Partial Migration In Steelhead Oncorhynchus Mykiss

Author : William Carson Matsubu
Publisher :
ISBN 13 :
Total Pages : 205 pages
Book Rating : 4.:/5 (112 download)

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Download or read book Tradeoffs of Juvenile Steelhead (Oncorhynchus Mykiss) Rearing in an Intermittently Closed Estuary, Northern California USA written by William Carson Matsubu and published by . This book was released on 2019 with total page 205 pages. Available in PDF, EPUB and Kindle. Book excerpt: Estuarine environments provide an essential habitat to many aquatic animals and, in some settings, can be susceptible to drastic environmental transformations caused by deviations in connectivity with the ocean. In these intermittently closed estuaries (ICEs), the presence or absence of a barrier beach, naturally controlled by wave action and river flow, determines the mouth state (closed or open). Depending on the frequency and duration of closures and reopenings, ICEs can create a conundrum for inhabitants, especially diadromous fish that must transit between marine and freshwater habitats to reproduce. Changes in connectivity to the ocean not only obstructs migration between the ocean and the river but can also cause deadly conditions, especially for juvenile pacific salmonids (Oncorhynchus spp.) that have a narrow range of physiological tolerances for the abiotic variables susceptible to large fluctuations in ICEs. Despite these hurdles, ICEs constitute essential habitats for threatened populations of steelhead (Oncorhynchus mykiss). This dissertation utilizes a threatened population of juvenile steelhead in the Russian River estuary, northern California, USA as a case study to examine an array of relevant topics about the efficacy of a juvenile anadromous fish occupying an ICE. The intersection of a threatened fish with a plastic and complex life history within an understudied system provides many challenges as well as opportunities. In addition to a brief introduction (Chapter 1) and conclusion (Chapter 6), the main chapters of this dissertation (Chapters 2-5) will address knowledge gaps specific to juvenile steelhead in the Russian River estuary. The second chapter addressed uncertainties regarding what abiotic conditions juvenile steelhead are exposed to and how steelhead may avoid physiologically stressful conditions. Specifically, we answered the following questions: (1) "What are the WQ habitats used by juvenile steelhead during open and closed conditions in an ICE?" and (2) "What behavioural change is evidenced between open and closed conditions that might alter the juvenile steelhead's risk of exposure to stressful WQ?" To answer these questions, we combined thermal sensor encoded acoustic telemetry and coincidental WQ sampling. Chapter two determined that, under open 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 conditions, they moved greater distances and were found to be aggregating near cool water refugia not used during open mouth conditions. These findings shed light on the abiotic conditions juvenile steelhead are exposed to and emphasize the importance of tributary hydrogeomorphic processes and groundwater linkages in subwatersheds that are sources of cool water refugia in ICEs. The purpose of the third chapter was to evaluate the vertical response of juvenile steelhead to the physiochemical conditions (i.e., temperature, DO, salinity) in the Russian River estuary. This chapter further explored the acoustic telemetry and simultaneously collected WQ data from chapter two to test the following hypotheses: (i) juvenile steelhead will shift their position in the water column based on prevailing physiochemical conditions; (ii) the degree to which juvenile steelhead adjust their position in the water column will vary with fish size; and (iii) smaller juvenile steelhead will risk potentially stressful conditions to take advantage of prey sources. The results indicated the depth of fish varied in response to the present abiotic conditions and typically reflected the occupation of more energetically and physiologically beneficial habitats, with smaller fish being shallower in the water column when proximate salinities are higher. Results indicated management activities that promote open mouth conditions may create challenges for steelhead that are not acclimated to saline conditions but reveal foraging strategies used by juvenile steelhead that are not yet tolerant of higher salinities. Macroinvertebrates are fundamental to the food webs, yet their response to management activities that affect connectivity to the ocean is not well understood. The fourth chapter used systematic sampling of the benthic and epibenthic macroinvertebrates to assess factors that affect the diversity and abundance of key food web taxa. Similar to other ICEs, the macroinvertebrate assemblage of the Russian River estuary was primarily composed of taxa that can deal with the variability, either physiologically or behaviourally. Furthermore, the prominent food web taxa were abundant during both open and closed mouth conditions and were found in large aggregations in habitat only inundated during closed mouth conditions. These increased densities in the closure-inundated habitat may reflect more expansive aggregations of key food web taxa that would lead to more efficient foraging for juvenile steelhead. Although the growth rates of juvenile steelhead in ICEs have been reported near the highest in literature, leading to increased marine survival, the specific factors contributing to this growth have not been specifically evaluated. The fifth chapter incorporated observed growth rates, a diet analysis, and thermal history of juvenile steelhead into a bioenergetics model to explore factors that most influence the growth potential of juvenile steelhead. In this chapter, I confirmed that growth rates of juvenile steelhead in the Russian River estuary rival the highest in literature and are attributed to ample foraging opportunities and the ability of juvenile steelhead to thermo-regulate behaviourally in the heterogeneous abiotic environment. Higher energetic costs due to higher temperatures in the upper reach were possibly buffered by the consumption of more energy dense prey. These results indicated that growth is likely not limiting the recovery of threatened steelhead in the Russian River estuary. With many populations of salmonids imperiled near their southern range, efforts for recovery could benefit from protecting habitats in ICEs and considering the impacts of management activities to the water quality conditions.