Aggregation of euphausiids and Pacific hake (Merluccius productus ...

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Aggregation of euphausiids and Pacific hake (Merluccius productus) along the outer continental shelf off Vancouver Island David L. Mackas, Robert Kieser, Mark Saunders, Douglas R. Yelland, Robin M. Brown, and Douglas F. Moore

Abstract: From spring through autumn, euphausiids and Pacific hake (Merluccius productus) form dense aggregations off the outer coast of British Columbia along regions of steeply sloping bathymetry such as the continental shelf break. Their spatial overlap is ecologically significant because of their very strong prey–predator interaction. We analyze high-resolution measurements of shelf-edge spatial distributions of euphausiid and Pacific hake biomass (by echo integration), water properties (from surface measurements and conductivity–temperature–depth (CTD)/Rosette profiles), and current patterns (from acoustic Doppler current profiler, CTD profiles, and nearby current meter moorings). Both euphausiids and hake share similar horizontal distributions, although separated vertically by tens of meters during daylight hours. Bathymetric and water property patterns provide good coarse-scale prediction of aggregation location. However, details of patch location are better explained by flow-field indices such as cross-shore position of the shelf break current, intensity of cross-isobath flow and upwelling at the depth of the euphausiid scattering layer, and doming of isopycnals. Under prevailing summer oceanographic conditions along the British Columbia coast, likely ecological consequences of aggregation in and beneath upwelling water include access to high food density in the overlying surface layer, reduced alongshore transport, and horizontal segregation between adult and larval euphausiids. Résumé : Du printemps jusqu’à l’automne, les euphausiacés et le merlu du Pacifique (Merluccius productus) forment des concentrations denses au large de la côte ouest de la Colombie-Britannique le long des régions qui, du point de vue bathymétrique, se caractérisent par une inclinaison abrupte, comme la rupture de pente du talus continental. Leur chevauchement spatial est important du point de vue écologique en raison de l’interaction proie–prédateur importante qui lie ces deux types d’organismes. Nous avons analysé des valeurs de mesure haute résolution des distributions spatiales en bordure du plateau de la biomasse d’euphausiacés et de merlus du Pacifique (par échointégration), les propriétés de l’eau (à partir de mesures en surface et des profils conductivité–température–profondeur (CTP)/Rosette) et les caractéristiques des courants (à l’aide de profileurs de courant Doppler acoustiques, des profils CTP et d’amarrages de courantomètre à proximité). Les euphausiacés et le merlu partagent des distributions horizontales semblables, bien que, verticalement, ils soient séparés par des dizaines de mètres pendant la journée. Les caractéristiques bathymétriques et les propriétés de l’eau ont une bonne valeur prédictive, à une échelle plus grossière, de l’emplacement des concentrations. Mais les détails sur les emplacements morcelés sont mieux déterminés par les indices de champ de courant comme la position transversale par rapport à la côte du courant à la rupture de pente du talus, l’intensité de l’écoulement transversal à l’isobathe et la remontée d’eau profonde à la profondeur de la couche de dispersion des euphausiacés, et le bombement des isopycnes. Dans les conditions océanographiques qui prédominent l’été le long de la côte de la Colombie-Britannique, les conséquences écologiques vraisemblables de la concentration dans les eaux de remontée ou en dessous comprennent l’accès à une densité alimentaire plus élevée dans la couche de surface sus-jacente, la réduction du transport le long de la côte et la ségrégation horizontale entre les euphausiacés adultes et larvaires. [Traduit par la Rédaction]

Introduction Euphausiids (Euphausia pacifica and Thysanoessa spinifera) are an important component of the zooplankton community off the outer coast of British Columbia (Fulton and LeBrasseur 1984; Simard and Mackas 1989; Mackas 1992, 1995). They are leading prey items for several finfish species (Outram and

Haegele 1972; Tanasichuk et al. 1991; Robinson et al. 1993; Ware and McFarlane 1995), including Pacific hake (Merluccius productus), spiny dogfish (Squalus acanthias), Pacific herring (Clupea pallasi), and Pacific salmon (Oncorhynchus spp.). Of these, Pacific hake exert the strongest predation pressure because of their large biomass (averaging >2 × 105 t during the summer feeding season; Ware and McFarlane 1995)

Received September 23, 1996. Accepted February 27, 1997. J13656 D.L. Mackas,1 D.R. Yelland, R.M. Brown, and D.F. Moore. Department of Fisheries and Oceans, Institute of Ocean Sciences, Sidney, BC V8L 4B2, Canada. R. Kieser and M. Saunder. Department of Fisheries and Oceans, Pacific Biological Station, Nanaimo, BC V9R 5K6, Canada. 1

Author to whom all correspondence should be addressed. e-mail: [email protected]

Can. J. Fish. Aquat. Sci. 54: 2080–2096 (1997)

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Fig. 1. Overlapping summer-season distributions of euphausiids (horizontal shading pattern) and Pacific hake (vertical shading pattern) along the southern Vancouver Island continental margin. Both euphausiids and hake aggregate along sloping bottom topography such as the shelf break and the edges of midshelf banks. Redrawn and simplified from Simard and Mackas (1989) and Ware and McFarlane (1995).

and because they typically obtain more than half of their ration from euphausiids (Tanasichuk et al. 1991). Spatial distributions of both euphausiids and hake are extremely patchy. Most of their total biomass is aggregated within a small fraction of the available ocean habitat, and biomass per unit surface area can vary by one or more orders of magnitude over separations of a few kilometers. Because of this intense patchiness, the degree of spatial overlap of predator and prey distributions is an important factor in determining the availability of prey to predator. Both euphausiids and hake are strong diel vertical migrants, moving upward into the surface layer at night and downward to form 5–20 m thick subsurface layers in daylight. The depth of these layers and the density of organisms in them are observable using acoustic backscatter (Simard and Mackas 1989; Cooke et al. 1992). We knew from earlier acoustic and trawl surveys (Simard and Mackas 1989; Ware and McFarlane 1995) that independently mapped horizontal distributions of euphausiids and Pacific hake off British Columbia are very similar (Fig. 1). Both form dense aggregations in regions of steeply sloping bathymetry such as the outer margin of the continental shelf and the edges of midshelf banks and basins. Aggregation of euphausiids near sloping sea bottom has been observed in other regions (Simard et al. 1986; Greene et al. 1988; Coyle et al. 1992). However, our initial acoustic mapping surveys off the British Columbia coast were too broad-brush to study the fine-scale spatial and temporal covariance between fish and plankton distributions and the physical and biological mechanisms responsible for their respective aggregations. In this paper, we report results of more detailed acoustic surveys of Pacific hake and euphausiid biomass (by echo integration) and subsurface water properties and current patterns (by conductivity–temperature– depth (CTD)/Rosette and underway acoustic Doppler current profiles (ADCP)). Our goal is to provide more detailed

information about distributions of euphausiids, Pacific hake, and oceanographic conditions within and very near the shelfbreak zone of high abundance. We selected the shelf break as a study area because it provides somewhat simpler geometry and more consistent patterns of environmental variability than the edges of midshelf banks. However, we anticipate that any knowledge we gain from the simpler system will be applicable to the more complex. For oceanographic conditions, we have emphasized the vertical and horizontal structure of flow velocity in addition to more traditional measurements of water properties and food distribution.

Methods We mapped distributions of macrozooplankton, fish schools, and subsurface currents in August of two different years (1988 and 1991) and at the three locations along the southern Vancouver Island continental shelf break shown in Fig. 2. In all cases, acoustic measurements of biomass and currents were made while the research vessel(s) steamed along a survey grid pattern. Occasional net tow samples of zooplankton biomass and size and species composition were obtained with a 0.25-m2 mouth area instrumented multiple net sampler (BIONESS; Sameoto et al. 1980). In 1991, Pacific hake biomass was estimated acoustically, and fish samples were obtained along and between the survey grids using a midwater trawl. Water property profiles were measured at 10–20 km intervals along CTD station lines. Survey design Details of survey design and sampling methods differed slightly between the two years. In 1988, acoustic sampling consisted of a single 5-h survey by the CSS Parizeau in the vicinity of the Nitinat submarine canyon (Fig. 3). A commercial fisherman had reported high fish and feed densities in this area a few days earlier. Acoustic survey lines intersected radially with the rim of the submarine canyon at a number of locations. Fish biomass was not quantified, but locations of fish aggregations were © 1997 NRC Canada

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Fig. 2. Map showing sampling locations for this study. Circles labelled A1 and E3 show sites of long-term current meter moorings. The 1988 sampling consisted of Nitinat Canyon 104-kHz acoustic survey (solid lines) and LB line CTDs. The 1991 sampling included CTD/Rosette lines (broken) along the LC and LH lines, followed immediately by underway acoustic sampling (104 kHz and 38 kHz) of repeated circuits along rectangular survey grids (solid lines) surrounding the shelf-break portion of the LC and LH CTD/Rosette lines. Arrows labelled T1–T4 show locations of 1991 midwater hake trawls. BIONESS tows in the euphausiid scattering layer were also done as part of the Nitinat, LC, and LH acoustic surveys.

noted qualitatively, as were regions of active midwater trawling for Pacific hake. Time constraints prevented synoptic CTD–rosette sampling in the Nitinat Canyon region. However, a CTD line was run through the region 12 days later at locations indicated in Fig. 3. Data from this line give a rough approximation of the vertical and crossshore distribution of water properties but cannot be compared in detail against the acoustic maps because of the time gap. In 1991, we obtained repeated and more extensive sampling of two narrow rectangular grids (Fig. 2, LC and LH). A motive for repeated sampling of each line was to examine tidal variability of currents and animal distributions. Two vessels were used to collect the data. The CSS Parizeau completed two consecutive surveys of the study area. The first consisted of CTD–Rosette cross-shore station lines through the center of each grid. The second repeatedly covered each grid 12–24 h later for euphausiid and flow field sampling using the 104kHz Ross echosounder, BIONESS, and 150-kHz ADCP. The fisheries research vessel W.E. Ricker measured Pacific hake distribution and biomass along the same acoustic lines using the towed 38-kHz Biosonics echosounder. During acoustic sampling, the two ships steamed in file along each leg of the survey grid at a speed of 8–9 knots separated by about 1 km. Time lag to cross the same site was ≈4 min. Typical along- and cross-track current speeds were 0–20 cm⋅s–1. Any between-vessel misalignment or offset of data caused by advective displacement of water and biota is therefore likely to be