Fish Ecology Along Modified Shorelines
Munsch, Stuart Harold
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Waterfronts are busy places. Ancient civilizations often formed along the water where people benefited from aquatic resources and trade. People have continued to develop waterfronts, and these areas are now major components of the global economy. They support not only international trade, but a diversity of local industries such as tourism and transportation. Waterfront development is occurring globally as the human population grows and increasingly locates in coastal settings. People have modified shorelines to support societal functions of waterfronts. These modifications have eliminated, restructured, and shaded shallow waters, which is concerning because many species of fish use shallow areas along shore, often during juvenile stages. Fish and their associated nearshore ecosystems often contribute to the value of waterfronts to people because they are culturally and economically significant. Thus, people and fish share waterfronts, and people will benefit by protecting nearshore ecosystems and the fish habitats that they support. In this dissertation, I examined effects of shoreline modifications (e.g., seawalls, piers) on fish and elucidated their natural history. The first three chapters are parts of a study that assessed fish habitat in Elliott Bay, WA. Our findings informed habitat rehabilitation along its urbanized shoreline as part of a seawall reconstruction. My participation in this study and the literature I read to prepare for my General Examination interested me in the behavior and habitat use of nearshore fish in Puget Sound. Therefore, in the fourth and fifth chapters, I assembled and analyzed data on fish behavior collected by the Wetland Ecosystem Team over the past decade throughout Puget Sound, WA. Following this work, I was interested in understanding how shoreline modifications affect fish habitats globally. I therefore wrote the sixth chapter to synthesize the current primary literature examining effects of shoreline modifications on estuarine fish and discuss how we can improve fish habitats along shore within constraints of human uses. In Chapter One, I quantified effects of seawalls and piers on fish assemblages and juvenile salmon feeding behavior in Elliott Bay. I found that (1) the composition of fish assemblages varied between (a) sites modified by seawalls and piers and (b) built beaches without piers, (2) fish abundances were lower in shaded areas under piers relative to sunlit areas, which also affected assemblage composition under piers, and (3) feeding behavior of juvenile Pacific salmon (Oncorhynchus spp.) was lower under piers relative to sunlit areas. In Chapter Two, I compared subtidal fish and crab assemblages between sites modified by intertidal seawalls and built beaches. I found that (1) the composition of fish assemblages varied between seawall sites and beaches and (2) species that selected for a substrate type (e.g., sand, rocks) often contributed to compositional differences. In Chapter Three, I compared the diets of juvenile Pacific salmon between seawall shorelines and reference beaches in Elliott Bay, WA. I found that (1) epibenthic copepods were less abundant along seawall shorelines and (2) small (<50 mm) chum salmon (O. keta) consumed less epibenthic copepods and more planktonic copepods along seawall shorelines. In Chapter Four, I quantified context-dependent behaviors of shallow water fish assemblages in Puget Sound. I found (1) smaller fish occupied shallower depths where predators were less abundant, (2) smaller fish schooled in larger groups, (3) pelagic fish schooled in larger groups in deeper water, (4) demersal fish schooled in larger groups when occupying the water column, (5) species partitioned habitats by depth and season, and (6) smaller fish were proportionally less abundant along deep shorelines created by intertidal armoring. In Chapter Five, I quantified the diurnal feeding behavior of juvenile Pacific salmon in Puget Sound, WA. I found that (1) juvenile salmon fed often and throughout the day, (2) their feeding intensity declined from dawn to late afternoon, and (3) this trend in behavior was not evident from examining diets alone. In Chapter Six, I synthesized a global understanding of effects of shoreline infrastructure on shallow fish assemblages and recommended how to research, manage, and rehabilitate shallow habitats along modified waterfronts. Overall, this dissertation suggests that built shorelines affect the ecology of fish, and that effects are primarily negative. It also suggests that we can protect nearshore ecosystems, but to do so requires that we understand their natural histories and appreciate their habitat functions and processes. Shoreline modifications are common worldwide, and the literature suggests that they affect the ecology of many nearshore systems. Many of these changes appear to affect basic habitat and fish attributes such as shelter, food availability, and vision. Thus, the findings in this dissertation may be generalizable elsewhere, or can at least provide a starting point for ecologists working in other systems. The fundamental challenge for managers and ecologists is to balance the protection of shallow fish habitats with the utility of waterfronts to people. I hope that my dissertation can guide efforts to reach this goal.
- Fisheries