Resource use and life history patterns of juvenile Coho and Chinook salmon in an Alaskan estuary

Thesis (Master's)--University of Washington, 2017-06Estuaries are valuable nursery grounds for anadromous Pacific salmon supplying diverse habitats, quality foraging grounds, and a transition between freshwater and saltwater environments. By providing alternative rearing habitats, estuaries may also bolster the life history diversity expressed in a population, thereby increasing population-level stability. Conversely, the degradation and loss of estuarine habitats has been partially responsible for declines in wild salmon populations. In Alaska, most natural systems are largely intact with minimal anthropogenic alterations. However, the human population and associated land use is increasing. Alaska has the opportunity to learn from the mistakes made in the Pacific Northwest and identify and protect the critical rearing habitat of these culturally, economically, and ecologically valuable species. To do so, we need an understanding of how juvenile salmon use Alaskan estuaries. In this thesis, I describe the resource use, residence, and movement patterns of juvenile salmon in the Anchor River estuary in southcentral Alaska. Specifically, I (1) assess the resource partitioning between juvenile Coho and Chinook salmon, (2) determine which age classes use the estuary and for how long, (3) examine the tidal movement of juvenile salmon, and (4) utilize a social network analysis to explore the social structure of juvenile Coho Salmon. Coho and Chinook salmon primarily partitioned resources spatially and to a lesser extent temporally, whereas their prey resources overlapped considerably. Varying abundances of Chinook Salmon provided a natural experiment that I used to determine that Coho Salmon interactively partition habitat resources by primarily occupying tidal marsh channels when Chinook Salmon are present in the main channel. Although juvenile Coho Salmon were present throughout the sampling period (May-Oct), they were most abundant later in the season (Aug) and individuals of all three age classes reared in the estuary for over 100 d. Some Coho Salmon tagged in 2015 were still present in the estuary in 2016. Chinook Salmon were most abundant earlier in the season (Jun-July) and did not remain in the estuary past mid-August. The longest individual residence time for a Chinook Salmon was 42 d. Channel connectivity influenced the tidal movement patterns of Coho Salmon. I used passive integrated transponder (PIT) tags and in-channel antennas to assess fish movement in two marsh channels. In the marsh channel that remained fully connected to the main channel at low tide, fish movement was mostly independent of the tidal stage or channel depth. In the marsh channel that was only marginally connected to the main channel, fish detections were greatest when the tidal stage was high enough to raise the channel depth. However, contrary to my expectations, fish were detected throughout the tidal cycle and at all channel depths, indicating that they used the marginal connection to move between habitat patches even at low tide. Juvenile Coho Salmon social relationships varied through time. I constructed static and dynamic social networks from the time-stamped observations of fish detections to explore the social structure of juvenile Coho Salmon. Fish did not preferentially associate with similarly sized fish. Fish did not appear to maintain stable relationships, but instead exhibited fission-fusion dynamics where social relationships were continually formed and dissolved, indicating that the frequent co-occurrence of individuals is likely due to mutual site fidelity and not social preference. Although the Anchor River is relatively small, the salmon populations it supports are highly important to the culture, economy, and ecology of the region. The results of this thesis demonstrate that the diverse habitats of the estuary support multiple species and life history types for prolonged periods. By appropriately identifying and conserving critical juvenile salmon rearing habitat, we can help maintain healthy salmon populations into the future. This thesis includes an animated dynamic social network (Video S1) provided as supplementary material