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Ocean Carrying Capacity Program

Effects of Consumer Size and Two Types of Zooplankton Prey on the Functional Response of Juvenile Pink and Chum Salmon

Feeding rate experiments were conducted for pink salmon (Oncorhynchus gorbuscha) fry (mean fork length (FL) 39 mm), juveniles (103-104 mm FL), and juvenile chum salmon (O. keta; 106-107 mm FL). Fishes were presented with small copepod (Tisbi sp.) or larger mysid shrimp (Mysidopsis bahia) prey at varying densities ranging from 1 to 235 prey/liter in feeding rate experiments conducted at water temperatures ranging from 10.5 to 12.0C under high light levels and low turbidity conditions.

Both salmon species demonstrated type II functional responses to zooplankton prey. When offered copepods, pink salmon fry fed at a higher maximum consumption rate (2.5 copepods/min) than larger juvenile pink salmon (0.4 copepods/min), whereas larger juvenile chum salmon exhibited the highest feeding rate (3.8 copepods/min). When feeding on mysids, the maximum feeding rate for larger juvenile pink (12.3 mysids/min) and chum (11.5 mysids/min) salmon were similar, and higher than feeding rates on copepods.

Functional response models parameterized for specific sizes of juvenile salmon and zooplankton prey can offer insight into the mechanistic limitations and prey preferences on planktivorous fish consumption rates given biological and physical conditions experienced by the fishes.

By Jamal Moss

Does Size Affect Survival Rates in Juvenile Sockeye Salmon?

We tested the hypothesis that larger juvenile sockeye salmon in Bristol Bay, Alaska, have higher marine stage survival rates than smaller juvenile salmon. The data consisted of archived scales from returning adults (33 years of data) and trawl samples of juveniles (n = 3,572) collected along the eastern Bering Sea shelf during August through September 2000-02.

The size of juvenile sockeye salmon mirrored indices of their marine stage survival rate (i.e., smaller fish had lower indices of marine stage survival rate). However, there was no relationship between the size of sockeye salmon and marine stage survival rate after their first year at sea, as estimated from archived scales and brood year survival; size was relatively uniform over the time series, possibly indicating size-selective mortality on smaller individuals during early marine residence. Variation in size, relative abundance, and marine stage survival rate of juvenile sockeye salmon is likely related to ocean conditions affecting their early marine migratory pathways along the eastern Bering Sea shelf.

By Ed Farley

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