The PICES CCCC (North Pacific Marine Science Organization, ClimateChange and Carrying Capacity program) MODEL Task Team achieved a consensus on the structure of a prototype lower trophic level ecosystem model for the North Pacific Ocean, and named it the North Pacific Ecosystem Model for Understanding Regional Oceanography, "NEMURO". Through an extensive dialog between modelers, plankton biologists and oceanographers, an extensive review was conducted to define NEMURO's process equations and their parameter values for distinct geographic regions. We present in this paper the formulation, structure and governing equations of NEMURO as well as examples to illustrate its behavior. NEMURO has eleven state variables: nitrate, ammonium, small and large phytoplankton biomass, small, large and predatory zooplankton biomass, particulate and dissolved organic nitrogen, particulate silica, and silicic acid concentration. Several applications reported in this issue of Ecological Modelling have successfully used NEMURO, and an extension that includes fish as an additional state variable. Applications include studies of the biogeochemistry of the North Pacific, and variations of its ecosystem's lower trophic levels and two target fish species at regional and basin-scale levels, and on time scales from seasonal to interdecadal.5

Large fish often inhabit colder waters than small fish. Using a simple bioenergetic model, we found that the optimal temperature for growth should decrease with increasing body size. We predicted that this mechanism would produce an ontogenetic change in thermal preference and then tested our predictions with Pacific salmon, Oncorhynchus spp. In a laboratory experiment, the slope of a regression of growth increment on initial size became steeper with increasing temperature, so that the optimal temperature for growth decreased with increasing body size. In field observations, larger and older salmon inhabited cooler areas, whereas smaller and younger salmon inhabited warmer areas. These patterns were consistent with a size‐dependent effect of temperature on condition factor, a parameter shown experimentally to be a measure of the most recent growth performance. Temperatures for maximising condition factor were lower for larger fish. Thus, an ontogenetic change in individual thermal preference toward cooler areas maximises the growth performance of fish, and the negative effects of climate warming on growth are hypothesised to be more severe for larger fish.

Seasonal, ontogenetic and bathymetric variations of diet were examined for walleye pollock Theragra chalcogramma based on a total of 6666 fish collected off the southeastern coast of Hokkaido Island, northern Japan (Doto area) during 1989 to 2000. Walleye pollock depended exclusively (> 99%) on pelagic prey and showed a clear ontogenetic dietary shift: smaller fish depended mainly upon mesozooplankton such as Neocalanus cristatus and Euphausia pacifica, and larger fish preyed upon pollock (cannibalism), myctophids Diaphus theta and firefly squid Watasenia scintillans. Seasonal variation was also evident. Smaller pollock depended mainly upon N. cristatus during spring then shifted gradually to E. pacifica during other seasons. For larger pollock, major prey shifted from pollock (cannibalism) during spring to micronekton during other seasons. Bathymetric variation was less pronounced, with cannibalism and Themisto spp. being more important in the shallow area (≤150 m). Feeding intensity, measured as stomach content index, was generally higher during spring and summer than autumn and winter, but showed incidentally high values during winter, suggesting sporadic but intense feeding opportunities. Fish condition fell during winter and then recovered rapidly during May and June. Recovery coincided with superabundance of N. cristatus during blooming, so that this species is essential for the recovery of pollock from wintering and spawning. KEY WORDS: Theragra chalcogramma · Food habits · Seasonality · Feeding intensity · ConditionResale or republication not permitted without written consent of the publisher

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An age-structured trophodynamic model was constructed to quantitatively analyze factors affecting postsettlement mortality and growth of walleye pollock (Theragra chalcogramma) in the Doto area, the main nursery ground of the Japan Pacific population. The model included (i) multiple age classes of pollock, (ii) a generic predator, (iii) fisheries, and (iv) major prey of pollock. Major processes considered were (i) recruitment, (ii) bottom-up control of somatic growth, (iii) mortality because of predation, cannibalism and fishing, (iv) size-selective prey selection, (v) temperature-dependent bioenergetics such as conversion efficiency and daily consumption rate, and (vi) production and advective supply of prey. By assuming that pollock select prey based upon both relative abundance and predator-prey size relationships, the model accurately simulated seasonal and ontogenetic variations in the diet. However considering ontogenetic segregation, the model showed that, due to cannibalism, newly recruited fish would be totally consumed within 6 months after settlement. By considering segregation (10% overlap during spring and 0.1% during other seasons), an agreement of diet between the simulation and empirical data averaged 82.7% for the different seasons and fish sizes. Euphausiids, the most important prey of pollock, suffered the highest predation impact (22.2 ± 5.3 WWg m )2 yr )1 ) exceeding annual production in the model domain (17.2 ± 0.1 WWg m )2 yr )1 ), indicating that an advective supply of prey is necessary to support the pollock population. The daily ration of pollock during spring and summer averaged at 1.2 and 0.6% BW day )1 for small (£200 mm) and large (>200 mm) pollock, respectively; this daily ration was reduced by half during autumn and winter.