NEPIP - Executive Summary

## Executive Summary

### U.S. GLOBEC Rationale in the Northeast Pacific

On a wide range of time scales (from seasonal to interdecadal), there are strongly correlated signals in physical and biological variables along the eastern boundaries of both gyres in the Northeast Pacific Ocean (NEP)--the currents of the Coastal Gulf of Alaska (CGOA) and the California Current System (CCS). Tide gauge and altimeter data suggest that the strengths of the boundary currents in these gyres covary out of phase on annual and interannual time scales (the equatorward CCS strengthens while the poleward and westward current in the CGOA weakens and vice versa [Chelton and Davis 1982]). Zooplankton volumes in the southern part of the CCS covary in phase with the interannual changes in the CCS transport, although the mechanisms responsible for the covariance are not clear (Chelton et al. 1982; Wickett 1967). On interdecadal time scales, there are data suggesting that zooplankton and salmon both covary out of phase in the two boundary currents (Roemmich and McGowan 1995; Brodeur and Ware 1992; Francis and Sibley 1991). Sardine in the CCS also covary in phase with salmon in the CGOA, but out of phase with salmon in the CCS (Kawasaki 1992). The interdecadal fluctuations of these populations, and others (Beamish, 1993), coincide with basin-scale physical changes in atmospheric forcing and surface ocean conditions (temperature, mixed-layer depth), although again the mechanisms responsible for the covariances are not known.

### Program Goals

- To understand the effects of climate variability and climate change on the distribution, abundance and production of marine animals (including salmon and other commercially important living marine resources) in the eastern North Pacific.

- To embody this understanding in diagnostic and prognostic models, capable of elucidating ecosystem dynamics and responses on a range of time scales, including major climatic fluctuations.

A focus of the first goal is to better understand the mechanism(s) responsible for the covarying, but out of phase, production dynamics of zooplankton and fish of the CGOA and CCS ecosystems. The target fish species for U.S. GLOBEC studies in the NEP are salmon. Salmon were selected due to the economic impact of changes in salmon abundance and because their populations vary concident with climate variability (Francis and Hare 1994). Zooplankton are important as indicators of the productivity of the coastal ecosystem. Moreover, zooplankton are directly linked to salmon as their prey, and indirectly by being alternate prey for some salmon predators (e.g., pollock, hake, some birds). Thus, the target species for process studies in the coastal regions of both gyres are the juvenile salmon and the dominant crustacean zooplankton (copepods and euphausiids) upon which salmon and other predators in the ecosystem rely. While the process studies will focus on these species, other elements of the program (modeling, retrospective analysis, monitoring) can address other species that could elucidate NEP ecosystem changes in response to climate change.

### Core Hypotheses

1. Production regimes in the Coastal Gulf of Alaska and California Current System covary, and are coupled through atmospheric and ocean forcing.

2. Spatial and temporal variability in mesoscale circulation constitutes the dominant physical forcing on zooplankton biomass, production, distribution, species interactions, and retention and loss in coastal regions.

3. Ocean survival of salmon is primarily determined by survival of the juveniles in coastal regions, and is affected by interannual and interdecadal changes in physical forcing and by changes in ecosystem food web dynamics.

### Approach

U.S. GLOBEC will study the effects of past and present climate variability on the population ecology and population dynamics of marine biota and living marine resources, and use this information as a proxy for how the ecosystems of the eastern North Pacific may respond to future global climate change. The program plans to use the strong temporal variability in the physical and biological signals to examine the biophysical mechanisms through which zooplankton and salmon populations respond to physical forcing and biological interactions in the coastal regions of the two gyres. Annual and interannual variability will be studied directly through monitoring activities (over a 5-7 year period) and detailed process studies (over a 5 year period); variability at longer time scales will be examined through retrospective analysis of directly measured and proxy data. Coupled bio-physical models of the ecosystems of these regions will be developed and tested using the process studies and data collected from the monitoring programs, then further tested and improved by hindcasting selected retrospective data series.

Process studies in the NEP will focus on the causes of salmon mortality in the nearshore region during the first part of their ocean residence, and will include investigations of bottom-up (zooplankton production, salmon diet) and top-down interactions (predation by other fish, birds, and mammals). The geographic locations for the studies will include three types of environments: 1) the predominantly downwelling environment of the CGOA (surface convergence toward shore); 2) the moderate upwelling environment off Oregon/Washington (Region I of the CCS, characterized by surface divergence from shore with a nearly linear alongshore jet that may bar movement offshore but increase movement alongshore); and 3), the strongly upwelling environment off northern/central California (Region II of the CCS, characterized by surface divergence from shore with a complex meandering jet and eddy system that may transport organisms far offshore).

Monitoring and retrospective components of the Northeast Pacific U.S. GLOBEC program will make use of a broader suite of species than the process studies, especially focusing on species that might serve as indicators of ecosystem variability in the boundary currents. Examples of such indicators are the small pelagic fishes and nearshore benthic invertebrates. Population sizes of small pelagic fishes have been documented to covary interannually and interdecadally with changes in the physical environment. These relationships can be studied using fishery records and proxy estimates of abundance recorded in anoxic sediments. Thus, small pelagic fishes are prime candidates for inclusion in retrospective studies. Salmon ocean survival (a component important in determining year-class strength) is believed to be determined during their earliest marine phase in the nearshore region. This is also the region where mortality of benthic invertebrate planktonic larvae affects their rates of successful settlement back to suitable nearshore adult habitat. Thus, nearshore settlement of benthic invertebrates from the plankton, which can be monitored inexpensively at shore (intertidal) sites, could provide finely resolved estimates of spatial and temporal variability in nearshore conditions--including physical processes (transport, near-shore retention) and biological processes (growth)--important to salmon growth and survival. The details of the mechanisms causing variable growth and mortality of benthic invertebrate larvae, holozooplankton, and juvenile salmon need to be better understood in terms of nearshore transports, mixing dynamics, production and food-web relations. In addition to examining a broader suite of species, monitoring and retrospective studies should also examine a wider range of geographic regions in order to encompass basin-scale (retrospective and monitoring) and multi-decadal (retrospective) climatic processes.

Modeling is a central element of the U.S. GLOBEC NEP program and should also encompass the broadest suite of species and geographic regions. At the largest scales, models must capture the basin-scale interannual and interdecadal climate fluctuations, and should reproduce the differential biological responses (inverse phasing) of the salmon and zooplankton populations and production in the northern (CGOA) and southern (CCS) domains. Regional models of the boundary currents must include details of the coastal circulation and biophysical interactions, with connections to the basin-scale fluctuations. Models are also needed to predict salmon growth and survival during their early ocean phase, emphasizing the role of ocean conditions, productivity and predator abundances in determining the year class strength on interannual and longer time scales--i.e., to provide a foundation for the prediction of salmon recruitment and, ultimately, better management of sustainable salmon harvests under "non-steady state" ocean conditions.