DOI: 10.7291/d1t97t
title: Steelhead passage at a mid-sized dam in California assessed using PIT-tags

description: Ecological traps can be caused when partial restoration leads organisms to
 make maladaptive habitat choices. One example of this is fishways (e.g.,
 fish ladders) that provide upstream passage at dams, but are not paired
 with adequate downstream passage. We tested the hypothesis that attracting
 anadromous fishes to spawn above a dam, but blocking downstream passage of
 their offspring leads to an ecological trap. Using passive integrated
 transponder (PIT) tags, we monitored the movements of steelhead
 (Oncorhynchus mykiss) at a dam and fishway on the central California
 coast. We found that downstream passage for juveniles and kelts was
 limited by four factors: migration delay, loss in the reservoir, avoidance
 of the downstream bypass, and water depths on the spillway. Based on the
 spillway-passage depth-thresholds, we estimated that the ability for fish
 to pass downstream was limited to only half of the migration season in 55%
 of the past 20 years (2002-2021). Our results support the ecological trap
 hypothesis, which may explain why restoration using fishways has failed to
 produce recovery gains in this population and elsewhere.
description: This study was conducted at the Los Padres dam in the Carmel
 River. The Carmel River watershed is 660 km<sup>2</sup> and
 drains a mountainous region of the south-central California coast,
 USA.  We captured juvenile steelhead using two
 different methods. In the spring of 2019, we captured outmigrating
 juveniles and smolts in a rotary screw trap (RST) located upstream of the
 reservoir (Figure 1). The RST has a large cone suspended between two
 floating pontoons. River flow rotates the cone and funnels the captured
 fish into an underwater holding tank at the back of the trap. Our RST had
 1x15-m pontoons and a 3-m cone. We operated the RST four days per week
 from March 1, 2019 to May 31, 2019, except for March 11, 2019 when
 operations were canceled due to high flows. We captured outmigrants at the
 RST for the full duration of operation: the first fish was captured on
 March 1, 2019 and the last fish was captured on May 31, 2019. We
 additionally captured juvenile steelhead during fall surveys in 2017 and
 2018. During these surveys we captured fish using electro-fishing over a
 100-m stream section. In 2017, we captured fish at three sites from
 October 10 to October 12, 2017 and in 2018 we captured fish at 12 sites
 from August 14 to October 31, 2018. We refer to all of the fish captured
 in the RST as 'outmigrants' because we cannot be certain which
 fish were migrating to the ocean as smolts or to the lower river as parr.
  All captured fish were processed in the same manner. We anesthetized each
 fish with MS-222, and tagged all fish &gt;65 mm FL with a PIT tag
 injected into the abdomen. Fish between 65-100 mm FL were tagged with
 12-mm PIT tags, and fish &gt;100 mm FL were tagged with 23-mm PIT
 tags. Staff of the dam operator (California American
 Water) captured spawners at the ladder trap from January 1, 2019 to May
 13, 2019. Observers regularly monitored the pool below the spillway to
 confirm that spawners were entering the trap. Trap operation was stopped
 in May only after confirming that no additional spawners were holding in
 the pool. To process the spawners, we transferred each captured spawner
 into a 0.5 by 1-m container filled with water and gently held the fish in
 place. We identified sex and tagged each spawner with a 23-mm PIT tag in
 the dorsal sinus. Staff from California American Water then transported
 each spawner around the dam and released it into the reservoir near the
 spillway. We tagged the first spawner on March 15, 2019 and the last on
 May 8, 2019. We used a series of PIT tag antennas to
 observe fish movements. The furthest upstream antenna was located 0.2 rkm
 upstream from the head of the reservoir ('ALP') and 0.1 rkm
 downstream of the RST. Downstream we placed one antenna inside the FWC
 entrance ('FWCE') and another on the end of the FWC outflow pipe
 ('FWCP'). The paired antennas on the FWC allowed us to observe
 if fish entered the FWC entrance but did not continue downstream through
 the FWC pipe. The next antenna was 0.6 rkm downstream below the Los Padres
 spillway ('BLP'). Three additional antennas were located in the
 lower river; two antennas were located at Sleepy Hollow ('SH')
 at rkm 28 and one antenna at Scarlett Well ('SW') located at rkm
 15. We did not have antennas on the spillway, so we inferred spillway
 passage if a fish was detected downstream of the spillway, but was not
 detected at the FWCE or FWCP antennas. To estimate the
 lowest spillway depths at which we observed fish passage (i.e., the
 threshold 276 passage depth), we used reservoir surface elevation (stage)
 data collected by MPWMD from 2002-2021. Reservoir surface elevation was
 measured at the Los Padres dam spillway crest (the ogee crest) on
 15-minute intervals using a pressure transducer. Under current conditions,
 water starts to flow over the spillway crest when the reservoir reaches
 317.000 m (NGVD 1929) elevation. We converted reservoir elevation to
 spillway crest water depth by subtracting 317.000 m from the reservoir
 elevation. We took the mean daily water depth at the spillway crest on the
 day a fish was detected at the BLP or FWC antennas to be the passage flow
 experienced by the fish.
description: README file is included. All missing values were deleted or
 indicated with NA.