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An Alaskan Pollock Fishery

the influence of physical features

on

a fishery population

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This schematic portrays mean or steady-state conditions for a northern hemisphere. We will discover that there are transient physical processes in the subpolar region that strongly influence the Alaskan Pollock fishery’s biology. Relationships exist between physical and biological processes that have important time intervals between one another, that is, there are important phase relationships between physical and biological events that influence the success of biological populations. These events can jointly function to maintain the fishery on a year-to-year basis yet can also contribute to a population’s fluctuations, if one or more are not present at an appropriate time.

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Variability in recruitment can start with early life stage success-or-not. Fisheries scientists consider feeding, predatory loss and transport as natural mechanisms bearing on a year-class’ numbers. Herein we will study the effect of transport by current and eddy on recruitment into a population.

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The walleye pollock [Theragra chalcogramma] fishery in Alaskan waters has produced an annual catch of over a million tons annually from within the United States Exclusive Economic Zone since 1986. Initially, the American consump-tion was near six million pounds [1981] and it rose twenty-fold by 1985. This fish is a rich source of protein with a very low level of cholesterol and is sold locally by super markets. Not that you would recognize the food products as that fish to the left ; but It brings in big $$$$.

Effective fisheries management required an assessment of the factors that cause the observed variability in annual recruitment. Most simply put, recruitment is the number of individuals added to a species’ population each year. Management goals are based on what-is-hoped-to-be accurate prediction of a given year-class’ strength. During early laval stages, strong positive and negative effects on survival are possible, especially when lavae must be transported from spawning grounds. Understanding any recruitment at the level of attempting actual, useful predictions means observing physical transport events, water mass mixing and larval growth, as well as predation and survival.

The Fisheries-Oceanography Coordinated Investigations [FOCI] scientists at the Pacific Marine Environmental Laboratory joined other government agencies, commercial entities, university scientists and the fishermen in efforts to study the variability of the physical and biological environment through field surveys, time series observations of the environment, satellite imagery, and factors that

are critical to egg and larvae survival. A great deal of information must be gathered and analyzed to understand a system such as a fishery. Correlations that address relationships between fluctuations in recruitment, meteorological and oceanographic data can be carried out because of time series initiated decades ago. These analyses and the possible resulting relationships are fixed by the space and time resolutions of the observations and are used to provide information on major shifts and trends. More recently, fluctuations in the initial, albeit limited, joint bio-physical data banks indicated the need for studies of shorter temporal and horizontal scales if effective annual management decisions were to be practiced. Sequences of joint field observations at appropriately small scales are hard to come by, expensive to obtain, and extremely scientist-intensive. There are not many of these data bases and efforts are underway to remedy the dearth of data since these particular relationships appear to be very important ones for the recruitment modelling needed to guide fishery efforts. So far, FOCI has gathered and processed a modest amount of such data.

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Local supermarkets occasionally offer authentic Alaskan King Crab Legs at prices that require a second mortgage on the homestead. Instead you may purchase, something called King Krab Legs and just lose the back forty. Check the small print, Krab is pollock. One method of processing pollock reduces the fish to white, tasteless flakes. By an infusion of actual king crab juices accumulated during processing of the real thing, then forming and coloring, there is king krab or crab delight for your table. Not bad actually, if you are not used to the real thing.

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Well, not really beautiful. What do you expect, their mother abandoned them on the bottom of the ocean before they were born. Pollock are part of the Cod family and are demersal fish. But they are planktonic for their first 8 to 12 weeks. Adults are about 16 inches long and weigh two to three pounds. Eggs and larvae are normally found in patches and larval patches in surface waters depend on currents to transport them away from the major spawning areas while keeping them from leaving the continental shelf so they reach the nursery grounds.

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Fishing for pollock in the Gulf of Alaska and the Bering Sea is a cooperative effort. Above, an American fishing vessel is approaching a larger Eastern Block ship to transfer a full net and then to receive an empty one.

The Pollock Fishery in the North Pacific finds its home In the Gulf of Alaska and the Bering Sea. Pollock hatched off Kodiak Island find their nursery as juveniles off the Semidi Islands on the narrow continental shelf southwest of Shelikof Strait. Young adults leave this location and most move through Unimak Pass into the Bering Sea. Many return to the Strait three years later as adults to spawn and can then be counted as recruited into the population. It is easier to count adults rather than larvae and so some counters wait. The Shelikof Strait region is but a small part of a larger fishery. The major portion of the fishery’s population is located within the Bering Sea. We study the Shelikof spawning site because it illustrates how physical and biological elements of an ecosystem combine in space and time to support a fishery.

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Nets can be drawn aboard a ship along a ramp at the stern. Note that the net contains [orange] bottom dwellers that were captured during the net’s tour over the bottom to catch the demersal pollock. There is always the possibility of environmental damage during the net tows.

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Here transfer is made of accumulated catch to a larger factory ship that will process, package and freeze so it can be ready for marketing when it reaches land. Some commercial operations will store frozen catch and processed products on land in foreign countries [eg, Chile] prior to shipment to a final destination [eg, Japan]. This type of operation keeps the producing ships at the fishing grounds for longer intervals of time.

11Advertisement for the “Bountiful Sea” concept. Nets are opened on deck and the aggregate sorted to separate pollock and by-catch.

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Just one more week …. One more catch …. Fishing late into the year in the Bering Sea can be a tad risky and a little uncomfortable for the grunts on board; as well as for the Coast Guard aerial patrols that have to account for everyone. At least one doesn’t have to use the freezers on board; leave the fish on the deck, its 20 below with a 40 knot polar easterly breeze, straight from the north pole.

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The counter-clockwise current in the eastern limb of the subpolar gyre is the Alaska Current. This flow is separated from the western portion, the Bering Sea, by the Alaskan Peninsula. One of the pollock spawning regions is in this eastern limb adjacent to the Peninsula [P] and Kodiak Island [K].

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A branch of the Alaska Stream [green trajectory] enters Shelikof Strait through Kennedy Entrance [K] and proceeds southwestward and now is known as the Alaskan Coastal Current [ACC]. The ACC flow and eddies generated from the flow are the physical features that move pollock larvae toward the fishery’s nursery. The ACC rejoins the Alaskan Stream at 156 degrees N and will most likely carry larvae to their death in the open waters of the Gulf, unless … near-shore eddies can transport the larvae to the safety of their nursery in the lee of the Shumagin Islands [159 - 160 degrees N ]… that is, if the eddies are generated at the right time in the right place, move along the Strait in the ACC, and arrive over the spawning grounds as the larvae hatch and rise, reaching vertically into the surface waters. Spawning, nutrients, light, photo-synthesis, phytoplankton, hatching larvae, larvae rising, current, eddies, entrainment, feeding, predation, transport and location and timing, all are central elements of a natural bio-physical process that sustains the fishery as it comes under the pressure of the fishing effort.

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Between Chirikof [C] and Semidi [S] islands, the blue line traces the lay of Shelikof Canyon past Wide Bay [W] and Cape Kekurnoi [K] into Shelikof Strait. Adult pollock follow the bottom of the canyon to two spawning sites, off Wide Bay and to the principle site off Cape Kekurnoi. The pollock larvae nursery is indicated on the chart by the letter [N].

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Winds in Shelikof Strait are cross-winds that reach speeds of 60 knots. There are two locations, at Wide Bay and the valley marked on the chart and aerial photograph, where air-sea interaction with surface waters of the Alaska Coastal Current can force waters across the Strait. At Wide Bay an elongated plume is formed while, adjacent to the valley, the interaction creates an instability in the ACC that can produce an eddy that can block the southwesterly ACC flow.

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The location and timing of pollock spawning in Shelikof Strait are both very consistent year-after-year. Adults arrive at the Strait through the Shelikof Canyon in late winter and spawning occurs there in early April near Cape Kekurnoi. Each female produces about half a million free-floating planktonic eggs [1.8 mm diameter] in about ten batches over a few weeks. The eggs remain near the bottom in depths of 150-200 meters for two weeks until they hatch; young larvae are 3-4 mm in length, relatively undeveloped, without eyes or mouth. Their life support is provide by material in the yolk sack that also acts as a buoyant life preserver as it empties. As they develop, they rise in the water column into the upper 50 m and larvae develop functional eyes, mouth and gut. They drift in currents during late April and May and grow about 0.2 mm/day. The larvae are visual feeders and eat mostly during the day. The best conditions for food and survival are shoreward and in transport, safely done in eddies, southwest to what effectively are nursery grounds at the Shumagin Islands. The larvae have now grown to the early juvenile stage and begin diel vertical migration over the first 15 to 20 m. By mid-summer they have reached the juvenile stage, are schooling, and continue to concentrate in the nearshore. The population continues to lose individuals by turbulent diffusion to offshore waters and is actually preparing to leave the Gulf and transit into the Bering sea through passes such as Unimak. While eddies provide an environment that keeps pollock nearshore into the late juvenile stage, concentration within makes them more vulnerable to predation [see above].

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This is a general summary slide that covers pollock spawning, hatching and movement of the larvae-to-juveniles stages as they are moved from Shelikof Strait to the Shumagin Islands. Notice the indication of loss to the offshore waters and the possiblity of return from offshore to nearshore waters by turbulent diffusion.

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The NOAA research vessel Miller Freeman used by FOCI to take the observations that are shown in the following slides.

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LEGEND

K Cape Kekurnoi

S Shumagin Islands

W Wide Bay

1,2,3 met stations

The permanent station locations occupied for oceanographic observations during each Miller Freeman cruise are shown by the solid white dots. The contoured data shown on the follow-ing slides were taken with the above spatial resolution.

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Distribution and Concentration of Phytoplankton from Sea Surface Chlorophyll

Spring bloom - early May - 1983

Note : the distribution of phytoplankton south of the Strait takes a circular form which indicates the plankton has been entrained during eddy generation. The white line through the distribution is the subtrack of the satellite that gathered the observations. The eddy was produced in the Strait northeast of its present position about two weeks prior and drifted to and over the spawning site as the larvae were rising into the surface layer of the water column. Dinner is served.

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Approximately 29 days later the pollock larvae distribution shows another single circular feature contoured from observations taken off Wide Bay. The feature implies another correlation between physical and biological events.

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Larvae have been studied in laboratories to determine growth rates and nutritional states. This is a photomicrograph of a first-feeding walleye pollock larva. Copepod prey tagged with fluor-escent red dye pigment are visible within its gut. RNA/DNA ratios are accurate measures of feeding conditions : higher ratios - better feeding.

Ear stones [Otoliths] are apparently present in a fish’s head for balance while swimming. They abide within three fluid-filled cham-bers of a fish’s head. When upright, gravity pulls the stones to the bottom of the chambers giving the fish a sense of up and down within the water column. A record of larva daily growth and survival can be accessed through examination of stained larvae otoliths. Cut into thin sections, the otoliths reveal growth through daily ring increments. They are accurate indicators of a fish’s age as well as the waters in which it has been swimming.

These larvae are fresh from the lower reaches of Shelikof Strait. The one on the left [Ralph, otoliths reveal a fish’s name as well] has partially consumed his yolk sack, the life preserver that assists in vertical ascent. Martha, on the right, is an older child that now has to fend for

herself as far as sustenance is concerned.

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During the first months of life after hatching the pollock must find themselves in a fairly food-rich environment that is relatively safe from predators. That means food and growth and transport to safety. For the Pollock fishery this means moving southwest-ward along the Peninsula in a cafeteria from Cape Kekurnoi [K] to the Shumagin Islands[ S]. And no being swept out into the Gulf of Alaska as the ACC moves down the Strait and back out to sea. So the cafeteria has to be on a bus and the bus route has to travel along the peninsula to the Shumagin Islands, the larvae’s nursery. Remember that the bus is a temporary danger as well as a refuge : it concentrates the larvae and they are more prone to predation.

The illustration above shows the mean distributions of walleye pollock eggs and larvae at four times in the first two months of their neritic life. The data represented above was gathered on ichthyoplankton surveys from 1987-92, broken into appropriate time intervals on the basis of mean ACC flow and treated statistically to yield best-fit ellipses for each chosen interval. The results show the progress of the eggs and larvae in ACC flow, first to the south-west and then south-southwestward in the ACC flow regime. Note that continued treatment in a mean sense on further and later intervals of data would most likely indicate export for the majority of the eggs and larvae to the open ocean of the Gulf of Alaska.

So, what is the bus and from whence comes the bus ? The transport of the [eggs and larvae], a population steadily shifting to [larvae and young juveniles], to the nursery grounds depends on the development of instability in the ACC to the north in the Strait about two weeks before the larvae rise into the surface layer and the generation of closed, turbulent features known as eddies. The circular motion around the periphery of an eddy suites the purpose of the pollock population [reach the nursery] as they drift along the Peninsula, inboard of the main ACC flow.

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E1 eddy stationary southwest of the Strait

E2 eddy stationary at Cape Kekurnoi [K]

P1 wind-driven plume out of Wide Bay

P2 wind-driven plume across the Strait

ACC Alaskan Coastal Current interrupted by the presence of E2 blocking

the Strait

ACC remnant south of E2 turning back toward the Strait allowing P1 to

exit Wide Bay to the south

Eddy E1 has exited Shelikof Strait after passing over the spawning regions at Cape Kekurnoi and Wide Bay. E1 was generated to the northeast within the Strait approxi-mately three weeks before reaching its present position

Eddy E2 is over the spawning region at Cape Kekurnoi and was generated approximately two weeks within the Strait before reaching the Cape

Plume P1 is over the spawning region off Wide Bay

Plume P2 is in the process of generating an additional eddy feature to the northeast

Shelikof Strait satellite infrared image #1

27Shelikof Strait satellite infrared image #2

The distribution of larva concentration at the left was obtained over a week-long interval. An overlay of the concentration distribution on an infrared satellite image taken during the ship’s survey week shows a plume of cooler water [blue] emerging [yellow trace] from Wide Bay over the spawning grounds and turning to the south-southwest north of the ACC [green] leaving the continental shelf. Along the trace of the plume’s motion, H’s mark high concentrations that have been entrained as the plume passes over larvae rising into the surface layer. This plume and the concentrations are another example of the bus analogy we have made.

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A clockwise drifter trajectory obtained during 11-13 May 1990 is shown in red overlayed on contours of abundance data taken coincidentally in a ship survey. L indicates the launch position. The correlation of trajectory and abundance emphasizes that an eddy can act to transport Pollock larvae in the Shelikof Strait region.

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This Bongo net tow was taken at station 27 in an eddy near the southern end of Shelikof Strait. Otolith anayses of larvae from the tow provided birth dates that were grouped in three day intervals from 17April through 16May. The graph of three-day intervals vs % of larvae hatched in each interval can be interpreted as showing two cohorts sets, one born from 23April through 04May and the other from 08May through 16May. A review of the ship’s track suggests that ear stones from tows could be used to identify larvae entrained by the eddy over the distinct spawning grounds at Cape Kekurnoi and at Wide Bay.

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how not to solve a whale of a problem

A TAIL OF INTRIGUE AND REVENGE

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END

POLLOCK FISHERY