Abstract

Juvenile Chinook salmon (Oncorhynchus tshawytscha) emigrating from natal tributaries of the Sacramento River may use a number of migration routes to negotiate the Sacramento-San Joaquin River Delta (hereafter, “the Delta”), each of which may influence their probability of surviving. We applied a mark-recapture model to data from acoustically tagged juvenile late-fall Chinook salmon that migrated through the Delta during the winter of 2009–10 (hereafter, 2010). This report presents findings from our fourth year of research.

We estimated route-specific survival for four release groups: two release groups that migrated through the Delta in December 2009 and January 2010, and two release groups that migrated during February 2010. Population-level survival through the Delta (S_Delta) ranged from 0.374 (SE = 0.040) to 0.524 (SE = 0.034) among releases. Although river flows for the February release groups were substantially higher (20,000–40,000 ft³/s at Freeport) than for the December release groups (about 10,000 ft³/s), S_Delta did not differ considerably between release groups. Among migration routes, fish migrating through the Sacramento River exhibited the highest survival, and fish entering the interior Delta exhibited the lowest survival. Fish entering Sutter and Steamboat Sloughs had lower survival than fish entering the Sacramento River during December, but similar survival during February. These patterns were consistent among release groups, and strikingly similar to patterns observed in previous years.

Migration routing varied among release groups partly because of differences in river discharge between releases. For the two December release groups, 26.5 and 28.9 percent of fish entered the interior Delta; for the two February release groups, 10.4 and 17.9 percent of fish entered the interior Delta. Differences in routing probabilities between December and February are partly related to the inverse relationship between flow and the fraction of discharge entering the interior Delta. The proportion of fish diverted into the interior Delta also can be affected by the status of the Delta Cross Channel’s gates. The fraction of fish entering Sutter and Steamboat Sloughs also varied considerably among release groups from 22.1 to 44.7 percent, and did not appear correlated to river discharge. For example, the lowest and highest proportion of fish entering Sutter and Steamboat Sloughs occurred during February. Because fish entering Sutter and Steamboat Sloughs bypass the entrance to the interior Delta, a high proportion of fish migrating into this route reduces the proportion of fish entering the interior Delta.

First posted September 6, 2012