Recurrent Die-Offs of Adult Coho Salmon Returning to Spawn in Puget Sound Lowland Urban Streams

Nathaniel L Scholz,Sarah G Mccarthy,Gina M Ylitalo,Katherine D Lynch,Barbara L French,Carla M Stehr,Mark S Myers,Tracy K Collier,Jenifer K Mcintyre,Cathy A Laetz,Dean Wilson,Steve Damm,Bill Mcmillan,Jana S Labenia,Jay W Davis,Laura Reed,Linda D Rhodes,Howard Browman

doi:10.1371/journal.pone.0028013

Abstract

Several Seattle-area streams in Puget Sound were the focus of habitat restoration projects in the 1990s. Post-project effectiveness monitoring surveys revealed anomalous behaviors among adult coho salmon returning to spawn in restored reaches. These included erratic surface swimming, gaping, fin splaying, and loss of orientation and equilibrium. Affected fish died within hours, and female carcasses generally showed high rates (>90%) of egg retention. Beginning in the fall of 2002, systematic spawner surveys were conducted to 1) assess the severity of the adult die-offs, 2) compare spawner mortality in urban vs. non-urban streams, and 3) identify water quality and spawner condition factors that might be associated with the recurrent fish kills. The forensic investigation focused on conventional water quality parameters (e.g., dissolved oxygen, temperature, ammonia), fish condition, pathogen exposure and disease status, and exposures to metals, polycyclic aromatic hydrocarbons, and current use pesticides. Daily surveys of a representative urban stream (Longfellow Creek) from 2002–2009 revealed premature spawner mortality rates that ranged from 60–100% of each fall run. The comparable rate in a non-urban stream was <1% (Fortson Creek, surveyed in 2002). Conventional water quality, pesticide exposure, disease, and spawner condition showed no relationship to the syndrome. Coho salmon did show evidence of exposure to metals and petroleum hydrocarbons, both of which commonly originate from motor vehicles in urban landscapes. The weight of evidence suggests that freshwater-transitional coho are particularly vulnerable to an as-yet unidentified toxic contaminant (or contaminant mixture) in urban runoff. Stormwater may therefore place important constraints on efforts to conserve and recover coho populations in urban and urbanizing watersheds throughout the western United States.

Highlights

In lowland Puget Sound, many urban streams in the vicinity of Seattle were a focus of extensive physical and biological restoration activities in the 1990s
Numerous adult coho carcasses were found in all monitored streams (Figure 1 and Table 2)
We did not observe corresponding die-offs of resident fish in urban streams, nor did we find the syndrome in other species of migratory salmon return to these same urban streams to spawn in the fall

Summary

Introduction

In lowland Puget Sound, many urban streams in the vicinity of Seattle were a focus of extensive physical and biological restoration activities in the 1990s. A related aim was to evaluate the extent to which adult salmon would return to spawn in the newly available and improved habitats This post-project effectiveness monitoring was carried out via fall spawner surveys that were conducted weekly from 1999–2001, with a primary focus on coho (Oncorhynchus kisutch), Chinook (O. tshawytscha) and chum (O. keta) salmon. These early monitoring efforts in 1999–2001 identified an unusual syndrome of pre-spawn mortality among adult coho returning to restoration sites to spawn. Coho spawning in Seattle-area streams are often a mix of hatchery and natural origins, with hatchery fish distinguishable by a clipped adipose fin and, less commonly, the presence of a rostralimplant coded wire tag

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Results

Conclusion