Science Stories: Adventures in Bay-Delta Data

All about the bass
  • February 16, 2021

By: Rosemary Hartman (CA Department of Water Resources), Pascale Goertler (Delta Science Program), Brian Mahardja (US Bureau of Reclamation), and Ted Sommer (CA Department of Water Resources).

A new synthesis study indicates Striped Bass could start swimming up the Sacramento River earlier in the year as climate change progresses. You probably know “stripers” as popular game fish, and die-hard fishermen will tell you all about the best times and places to try and catch a big one. However, Striped Bass do not stay put. Stripers are “anadromous”, meaning that just like salmon, they begin their life in fresh water, migrate to the estuary and ocean, and eventually return to freshwater as adults to spawn. This study, by IEP Scientists Pascale Goertler, Brian Mahardja, and Ted Sommer, looked at one of our oldest data sets to examine fish migration patterns. Scientists in other systems have found a number of changes to migration timing of many species linked to climate change, and Pascale, Brian, and Ted were curious whether there had been any changes in our system. They found that adult Striped Bass migrated into the Central Valley later in the year when there was higher Delta outflow and cooler sea surface temperatures (Figure 1).

Diagram of striped bass migrating from the ocean or estuary up into a river under different environmental conditions such as flow and ocean temperatures.
Figure 1. Diagram of how environmental conditions influence Striped Bass Migration. In the top panel, warm ocean temperatures and low freshwater flow correlate with earlier upstream migration. In the bottom panel, lower ocean temperature and higher flow mean Striped Bass hang around in the ocean or estuary for longer before migrating.

In order to figure this out, the team started with data from the CDFW Adult Striped Bass Survey. This survey has captured and tagged Stripers using gill nets and fyke traps throughout the Delta and the Sacramento River since 1969 (Figure 2). Having a really long-term data set let them correlate migration to climate factors, which has not been studied very often in migratory fishes, but it also caused problems. This is a “presence-only” data set, which means the researchers didn’t know whether there was no record for a given date because the monitoring crews didn’t sample, or because they sampled, but failed to catch any fish. The monitoring crews also didn’t record how long they were out in a consistent way. Did they only catch one fish because there weren’t many fish? Or because they only put the net in the water for five minutes instead of an hour? They didn’t have the crews from the 1970s to ask.

Two men using a long-handled fish net to remove a large striped bass fish out of a large wire (Fyke) trap stationed along the water's edge.
Figure 2. CDFW scientists have been monitoring Striped Bass populations using fyke traps such as this one since the 1960s. CDFW photo.

While the patterns Pascale and her colleagues found have held true over the past forty years of the data set they analyzed, migration timing could change in the future. Based on trajectory of climate change, we expect warmer sea surface temperature and lower outflow during late spring and early summer (as snowpack level decreases), which could mean earlier bass migration. Striped Bass are a voracious predator, so adult fish entering the estuary earlier in the year could mean a head-on collision with juvenile salmon moving out of the estuary. While Striped Bass have coexisted with salmon for the past 150 years, they are not native to California, and changes in predation patterns could affect already stressed salmon populations.

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