Three Years of PIT tagging – What have we learned?

In 2013, the Fall River Conservancy, California Trout, the UC Davis Center for Watershed Sciences, and the California Department of Fish and Wildlife launched the Fall River Wild Trout Pit Tagging Program. It’s been three years since the PIT tagging program was launched and thus far it has provided a wealth of interesting data that will be useful for management decisions to keep this fishery healthy.

How much data have we collected?

  • Completed 5 PIT Tagging events, and tagged 1859 native Fall River trout with Passive Integrated Transponders (PIT tags)
  • Measured all 1859 trout’s length, weight, and growth rates. The average fork length for Fall River rainbow trout to date is 12 inches!
  • Photographed each fish, and taken fin samples for genetic analysis at the UC Davis genetic lab
  • In 2015-16, four additional PIT detection arrays have been installed at strategic locations on the Fall River.
Photo Credit: Val Atkinson

Program Findings: Genetically different sub-populations of Fall River Trout

Analysis of the genetic data has determined that there are genetically different sub populations of Rainbow trout in the Fall River.  These sub-populations encounter each other in the main Fall River system, but are segregated in spawning locations with very little gene flow between populations.

UC Davis researchers estimate that the Fall River trout system has two distinct spawning locations:

  • 70-100% fish that spawn in spring-fed habitat and
  • 0-30% fish that spawn in snowmelt-dominated habitat, depending on water year and Bear Creek spawning ground availability.

Different Spawning Timing between Bear Creek and Spring Sub-Populations:

The PIT detection arrays have observed 328 fish (17.6%) of the PIT tagged fish, in over 250,000 individual detections by these fish.  This has allowed researchers to estimate spawning initiation and duration periods.

  1. The Bear Creek subpopulation has been observed spawning in the standard spring time spawning period for rainbow trout – late February through March.
  2. However, in the spring-fed spawning group in the Fall River, spawning has been observed from September to June.

As a result of these spawning patterns, we see distinct age classes in the size distributions of snowmelt spawning fish, but not the spring-fed spawning fish, which are able to breed over a longer period.

What is driving distinctly different spawning periods for the two sub-populations of trout?

The genetically distinct sub-populations of Fall River fish experience different hydrological conditions, and understanding these conditions will ultimately lead to better management.

The spring-fed system has a consistent year-round flow of cool, nutrient-rich water that has allowed diversification of spawning timing in the spring-fed spawning group. This group is constrained by area but not by water condition. With good water conditions and access to spawning grounds year-round, this population is resilient to climatic conditions and can support the fishery during harsh droughts.

The snowmelt spawning group, in contrast, is constrained by the timing of snowmelt, but has additional spawning habitat. The reliance on snowmelt on variable availability of water leads to a “boom or bust” style pattern which leaves this sub-population especially vulnerable to prolonged drought and changing climate.

Multi-year drought has caused perilous conditions for Bear Creek fish, as the creek was disconnected from the Fall River early in the season. This stranded many adults and it is likely that there was too short of a time period for juveniles from Bear Creek to outmigrate. The years 2013, 2014, and 2015 could be a little to zero recruitment years for Bear Creek. UC Davis researchers collected fin clips for genetic analysis on adult mortalities retrieved by DFW, and the morts were scanned for PIT tags – surprisingly, none had PIT tags.

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