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.

Biomass fluctuations of small coastal-pelagic fishes represent perhaps the most iconic temporal record of the impacts of natural climate variability on marine ecosystems1,2,3. These fishes are key constituents of the marine pelagic food web as primary feeders on plankton, prey to higher trophic level foragers such as birds, marine mammals, piscivore fishes4,5 and valued for human consumption and industry6. Despite over a century of research, the mechanisms governing their population volatility remain elusive7,8. Here, we use a 45-year record of nitrogen stable isotopes measured in larvae of Northern Anchovy (Engraulis mordax) in the California Current to assess patterns in food chain length. Larval trophic efficiency associated with a shortened food chain increased for boom periods of high adult biomass, during which the ratio of large to small larvae decreased by an order of magnitude. In contrast, the ratio increased during periods of low adult biomass, likely reflecting a higher rate of early larval mortality. For the first time, we connect energy transfer efficiency to early larval survival, which in turn regulates the decadal-scale volatility of anchovy biomass, leading to the proposal of the Trophic Efficiency in Early Life (TEEL) hypothesis. Our findings illustrate a potential for trophic indicators to inform on the condition and recruitment to coastal-pelagic fish populations.