Roger A. Tabor

Ontogenetic Shifts in Habitat and Diet of Cutthroat Trout in Lake Washington, Washington

Abstract Salmonids often display a series of ontogenetic shifts in habitat, and these may also be associated with changes in diet. For example, adfluvial populations rear in streams for several years and then migrate to lakes. The patterns of habitat use, trophic ecology, and movements of such populations are commonly studied during the riverine stages. The lacustrine period is typically less well known, but salmonids may play an important ecological role as lake piscivores. In Lake Washington, Seattle, Washington, cutthroat trout Oncorhynchus clarki are a top native piscivore and may affect the dynamics of the fish and zooplankton upon which they prey. Our objective was to study the growth, diet, and size distribution of cutthroat trout in littoral and limnetic habitats of Lake Washington, with emphasis on consumption of two of the lakes dominant pelagic planktivores: juvenile sockeye salmon O. nerka and longfin smelt Spirinchus thaleichthys. Cutthroat trout entered the lake at approximately age 2. As t...

The Effect of Light Intensity on Sockeye Salmon Fry Migratory Behavior and Predation by Cottids in the Cedar River, Washington

Abstract We examined the relationship between light intensity, migratory behavior of sockeye salmon Oncorhynchus nerka fry, and predation by cottids Cottus spp. We tested the hypothesis that above-natural intensities of nighttime light would increase cottid predation of sockeye salmon fry. In circular tank experiments under controlled laboratory conditions, we tested the ability of cottids to prey on sockeye salmon fry under six different light intensities using minimal water circulation to separate the effect of the migratory behavior of fry from the ability of cottids to capture them. We found that cottids preyed most effectively in complete darkness, whereas the lowest predation occurred at the brightest light intensity. We next tested the predation ability of cottids at four light intensities in a pair of artificial streams to simulate more natural conditions. In experiments without cottids, the majority of fry passed quickly through the artificial streams under complete darkness, but as light intensi...

Smallmouth Bass and Largemouth Bass Predation on Juvenile Chinook Salmon and Other Salmonids in the Lake Washington Basin

Abstract We assessed the impact of predation by smallmouth bass Micropterus dolomieu and largemouth bass M. salmoides on juveniles of federally listed Chinook salmon Oncorhynchus tshawytscha and other anadromous salmonid populations in the Lake Washington system. Bass were collected with boat electrofishing equipment in the south end of Lake Washington (February–June) and the Lake Washington Ship Canal (LWSC; April–July), a narrow waterway that smolts must migrate through to reach the marine environment. Genetic analysis was used to identify ingested salmonids to obtain a more precise species-specific consumption estimate. Overall, we examined the stomachs of 783 smallmouth bass and 310 largemouth bass greater than 100 mm fork length (FL). Rates of predation on salmonids in the south end of Lake Washington were generally low for both black bass species. In the LWSC, juvenile salmonids made up a substantial part of bass diets; consumption of salmonids was lower for largemouth bass than for smallmouth bass....

Ontogenetic Diet Shifts of Prickly Sculpin in the Lake Washington Basin, Washington

Abstract We studied the ontogenetic diet shifts of prickly sculpin Cottus asper (the largest North American freshwater cottid) in the Lake Washington basin from a variety of habitat types, including fluvial and lacustrine. In all habitats, prickly sculpin progressively shifted to larger prey, such as fish and crayfish (Decapoda), as they increased in size. In offshore areas of Lake Washington, amphipods were the dominant prey by weight consumed by prickly sculpin of 75-124 mm total length (TL). Although generally uncommon numerically in the diet, fish (primarily small cottids) made up a large percentage of the diet by weight for prickly sculpin larger than 125 mm TL. In the lower end of the Cedar River, juvenile sockeye salmon Oncorhynchus nerka (23-30 mm TL) were most commonly observed in the diets of 50-99-mm prickly sculpin, while larger prickly sculpin tended to consume larger fish, including adult longfin smelt Spirinchus thaleichthys, lampreys Lampetra spp. (ammocoetes and adults), and small cottids...

Habitat Use by Juvenile Chinook Salmon in the Nearshore Areas of Lake Washington: Effects of Depth, Lakeshore Development, Substrate, and Vegetation

Abstract Juvenile Chinook salmon Oncorhynchus tshawytscha in nearshore areas of Lake Washington, Washington, were studied to determine their depth distribution, substrate associations, and use of overhanging vegetation (OHV) and shoreline armoring. From March to May, juvenile Chinook salmon progressively shifted to deeper waters as they increased in size. At night, they were associated with small substrates, whereas no association with a particular substrate type was detected during daytime. Juveniles were commonly found in open areas or within 5 m of overwater structures (e.g., piers and docks) but were rarely found directly under such structures. The number of juveniles associated with armored shorelines was lower than expected given the availability of this habitat type. Large numbers of small juveniles often used OHV during the day; however, at night they moved away from cover and occupied open areas with no structure. Larger juveniles did not show a strong affinity to overhead cover. Our results sugg...

Historical and contemporary forces shape genetic variation in the Olympic mudminnow (Novumbra hubbsi), an endemic fish from Washington State, USA

Genetic data have become increasingly useful for conservation planning when data regarding population status and long-term viability is limited. The Olympic mudminnow is the only fish species endemic to Washington State, USA. The species is an increasing priority for conservation given its limited distribution and increasing habitat loss. Presently, information important for developing conservation plans including population abundance data, knowledge of population boundaries, and estimates of gene flow among populations are limited. We used microsatellite markers to assess the level of genetic variation within and among Olympic mudminnow collections from 23 sites across the species range. Genetic variation within collections ranged widely and was greatest within the Chehalis River Basin, a former glacial refugium. Analysis of population boundaries showed that each collection site represented a unique population with the exception of collections made within two large wetland and stream complexes. Genetic variation among populations appears to be strongly influenced by glacial history and the species’ life history. Populations originating from the Chehalis River glacial refugium clustered together in multiple analyses and populations from the Olympic Coast, which persisted in separate refugia and have limited capacity for dispersal, showed a high level of differentiation. Competing theories existed regarding the origins of disjunct populations in east Puget Sound and genetic data showed that these populations represent undocumented introductions rather than a glacial remnant or historic colonization from the Chehalis refugium. Data presented in this study will help fill important information gaps and advance conservation planning for this species.

Characterization of tetranucleotide microsatellite loci for Olympic mudminnow (Novumbra hubbsi)

We describe the isolation and amplification of 21 potential tetranucleotide microsatellites for the Olympic mudminnow (Novumbra hubbsi). After initial testing, eight microsatellites were removed from future analysis for non-resolvable electropherograms. The remaining 13 loci were screened against a total of 772 individuals from 12 populations. The thirteen polymorphic loci amplified between four and forty-eight alleles per locus with expected heterozygosities ranging from 0.41 to 0.95. These markers should aid in conservation and the description of population structure of this state sensitive species.

Diet of Adult Olympic Mudminnow, Novumbra hubbsi

Abstract Olympic mudminnow (Novumbra hubbsi) is a small species endemic to western Washington State that inhabits marshy, wetland-type areas with abundant aquatic macrophytes. Because they can be locally abundant and may be the only fish species present, they may have important effects on the aquatic community. However, little is known about their diet under natural conditions. To get a detailed account of their feeding ecology, Olympic mudminnow were sampled from six sites. Three sites were sampled monthly from February to August 2012 and the other three sites were sampled once in June 2012. Gastric lavage was used to collect stomach samples from fish ≥ 40 mm total length (TL). Stomach samples were collected from 477 fish (40–75 mm TL). Olympic mudminnow diet was comprised primarily of aquatic invertebrates. By weight, oligochaetes and chironomid larvae were the predominant prey types but by number, copepods were the predominant prey type. Of the three monthly sample sites, diet overlap between seasons was high at one site, low at another site, and variable at the third site. Diet overlap was generally low between sites. Olympic mudminnow 40–49 mm TL primarily displayed a generalist feeding strategy, whereas larger fish displayed more specialization. A consistent pattern of ontogenetic diet shifts was not apparent. Results of this study and an earlier study indicate Olympic mudminnow consume a wide variety of types and sizes of aquatic invertebrates and their diet can vary widely between sites and seasons.

Seasonal Movements of Smallmouth Bass in the Lake Washington Ship Canal, Washington

Abstract The seasonal movement patterns of adult smallmouth bass (Micropterus dolomieu) were examined in the Lake Washington Ship Canal (LWSC), a long, narrow water body that connects Lake Washington with Puget Sound. Seventy-two smallmouth bass were captured, implanted with acoustic tags, and released back into the LWSC where they were captured. Smallmouth bass displayed a strong seasonal migration pattern between the LWSC and Lake Washington. Out of 57 smallmouth bass tracked for seasonal movement information, 82% migrated from the LWSC to Lake Washington sometime between June and October. Departure from the LWSC may be related to a combination of factors, such as cessation of spawning activity, reduced water quality conditions, and reduced prey availability. Smallmouth bass < 350 mm FL were more likely to overwinter in the LWSC than larger fish and if they did migrate to Lake Washington, they migrated later in August–October. After overwintering in Lake Washington, smallmouth bass moved back into the LWSC between early-March and mid-April. Smallmouth bass returned to the LWSC at the beginning of the spring warming phase, a behavior likely related to spawning activity. Smallmouth bass often showed some degree of site fidelity between years for both spring/summer locations in the LWSC and summer/fall/winter locations in Lake Washington. Because smallmouth bass often have a high degree of spawning site fidelity, there can be different populations within the same water body. Our results provide some preliminary evidence that LWSC smallmouth bass are reproductively isolated from smallmouth bass that inhabit Lake Washington year-round.

Use of Nonnatal Tributaries for Lake-Rearing Juvenile Chinook Salmon in the Lake Washington Basin, Washington

Abstract Ocean-type juvenile Chinook salmon (Oncorhynchus tshawytscha) can be present in the nearshore areas of Lake Washington and Lake Sammamish, WA for 4–5 months (January–June) and may encounter nonnatal tributaries. The use of these tributaries is not well known. We determined the presence of juvenile Chinook salmon in 12 tributaries through nighttime snorkel surveys from March to June. At one heavily-used tributary, we determined their temporal and spatial distribution by conducting surveys every two to three weeks from February to June. Additionally, we determined whether delta areas of tributaries are used by juvenile Chinook salmon by comparing their density and diet to other lakeshore sites. Of 12 streams surveyed, juvenile Chinook salmon were observed in eight. The abundance of Chinook salmon appeared to be related to a variety of factors including proximity to the natal stream, stream gradient, and stream size. In an intensively-monitored stream, juvenile Chinook salmon were found primarily in shallow areas in February and March and then shifted to deeper pools as juveniles increased in size. Within the lake nearshore area, juvenile Chinook salmon commonly used delta areas of nonnatal tributaries and their abundance was frequently greater than other nearby shoreline sites. Diet analysis indicated nonnatal streams are also a source of prey for lake-dwelling juvenile Chinook salmon, especially during rain events. Nonnatal tributaries in lake systems appeared to be valuable habitat features for juvenile Chinook salmon and can function in a variety of ways, including providing both stream and delta habitat.