NEPIP - Process Studies - CGOA

## Coastal Gulf of Alaska
Process Studies

U.S. GLOBEC developed a list of nineteen research questions for the CGOA (see pp. 64-65 of U.S. GLOBEC Report No. 15, 1996). In summary the questions addressed a few key themes: 1) the relationship between atmospheric forcing (including wind and precipitation [e.g., buoyancy effects]) and coastal circulation, mixed-layer depth and temperature, retention time scales, and cross-shelf transport; 2) the factors, both physical and biological, controlling primary productivity and the composition and production of plankton in the CGOA; 3) the impact of climate change on trophic phasing within the ecological food web of the region and especially its effect on over-wintering plankton distribution and biomass; and 4) the impacts of climate change on higher trophic levels (fish, birds, mammals), especially on their distribution, patchiness, growth, survival, reproduction, and seasonality.

A smaller group of scientists met later to further focus the discussions of the April 1995 workshop and produced the following overarching hypothesis for U.S. GLOBEC Coastal Gulf of Alaska research (U.S. GLOBEC, 1996b):

> Ocean survival of salmon is determined primarily by survival of juvenile salmon in coastal regions, and is affected by interannual and interdecadal changes in Gulf of Alaska physical forcing.

Detailed, process-oriented research and surveys in a focused coastal study were designed to address this hypothesis. In this implementation plan, U.S. GLOBEC emphasizes studies of the physical environment, plankton environment, and juvenile salmon. The specific questions that U.S. GLOBEC proposes to address through process-studies in the CGOA are:

- How does changing climate, especially its impacts on local wind forcing and basin-scale currents, affect spatial and temporal variability in mesoscale circulation?

- How do mesoscale features in the Gulf of Alaska impact zooplankton biomass, production, and distribution, and the retention and loss of zooplankton from coastal regions?

- Is the cross-shelf import of large zooplankton (e.g., copepods and euphausiids) from deeper offshore regions to nearshore shallow waters in the spring required for rapid growth and high survival of juvenile pink salmon in the coastal Gulf of Alaska?

- To what extent is high and variable predation mortality on juvenile pink salmon in the coastal region of the Gulf of Alaska responsible for the large interannual variation in adult pink salmon populations?

The first two questions are identical to the first two specified for the CCS. They consider climatic forcing and variability to variation in mesoscale circulation, which in turn may be a significant factor impacting plankton production dynamics and transport. As was the case in the CCS, the last two questions relate salmon survival in the ocean to bottom-up and top-down controls operating during the juvenile period while the salmon are nearshore, although because the CGOA is predominantly a downwelling system, the mechanistic details differ (especially for bottom-up control). U.S. GLOBEC recommends that Year 4 studies in the CGOA focus primarily on bottom-up control, and the plan detailed below reflects that selection. The emphasis on bottom-up processes, however, does not mean that concurrent data on top-down processes should not be collected during Year 4. Such data should be obtained if it does not interfere with the primary research. Moreover, data on the structure and variation of the physical environment and the plankton environment should be collected in all process-study years. With these thoughts in mind, we provide background related to potential bottom-up control of juvenile pink salmon survival in the CGOA.

In Prince William Sound, Alaska, a mid-spring bloom of Neocalanus biomass corresponds closely to the timing of outmigrating pink salmon fry (Cooney et al. 1995). The diet of the juveniles over the shelf after they leave PWS is not well known, but it is likely that the large interzonal copepods and euphausiids are important forage items for many species residing on the shelf. Cooney (1986b, p. 293-294) notes,

> "Cooney (1986a) demonstrates the seasonal presence of the oceanic interzonal copepods over the shelf of the northern Gulf of Alaska. This presence is associated both with the time these species reside in the wind-influenced surface layer of the bordering ocean and with the duration of the shelf convergence season that lasts from October to April each year (Royer 1981). These and other oceanic zooplankters are dominant members of the shelf and coastal communities, a fact that adds support to the notion that the bordering ocean may be the source for substantial amounts of organic matter that is advected shoreward in the seasonally persistent onshore Ekman flow (Cooney 1984). . . . The considerably narrower shelf of the Gulf of Alaska has a much more advective environment due to influences by both the Alaska Current over and along the shelf break, and by the Alaska Coastal Current (ACC) that occupies the first 40 km from the beach seaward. . . . Interactions between these two currents (where the shelf is < 50 km wide) presumably provides a mechanism to mix and transport the coastal and oceanic faunas over and along the shelf. This mechanism, combined with the wind-induced onshore Ekman flow, assures that near-surface (upper 200 m) zooplankters of oceanic origin become a seasonal part of the shelf/coastal zooplankton communities."

The exact mechanism of the coupling between the Neocalanus and salmon juveniles on the shelf, and whether it is direct or indirect, is not known. The impact could be direct in that the copepods (esp. Neocalanus) transported onshore, or their progeny, are important prey of the juvenile pink salmon, thus promoting rapid growth and higher survival. Alternatively, the import of zooplankton biomass may have an indirect effect by presenting an alternative prey to potential consumers of juvenile salmonids, such as herring and pollock. In that case, large quantities of zooplankton prey may improve juvenile salmon survival by reducing predator related mortality. The primary physical processes which contribute to influx, retention and exchange of water, nutrients and plankton on the continental shelf of Alaska are the buoyancy-driven and wind-forced transports near-shore, and interaction with the gyre-scale Alaska Current off the shelf.

U.S. GLOBEC recommends that a process-oriented investigation of the food web shown in Figure 12, focusing particularly on pink salmon, their prey and predators, be conducted on the shelf region outside Prince William Sound in the northern part of the Gulf of Alaska. Prince William Sound has large wild and hatchery-released stocks of pink salmon. Approximately 450 million hatchery fry with distinctively thermally marked otoliths are released each year from the hatcheries within PWS. These join an approximately equal number of wild out-migrants from adjacent natal areas. It is believed that the fish remain in PWS for up to about two months before exiting onto the Alaskan continental shelf proper (Cooney 1993). Their residence time, food habits (diet), and the magnitude and sources of mortality while they are on the shelf are not known. The marked fish can be used to estimate survival from the time of release to 1) the fish exiting the Sound, and 2) to hatchery return. These hatcheries are the only ones in this region using thermal tags, thus it provides positive identification of the pink salmon source. Although Cooney and Willette (1996) observed similarly phased marine survivals of hatchery and native pink salmon stocks from PWS, studies of survival of wild and hatchery stocks of salmon from other regions indicate higher survival of wild salmon, perhaps due to different behaviors. Thus, some consideration of potential differential survival of hatchery reared and wild salmon may be needed during U.S. GLOBEC studies.

Collaboration with other programs is essential to meeting the objectives set for this year. Potential collaborators include the Exxon Valdez Oil Spill (EVOS) Trustees, that are currently funding the Sound Ecosystem Assessment (SEA) project investigation of the pelagic food web in Prince William Sound, AK, and the Ocean Carrying Capacity (OCC) program which is currently conducting annual shelf-wide trawling surveys for salmonids. U.S. GLOBEC proposes to conduct studies similar to those being conducted by SEA in PWS, but over a much larger region on the shelf (outside PWS), ranging from approximately 143°-150°W. The Alaska Coastal Current, which dominates the circulation on the shelf in this region flows from east to west in this region (Fig. 13). The box delimiting the study region is approximately 300 km alongshore and 150 km in the cross-shore direction. Reports from the OCSEAP (Outer Continental Shelf Environmental Assessment Program) program conducted during the mid-to-late 1970s summarize much of what is known about the ocean conditions and biology of this region (e.g., see the papers in Hood and Zimmerman 1986). The westernmost transect shown on Figure 13 is the Gulf of Alaska (GAK) line. Significant data on ocean physics exist for that line, with especially the innermost station (GAK1) having been sampled very frequently since 1970 (Royer, 1993). A hydrodynamic model of flow into, within and exiting PWS is being developed within the SEA program. Seeding of PWS with Neocalanus populations from offshore is of interest to that program. Neocalanus intrudes into PWS along with the other interzonal copepod, Eucalanus bungii (Cooney, 1986a), demonstrating a connection with the adjacent shelf/ocean. A U.S. GLOBEC investigation focused on the region identified above will elucidate the mechanisms by which these interzonal copepods, which overwinter in the deep-water off the shelf, recruit onto the coastal shelf (U.S. GLOBEC's interest) and into PWS (SEA's interest). Mesoscale features are observed on the Alaskan continental shelf (Fig. 14); for instance, there is a permanent eddy on the shelf, west of Kayak Island (Fig. 13), which may be important in determining residence times of some of the organisms on the shelf, even though it is "upstream" of PWS. Drifters deployed during the OCSEAP program in the ACC upstream of the eddy and PWS, made several loops of the eddy before being eventually advected further west and into PWS (Fig. 15). Clearly, transport along the coast of the region in the ACC can be complex and introduce water and organisms from the outer shelf (and perhaps further offshore) into inner shelf regions and fjords like PWS.

Although the focus of the study is on the higher trophic levels (pink salmon; the zooplankton upon which they feed; and, predators and competitors of the juvenile salmon), observations during the process studies should include nutrient and phytoplankton concentrations, to the extent possible. These fields will provide some understanding of the lower trophic levels of the food web. The combination of strong buoyancy inputs and downwelling-favorable winds should inhibit upward motion and lead to low nutrient concentrations after any spring bloom. Thus, there is a special interest in how and where vertical fluxes of nutrients may be found in coastal downwelling systems, which have been much less frequently studied than upwelling systems (like the CCS).

Studies of the CGOA will use many of the same methods required (and described earlier) for studying the CCS ecosystem (e.g, moorings, ships, drifters, remote sensing, surveys, etc.). Rather than repeat those details here, we describe some specific types of studies that should be conducted during Year 4:

- broad-scale surveys of the environment--SEASOAR or similar technology should be used to provide the physical context and map some biological parameters (e.g., fluorescence, multiple frequency acoustics, and optics) for a region encompassing ca. 150 km cross-shelf and 300 km alongshore. Weather permitting, a SEASOAR survey of that region might require 7-10 days of shiptime. Hotspots (e.g., aggregations of fish; euphausiid swarms) in the acoustics or bio-optics should be sampled using conventional (e.g., MOCNESS) nets to provide specific information on the prey field. Acoustics (supplemented by appropriate in situ collections [e.g., trawling, seining]) should be used to measure the abundance of pink salmon and other fish species, and net sampling will be needed to collect biological specimens for diet studies and to estimate growth rates [see next two bullets].

- diet of juvenile salmonids and competitors and predators--These will consider differences in the availability of different prey types, and the effects of mesoscale variability.

- determine growth rates of juvenile salmon during their residence in the coastal environment--Growth can be evaluated from marked fish using scales or otoliths; condition of the fish should be evaluated.

Juvenile salmonid habitat utilization and diet and growth rates should be studied in a Lagrangian sense by deploying one or more drifters and sampling physical and biological conditions semi-continuously along the drift trajectory for up to one week. This might be repeated multiple times during each cruise. Finally, a second Eulerian survey would be conducted of the entire region at the end of each cruise.

Ideally, this sampling program would provide information on 1) the prey density, distribution and availability to the juvenile salmon, 2) the abundance of juvenile salmonids and other fish, 3) the diet of the fish species, but especially juvenile pink salmon, 4) growth rates of juvenile salmon during their residence in the coastal environment, and 5) the physical environment. Additionally, some information will be learned in this year about the dominant sources of juvenile salmon mortality, but the intensive studies of predator abundances, distributions, and foraging rates will be the primary focus of the CGOA studies in Year 6 (see paragraphs below). Process studies in both years should be coordinated with estimates of return rates (survival) of hatchery released fish (obtained from the hatchery, and fishery collections of thermally tagged fish), and with estimates of growth determined from analysis of scales and/or otoliths from returning or fishery captured fish. Because pink salmon have a short life span (2 years), and a short freshwater residence period (i.e., they enter the marine environment at a young age and small size), they are more likely than other salmon species to have survival or growth rates impacted by interannual or interdecadal variability in coastal conditions.

Year 6 studies in the CGOA will focus specifically on documenting the sources and rates of juvenile pink salmon predation mortality. In addition to the types of observations described above for Year 4 studies of the CGOA, the following should be emphasized in Year 6 studies:

- identify and estimate abundance of predators on juvenile salmon; this would include other fishes, birds and mammals. This may require directed studies encompassing a larger region, perhaps including the Shelikof Strait, than some of the broad-scale surveys done in Year 4.

- determine the predation rate of various predators on the juvenile salmon

The focus of process studies in Year 6 will be to determine mortality of juvenile pink salmon as they transit the coastal zone from the vicinity of PWS until they depart the shelf for deep water. This period, along with the period spent in PWS, is thought to be the time when overall year-class strength is determined. Mortality of the juvenile salmon during this period is likely due to predation, and thus is sensitive to the number, distribution, and feeding rates of salmonid predators. Since it is not known how far to the west the pink salmon emigrating from PWS reside on the shelf in the Alaska Coastal Current before moving further offshore, we recommend that the sampling surveys for the migrating juveniles (and their predators) extend further to the west--into the Shelikof Strait--than the process studies proposed for Year 4. Information on the juvenile pink's residence time in the shelf environment, and their migration pathway to the deep ocean is very important because predator (esp. adult pollock and bird) abundances in the Shelikof Strait region, north and west of Kodiak Island, are significantly higher than they are on the shelf region outside of Kodiak Island and PWS. If the juvenile salmon pass through Shelikof Strait, depending on the time of the year, mortality from pollock predation may be very high. Some spatial and temporal resolution within the sampled area will be sacrificed to sample over a larger region in Year 6.

U.S. GLOBEC studies in this year must be coupled/coordinated with the shelf-wide trawling surveys for salmonids of the Ocean Carrying Capacity program and the NMFS acoustic surveys of adult pollock. The goal of the U.S. GLOBEC process studies in Year 6 will be to track the thermally-tagged hatchery released juvenile pink salmon as they are advected (and/or migrate) westward and southward, passing either south of Kodiak Island or through the Shelikof Strait, until they leave the shelf and enter deep water. Along this trajectory observations and sampling of suspected salmon predators will be done to estimate the sources and rates of natural mortality of the advected/migrating salmon cohort. The rate of natural mortality is not constant for all ages within the salmon cohort, and will vary both spatially and temporally as predator abundances vary. Mortality rates of marine fish are usually highest in early life (as larvae and juveniles), and decrease with growth, prior to final spawning and senescence. Predation by a variety of species (fishes, mammals and birds) is the most frequent cause of natural mortality in most marine fishes. Identifying the dominant predators, their distributions, which are probably patchy, determined somewhat by oceanographic conditions, and quantifying predation rates are given high priority in this year. Predator distributions, abundances and temporal phasing to prey populations may respond to interannual and longer-term variations in ocean conditions.