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Ocean Carrying Capacity Program

Marine Growth of Sockeye Salmon From Karluk Lake in Relation to the Ocean Abundance of Maturing Sockeye Salmon, 1925-2000

Understanding factors that influence the marine growth of salmon is key to developing models for estimating the carrying capacity of salmon in the North Pacific Ocean. Annual growth patterns on salmon scales provide clues to factors that influence the ocean growth of salmon. Inverse patterns between scale growth and salmon abundance provide evidence that density-dependent factors affect the marine growth of salmon. For example, a trophic mechanism that may affect growth is competition for food. Responses among salmon to higher salmon abundances often include reduced feeding, changes in migration, and changes in prey selection that in turn reduce growth. However, little is known about the ocean growth of salmon prior to the mid-1970s.

In this study, we compiled and measured a 76-year historical time series of age-2.2 sockeye salmon scales collected at the weir on Karluk River, Alaska, from 1925 to 2000. Strong inverse patterns occurred between scale growth in the final year in the ocean (M3) and the abundance of maturing southern Alaska sockeye salmon (Fig 2 below).

Figure 2, see caption
Figure 2.  Ocean regime comparison of maturing sockeye salmon growth and maturing sockeye salmon abundance in southern Alaska waters, 1925-2000.

A shift to a higher magnitude of competition coincided with the 1976-77 ocean regime shift in the North Pacific Ocean. Regression analysis for time series data (TSMARS) showed that the contributions of marine growth in sockeye salmon on marine growth during their final year in the ocean were negative and significant (Fig. 3 below). Sockeye salmon abundance was also a negative contributor to marine growth of sockeye salmon during their final year in the ocean (Fig. 4 below).

Figure 3, see caption
Figure 3.  Changes in additive contribution of the marine growth of maturing sockeye salmon in year t-1 on the marine growth of maturing sockeye salmon in year t, as M3 lag 1 increases from 0.240 mm to 0.440 mm.
Figure 4, see caption
Figure 4.  Changes in the additive contribution of abundance of southern Alaska sockeye salmon (SAS) on the marine growth of maturing Alaska sockeye salmon M3, as SAS increases from 2 million to 22 million.

We conclude that the marine growth of maturing sockeye salmon is limited by the abundance of maturing sockeye salmon. Our results indicate that a possible carrying capacity exists for maturing salmon in coastal waters of the North Pacific Ocean.

By Ellen Martinson and Jack Helle

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