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.

This study was conducted at the Los Padres dam in the Carmel River. The Carmel River watershed is 660 km² 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 >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 >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.

README file is included. All missing values were deleted or indicated with NA.