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Marine Ecosystem

After hatching and a year or so in freshwater, sockeye salmon begin several years of ocean cruising, taking them over a thousand miles through the North Pacific. Due to diverse circulation patterns and the upwelling of nutrient-rich waters from the continental shelf, Alaska’s waters are among the most productive in the world. Here salmon follow major ocean currents that circulate mostly in a counterclockwise direction, swimming anywhere from ten to thirty miles per day, generally staying within thirty feet or less of the surface. In cool winters, they dip down to just above the fortieth parallel, the latitude of both Japan and northern California. In warm winters, they enter the Bering Sea north of the Aleutian Islands. As they roam, they mix with sockeye from Asian rivers, tens of millions of salmon participating in a great circuitous voyage only slightly smaller than the continental United States.

Wherever they travel, they constantly feed on the prolific marine life—mostly squid, krill, copepods, amphipods, and juvenile fish—which in turn feast on the rich phytoplankton blooms that thrive on nutrient upwellings found in this turbulent area of the Pacific. Salmon feed on large zooplankton at night, while during the day crab larvae and small fish serve as the main entrées. Zooplankton in particular abound in carotenoid pigment, giving sockeyes their rich muscle color. By the time a sockeye reaches maturity, usually during its second or third year at sea, it commonly weighs between 4.5 and 6.5 pounds.

Now the final and most astounding part of the journey begins. Using what appears to be a precise navigational system, mature sockeye forsake the sea and begin the journey back to their natal waters. Their timing could scarcely be more accurate. After ranging as far as Japan or Russia, as many as 80 percent of returning Bristol Bay sockeye reach their home saltwater estuary within a two-week period, usually peaking during the first week of July.

Even then they are often subjected to intense predation by orca and beluga whales. Pods of beluga whales, with their white, plump bodies ghosting beneath the surface, eagerly feed on both smolt and adult salmon. These fourteen-foot behemoths have been observed as far upstream as Igiugig, a small Yup’ik village some thirty miles up the Kvichak River, at the mouth of Lake Iliamna. At times, belugas become prey themselves. In spring 1989, at the mouth of the Naknek River, a dozen orca whales lived up to their nickname, “wolves of the sea,” and attacked some fifty belugas gathered there to feed on descending smolt. As several awestruck fishermen watched, a massive, thirty-foot bull orca flipped the smaller belugas out of the water and up onto the beach. One observer described the attack as “a feeding frenzy with the water just boiling.”