Synchrony of movement of anadromous fish in California’s Central Valley

Background/Question/Methods

: In anadromous fish, the period of seaward migration is of crucial management concern, particularly in systems with greatly modified flow regimes. In these systems, survivorship can depend heavily on matching timings between heightened flow and migrations. While we have some understanding of the phenology of outmigration, methods did not previously exist to quantify the synchrony of movement of fish as they traverse a network of detectors along a river system, nor did methods exist to compare synchrony between different sets of fish. We produced such methods, using a path segmentation and sequence-alignment-based approach. One of our new metrics measures “direct synchrony” between the out-migrations of two fish, essentially whether the fish travel consistently nearby each other. Another new metric assesses “lagged synchrony,” which is similarity of out-migration timings while allowing for possible time delays between fish. We then derived metrics for population-level synchrony and clustering of fish into synchrony groups. We applied our methods to steelhead trout kelt and chinook salmon smolt telemetry data from the Sacramento River to investigate how movement synchrony of outmigration varies across types of migrating fish as well as by release timing.

Results/Conclusions

: We assessed synchrony of outmigration within three sets of fish: one release from 2007 of steelhead kelts and two releases about three weeks apart at the same location from the winter of 2010-2011 of chinook smolts. For the steelhead, we found no significant direct synchrony, but we found significant lagged synchrony, possibly indicating that timings within outmigration are conserved across fish even while onset of outmigration may be less conserved. For the earlier of our two chinook releases, we found significant direct synchrony, but for the later release we found no significant synchrony. Thus synchrony of distinct chinook releases can vary substantially even when the releases happen nearby in space and time. Our methods may eventually facilitate better understanding of drivers of synchrony that may help improve management. Our methods provide a statistically appropriate means of assessing synchrony of movements measured with telemetry data. Our results indicate that out-migrations of anadromous fish can be significantly synchronous with one another, and that synchrony can be context-dependent.