Christopher C. Caudill

Homing and straying by anadromous salmonids: a review of mechanisms and rates

There is a long research history addressing olfactory imprinting, natal homing, and non-natal straying by anadromous salmon and trout (Salmonidae). In undisturbed populations, adult straying is a fundamental component of metapopulation biology, facilitating genetic resilience, demographic stability, recolonization, and range expansion into unexploited habitats. Unfortunately, salmonid hatcheries and other human actions worldwide have affected straying in ways that can negatively affect wild populations through competitive interactions, reduced productivity and resiliency, hybridization and domestication effects, and outbreeding depression. Reduced adult straying is therefore an objective for many managed populations. Currently, there is considerable uncertainty about the range of ‘natural’ stray rates and about which mechanisms precipitate straying in either wild or human-influenced fish. Research in several disciplines indicates that adult straying is affected by endocrine physiology and neurological processes in juveniles, incomplete or interrupted imprinting during rearing and emigration, and by complex interactions among adult maturation processes, reproductive behaviors, olfactory memory, environmental conditions during migration, and senescence physiology. Reported salmonid stray rates indicate that the behavior varies among species, among life-history types, and among populations within species. Most strays enter sites near natal areas, but long-distance straying also occurs, especially in hatchery populations that were outplanted or transported as juveniles. A majority of past studies has estimated straying as demographic losses from donor populations, but some have estimated straying into recipient populations. Most recipient-based estimates have substantiated concerns that wild populations are vulnerable to swamping by abundant hatchery and farm-raised strays.

EMPIRICAL EVIDENCE FOR NONSELECTIVE RECRUITMENT AND A SOURCE-SINK DYNAMIC IN A MAYFLY METAPOPULATION

Dispersal among spatially subdivided populations and variability in local habitat patch quality can strongly affect local and regional population dynamics. A meta- population of mayflies ( Callibaetis ferrugineus hageni) was studied in which larvae de- veloped in beaver ponds and emerged to the terrestrial habitat as adults; adult females had the potential to disperse among ponds before oviposition. Ponds (patches) differed in the presence or absence of trout. Densities of late instar larvae were more than an order of magnitude greater in fishless ponds. Estimates of adult production, migration, and oviposition rate were used to determine whether the difference in larval abundance between pond types was more consistent with a source-sink or balanced dispersal model. Patch quality among ponds (estimated by the emergence rate of adults) varied from 0 to ;3900 individuals·m 22 ·yr 21 , and was significantly lower in ponds with trout. Survivorship of adult females in the regional terrestrial habitat was low (1.7%). Consistent with the predictions of source-sink theory, local recruitment to the next generation was not related to local emergence. Moreover, comparison of emer- gence and recruitment revealed that some ponds were net exporters of females (sources) and others were net importers (sinks). Ovipositing females did not avoid ovipositing in ponds with trout. Overall, the data were consistent with a source-sink dynamic because patches differed in quality, and because there was net migration of individuals from sources to sinks. The results support the hypothesis that local and regional population dynamics are influenced by spatial variation in patch quality, and by the ability of individuals to both disperse among and assess the quality of habitat patches.

TRANSPORTING JUVENILE SALMONIDS AROUND DAMS IMPAIRS ADULT MIGRATION

Mitigation and ecosystem-restoration efforts may have unintended consequences on both target and nontarget populations. Important effects can be displaced in space and time, making them difficult to detect without monitoring at appropriate scales. Here, we examined the effects of a mitigation program for juvenile salmonids on subsequent adult migration behaviors and survival. Juvenile chinook salmon (Oncorhynchus tshawytscha) and steelhead (O. mykiss) were collected and uniquely tagged with passive integrated transponder (PIT) tags at Lower Granite Dam (Washington State, USA) on the Snake River and were then either transported downstream in barges in an effort to reduce out-migration mortality or returned to the river as a control group. Returning adults were collected and radio-tagged at Bonneville Dam (Washington-Oregon, USA) on the Columbia River 1-3 years later and then monitored during approximately 460 km of their homing migrations. The proportion of adults successfully homing was significantly lower, and unaccounted loss and permanent straying into non-natal rivers was higher, for barged fish of both species. On average, barged fish homed to Lower Granite Dam at rates about 10% lower than for in-river migrants. Barged fish were also 1.7-3.4 times more likely than in-river fish to fall back downstream past dams as adults, a behavior strongly associated with lower survival. These results suggest that juvenile transport impaired adult orientation or homing abilities, perhaps by disrupting sequential imprinting processes during juvenile out-migration. While juvenile transportation has clear short-term juvenile-survival benefits, the delayed effects that manifest in adult stages illustrate the need to assess mitigation success throughout the life cycle of target organisms, i.e., the use of fitness-based measures. In the case of Snake River salmonids listed under the Endangered Species Act, the increased straying and potential associated genetic and demographic effects may represent significant risks to successful recovery for both target and nontarget populations.

Migration Timing of Columbia River Spring Chinook Salmon : Effects of Temperature, River Discharge, and Ocean Environment

Abstract In an effort to improve run timing forecasts for Columbia River spring Chinook salmon Oncorhynchus tshawytscha, we examined relationships among regional ocean climate indices, in-river environmental conditions, and full run and stock-specific migration timing metrics. Results consistently indicated that adult Chinook salmon arrived earliest in years with low river discharge or warm water temperatures and arrived latest in years of cold water temperatures and high flows. As single predictors, in-river conditions generally explained more interannual variability in salmon return timing than did air temperature, the Pacific Decadal Oscillation, or the North Pacific Index. However, best-fit multiple-regression models included a combination of in-river and climate predictors. While spatial and temporal scales of the analyses were relatively coarse (i.e., monthly values were used for all predictors), clear patterns emerged that can be used to improve pre- and in-season run timing forecasting models for ...

LACK OF APPROPRIATE BEHAVIORAL OR DEVELOPMENTAL RESPONSES BY MAYFLY LARVAE TO TROUT PREDATORS

Many organisms living in heterogeneous environments alter behaviors or morphology when developing in the presence of predators and subsequently incur associated sublethal fitness costs. Larvae of the mayfly Callibaetis ferrugineus hageni develop in beaver ponds with or without trout predators. We examined the potential sublethal fitness costs of developing in the presence of trout predators by comparing patterns of timing and size at emergence from ponds over two years, before and after manipulating trout densities. In addition, the behavior, timing, and size at emergence of larvae reared in mesocosms were compared between treatments with and without trout chemical cues. Timing of mayfly emergence and adult size was not affected by the manipulation of trout in the field, or by the presence of trout chemical cues in mesocosms. Observations of mayflies in mesocosms provided no evidence that predator cues induced antipredator behaviors, such as reduced activity or increased crypsis. Surprisingly, late instar larvae swam more frequently in the presence of trout cues during the middle of the day, a behavior that could increase their vulnerability to visually feeding predators. Thus, larvae did not exhibit any traits expected to increase survival in the presence of trout cues. The apparently maladaptive responses to trout may result from phylogenetic inertia or conflicting selection pressures encountered when developing in fishless habitats. The evolution of plasticity to trout in Callibaetis may be inhibited by frequent dispersal from fishless source populations.

Movement of Radio-Tagged Adult Pacific Lampreys during a Large-Scale Fishway Velocity Experiment

Abstract Optimization of fishways to pass multiple species is challenging because life history, swimming ability, and behavior often differ among species. For example, high fishway water velocities designed to attract adult Pacific salmon Oncorhynchus spp. at Columbia River dams inhibit fishway entrance and passage success of adult Pacific lampreys Lampetra tridentata, a species of conservation concern. We tested whether reduced water velocities (∼1.2 m/s, 0.15 m of head) at Bonneville Dam fishway openings improved entrance efficiency and other passage metrics for radio-tagged Pacific lampreys compared with control velocities (>1.98 m/s, 0.46 m of head) and near-zero (“standby”) velocities. Lamprey entrance efficiencies were significantly higher in the reduced-velocity treatment (26–29%) than in the control (13–20%) or standby (5–9%) treatment. In some years, significantly more Pacific lampreys passed through fishway collection channels and transition pools and reached the fish ladder during reduced-veloc...

Context-dependent diel behavior of upstream-migrating anadromous fishes

Variability is a hallmark of animal behavior and the degree of variability may fluctuate in response to environmental or biological gradients. For example, diel activity patterns during reproductive migrations often differ from those in non-breeding habitats, reflecting trade-offs among efficient route selection, reproductive phenology, and risk avoidance. In this study, we tested the hypothesis that diel movements of anadromous fishes differ among freshwater migration habitats. We analyzed diel movement data from ~13 000 radio-, PIT-, and acoustic-tagged adult fishes from five Columbia River species: Chinook salmon, Oncorhynchus tshawytscha; sockeye salmon, O. nerka; steelhead, O. mykiss; Pacific lamprey, Entosphenus tridentatus; and American shad, Alosa sapidissima. All five species were active during most of the diel cycle in low-gradient, less hydraulically complex reservoir and riverine habitats. Movement shifted to predominantly diurnal (salmonids and American shad) or nocturnal (Pacific lamprey) at hydroelectric dam fishways where hydraulic complexity and predator density were high. Results suggest that context-dependent behaviors are common during fish migrations, and that diel activity patterns vary with the degree of effort or predation risk required for movement.

Migration depths of adult spring and summer Chinook salmon in the lower Columbia and Snake Rivers in relation to dissolved gas supersaturation

Abstract High spill volume at dams can create supersaturated dissolved gas conditions that may have negative effects on fish. Water spilling over Columbia and Snake River dams during the spring and summer creates plumes with high dissolved gas that extend downstream of dam spillways and throughout reservoirs and creates gas-supersaturated conditions throughout the water column. During the spring and summer of 2000, 228 adult Chinook salmon Oncorhynchus tshawytscha were tagged at Bonneville Dam with archival radio data storage transmitters (RDSTs) that recorded depth and water temperature as the fish migrated through dams and reservoirs of the lower Columbia and Snake rivers. Swimming depths from 131 of the 228 adult spring and summer Chinook salmon tagged with RDSTs were used to estimate the potential for gas bubble formation given in-river dissolved gas concentrations and hydrostatic compensation. We found that adult spring and summer Chinook salmon spent a majority of the time at depths that would have ...

Fallback by Adult Sockeye Salmon at Columbia River Dams

Abstract We implanted radio transmitters into sockeye salmon Oncorhynchus nerka in 1997 to determine the (1) fallback percentage and rate at eight Columbia River dams, (2) effect of fallback on adult counts at each dam, (3) relations between spillway discharge and fallback, (4) relations between injuries and fallback, and (5) relations of fallback and survival to spawning tributaries. The rate of fallback, that is, the total number of fallback events at a dam divided by the number of fish known to have passed the dam, ranged from 1.9% to 13.7% at the eight dams. The rate of fallback was highest at Bonneville Dam, the dam with the most complex fishway. Fallback produced overcounts of 2% to 7% at most dams. Fallback was weakly related to spill volume at Bonneville Dam. Significantly more sockeye salmon with head injuries fell back than fish without head injuries. About 40% of the sockeye salmon had injuries from marine mammals, but these injuries were not associated with the rate of fallback. The rate of su...

Overwintering Distribution, Behavior, and Survival of Adult Summer Steelhead: Variability among Columbia River Populations

Abstract Unlike most anadromous salmonids, summer steelhead Oncorhynchus mykiss overwinter in rivers rather than the ocean for 6–10 months prior to spring spawning. Overwintering in rivers may make summer steelhead more vulnerable to harvest and other mortality sources than are other anadromous populations, but there has been little systematic study of this life history strategy. Here, we used a large-scale radiotelemetry study to examine the overwintering behaviors and distributions of 26 summer steelhead stocks within the regulated lower Columbia–Snake River hydrosystem. Over 6 years, we monitored 5,939 fish, of which 3,399 successfully reached spawning tributaries or the upper Columbia River basin and were assigned to specific populations. An estimated 12.4% of fish that reached spawning areas overwintered at least partially within the hydrosystem (annual estimates = 6.8–19.6%), while the remainder overwintered in tributaries. Across all populations, later-arriving fish were more likely to overwinter i...