Ultrasonic telemetry reveals seasonal variation in depth distribution and diel vertical migrations of sub-adult Chinook and coho salmon in Puget Sound

Abstract

Many aquatic organisms display seasonal and diel vertical migration (DVM) pat- terns, which are influenced by complex combinations of biotic and abiotic factors. Here, we exam- ined the vertical distributions of sub-adult coho Oncorhynchus kisutch and Chinook salmon O. tshawytscha in Puget Sound, Washington, USA, using acoustic telemetry to (1) compare the depths occupied by each species, (2) determine whether DVM occurred, (3) ascertain if depth dis- tributions changed seasonally, and (4) consider hypotheses regarding abiotic and biotic factors that could affect these behavior patterns. We modeled the data on individual fish depths and detected significant interactions among species, diel period, and season. Coho salmon spent more time near the surface than Chinook salmon overall, and exhibited DVM during the spring, being farther below the surface at night and closer during the day. This reversal of the typical DVM pat- tern was not evident in other seasons. Chinook salmon showed no evidence of diel movement, only a seasonal shift from being closest to the surface in spring, deeper in summer, deeper yet in fall, and deepest in winter. The proximity of Chinook salmon to the surface coincided with peak productivity measured as chlorophyll a, which could affect the salmon through decreased water clarity or some ecological process. The DVM exhibited by coho salmon in spring may be related to water clarity and avoidance of predatory mammals but these hypotheses could not be tested with the available data. Our results emphasize the complexity of diel activity patterns among closely related species, and even among individuals.