San Joaquin River Predator Population Study


The fish assemblages in the San Joaquin River and the Sacramento-San Joaquin Delta are heavily dominated by non-native species. However, the population sizes of many of these species are unknown. To help fill in this information gap, FISHBIO is conducting a study to count five species of bass and catfish in the San Joaquin River to help estimate their population sizes. These species are known to eat juvenile salmon and trout, and better estimates of their abundance will help assess the risk of predation to young salmonids in the San Joaquin River, as well as whether this risk may be limiting salmon and steelhead recovery in the region.

The objective of this study is to use a mark-and-recapture method to estimate the population sizes of striped bass, largemouth bass, smallmouth bass, white catfish, and channel catfish. To catch these fishes, five large fyke traps are deployed over 63 km of the lower San Joaquin River. A fyke trap has a funnel-shaped opening that narrows toward the interior of the trap. Migrating fish that swim into the funnel are trapped in the compartment at the back of the fyke. The California Department of Fish and Wildlife has been using these fyke traps to sample striped bass in the Sacramento River basin since 1951, but a similar effort has not yet been carried out in the San Joaquin River. This study builds on a pilot effort FISHBIO conducted in 2013 to examine the feasibility of fyke traps to capture striped bass.

Individuals of the five target species that are caught in the fyke traps are tagged with Passive Integrated Transponder (PIT) tags, as well as pink external tags (either long spaghetti tags or round disc tags), which may be turned in for a reward if recaptured. Stomach samples are also collected to help examine the similarities and differences between the diets of the different species. So far, anglers have reported catching marked fishes up to 230 miles away from where they were tagged (see map below). This project is funded by the Banta Carbona, Patterson, and West Stanislaus Irrigation Districts, and will help illuminate the movements and relative abundance of several non-native species, as well as how their population dynamics may impact the survival of native salmonids in the San Joaquin River Basin.

Map of Fyke project tag recaptures

How Many Bass and Catfish are in the San Joaquin River?

One fish, two fish – Using mark-recapture to estimate population size