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Development of coupled physical and
biological models
Circulation (Hermann, Stabeno)
- Semispectral Primitive Equation Model
(SPEM) of Haidvogel et al.
- three-dimensional, eddy resolving (4 km resolution)
- driven by 12-hour winds and monthly runoff
Prey (Hinckley, Napp) - Nutrient- Phytoplankton- Zooplankton model (NPZ)
- spatially explicit; fixed spatial grid (Eulerian)
- zooplankton segregated by stage and/or species
- includes relevant prey items for pollock
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Arrows indicate typical paths of pre-spawning adults, larvae, and early
juvenile starges. Inset map shows regional circulation. Hatched
area indicates region of spawning.
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Pollock (Hinckley) - Individual Based Model (IBM)
- spatially explicit; follow representative individuals through space and time (pseudo-Lagrangian)
- stochastic
- includes egg, larval, and juvenile stages
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Stored output from the physical model is used to drive biological models.
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Hindcasting of circulation and biology in
the coastal Gulf of Alaska
- After tuning the physical model with hydrographic, current meter,
and drogued drifter data, we ran hindcasts of circulation and
biology for 1978, 1987, 1988, 1989, 1991, and
1994 (movie). These
years span a broad range of wind, runoff and recruitment
conditions.
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Salinity (psu) and surface velocity (m s-1) from
physical model formid-July, 1987
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- Mesoscale statistics of the circulation field (frequency and
rotational sense of eddies) and mean currents are captured
by the physical model.
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Depth-integrated of larvae (#/m2) in mid-May of 1987 and 1989.
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- Measured interannual differences in pollock larval density are
captured by the individual-based model. Weaker advection in
1989 led to a higher concentration of larvae near the exit of
Shelikof Strait in mid-May, relative to 1987.
- Measured spatial pattern of surface chlorophyll-a (spring
climatology from Coastal Zone Color Scanner archives) is similar
to spatial pattern from NPZ model (spring 1987). Higher densities
of phytoplankton (and zooplankton) occur near the southwest
exit of Shelikof Strait.
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Cross section of the mean alongshore current (cm s-1) off Cape Kekurnoi
for the period April 8-September 30, 1991. Current meter locations are
indicated at the top.
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Surface chlorophyll (ug/l) from the NPZ model,versus CZCS data. |
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Exploring the effects of spawning location
on early life history
- Why does a population of walleye pollock spawn at the exit of
Shelikof Strait each spring? One hypothesis is that this population
evolved to optimize physical transport to the juvenile nursery area
near the Shumagin Islands 375 km to the southwest.
Alternatively, factors other than physical transport (e.g. density
of prey) may be significant.
- We addressed these hypotheses with the coupled suite of physical
and biological models, driven by winds and runoff appropriate to two
years of good recruitment, 1978 and 1994. Five regions (1-5) and four
spawning times (Early, Middle, Late, Very Late) were considered.
"1-Middle" represents typical observed spawning.
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Initial locations of spawned populations in the experiment. Transport
of fish to the juvenile nursery area promotes successful recruitment for
a given year.
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- Animated results
show that fish spawned to the south of Kodiak Island
(3-Middle) or much earlier or later than the observed spawning
period (e.g. 1-Very Late) do not reach the Shumagin Island nursery
area as juveniles by early September. However, the region and time of
spawning which did allow successful transport to the nursery
area (e.g. 4-Late) was much broader than the observed region and
time. Hence factors other than physical transport alone must be
considered to explain the spawning location and
timing of this stock.
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Final locations of spawned populations for 1994.
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