Rodney J. Nakamoto

Reduced streamflow lowers dry-season growth of rainbow trout in a small stream

Abstract A wide variety of resource management activities can affect surface discharge in small streams. Often, the effects of variation in streamflow on fish survival and growth can be difficult to estimate because of possible confounding with the effects of other variables, such as water temperature and fish density. We measured the effect of streamflow on survival and growth of rainbow trout Oncorhynchus mykiss in a small stream in northwestern California by manipulating the flow entering four of eight enclosed stream sections (9.0–15.3 m long) containing one pool and 2.5–4.0 m of upstream riffle habitat. In the four manipulated experimental units, we reduced inflow by 75–80% over a 6-week period in summer 2003. Flow diversion substantially decreased water velocity in riffle–pool transition areas but did not strongly affect habitat volume or water temperature. Fish in control units grew about 8.5 times as much as those in units with reduced streamflow; however, discharge manipulation did not affect sur...

AN EMERGENT MULTIPLE PREDATOR EFFECT MAY ENHANCE BIOTIC RESISTANCE IN A STREAM FISH ASSEMBLAGE

While two cyprinid fishes introduced from nearby drainages have become widespread and abundant in the Eel River of northwestern California, a third nonindigenous cyprinid has remained largely confined to ≤25 km of one major tributary (the Van Duzen River) for at least 15 years. The downstream limit of this species, speckled dace, does not appear to correspond with any thresholds or steep gradients in abiotic conditions, but it lies near the upstream limits of three other fishes: coastrange sculpin, prickly sculpin, and nonindigenous Sacramento pikeminnow. We conducted a laboratory stream experiment to explore the potential for emergent multiple predator effects to influence biotic resistance in this situation. Sculpins in combination with Sacramento pikeminnow caused greater mortality of speckled dace than predicted based on their separate effects. In contrast to speckled dace, 99% of sculpin survived trials with Sacramento pikeminnow, in part because sculpin usually occupied benthic cover units while Sacramento pikeminnow occupied the water column. A 10-fold difference in benthic cover availability did not detectably influence biotic interactions in the experiment. The distribution of speckled dace in the Eel River drainage may be limited by two predator taxa with very different patterns of habitat use and a shortage of alternative habitats.

Technical and Biological Considerations for the Analysis of Brain Cholinesterase of Rainbow Trout

Abstract This study was conducted in order to determine how various technical and biological factors affect brain cholinesterase (ChE) activity of rainbow trout Salmo gairdneri. Brain ChE activity increased in proportion to increasing temperature to 20°C, but at higher temperatures, the ChE activity increased at a slower rate and was absent at 45°C. Brain ChE activity remained stable for a long time when the rainbow trout were stored at −20°C or lower, and short (2-d) storage at 4°C was not detrimental; dead fish left 1 d at ambient water temperature also did not have lower ChE activities. Habitat temperature did not affect brain ChE activity significantly. Small rainbow trout had markedly higher brain ChE activities than large fish.

Effects of Steelhead Density on Growth of Coho Salmon in a Small Coastal California Stream

Abstract Weight change in age-0 coho salmon Oncorhynchus kisutch at about natural density was negatively related to the density of juvenile steelhead (anadromous rainbow trout O. mykiss) in a 6-week experiment conducted in July–August 1993 in the north and south forks of Caspar Creek, California. The experiment used 12 enclosed stream sections, each containing a pool and a portion of upstream riffle, with two replicates of three steelhead densities—zero, natural density (1×), and twice the natural density (2×)—on both the north and south forks. The natural density of coho salmon was about one-sixth the density of steelhead. Coho salmon survival was high (87% overall) and not related to treatments. In the north fork, coho salmon weight change was positive in zero density steelhead treatments, zero in 1× treatments, and negative in 2× treatments. Coho salmon weight change in the south fork was less favorable than in the north fork but was also negatively related to the density of steelhead. These results in...

Small founding number and low genetic diversity in an introduced species exhibiting limited invasion success (speckled dace, Rhinichthys osculus)

Molecular evaluations of successful invaders are common, however studies of introduced species that have had limited invasion success, or have died out completely, are rare. We studied an introduced population of speckled dace (Rhinichthys osculus) from northern California, USA that has rapidly increased in abundance but remained restricted to a 25-km stretch of river since its introduction in the mid-1980s. Field and laboratory analyses indicate that invasion success of speckled dace is constrained by the combined effects of multiple predators. The role of bottleneck effects associated with the introduction has not been studied. We assayed variation in seven microsatellite loci and one mitochondrial DNA gene in the introduced population and nine putative source populations to identify the source population and evaluate bottleneck effects. The Trinity River system was supported as the source owing to its genetic similarity and geographic proximity to the introduced population. Consistent with a bottleneck, the introduced population exhibited reduced allelic and haplotype richness in comparison to source populations. Estimates of the genetically effective number of individuals founding the introduced population using nuclear coalescent analyses and a mitochondrial simulation procedure were highly concordant in suggesting that the initial colonizing group was comprised of about 10 individuals. A bottleneck effect in an exotic species exhibiting limited invasion success has rarely been documented and thus introduction of speckled dace represents an important model system for future investigation. Establishing a relationship between genetic diversity and factors limiting invasion success in this system (e.g., predator avoidance) will help determine the extent to which genetic diversity loss has constrained invasion success in speckled dace.

Habitat Relationships and Larval Drift of Native and Nonindigenous Fishes in Neighboring Tributaries of a Coastal California River

Abstract Motivated by a particular interest in the distribution of the nonindigenous, piscivorous Sacramento pikeminnow Ptychocheilus grandis, we examined fish-habitat relationships in small tributaries (draining 20-200 km2) in the Eel River drainage of northwestern California. We sampled juvenile and adult fish in 15 tributaries in both the summer and fall of 1995 and attempted to relate the densities of the most abundant species to physical variables. To determine which species used small tributaries for spawning, we also collected drifting larval fish during the spring of 1996 and 1997. Water temperature, as measured by maximum weekly average temperature, dominated the relationships between physical variables and the densities of age-0 Sacramento pikeminnow, age-0 steelhead Oncorhynchus mykiss, California roach Hesperoleucus symmetricus (also known as Lavinia symmetricus), and Sacramento sucker Catostomus occidentalis. Of these groups, only age-0 steelhead were most abundant in cool tributaries. In con...

The Effect of Deposited Fine Sediment on Summer Survival and Growth of Rainbow Trout in Riffles of a Small Stream

Abstract Elevated fine-sediment inputs to streams can alter a variety of conditions and processes, including the amount of fine sediment stored in riffles. We sought to measure the influence of deposited fine sediment on the survival and growth of juvenile rainbow trout Oncorhynchus mykiss (106–130 mm fork length) using a field experiment that included 18 enclosures in riffles of a small northwestern California stream. The experiment included six replicates of three levels of deposited fine sediment (low, background, and high) that embedded riffle cobbles at 0, 50, and 100%, respectively. Only 1 of 12 fish survived in high-sediment enclosures, while survival of fish in low- and background-sediment treatments equaled or exceeded 50%. Low- and background-sediment treatments could be distinguished from each other by a difference in fish growth: fish in the low-sediment treatment gained mass, on average, while all surviving fish in the background-sediment treatment lost mass. In addition to providing relative...

Seasonal and Among-Stream Variation in Predator Encounter Rates for Fish Prey

Abstract Recognition that predators have indirect effects on prey populations that may exceed their direct consumptive effects highlights the need for a better understanding of spatiotemporal variation in predator–prey interactions. We used photographic monitoring of tethered Rainbow Trout Oncorhynchus mykiss and Cutthroat Trout O. clarkii to quantify predator encounter rates for fish in four streams of northwestern California during winter–spring and summer. To estimate maximum encounter rates, provide the clearest contrast among streams and seasons, and provide an empirical estimate of a key parameter in an individual-based model of stream salmonids, we consistently placed fish in shallow microhabitats that lacked cover. Over 14-d periods, predators captured fish at 66 of the 88 locations where fish were placed. Eight species of birds (including two species of owls) and mammals were documented as capturing fish. Thirty-six percent of the predator encounters occurred at night. Predator encounter rates va...

Local-scale invasion pathways and small founder numbers in introduced Sacramento pikeminnow (Ptychocheilus grandis).

Given the general pattern of invasions with severe ecological consequences commonly resulting from multiple introductions of large numbers of individuals on the intercontinental scale, we explored an example of a highly successful, ecologically significant invader introduced over a short distance, possibly via minimal propagule pressure. The Sacramento pikeminnow (Ptychocheilus grandis) has been introduced to two coastal rivers in northern California where it poses a risk to threatened and endangered fishes. We assayed variation in seven microsatellite loci and one mitochondrial DNA gene to identify the source populations and estimate founder numbers for these introductions. Our analysis suggests that successful invasion of the Eel River was likely the result of a single transfer of 3–4 effective founders from nearby within the species’ native range: Clear Lake or its outflow Cache Creek. The other introduced population (Elk River), known from only seven individuals, likely represents secondary expansion from the introduced Eel River population. Our findings highlight the threat posed by close-range invaders and the ability of some fishes to rapidly invade ecologically suitable areas despite small effective founding numbers.