EXECUTIVE SUMMARY

Federally endangered Lost River sucker (Deltistes luxatus) and shortnose sucker (Chasmistes brevirostris) were once abundant throughout their range but populations have declined. They were extirpated from several lakes in the 1920s and may no longer reproduce in others. Poor recruitment to the adult spawning populations is one of several reasons cited for the decline and lack of recovery of these species and may be the consequence of high mortality during juvenile life stages. High larval and juvenile sucker mortality may be exacerbated by an insufficient quantity of suitable or high quality rearing habitat. In addition, larval suckers may be swept downstream from suitable rearing areas in Upper Klamath Lake into Keno Reservoir, which is seasonally anoxic.

The Nature Conservancy flooded about 3,600 acres (1,456 hectares) to the north of the Williamson River mouth (Tulana Unit) in October 2007 and about 1,400 acres (567 hectares) to the south and east of the Williamson River mouth (Goose Bay Unit) a year later to retain larval suckers in Upper Klamath Lake, create nursery habitat, and improve water quality. The U.S. Geological Survey joined a long-term research and monitoring program in collaboration with The Nature Conservancy, the Bureau of Reclamation, and Oregon State University in 2008 to assess the effects of the Williamson River Delta restoration on the early life-history stages of Lost River and shortnose suckers. The primary objectives of the research were to describe habitat colonization and use by larval and juvenile suckers and non-sucker fishes and to evaluate the effects of the restored habitat on the health and condition of juvenile suckers. This report summarizes data collected in 2009 by the U.S. Geological Survey as a part of this monitoring effort.

The Williamson River Delta appeared to provide suitable rearing habitat for endangered larval Lost River and shortnose suckers in 2008 and 2009. Larval suckers captured in this delta typically were larger than those captured in the adjacent lake habitat in 2008, but the opposite was true for larval shortnose suckers in 2009. Mean sample density was greater for both species in the Williamson River Delta than adjacent lake habitats in both years. Larval suckers captured in the restoration area, however, had less food in their guts compared to those captured in Upper Klamath or Agency Lakes.

Differential distribution among sucker species within the Williamson River Delta and between the delta and adjacent lakes indicated that shortnose suckers likely benefited more from the restored Williamson River Delta than Lost River or Klamath largescale suckers (Catostomus snyderi). Catch rates in shallow-water habitats with vegetation within the delta were higher for shortnose and Klamath largescale suckers than for larval Lost River suckers in 2008 and 2009.However, catch rates at the mouth of the Williamson River in 2008 and in Upper Klamath Lake in 2009 were higher for larval Lost River suckers than for larvae identified as either shortnose or Klamath largescale suckers. Shortnose suckers also comprised the greatest portion of age-0 suckers captured in the Williamson River Delta in 2008 and 2009. The relative abundance of age-1 shortnose suckers was high in our catches compared to age-1 Lost River suckers in 2009 in the delta and adjacent lakes, which may or may not indicate shortnose suckers experienced better survival than Lost River suckers in 2008.

Age-0 and age-1 suckers were similarly distributed throughout the Williamson River Delta in 2008 and 2009. Age-0 suckers used shallow vegetated and unvegetated habitats primarily in mid- to late July in both years. A comparison of catch rates between our study and a concurrent study in Upper Klamath Lake indicated that Goose Bay was the most used habitat in 2009 and the Tulana Unit was the one of the least used habitats in 2008 and 2009 by age-0 suckers. Catch rates for age-1 suckers, however, indicated that both of these areas were heavily used by age-1 suckers in May 2009. Age-1 sucker catches shifted from the shallow water (about 0.5–1.5 meters deep) eastern side of the Williamson River Delta in May, to mid-depth (about 1.5–2.7 meters deep) environments in June, and then to deep (about 4–6 m deep) open water areas within the delta, Agency Lake, and Upper Klamath Lake at the end of June and early July in both years.

A pilot project conducted in 2009 demonstrated the potential for passive integrated transponder tags to be used to study mortality and movement of age-1 and age-2 suckers in Upper Klamath Lake and the Williamson River Delta. We injected 132 age-1 and age-2 suckers with passive integrated transponder tags, between mid-April and late June. Eight percent of these tags were detected on remote underwater antennas arrays and at piscivorous bird colonies. Although higher detection rates are desirable for estimating mortality rates, movement and sources of mortality could be inferred if more tags were used and we achieved a similar detection rate. A concurrent study by The Nature Conservancy indicated that periods of high pH, low dissolved-oxygen concentrations, and relatively high un-ionized ammonia concentrations occurred in our study area in 2008 and 2009, and were associated with blooms of cyanobacteria. Conditions were more severe and lasted longer in 2008 than in 2009 in the study area. Levels of pH and high water temperatures exceeded published tolerance limits for juvenile suckers in parts of the delta but apparently did not influence distribution. High concentrations of un-ionized ammonia and low concentrations of dissolved-oxygen were associated with low numbers of juvenile suckers in our catches.

Only two piscivorous fishes that appeared to pose a meaningful threat of predation to suckers were captured in 2009—fathead minnows (Pimephales promelas) and yellow perch (Perca flavescens). Fathead minnows that prey on larval but not juvenile suckers dominated catches in all strata, but made up a small portion of fish caught in the shallow-water Goose Bay stratum in 2009 and Tulana Emergent stratum in 2008 and 2009, where larval shortnose suckers primarily were captured. Two cohorts of yellow perch were detected in our samples. Based on their gape size and patterns of co-occurrence, the smaller cohort was capable of preying on larval suckers and the larger cohort was capable of preying on age-0 juvenile suckers. Larval yellow perch were captured throughout our study area in 2009 but not in 2008. This could indicate that yellow perch only spawned in the delta and adjacent lake habitat during the second year of our study.

The first two years of this study indicated the Williamson River Delta restoration project successfully created suitable rearing habitat for larval and juvenile suckers. However, it is unknown weather restoration will ultimately increase the number of adults. Because the estimated age of initial spawning is between 6 and 14 years, we do not expect to see population level effects of the Williamson River Delta restoration for some time. Until then, it will be important to continue to monitor the use of the delta by larval and juvenile suckers. Because juvenile suckers do not spend all their time in the Williamson River Delta, it also is important to determine the causes of juvenile sucker mortality to understand poor juvenile to adult survival.

First posted September 8, 2010