Search

The viability of populations is influenced by driving forces such as density dependence and climate variability, but most population viability analyses (PVAs) ignore these factors because of data limitations. Additionally, simplified PVAs produce limited measures of population viability such as annual population growth rate (lamda) or extinction risk. Here we developed a "mechanistic" PVA of threatened Chinook salmon (Oncorhynchus tshawytscha) in which, based on 40 years of detailed data, we related freshwater recruitment of juveniles to density of spawners, and third-year survival in the ocean to monthly indices of broad-scale ocean and climate conditions. Including climate variability in the model produced important effects: estimated population viability was very sensitive to assumptions of future climate conditions and the autocorrelation contained in the climate signal increased mean population abundance while increasing probability of quasi extinction. Because of the presence of density dependence in the model, however we could not distinguish among alternative climate scenarios through mean lamda values, emphasizing the importance of considering multiple measures to elucidate population viability. Our sensitivity analyses demonstrated that the importance of particular parameters varied across models and depended on which viability measure was the response variable. The density-dependent parameter associated with freshwater recruitment was consistently the most important, regardless of viability measure, suggesting that increasing juvenile carrying capacity is important for recovery.

Abstract: Snake River stream-type Chinook salmon smolts migrate >1000km from rearing habitats to the Pacific Ocean and return 1–3 years later for their upstream spawning migration. Construction of 8 mainstem dams on the Snake/Columbia River that fish must pass has greatly altered the connectivity between their freshwater spawning and rearing habitats and the ocean. In addition to direct mortality to smolts passing through turbines, these dams along with over 200 additional dams and storage reservoirs above them affect the volume, timing and turbidity of river flows and the size of the plume entering the ocean. At mainstem dams, improvements to fish ladders have largely eliminated problems for upstream migrants, while construction of screened bypass systems, a spill program, and transport of smolts by barge have greatly improved direct survival of juveniles. However, smolt-to-adult returns have not shown the same improvement and have been highly variable in recent years. While direct survival for juveniles passing 8 dams is now as high or higher than historically when they passed only 4, survival downstream of the last dam has changed in part due to altered timing and condition of smolts upon ocean entry. Recent additions of surface passage structures at dams have reduced travel time through the system to more closely approach historical rates prior to dam construction. However, substantial additional improvement in direct survival of smolts through the hydropower system does not appear achievable with existing knowledge and technology. Restoring conditions in the plume might improve survival but would require increased flow volume. [Copyright &y& Elsevier]

Effective conservation and management of natural resources requires accurate predictions of ecosystem responses to future climate change, but environmental science has largely failed to produce these reliable forecasts. The future response of Pacific salmon ( Oncorhynchus spp.) to a changing environment and continued anthropogenic disturbance is of particular interest to the public because of their high economic, social, and cultural value. While numerous retrospective analyses show a strong correlation between past changes in the ocean environment and salmon production within the north Pacific, these correlations rarely make good predictions. Using a Bayesian time-series model to make successive 1-yr-ahead forecasts, we predicted changes in the ocean survival of Snake River spring/summer chinook salmon ( O. tshawytscha) from indices of coastal ocean upwelling with a high degree of certainty ( R2 = 0.71). Furthermore, another form of the dynamic times-series model that used all of the available data indicated an even stronger coupling between smolt-to-adult survival and ocean upwelling in the spring and fall ( R2 = 0.96). This suggests that management policies directed at conserving this threatened stock of salmon need to explicitly address the important role of the ocean in driving future salmon survival. [ABSTRACT FROM AUTHOR]

To mitigate negative impacts of delayed migration it is necessary to understand the causes of avoidance exhibited by animals at behavioural barriers. For downstream migrating juvenile salmon, avoidance of velocity gradients at anthropogenic structures may compromise fitness. Building on previous experimental investigations on salmonid response to velocity gradients, this study aimed to quantify impacts of behaviour on subsequent passage in the presence and absence of visual cues. In an experimental flume, downstream moving juvenile Chinook salmon, Oncorhynchus tshawytscha, encountered either a high or a low velocity gradient created by an orifice weir, under light (95lx) or dark (infrared illumination only) conditions. The majority of fish exhibited an observable response on encountering accelerating velocity, with avoidance behaviour elevated when light (45%) in comparison to when dark (12%). More time was spent facing the flow when the velocity gradient was high. Fish that exhibited avoidance were delayed by approximately eight-fold, travelled 3.5 times further, and experienced a higher mean cumulative velocity gradient across the body length (spatial velocity gradient) prior to successful downstream passage. This study highlights the impact of variation in behaviour on fish passage, and the potential for combined multimodal signals (in this instance visual and mechanosensory) to be used to repel fish, for example from hazardous areas such as turbine intakes. Conversely, by limiting information available, undesirable delay, for example at entrances to downstream fish bypasses, may be reduced. [Copyright &y& Elsevier]