difference in nitrogen stable isotope levels is generally considered to be 3.0 to 3.5 per

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mil (x) (DeNiro and Epstein, 1981). That is, if a primary producer had a value of 0x, a primary consumer that feeds on it should have a value of 3.0x to 3.5x. A secondary consumer feeding on the primary consumer would be expected to have a value of 6.0x to 7.0x, and so on. Hill and Wheeler (2002) examined organic carbon and nitrogen in surface waters off of Oregon and Washington to characterize three distinct water sources: oligotrophic offshore water, the Columbia River plume, and the coastal upwelling region. They found that the river plume had the highest levels of both total and dissolved organic carbon. Previous studies have shown that potential sources of carbon in marine and coastal ecosystems will have different d13C signatures, and they used the carbon isotopic composition of suspended particulate organic matter (POM) and sediments as a means of calculating the amount of terrestrial carbon that is contributed to freshwater and marine systems (Fry and Sherr, 1984). A prerequisite for using isotope ratios as tracers in food webs is a detectable and consistent difference in the isotopic signature between its components. In areas of the oceans where there are different sources of primary production, along with diverse assemblages of consumers, such as along a continental shelf or slope, stable isotopes can be used in conjunction with traditional methods (i.e. stomach content analysis) to study feeding relationships and the relative contributions of different primary producers (Perry et al., 1999; Polunin et al., 2001; Davenport and Bax, 2002). Astoria Canyon is a highly productive fishery region. The canyon is home for many pelagic fish species, and many years of surveys have found extensive groundfish resources there as well. Despite the importance of its fisheries resources, we know little about the pathways leading from primary nutrients to higher trophic levels in Astoria Canyon. To date, the only study of the food habits of fishes in Astoria Canyon was that of Pereyra et al. (1969), which examined the feeding ecology of adult yellowtail rockfish that utilized mesopelagic prey resources. In this work, we report on biological, chemical, and physical measurements taken in Astoria Canyon aimed at further elucidating feeding relationships and the physical environment in which these trophic transfers occur.

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2. Materials and methods 2.1. Study area This study was conducted during a NOAA Ocean Exploration cruise aboard the R/V Ronald H. Brown from June 27 through July 3, 2001 in Astoria Canyon. The head of the canyon begins at a bottom depth of 120 m just 16 km west of the present river mouth (Fig. 1). The axis of the canyon extends from well within the continental shelf zone, west-southwest for approximately 110 km, where, at a bottom depth of more than 2000 m, it transitions into a deep-sea channel. The width of the canyon ranges from 9 km where it cuts across the shelf to 70 km where it transitions into Astoria Channel (Carlson, 1968). Astoria Canyon is the southern-most submarine canyon of a series of canyons that bisect the continental margin of the Pacific Northwest off British Columbia and Washington. Rogue Canyon, far to the south, is the only other significant canyon feature on the Oregon margin (Underwood, 1991). Astoria Canyon lies within a major upwelling region and underlies the Columbia River Plume, a major hydrographic feature along the coasts of Oregon and Washington (Hickey, 1989). Our study was confined to the eastern portion of the canyon. 2.2. Physical measurements Two taut-wire moorings were deployed in the canyon on June 29 to measure currents and temperatures. The moorings collected data during the intensive 1-week biological sampling period (June 27 – July 3) and remained in the canyon through August 2 in order to sample during one entire lunar cycle. The Aanderaa RCM-7 current meters recorded speed, averaged over the sampling interval, and instantaneous direction half-hourly. Moorings M-118 and M-119 were located at 46j13.8VN, 124j27.0VW and 46j11.0VN, 124j39.40VW, respectively. M-118, in 337-m water depth, recorded currents at a depth of 275 m, and M-119 recorded currents at 50, 275, and 325 m in 400-m water depth. M-118 was located nearer the head of the canyon, and M-119 was located where the canyon and upper slope intersect (Fig. 1). Temperatures were recorded using Miniature Temperature Recorders (MTR) and Miniature Autonomous

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Fig. 1. Sampling locations in Astoria Canyon. The CTD stations are indicated by single upper-case letters/open circles; the IKMT trawl sites begin with the letter ‘‘I’’/solid star; the Sea Eagle trawl sites begin with the letter ‘‘R’’/solid triangle (midwater rope trawl) or ‘‘B’’/open square (bottom trawl); and the mooring locations begin with the letter ‘‘M’’/solid square. Bathymetry is adapted from NOAA National Ocean Survey Seamap Series for the North Pacific Ocean, no. 12042-12B. Depth contours are in meters. Only the 150-, 1000-, and 2000-m contours are featured on the inset map to show the relative position of the shelf break.

Plume Recorders (MAPR) (Baker and Milburn, 1997). On M-118, temperatures were sampled halfhourly at depths of 60, 100, 125, 160, 200, 250, 290, and 340 m, and on M-119, at depths of 100, 150, 190, and 270 m. Wind speed and direction 5 m above sea level were sampled hourly from Buoy 46029 by the NOAA National Data Buoy Center. Buoy 46029 at 46j07V00U N and 124j30V36U W in 128 m of water is located 13 km SSW of M-118 and 13 km ESE of M119. The buoy’s location makes its measured winds representative of those over the canyon (Fig. 1). Spectral analysis of time series involved removing the mean and a linear trend from each time series, cosine tapering 10% of both ends of the resulting record, Fourier analyzing, and then band smoothing the resulting periodograms using a Hanning spectral window with an 11-point width (e.g. Emery and Thomson, 1997). CTD casts were made at several stations across the canyon and along its axis from the R/V Brown. A CTD rosette containing multiple Niskin bottles was lowered at each station and water samples were collected from just below the sea surface, from the midwater (based

on station depth) and from just above the sea floor. Replicate water samples were first filtered through 28Am mesh to remove larger particulates and then through Whatman GF/F filters to collect particulate organic matter (POM). The filters were frozen for further processing on land for isotopic analysis (see below). 2.3. Biological sampling Most of the biological samples and data were collected from the R/V Brown. Multi-frequency acoustic data were collected from continuous transects across the canyon using an EK-500 echosounder receiving data from 38-, 120-, and 200-kHz transducers. Complete analyses of these acoustic data are beyond the scope of this paper and will be presented elsewhere. The acoustics were employed to target midwater and bottom trawling on particular signals and acoustic layers. Three horizontal tows for micronekton were made with an IKMT (Isaacs –Kidd midwater trawl) with a mouth area of 5.4 m2. The main body of the net was comprised of 10-mm stretch

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mesh, with 500-Am mesh in the codend. A large V-fin depressor situated at the front of the net increased the sampling depth for the given amount of wire out. A calibrated General Oceanicsk flow meter suspended in the mouth of the net provided data on the amount of water filtered by the trawl. The depth of each tow was determined using a MAPR device. All fishes and invertebrates caught were identified by species and weighed in the laboratory to determine biomass. Selected species from these tows had white muscle tissue dissected for stable-isotope analysis and stomachs dissected for content analysis and were handled as described above. Whole invertebrates also were frozen for further processing and isotopic analysis. Some biological sampling was carried out from a commercial fishing vessel, the F/V Sea Eagle, during the same time period. Fishes and invertebrates were collected from the Sea Eagle using a bottom trawl that featured a ‘‘rock hopper’’ footrope (number of tows = 2) and using a Nordic 264 rope trawl (n = 7) to assess species diversity and abundance around the canyon. Fish that were collected in the trawls were sorted by species and then weighed, measured, and counted. Selected species had white muscle tissue dissected for stable-isotope analysis and stomachs dissected for content analysis. The muscle tissue was dissected from just below the dorsal spines or fins of each fish and then frozen. The stomachs were preserved in 10% formalin. Whole invertebrates also were sorted by species, and selected animals were frozen for further processing and isotopic analysis. 2.4. Laboratory analyses Prior to analysis, muscle-tissue samples were first thawed and then dried completely at 55 jC. The samples were ground using a Wig-L-Bugk (Dentsply) automated mortar and pestle and then loaded into tin capsules. Filter samples also were thawed and dried completely at 55 jC and then acidified with 1 N HCl to remove any inorganic carbon. The stableisotope analysis was carried out using a Costech elemental analyzer coupled to a Thermo Finnegan stable-isotope-ratio mass spectrometer in the continuous-flow mode, with ultra-high-purity helium as the carrier gas. The stable-isotope ratios (15N/14N and 13 12 C/ C) were reported as d15N and d13C, with units of per mil (x) difference relative to standards; N2 in

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air for nitrogen and PDB (Peedee belemnite) (Craig, 1957) for carbon. Lipids are depleted in 13C relative to other biochemical fractions (DeNiro and Epstein, 1977, 1978; Tieszen et al., 1983). Since lipids were not removed prior to the analysis of muscle-tissue samples, final fish and invertebrate d13C values were normalized (McConnaughey and McRoy, 1979) to account for the depletion effect of lipids.

3. Results 3.1. Physical environment of the canyon Surface winds measured south of the canyon had a vector-averaged speed of 3.4 m s 1 and direction of 149j (Fig. 2A), typical of southward winds along the Oregon coast during spring and summer (Hickey, 1997). The spectra of wind over the interval June – August 2001 exhibited both diurnal and semidiurnal peaks. Currents measured at 50-m depth on M-119 (Fig. 2C) showed a strong vector-averaged flow of 16.4 cm s 1 to the south (181j), with no indication of flow reversal. Spectra analysis of the record confirmed a strong semi-diurnal, nearly linear, oscillatory component, a substantial inertial frequency signal, but a very weak diurnal oscillation. Currents at 225 m (Fig. 2D) and 325 m (not shown) at M-119 flowed primarily to the NNE (24j and 17.6j, respectively) along the trend of the slope isobaths at vector-averaged speeds of 6.8 and 3.6 cm s 1, possibly showing evidence of the influence of the California undercurrent. Currents in the upper canyon (M-118) at 225-m depth (Fig. 2B) were weaker (mean speed = 0.8 cm s 1) and more variable in direction, with the overall vector-averaged direction of 96j in the direction of the canyon wall. Spectra of data from the three deepest meters showed almost no inertial oscillation, a strong rectilinear semi-diurnal signal, and strong diurnal signals as well. Diurnal motions, which were clearly intensified by the canyon topography, were rectilinear at 225 and 275 m but nearly circular at the 325-m deep site (not shown). Temperatures (T) recorded at M-118 (Fig. 3A) showed substantial temporal variability and sizeable vertical gradients. Between 50 and 325 m, the temporal mean of T differed by 1.5 jC (Fig. 3B), roughly

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Fig. 2. Vector time series of (A) winds measured offshore, just south of Astoria Canyon (46j07V, 124j30.6VW); (B) currents in the upper part of the canyon at M-118 and 275-m depth; (C) currents in the upper water column (50-m depth) over the canyon but farther offshore at M-119; (D) currents at M-119 but below shelf depth (225 m).

equivalent to the magnitude of the temporal variance around that mean at each depth. The sequence of spectra (Fig. 3C) showed a semi-diurnal signal at all depths, and a diurnal signal that was very weak at the upper meters but intensified below shelf depth in the canyon. Tidal oscillations in T recorded at 200-m depth on the upper canyon mooring (M-118) were made more clearly visible in Fig. 4 (solid line). Tidal oscillations in the north – south component of currents measured in the near surface waters (50m depth) at mooring M-

119, nearer the canyon mouth, were even more apparent (dotted line). CTD observations showed high spatial (vertical and horizontal) and temporal variability in both the hydrography and turbidity of the canyon waters. In the upper canyon, a broad layer of intense light scattering was common between 200- and 300-m depth, likely a remnant of resuspension and advection of adjacent shelf sediments. Just above the canyon floor, a 50-m-thick layer of sharply increased light scattering, lower temperature, and increased salinity

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Fig. 3. (A) Eight time series of temperatures (T) recorded at M-118 at depths (starting with the uppermost series) of 60, 100, 125, 160, 200, 250, 290, and 340 m. (B) Profiles of temperature representing the mean, maximum, and minimum T from each of the time series. (C) Spectra of time series each successively offset by one unit on the y-axis. The lowest curve (not offset) represents data from 60-m depth.

suggest up-canyon flow vigorous enough to create a thick layer of resuspended bottom sediments. Lightscattering values in this layer increased with decreasing canyon depth. Even in the upper canyon, vertical stratification was sufficient to maintain pronounced layering of the fine-grained suspended particulate matter. The vertical and horizontal variability was not as evident in the d13C data from the POM that was collected, but there were some measurable differences

(Table 1). Overall though, the POM data varied little between the stations around the canyon, regardless of depth. The mean d13C was 23.82x with a standard deviation (S.D.) of 1.10x (n = 22). The station with the greatest d13C variability between surface and bottom POM was station J, which ranged from a low of 25.25x near the bottom to a high of 21.40x near the surface. Station J was the deepest station sampled and was located in the heart of the canyon (see Fig. 1). Several stations had surface POM

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Fig. 4. Along-shore current speeds (dotted line) at 50-m depth on M-119, plotted over temperatures (solid line) measured in the upper part of Astoria Canyon on M-118 at 200-m depth.

d13C that were nearly identical to bottom POM d13C. One was station G, located just up-canyon from station J, with a depth of more than 700 m. Two of these were stations B and F, which were the northernmost and southernmost stations, respectively, along a cross-canyon transect and were among the shallowest sites sampled. 3.2. Biological observations An example of the bioacoustic data collected for choosing trawl locations is shown in Fig. 5. In this example (from a 120 kHz profile, taken over the north wall of the canyon), several distinct layers at shelf depth, along the canyon wall, over a pinnacle at f 225-m depth and at approximately the same depth

Table 1 Particulate organic matter d13C values from water samples collected at stations located around Astoria Canyon at the surface, midwater, and near the bottom Surface Station Station Station Station Station Station Station Station Station

A B C D E F G I J

22.83 23.38 22.37 – – 22.27 24.48 23.96 21.40

Midwater

Bottom

22.95 23.14 23.79 – 24.43 24.29 – 25.24 23.66

Dashed lines represent locations that could not be sampled.

24.44 23.81 24.94 25.14 22.48 24.63 25.06 25.25

within the canyon proper can be seen. On the basis of the IKMT and trawl data (described below), the layers were later found to be correlated with catches of rockfishes in close proximity to the canyon walls and pinnacles and with euphausiids and Pacific hake (Merluccius productus) in the layers farther away from these topographic features, but still within the canyon. Two IKMT trawls were made along the northern side of the canyon and one on the south side (Fig. 1). Tow 1 (to 89 m) targeted shallow acoustic layers along the northern canyon wall, and caught mostly larger zooplankton (decapod larvae, euphausiids, hyperiids, chaetognaths, and fish larvae), but no large nekton (Table 2) were collected in this tow. The two deeper tows (to 222 and 264 m) caught a variety of large mesopelagic organisms, including midwater shrimps, squids, and fishes (Table 2). The northern lampfish (Stenobrachius leucopsarus) dominated the catch by number and weight in tow 3 (Table 2). However, in tow 2, the catch was predominantly made up of the offshore species of euphausiid, Euphausia pacifica; the trawl apparently passed through a layer of this species. Nine fish/decapod trawls (Fig. 1) were made from the Sea Eagle. Data from only three of the hauls (R1, B1, and B2) are reported here (Table 3) because they were the only ones from which tissue samples and stomachs were collected for comparing stable-isotope data to stomach content data. Similar to the IKMT tows, tow R1 fished in midwater and caught a low diversity of taxa, which with the exception of some

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Fig. 5. A 120-kHz bioacoustic profile taken on a north-to-south transect across the north wall of Astoria Canyon, Oregon. The vertical gridlines represent 10-min time intervals.

Pacific hake, was comprised entirely of rockfishes of the genus Sebastes. The deepest bottom tow, B1 (max. depth = 549 m), caught a relatively high diversity of fishes, including grenadiers, thornyheads (Sebastoloobus alascanus, Sebastolobus altivelis), sablefish (Anoplopoma fimbria), viperfish (Chauliodus macouni), and snailfishes (Family Liparididae), but no rockfishes. There was also a fairly substantial catch of tanner crabs (Chionoecetes spp.) in that haul. Finally, the last tow (B2) contained mostly thornyheads, eelpouts (Family Zoarcidae), and Dover sole (Microstomus pacificus) but few other species. The stomach content data are presented in Table 4. Of the rockfish stomachs that were analyzed, widow rockfish (Sebastes entomelas) were found to have primarily unidentifiable gelatinous material; yellowtail rockfish (Sebastes flavidus) were found to have euphausiids, myctophids, and squids; and bocaccio rockfish (Sebastes paucispinis) were found to have only Pacific ocean perch (Sebastes alutus) and other fish remains in their stomachs. The stomach contents of the myctophid, California headlightfish (Diaphus theta), were found to contain primarily euphausiids (E. pacifica) and hyperiid amphipods, while the

northern lampfish (S. leucopsarus) had a similar diet but also consumed copepods to some extent (Table 4). 3.3. Stable-isotope analyses The carbon and nitrogen stable-isotope data for individual fishes and invertebrates are presented in Fig. 6A. The isotopic values of most of the fishes that were sampled were enriched in heavier isotopes of both carbon and nitrogen relative to the invertebrates that were collected. To better compare the trophic position of the rockfishes, d13C and d15N values of only the Sebastes species are presented in Fig. 6B. Both the d13C and d15N values were significantly different between rockfish species ( p < 0.0001, SAS General Linear Model). Of the five species of rockfish that were sampled in the pelagic tow, bocaccio rockfish (n = 9) was the most enriched in 13C and 15N, which is in agreement with the findings of the stomach-content analysis indicating substantial piscivory by this species. A post-hoc, pairwise comparison determined that bocaccio rockfish d13C values were significantly enriched relative to both yellowtail rockfish (n = 17) and widow rockfish (n = 17), and canary

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Table 2 Standardized densities (number per 1000 m3) and biomass (grams per 1000 m3) of zooplankton and micronekton collected in each Isaacs – Kidd Midwater Trawl (IKMT) in Astoria Canyon in 2001

Cnidaria Hydromedusae unidentified Atolla sp. Siphonopora Siphonophora unidentified Cephalopoda Gonatus onyx Taonius pavo Crustacea Cancer magister Munida quadrispina Sergestes similis Pasiphaea pacifica Pandalus jordani Thysanoessa spinifera Euphausia pacifica Boreomysis califonica Holmesiella anomola Phronima sedentaria Paraphronima gracilis Primno macropa Themisto pacifica Cyphocaris challengeri Ampelisca sp. Karoga megalops Stilipes distincta Chaetognatha Eukhronia hamata Sagitta elegans Sagitta scrippsae Osteichthyes Sebastes spp. Stenobrachius leucopsaurus Stenobrachius nannochir Diaphus theta Tarletonbeania crenularis Bathylagus stilbius Chauliodus macouni Lycodapus spp. Nectoliparis pelagicus Lyopsetta exilis Glyptocephalus zachirus

Tow 1 (max. depth = 89 m)

Tow 2 (max. depth = 222 m)

Tow 3 (max. depth = 264 m)

Density

Density

Density

Biomass

2.562

0.914

0.229

0.114 0.686 0.343 0.914

0.038

0.001 0.008 0.010 0.006

0.001 0.003 0.002

0.114

0.007

0.016

rockfish (Sebastes pinniger, n = 4) d13C values were also significantly enriched relative to widow rockfish (Tukey’s Studentized range test, a = 0.05). The d15N values of bocaccio rockfish were also significantly enriched relative to both yellowtail rockfish and widow rockfish.

Biomass

1.209

0.213

0.341

0.384 0.085

0.207 0.437

2.050 2.263 11.785 3.886

0.098 0.000 0.805 4.699 0.360

320.239

47.881

0.128

0.013

0.085 0.085 0.171

0.003 0.000 0.009

0.011

0.114 1.942 0.686

0.114

Biomass

6.917

11.060

0.384

1.077

0.043

0.015

0.128 0.043 0.043

0.018 0.003 0.011

0.031

0.010

0.063 0.094

0.124 0.044

0.283 4.534 1.385 1.354 1.228 1.008 0.031 0.063 0.252

0.045 1.871 2.725 0.688 0.075 0.057 0.003 0.006 0.028

0.031 0.126 0.220

0.001 0.010 0.024

8.533 0.031 0.063 0.031 0.189 0.031 0.031 0.063

11.710 0.006 0.018 0.019 0.056 0.017 0.010 0.001

4. Discussion 4.1. Currents and hydrography Current and temperature time series in Astoria Canyon exhibit strong, semi-diurnal components at

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Table 3 Standardized densities (number per 106 m3) of fish and decapods collected in midwater (R) and bottom (B) trawls in Astoria Canyon in 2001 Common name

Scientific name

Tanner crab Myxinidae Pacific hagfish Scyliorhinidae Brown cat shark Alepocephalidae California slickhead Stomiidae Pacific viperfish Neoscopelidae Blackchin Pacific blackchin Moridae Pacific flatnose Gadidae Pacific hake Macrouridae Giant grenadier Grenadiera Scorpaenidae Rougheye rockfish Brown rockfish Darkblotched rockfish Widow rockfish Yellowtail rockfish Bocaccio Canary rockfish Shortspine thornyhead Longspine thornyhead Anoplopomatidae Sablefish Cyclopteridae Unidentified snailfish Zoarcidae Snakehead eelpout Black eelpout Unidentified eelpout Pleuronectidae Dover sole Deepsea sole Arrowtooth flounder

Chionoecetes spp.

a

Tow R1 (max. depth = 89 m)

Tow B1 (max. depth = 549 m)

Tow B2 (max. depth = 340 m)

Density

Density

Density

22.01

0.54

Eptatretus stouti

0.51

0.54

Apristurus brunneus

5.63

0.54

Alepocephalus tenebrosus

0.51

Chauliodus macouni

65.52

Neoscopelus macrolepidotus Scopelengys tristis

1.09 0.10

Antimora microlepis Merluccius productus

0.51 19.30

Albatrossia pectoralis Coryphaenoides spp. Sebastes aleutianus Sebastes auriculatus Sebastes crameri Sebastes entomelas Sebastes flavidus Sebastes paucispinis Sebastes pinniger Sebastolobus alascanus Sebastolobus altivelis

1.09

8.70 748.32 0.27 19.99 0.69 31.02 51.70 4.14 2.07 1.33 149.66

12.75 13.29

Anoplopoma fimbria

12.80

2.17

Cyclopteridae

11.26

0.27

2.56 6.65

2.17 4.07 3.80

Lycenchelys crotalinus Lycodes diapterus Zoarcidae Microstomus pacificus Embassichthys bathybius Atheresthes stomias

1.02 1.02

4.07 0.27 0.27

Possible complex of two species: Coryphaenoides acrolepis (Pacific grenadier) and C. cinerus (popeye grenadier).

all measured locations, have inertial frequency components only above shelf depth, and have a diurnalfrequency content that is weak above but strong within the canyon. The topography of Astoria Canyon, like other deep, narrow canyons, clearly intensifies diurnal

motions (Fig. 3C). Results are consistent with the much more extensive physical oceanographic investigations of Astoria Canyon by Hickey (1997). One interesting aspect of Fig. 4 is the apparent correlation of the subtidal signals in the two records, with a slight time lead of current on T changes. Both

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K.L. Bosley et al. / Journal of Marine Systems 50 (2004) 21–37

Table 4 Prey composition (percent of total weight) of three species of rockfishes (Sebastes spp.) and two mesopelagic myctophids from Astoria Canyon collected in midwater and bottom trawls in June 2001 Prey items

Copepoda Calanus marshallae Clausoalanus spp. Candacia spp. unidentified calanoid Hyperiidae Themisto pacifica Primo macropa unidentified Euphausiacea Euphausia pacifica unidentified Decapoda Pasiphaea pacifica Cephalopoda unidentified squid Siphonophora Other gelatinous material Osteichthyes Nectoliparis pelagicus Stenobrachius leucopsarus Sebastes alutus unidentified fish remains Unidentified material

Predator

S. entomelas

S. flavidus

S. paucispinis

D. theta

S. leucopsarus

Number examined

16

18

6

25

27

Length range (mm)

341 – 398

384 – 496

382 – 415

48 – 81

53 – 96 2.41 1.12 1.12 4.18

0.43 0.02 1.33 1.32 62.06 0.14

3.75

2.94

18.43

2.94

57.68 19.45

29.41 35.29

2.98 8.46 0.06 95.27 1.44

Hickey (1997) and Allen et al. (2001) have noted a similar correlation in their measurements. The explanation begins by noting that currents in the near-surface layer should be in approximate geostrophic balance with the cross-shore pressure gradient (dp/dx). The subtidal, along-shore, near-surface current record can serve as a surrogate for dp/dx. Fluid in the canyon is subject to overlying pressure gradients (dp/dx) and other forces, but as Freeland and Denman (1982) note, the walls of the canyon prevent the primary force balance in the canyon from being geostrophic as it is near-surface. Instead, dp/dx forcing in the canyon opposes frictional forces and along-axis baroclinic pressure gradients, which together determine alongaxis flow. Changes in dp/dx can therefore alter the strength and possibly the direction of along-axis flow, with one consequence being changes in T. Larger, negative dp/dx drive stronger, up-canyon flow, causing isopycnals to bend upward and T at a fixed location to

23.39

3.10

91.02 8.74 0.25

0.68 20.59

decrease. The correlation of surface currents and T in the canyon shown in Fig. 4 occurs in this way. The correlation of subtidal T at 200 m and alongaxis currents at M-118 at 225 m was not nearly as good. The poor correlation of those two time series (not shown) makes it quite unlikely that changes in T in Fig. 4 were caused by horizontal advection past the measurement site. The reason instead must be the vertical displacement of isopycnals, a displacement documented by Hickey (1997) using CTD measurements. Therefore, in the upper canyon, T primarily recorded the vertical movement of isotherms in response to up- or down-canyon flow. Temperature at M-118 at 200-m depth over the time period June 29th to July 6th decreased by as much as 0.4 jC (Fig. 4). Using a dT/dz of 4.1  10 3 jC m 1 at 200-m depth estimated from the central profile of Fig. 3A, the corresponding vertical displacement of isopycnals at M-118 would be 98 m.

K.L. Bosley et al. / Journal of Marine Systems 50 (2004) 21–37

33

Fig. 6. (A) d13C vs. d15N of all of the fish and invertebrates that had tissue samples collected from sites in and around Astoria Canyon. Species that are followed by a (B) are benthic and those that are followed by a (BP) are benthopelagic. (B) d13C vs. d15N of rockfish that had tissue samples collected.

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K.L. Bosley et al. / Journal of Marine Systems 50 (2004) 21–37

Typical southward summer winds during the measurement period signal a persistent, but variable, onshore atmospheric-pressure gradient. Subtidal currents in the upper water column move southward over the canyon, and currents in the canyon should upwell as a result. An expectation based on previous measurements and models for this and other deep, narrow canyons (e.g. Allen, 1996; Hickey, 1997; She and Klinck, 2000) is that deep, upwelled water and the nutrients contained therein will continue upward, spill onto the shelf, and nourish the shelf downstream of the southern canyon edge, resulting in elevated productivity there. For example, it was along the southern side of Astoria Canyon where Pereyra et al. (1969) reported much higher catches of fish. The subtidal current and T record in Fig. 4 suggests that the rate of upwelling varies. Hickey (1997) shows from measurements in Astoria Canyon that under such conditions, upwelling is not laterally uniform. In fact, at times before and after maximum upwelling, down-canyon flow occurs on the upstream side of the canyon, leading to a cyclonic eddy that extends downward into it and as much as 50 m above it near the canyon head. This recirculation (Hickey, 1997) may prolong the residence time of nutrients and biota within the canyon. 4.2. Water chemistry The d13C values of POM were typical of marine phytoplankton, which have been shown to range anywhere from 24x to 18x (Fry and Sherr, 1984; Rau et al., 1990). The station (J) with the greatest variability between d13C values of surface and bottom POM was the deepest site that was sampled and was located along the axis of the canyon. With the exception of Station G (which had nearly identical d13C values of surface and bottom POM), the other stations along the axis of the canyon that had samples collected from multiple depths (C and A) had a 1.6x to 2.5x difference between surface and bottom POM. The shallow stations on the north and south sides of the canyon showed the greatest homogeneity between d13C values of surface and bottom POM. Assuming 1 – 2x enrichment in 12C between trophic levels (DeNiro and Epstein, 1978), it appears that many of the animals at higher trophic levels, particularly invertebrates and small fish, were probably deriv-

ing their carbon from localized primary production rather than from distant sources, based on the range of POM d13C values that were measured. This determination is dependant on the length of time necessary to incorporate the isotopic signature of a diet. A longlived rockfish would probably reflect an integration of isotopic signatures over a longer period of time, whereas invertebrates and smaller (i.e. micronektonic) fish would reflect integration over shorter time periods. The importance of localized primary production to the food web around the canyon may have been elevated during 2001. Terrestrial and riverine inputs of carbon were expected to be anomalous as it was a drought year. Indeed, the average monthly flow of the Columbia River in June 2001 was 4296 m3 s 1, which makes it one of the lowest on record. River flow was substantially lower than the June averages from the previous 10 years (1991 – 2000 ranged from 5775 to 14570 m3 s 1; data from United States Geological Survey station at Beaver, Oregon). 4.3. Biological sampling Many of the zooplankton and micronekton collected in the IKMT tows in the canyon are known to occupy the epipelagic (upper 200 m) layers of the ocean off Oregon although a number of these taxa are known to vertically migrate down to the mesopelagic zone during daytime (Pearcy and Laurs, 1966; Krygier and Pearcy, 1981). However, many of the micronektonic fishes are rarely found over the continental shelf (Brodeur et al., 2003); some (e.g. Stenobrachius nannochir) are not known to migrate up into epipelagic waters (Pearcy et al., 1979) and are not likely to occur this close to the coast except in canyons or other areas with substantial on-shelf transport. Brodeur et al. (2003) found that the distribution of several of the dominant myctophids we collected in this study was displaced farther onto the shelf on a transect near Astoria Canyon than on other transects along the coast of Oregon, and speculated that they may be advected onshore by way of the canyon. Although our micronekton sampling in Astoria Canyon was limited, we observed densities and biomasses of some taxa that were high relative to nearby areas (Pearcy and Laurs, 1966; Kalish et al., 1986), indicating perhaps an interaction between behavior of these organisms and currents affected by the canyon topography (see also

K.L. Bosley et al. / Journal of Marine Systems 50 (2004) 21–37

Greene et al., 1988; Macquart-Moulin and Patriti, 1996; Mackas et al., 1997). These high densities were corroborated by cross-canyon acoustic transects, which indicated substantial scattering layers in the canyon and which often intersected the canyon walls, especially during daytime (Brodeur, personal observation and example in Fig. 5). There appear to be trophic effects associated with the cross-canyon horizontal structure in Astoria Canyon. High densities of several species of pelagic rockfishes were collected in our shallow trawls and were often observed in remotely-operated-vehicle video transects during this cruise (Wakefield, unpublished data). Dense swarms of euphausiids were also observed near the bottom at the rim of the canyon during daytime deployments (Go´mez-Gutie´rrez et al., 2003). Stable-isotope and dietary data suggest that the rockfish species may utilize either euphausiid aggregations or mesopelagic prey that are advected up the canyon and toward the canyon walls, where the shallow bottom blocks their vertical descent (Genin, 2004), or where they are then advected onto the adjoining shelf and trapped, making them vulnerable to predation by rockfishes (Pereyra et al., 1969). This trophic interaction between rockfishes and mesopelagic prey has been observed numerous times in the North Pacific, both on offshore banks (Brodeur and Pearcy, 1984; Genin et al., 1988) and in canyons (Pereyra et al., 1969; Lorz et al., 1983; Brodeur, 2001), which may point to these areas as being critical habitats for these heavily exploited rockfish species (Yoklavich et al., 1999). It is also clear from the stable-isotope and dietary data that some rockfish species occupy a higher trophic position relative to other rockfish species feeding in the same general area. These data show that adult bocaccio rockfish were at a higher trophic position than any of the other rockfish species that were sampled, consistent with other dietary studies of this species (summarized by Love et al., 2002). The lower-trophic positions of widow rockfish and yellowtail rockfish relative to bocaccio, as indicated by their significantly lower d13C and d15N values, are also consistent with other dietary studies (Love et al., 2002; Lee, 2003) that found their diets to be comprised of gelatinous zooplankton and micronekton, including smaller fishes, euphausiids and amphipods.

35

The d15N values of myctophids, sergestid and pasiphaed shrimp, and squid (Gonatus onyx) were clustered between the other fish species and most of the other invertebrates, indicating that they all play an intermediary role in energy flow within the canyon food web. In an earlier study off Oregon, Tyler and Pearcy (1975) observed three species of myctophids (two in common with the current study, D. theta and Stenobrachius leucopsaurus) to feed primarily on euphausiids, copepods, and hyperiid amphipods. Our stable-isotope data, along with diet studies off Oregon (Nishida et al., 1988), indicate that shrimp and squid may also represent a link between zooplankton that graze on primary producers and higher trophic levels.

5. Conclusions This study illustrates the potential of physical processes to concentrate marine organisms in and around undersea canyons. Recirculation (Hickey, 1997), for example, favors a prolonged residence time of nutrients and biota within the canyon. At the time of this study, local primary production appeared to have a predominant effect on the canyon food web compared to other potential sources. Species composition was shown to vary both vertically in the water column and spatially. Stable-isotope and dietary data suggest that rockfish may utilize either euphausiid aggregations or mesopelagic prey that are advected up the canyon and towards the canyon walls, and show species-specific differences in the trophic position of fishes. Fish that were found to be primarily piscivorous (based on gut contents) had d15N and d13C values that were enriched relative to fish that were found to be omnivorous. Fish that were shown to be entirely planktivorous had d15N values that were depleted relative to other fish. It is hoped that the information presented here may serve to support future studies in this and possibly other highly productive undersea canyons.

Acknowledgements Funding for this work was provided by the NOAA Northwest Fisheries Science Center, NOAA Office of Ocean Exploration, NOAA Pacific Marine Environ-

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K.L. Bosley et al. / Journal of Marine Systems 50 (2004) 21–37

mental Laboratory, the West Coast and Polar Undersea Research Center of NOAA’s National Undersea Research Program, Oregon State University, and NOAA Oceanic and Atmospheric Research Office of Marine and Aviation Operations for R/V Brown ship time. The authors wish to thank the following individuals for assisting with various aspects of this project: Doug Burrows, Julia Clemons, Bob Embley, Chris Goldfinger, Chris Harvey, Gordon Hendler, Greg Krutzikowsky, Phil Levin, Bruce McCain, Susan Merle, Todd Miller, William Pearcy, Bill Peterson, Kevin Piner, Bill Reichert, Joe Resing, Josie Thompson and Sharon Walker. Additional thanks go to the Canadian Scientific Submersible Facility; Capt. Dan Parker and the crew of the F/V Sea Eagle; Jennifer Bloeser and Dave Douglas for assistance with sample collection on the Sea Eagle, the officers and crew of the R/V Ronald H. Brown, and Bill Pearcy, Bill Peterson, Karen Bosley and three anonymous reviewers for comments on earlier versions of the manuscript. This is contribution no. 2542 from NOAA Pacific Marine Environmental Laboratory.

References Allen, S.E., 1996. Topographically generated subinertial flows within a finite-length canyon. J. Phys. Oceanogr. 26, 1608 – 1632. Allen, S.E., Vindeirinho, C., Thomson, R.E., Foreman, M.G.G., Mackas, D.L., 2001. Physical and biological processes over a submarine canyon during an upwelling event. Can. J. Fish. Aquat. Sci. 58, 671 – 684. Baker, E.T., Milburn, H.B., 1997. MAPR: a new instrument for hydrothermal plume mapping. Ridge Events 8, 23 – 25. Brodeur, R.D., 2001. Habitat-specific distribution of Pacific ocean perch (Sebastes alutus) in Pribilof Canyon, Bering Sea. Cont. Shelf Res. 21, 207 – 224. Brodeur, R.D., Pearcy, W.G., 1984. Food habits and dietary overlap of some shelf rockfishes (Genus Sebastes) from the northeastern Pacific Ocean. Fish. Bull. 82, 269 – 293. Brodeur, R.D., Pearcy, W.G., Ralston, S., 2003. Abundance and distribution patterns of nekton and micronekton in the Northern California current transition zone. J. Oceanogr. 59, 515 – 535. Carlson, P.R., 1968. Marine Geology of Astoria Submarine Canyon. PhD thesis, Oregon State University, Corvallis, OR, 259 pp. Carter, G.S., Gregg, M.C., 2002. Intense, variable mixing near the head of Monterey Submarine Canyon. J. Phys. Oceanogr. 32, 3145 – 3165.

Craig, H., 1957. Isotopic standards for carbon and oxygen and correction factors for mass spectrometric analysis of carbon dioxide. Geochim. Cosmochim. Acta 12, 133 – 149. Davenport, S.R., Bax, N.J., 2002. A trophic study of a marine ecosystem off southeastern Australia using stable isotopes of carbon and nitrogen. Can. J. Fish. Aquat. Sci. 59, 514 – 530. DeNiro, M.J., Epstein, S., 1977. Mechanism of carbon isotope fractionation associated with lipid synthesis. Science 197, 261 – 263. DeNiro, M.J., Epstein, S., 1978. Influence of diet on the distribution of carbon isotopes in animals. Geochim. Cosmochim. Acta 42, 495 – 506. DeNiro, M.J., Epstein, S., 1981. Influence of diet on the distribution of nitrogen isotopes in animals. Geochim. Cosmochim. Acta 45, 1885 – 1994. Emery, W.J., Thomson, R.E., 1997. Data Analysis Methods in Physical Oceanography. Pergamon Press, New York, p. 634. Freeland, H.J., Denman, K.L., 1982. A topographically controlled upwelling center off Southern Vancouver Island. J. Mar. Res. 40, 1069 – 1093. Fry, B., Sherr, E., 1984. d13C measurements as indicators of carbon flow in marine and freshwater ecosystems. Contrib. Mar. Sci. 27, 13 – 47. Gardner, W.D., 1989. Periodic resuspension in Baltimore Canyon by focusing of internal wavers. J. Geophys. Res. 94, 18185 – 18194. Genin, A., 2004. Trophic focusing: the role of bio-physical coupling in the formation of animal aggregations over abrupt topographies. J. Mar. Syst. (this issue). Genin, A., Haury, L., Greenblatt, P., 1988. Interactions of migrating zooplankton with shallow topography: predation by rockfishes and intensification of patchiness. Deep-Sea Res. 35, 151 – 175. Go´mez-Gutie´rrez, J., Peterson, W.T., De Robertis, A., Brodeur, R.D., 2003. Mass mortality of krill caused by parasitoid ciliates. Science 301, 339. Greene, C.H., Wiebe, P.H., Burczynski, J., Youngbluth, M.J., 1988. Acoustical detection of high-density krill demersal layers in the submarine canyons off Georges Bank. Science 241, 359 – 361. Hickey, B.M., 1989. Patterns and processes of circulation over the shelf and slope. In: Landry, M.R., Hickey, B.M. (Eds.), Coastal Oceanography of Washington and Oregon. Elsevier, Amsterdam, pp. 41 – 116. Hickey, B., 1995. Coastal submarine canyons. In: Mueller, P., Henderson, D. (Eds.), Proceedings Hawaii Winter Workshop, January 17 – 10, 1995, Univ. Hawaii, pp. 95 – 100. Hickey, B., 1997. The response of a steep-sides, narrow canyon to time variable wind forcing. J. Phys. Oceanogr. 27, 697 – 726. Hickey, B., Baker, E., Kachel, N., 1986. Suspended particle movement in and around Quinault submarine canyon. Mar. Geol. 71, 35 – 83. Hill, J.K., Wheeler, P.A., 2002. Organic carbon and nitrogen in the northern California current system: comparison of offshore, river plume, and coastally upwelled waters. Prog. Oceanogr. 53, 369 – 387. Hunkins, K., 1988. Mean and tidal currents in Baltimore Canyon. J. Geophys. Res. 93, 6917 – 6929.

K.L. Bosley et al. / Journal of Marine Systems 50 (2004) 21–37 Kalish, J.M., Greenlaw, C.F., Pearcy, W.G., Holliday, D.V., 1986. The biological and acoustical structure of sound scattering layers off Oregon. Deep-Sea Res. 33, 631 – 653. Kampf, J., Fohrmann, H., 2000. Sediment-driven downslope flow in submarine canyons and channels. J. Phys. Oceanogr. 30 (9), 2302 – 2319. Koslow, J.A., Ota, A., 1981. The ecology of vertical migration in three common zooplankters in La Jolla Bight, April – August 1967. Biol. Oceanogr. 1, 107 – 134. Krygier, E.E., Pearcy, W.G., 1981. Vertical distribution and biology of pelagic decapod crustaceans off Oregon. J. Crustac. Biol. 1, 70 – 95. Kunze, E., Rosenfeld, L.K., Carter, G.S., Gregg, M.C., 2002. Internal waves in Monterey submarine canyon. J. Phys. Oceanogr. 32, 1890 – 1913. Lafuente, J.G., Sarhan, T., Vargas, M., Plaza, F., 1999. Tidal motions and tidally induced fluxes through La Linea Submarine Canyon, western Alboran Sea. J. Geophys. Res. Oceans 104, 3109 – 3119. Lee, Y.W., 2003. Oceanographic effects on the dynamics of food habits and growth condition of some groundfish species of the Pacific Northwest. PhD thesis, Oregon State University. Lorz, H.V., Pearcy, W.G., Fraidenburg, M., 1983. Notes on the feeding habits of the yellowtail rockfish, Sebastes flavidus, off Washington and in Queen Charlotte Sound. Calif. Fish Game 69, 33 – 38. Love, M.S., Yoklavich, M., Thorsteinson, L., 2002. The Rockfishes of the Northeast Pacific. The University of California Press, Berkeley, CA. Lueck, R.G., Osborn, T.R., 1985. Turbulence measurements in a submarine canyon. Cont. Shelf Res. 4, 681 – 695. Mackas, D.L., Kieser, R., Saunders, M., Yelland, D.R., Brown, R.M., Moore, D.F., 1997. Aggregations of euphausiids and Pacific hake (Merluccius productus) along the outer continental shelf off Vancouver Island. Can. J. Fish. Aquat. Sci. 54, 2080 – 2096. Macquart-Moulin, C., Patriti, G., 1996. Accumulation of migratory micronekton crustaceans over the upper slope and submarine canyons of the northwestern Mediterranean. Deep-Sea Res. 43, 579 – 601. McConnaughey, T., McRoy, C.P., 1979. Food-web structure and the fractionation of carbon isotopes in the Bering Sea. Mar. Biol. 53, 257 – 262. Nishida, S., Pearcy, W.G., Nemoto, T., 1988. Feeding habits of mesopelagic shrimps collected off Oregon. Bull. Ocean Res. Inst. 26 (II), 99 – 108. Noble, M., Butman, B., 1989. The structure of subtidal currents within and around Lydonia Canyon, Evidence for enhanced cross-shelf fluctuation over the mouth of the canyon. J. Geophys. Res. 94, 8091 – 8110. Owens, N.J.R., 1987. Natural variations in 15N in the marine environment. Adv. Mar. Biol. 24, 390 – 451.

37

Pearcy, W.G., Laurs, R.M., 1966. Vertical migration and distribution of mesopelagic fishes off Oregon. Deep-Sea Res. 13, 153 – 165. Pearcy, W.G., Nemoto, T., Okiyama, M., 1979. Mesopelagic fishes of the Bering Sea and adjacent northern North Pacific Ocean. J. Oceanogr. 35, 127 – 135. Pereyra, W.T., Pearcy, W.G., Carvey Jr., F.E., 1969. Sebastodes flavidus, a shelf rockfish feeding on mesopelagic fauna, with consideration of the ecological implications. J. Fish. Res. Board Can. 26, 2211 – 2215. Perry, R.I., Thompson, P.A., Mackas, D.L., Harrison, P.J., Yelland, D.R., 1999. Stable carbon isotopes as pelagic food web tracers in adjacent shelf and slope regions off British Columbia, Canada. Can. J. Fish. Aquat. Sci. 56, 2477 – 2486. Peterson, B.J., Fry, B., 1987. Stable isotopes in ecosystem studies. Ann. Rev. Ecolog. Syst. 18, 293 – 320. Petruncio, E.T., Rosenfeld, L.K., Paduan, J.D., 1998. Observations of the internal tide in Monterey Canyon. J. Phys. Oceanogr. 28, 1873 – 1903. Plank, W.S., Zaneveld, J.R., Pak, H., 1974. Temporal variability of suspended matter in Astoria Canyon. J. Geophys. Res. 79, 4536 – 4541. Polunin, N.V.C., Morales-Nin, B., Pawsey, W.E., Cartes, J.E., Pinnegar, J.K., Moranta, J., 2001. Feeding relationships in Mediterranean bathyal assemblages elucidated by stable nitrogen and carbon isotope data. Mar. Ecol., Prog. Ser. 220, 13 – 23. Rau, G.H., Teyssie, J.-L., Rassoulzadegan, F., Fowler, S.W., 1990. 13 12 C/ C and 15N/14N variations among size fractionated marine particles: implications for their origin and trophic relationships. Mar. Ecol., Prog. Ser. 59, 33 – 38. She, J., Klinck, J.M., 2000. Flow near submarine canyons driven by constant winds. J. Geophys. Res. 15, 28671 – 28694. Sobarzo, M., Figueroa, M., Djurfeldt, L., 2001. Upwelling of subsurface water into the rim of the Biobio submarine canyon as a response to surface winds. Cont. Shelf Res. 21, 279 – 299. Tieszen, L.L., Boutton, T.W., Tesdahl, K.G., Slade, N.A., 1983. Fractionation and turnover of stable carbon isotopes in animal tissues: implications for d13C analysis of diet. Oecologia 57, 32 – 37. Tyler Jr., H.R., Pearcy, W.G., 1975. The feeding habits of three species of lanternfishes (Family Myctophidae) off Oregon, USA. Mar. Biol. 32, 7 – 11. Underwood, M.B., 1991. Submarine canyons, unconfined turbidity currents, and sedimentary bypassing of forearc regions. Rev. Aquat. Sci. 4, 149 – 200. Yoklavich, M., Greene, G., Calliet, G.M., Sullivan, D.E., Lea, R.N., Love, M.S., 1999. Habitat associations of deep-water rockfishes in a submarine canyon: an example of a natural refuge. Fish. Bull. 98, 625 – 641.