Juvenile Steelhead Locate Coldwater Refugia in an Intermittently Closed Estuary

Abstract

Many coastal estuaries in Mediterranean climates are susceptible to inlet closures resulting from barrier beach formation. These closures are ecologically important because they eliminate tidal exchange and connectivity of nekton movement to the coastal ocean and, depending on closure duration, can convert a dynamic estuary into a quiescent lagoon. Although closures can create lethal or stressful conditions for nekton and benthic communities, especially obligate diadromous species, under some conditions they can enhance survival of juvenile steelhead Oncorhynchus mykiss. However, the mechanisms explaining how closed conditions enhance the growth of juvenile steelhead and how inhabitants avoid physiologically stressful conditions remain unknown. In the present study, recent technological advances in sensor-encoded acoustic telemetry provided the ability to simultaneously locate and determine the temperature of juvenile steelhead as small as 93 mm FL by using mobile and stationary tracking. In the Russian River estuary, an intermittently closed estuary in northern California, we used acoustic telemetry to infer water quality exposure by linking the temperature of individually tagged fish with water quality profiles collected in close proximity. Under open-inlet conditions, juvenile steelhead experienced primarily brackish and saline water in the lower and middle reaches and warm freshwater in the upper reach, whereas under closed-inlet conditions they experienced warm freshwater in the middle and upper reaches. During closed conditions, juvenile steelhead displayed behavior that suggested the ability to mediate stressful environmental conditions; specifically, they responded to closed conditions by moving greater distances and aggregating near thermal refugia. Our findings show the importance of recognizing these strategies when contemplating changes to estuary management and highlight the significance of tributary hydrogeomorphic processes and groundwater linkages in subwatersheds that are sources of cool water for thermal refugia in intermittently closed estuaries.

Received August 5, 2016; accepted February 27, 2017 Published online May 11, 2017

ACKNOWLEDGMENTS

We are extremely grateful for the assistance provided by the SCWA (Contract Number 13/14-033, Order Number 7339-C5). J. Martini-Lamb’s (SCWA) involvement in implementation of the RRE Management Project was crucial to study success. This project would not have been possible without the fieldwork and technical assistance of A. Moratto, D. Favela, E. McDermott, J. Beaton, J. Smith, A. Dockham, A. Iwaki, B. Bei, B. Brawley, and many more. D. Cook’s willingness to conduct standardized seining operations around our schedule proved extremely helpful. D. Manning and J. Fuller generously provided valuable input to project design, logistics, and telemetry operations. We greatly appreciate discussions with J. Largier and D. Behrens for their insights into inlet morphology and estuary circulation. We thank D. Beauchamp, J. Hall, G. Marston, T. Quinn, and A. Whitfield for their reviews and helpful comments on the manuscript.