Seasonal Distribution and Abundance of Larval and Juvenile Lost River and Shortnose Suckers in Hanks Marsh, Upper Klamath National Wildlife Refuge, Upper Klamath Lake, Oregon: 2007 Annual Report

Abstract

In the summer of 2007, we undertook an assessment of larval and juvenile sucker use of Hanks Marsh in Upper Klamath Lake, Oregon. This 1,200-acre marsh on the southeastern shoreline of the lake represents part of the last remaining natural emergent wetland habitat in the lake. Because of the suspected importance of this type of habitat to larval and juvenile endangered Lost River and shortnose suckers, it was thought that sucker abundance in the marsh might be comparatively greater than in other non-vegetated areas of the lake. It also was hoped that Hanks Marsh would serve as a reference site for wetland restoration projects occurring in other areas of the lake. Our study had four objectives: to (1) examine seasonal distribution and relative abundance of larval suckers in and adjacent to Hanks Marsh in relation to habitat features such as depth, vegetation, water quality, and relative abundance of non-sucker species; (2) determine the presence or absence and describe the distribution of juvenile suckers [35 to 80 mm standard length (SL)] along the periphery of Hanks Marsh; (3) assess spatial and temporal overlap between larval suckers and their potential predators; and (4) assess suitability of water quality throughout the summer for young-of-the-year suckers. Due to the low number of suckers found in the marsh and our inability to thoroughly sample all marsh habitats due to declining lake levels during the summer, we were unable to completely address these objectives in this pilot study. The results, however, do give some indication of the relative use of Hanks Marsh by sucker and non-sucker species. Through sampling of larval and juvenile suckers in various habitat types within the marsh, we determined that sucker use of Hanks Marsh may be very low in comparison with other areas of the lake. We caught only 42 larval and 19 juvenile suckers during 12 weeks of sampling throughout the marsh. Sucker catches were rare in Hanks Marsh, and were lower than catch rates in other marshes of Upper Klamath Lake and in other nearshore and offshore areas of the lake. Based on the few suckers we did capture in Hanks Marsh, larvae tended to be found more often in vegetated habitats. A modified sampling design and approach may be necessary to address the objectives in this study, given that declining lake-surface elevation prevented us from adequately sampling all portions of the marsh throughout the sampling season. Common non-sucker species in Hanks Marsh included juvenile and adult brown bullhead, larval blue chub, tui chub, fathead minnow, and yellow perch. This species composition was similar to that of other marshes in Upper Klamath Lake but most species were found in lower numbers in Hanks Marsh than other marshes. It may be that use of Hanks Marsh is limited by poor water quality, which we found to exist at many sites after June. It also may be that access to or habitat in the marsh is limited at certain times of the year by low water. Although the results from this initial study of Hanks Marsh indicate that the area may have little direct benefit for sucker species, indirect benefits for these species possibly may come from its positive influence on some aspects of water quality in the lake, such as regulation of pH. It also may be the case that use of Hanks Marsh may vary by year and conditions; however, under the current scope of the study, we were unable to investigate inter-annual variability.