By Rosi Dagit
On our return to Sepulveda Basin, to continue the upper river fish survey, we captured 203 fish, a far cry from the more than 3,600 tilapia fry caught this time last year.
There had been the first major rain of the season the day before, on Monday, Nov. 21, with between 0.75-1.25 inches over the LA River basin. Sediment and the remains of several homeless encampments were in our car park site. Muck was unconsolidated and varied from 1-6 inches deep. We couldn’t walk far upstream, starting 30 yards to the underpass at Burbank Blvd. because the water was too deep but flow in the main stem concrete area where the sample sites are located was shallow. Most of the vegetation along the banks was gone with just a few patches of water primrose (Ludwigia hexapetala), a few cattails and some willows closer to the bridge were all that remained.
The majority of fish were non-native juvenile Tilapia sp. and Gambusia affinis. Both of these species are common and abundant throughout the LA River, where they were initially released to control mosquito larvae. Samples of both species, as well as fathead minnow, juvenile red swamp crayfish and Asiatic clams were also collected and frozen for toxicology testing.
A single adult Plecostomus (Hypostomus plecostomus), which is a common aquarium fish, was also found washed up on the concrete bank at site 2. It recovered when placed into cool water, but was collected for toxicology testing.
An Army Corps ‘dozer and dump truck moved sediment and debris out of the concrete channel, according to their management permit. The site supervisor kindly allowed us to complete our site 1 and 2 sampling before they moved heavy equipment into that area. The rest of our six sites were upstream of their work area.
Following collection of what looked like an arroyo chub (but was later confirmed to be a fathead minnow) at site 1, just downstream of the disturbance in the channel, we contacted the California Department of Fish and Game about this potential threat to any other arroyo chub that might have washed down.
Dagit is the Senior Conservation Biologist, Resource Conservation District of the Santa Monica Mountains.