Changes In Fish Distribution Research Articles

Machine-learning aiding sustainable Indian Ocean tuna purse seine fishery

Among the various challenges facing tropical tuna purse seine fleet are the need to reduce fuel consumption and carbon footprint, as well as minimising bycatch of vulnerable species. Tools designed for forecasting optimum tuna fishing grounds can contribute to adapting to changes in fish distribution due to climate change, by identifying the location of new suitable fishing grounds, and thus reducing the search time. While information about the high probability to find vulnerable species could result in a bycatch reduction. The present study aims at contributing to a more sustainable and cleaner fishing, i.e. catching the same amount of target tuna with less fuel consumption/emissions and lower bycatch. To achieve this, tropical tuna catches as target species, and silky shark accidental catches as bycatch species have been modelled by machine learning models in the Indian Ocean using as inputs historical catch data of these fleets and environmental data. The resulting models show an accuracy of 0.718 and 0.728 for the SKJ and YFT, being the absences (TPR = 0.996 for SKJ and 0.993 for YFT, respectively) better predicted than the high or low catches. In the case of the BET, which is not the main target species of this fleet, the accuracy is lower than that of the previous species. Regarding the silky shark, the presence/absence model provides an accuracy of 0.842. Even though the model's performance has room for improvement, the present work lays the foundations of a process for forecasting fishing grounds avoiding vulnerable species, by only using as input data forecast environmental data provided in near real time by earth observation programs. In the future these models can be improved as more input data and knowledge about the main environmental conditions influencing these species becomes available.

Policy-oriented adaptation strategies for artisanal marine fisheries under climate and non-climate stressors in San Andres de Tumaco, Nariño, Colombia

Climate and non-climate stressors significantly impact artisanal fishing in San Andrés de Tumaco, Nariño, Colombia, a crucial activity for local food security and employment. Our study aimed to prioritize these stressors on artisanal Fisheries Economic Units (FEUs) and propose policy-oriented adaptation strategies. We adopted an integrated approach, using diverse evidence sources and techniques such as complex network theory and Multi-Criteria Decision Analysis. Key challenges identified include reduced fish catch, abnormal climatic conditions, and changes in fish distribution. The community prioritizes educational, counseling, cultural, and behavioral adaptation measures. Crucial policy-oriented strategies encompass researching fish species' vulnerability to climatic changes, predicting commercial fish distribution considering climatic shifts, and developing a WebGIS for accessible regional climatic information; strategies that are consequently aligned with the Integral Policy for the Development of Sustainable Fisheries in Colombia. Our integrated approach assists decision-making, fostering adaptive capacity within these communities. It underscores the need for policy-driven, locally-sensitive, and strategic action to ensure the long-term resilience of artisanal fisheries against climate change and non-climate stressors.

Freshwater Fish Biodiversity in a Large Mediterranean Basin (Guadalquivir River, S Spain): Patterns, Threats, Status and Conservation

The Guadalquivir River Basin is one of the largest in the Iberian Peninsula and has a remarkable freshwater biodiversity. Although many studies on hydrological regimes or water quality have been conducted in this basin the biodiversity of freshwater fish, as well as their distribution and conservation status, has never been globally addressed as in other Iberian basins. In this context, we synthesized information on freshwater fish using field procedures and a bibliographic search. Fish distribution patterns at different spatial scales and general environmental conditions were analyzed as well as the conservation status of the fish community. We documented the presence of 40 species (20 native and 20 exotic) in the basin during the 20th century until today. However, we only captured 18 species during the field sampling, with a prevalence for any native species of less than 23% (except Luciobarbus sclateri). The highest species richness was found in mid reaches, while the lower reaches had very low diversity values. Around 50% of species are threatened; according to the IUCN, several species are declining at an alarming rate and others are probably extinct and/or their current status is unknown. Human disturbances during the last few decades have caused serious changes in fish distribution and consequently to their conservation status. Hydrological alterations, intensive agriculture and introduced species are probably the principal reasons for Guadalquivir’s ichthyofauna imperilment. Our study indicates an urgent and real need to identify important areas for fish conservation to guarantee a minimum fish biodiversity conservation over the long term, as well as effective strategies for fish recovery where it still is possible.

Reconnecting the Elwha River: Spatial Patterns of Fish Response to Dam Removal

The removal of two large dams on the Elwha River was completed in 2014 with a goal of restoring anadromous salmonid populations. Using observations from ongoing field studies, we compiled a timeline of migratory fish passage upstream of each dam. We also used spatially continuous snorkeling surveys in consecutive years before (2007, 2008) and after (2018, 2019) dam removal during summer baseflow to assess changes in fish distribution and density over 65 km of the mainstem Elwha River. Before dam removal, anadromous fishes were limited to the 7.9 km section of river downstream of Elwha Dam, potamodromous species could not migrate throughout the river system, and resident trout were the most abundant species. After dam removal, there was rapid passage into areas upstream of Elwha Dam, with 8 anadromous species (Chinook, Coho, Sockeye, Pink, Chum, Winter Steelhead, Summer Steelhead, Pacific Lamprey, and Bull Trout) observed within 2.5 years. All of these runs except Chum Salmon were also observed in upper Elwha upstream of Glines Canyon Dam within 5 years. The spatial extent of fish passage by adult Chinook Salmon and Summer Steelhead increased by 50 km and 60 km, respectively, after dam removal. Adult Chinook Salmon densities in some previously inaccessible reaches in the middle section of the river exceeded the highest densities observed in the lower section of the river prior to dam removal. The large number (>100) of adult Summer Steelhead in the upper river after dam removal was notable because it was among the rarest anadromous species in the Elwha River prior to dam removal. The spatial extent of trout and Bull Trout remained unchanged after dam removal, but their total abundance increased and their highest densities shifted from the lower 25 km of the river to the upper 40 km. Our results show that reconnecting the Elwha River through dam removal provided fish access to portions of the watershed that had been blocked for nearly a century.

European small pelagic fish distribution under global change scenarios

AbstractThe spectre of increasing impacts on exploited fish stocks in consequence of warmer climate conditions has become a major concern over the last decades. It is now imperative to improve the way we project the effects of future climate warming on fisheries. While estimating future climate‐induced changes in fish distribution is an important contribution to sustainable resource management, the impacts on European small pelagic fish—representing over 50% of the landings in the Mediterranean and Black Sea between 2000 and 2013—are yet largely understudied. Here, we investigated potential changes in the spatial distribution of seven of the most harvested small pelagic fish species in Europe under several climate change scenarios over the 21st century. For each species, we considered eight Species Distribution Models (SDMs), five General Circulation Models (GCMs) and three emission scenarios (the IPCC Representative Concentration Pathways; RCPs). Under all scenarios, our results revealed that the environmental suitability for most of the seven species may strongly decrease in the Mediterranean and western North Sea while increasing in the Black and Baltic Seas. This potential northward range expansion of species is supported by a strong convergence among projections and a low variability between RCPs. Under the most pessimistic scenario (RCP8.5), climate‐related local extinctions were expected in the south‐eastern Mediterranean basin. Our results highlight that a multi‐SDM, multi‐GCM, multi‐RCP approach is needed to produce more robust ecological scenarios of changes in exploited fish stocks in order to better anticipate the economic and social consequences of global climate change.

An impact and adaptation assessment of changing coastal fishing grounds and fishery industry under global change

Changes in fish distribution and migration patterns have occurred in mid- and high-latitude oceans worldwide in response to global change. Since the 1980s, the sea surface temperature (SST) of the Sea of Japan has increased significantly. The mechanisms behind these changes in migratory fish assemblages are difficult to determine from general capture fishery databases. This study collected a long-term dataset of set-net catches reported from Toyama Bay in western Japan to analyse catch compositions. The results indicated that in the coastal zone at the front of the Tsushima warm current and Liman cold current, increased SSTs have caused fluctuations both in the presence of cold-water and warm-water fish and in the timing of fishing seasons. Additionally, the results based on multi-dimensional scaling and cluster analyses showed that the study period can be divided into 5 clusters: (1) 1963–1964, (2) 1966–1972, (3) 1973–1998, (4) 1999–2006, and (5) 2007–2013; the similarity value was 89%. The boundaries between these clusters were similar to the timing of changes in SST, Pacific decadal oscillation, and North Pacific gyre oscillation. A species composition change analysis of these clusters showed that clustering was associated with changes in the intensities of the Tsushima warm current and Liman cold current. A northward expansion of low-latitude fish species, especially small- and medium-sized fish, was observed in Toyama Bay, similar to the expansion of high-latitude fish species into polar regions. Based on the principles of risk management and these research results, the adaptation strategies recommended in this study were differentiated into two categories: proactive adaptation and planned adaptation. (1) Establishment of pre-disaster adaptation capability: Proactive adaptation emphasizes countering the uncertainty in marine fishery production and improving the resilience of local communities. (2) Establishment of resilience during and after climate disasters: Planned adaptation can be used to manage uncertainty in the seafood supply by implementing adjustments in production and marketing and also to mitigate the impact of climate variation on the marine fishery industry and local coastal communities.

Can we project changes in fish abundance and distribution in response to climate?

Large-scale and long-term changes in fish abundance and distribution in response to climate change have been simulated using both statistical and process-based models. However, national and regional fisheries management requires also shorter term projections on smaller spatial scales, and these need to be validated against fisheries data. A 26-year time series of fish surveys with high spatial resolution in the North-East Atlantic provides a unique opportunity to assess the ability of models to correctly simulate the changes in fish distribution and abundance that occurred in response to climate variability and change. We use a dynamic bioclimate envelope model forced by physical-biogeochemical output from eight ocean models to simulate changes in fish abundance and distribution at scales down to a spatial resolution of 0.5°. When comparing with these simulations with annual fish survey data, we found the largest differences at the 0.5° scale. Differences between fishery model runs driven by different biogeochemical models decrease dramatically when results are aggregated to larger scales (e.g. the whole North Sea), to total catches rather than individual species or when the ensemble mean instead of individual simulations are used. Recent improvements in the fidelity of biogeochemical models translate into lower error rates in the fisheries simulations. However, predictions based on different biogeochemical models are often more similar to each other than they are to the survey data, except for some pelagic species. We conclude that model results can be used to guide fisheries management at larger spatial scales, but more caution is needed at smaller scales.

Fish distributions reveal discrepancies between zonal attachment and quota allocations

AbstractThe oceans’ fisheries contribute to human wellbeing by providing essential nutrients, employment, and income. Changes in fish distribution, due to climate change or stock expansion, jeopardize conservation objectives because fishers catch more than is allocated as quota. Quotas, or catch shares, should, therefore, correspond to the share of the fish stock biomass present within a country's Exclusive Economic Zone, a concept known as Zonal Attachment. Here, we assess the Zonal Attachment of transboundary fish stocks present in northern Europe, in the waters of the United Kingdom, the European Union (without the United Kingdom), and Norway. In 12 of 14 important fish stocks, estimates of Zonal Attachment to the United Kingdom were significantly higher than current quota allocations, explaining the country's substantial discard problem. With environmental change, and stock recovery under improved fisheries conservation, scientific evidence should be used not only to set catch limits, but also to re‐examine catch shares.

Changing fish distributions challenge the effective management of European fisheries

Changes in fish distribution are being observed across the globe. In Europe's Common Fisheries Policy, the share of the catch of each fish stock is split among management areas using a fixed allocation key known as ‘Relative Stability’: in each management area, member states get the same proportion of the total catch each year. That proportion is largely based on catches made by those member states in the 1970s. Changes in distribution can, therefore, result in a mismatch between quota shares and regional abundances within management areas, with potential repercussions for the status of fish stocks and the fisheries that depend on them. Assessing distribution changes is crucial to ensure adequate management and sustainable exploitation of our fish resources. We analysed scientific survey data using a three‐tiered analytical approach to provide, for the first time, an overview of changes in distribution for 19 northeast Atlantic fish species encompassing 73 commercial stocks over 30 yr. All species have experienced changes in distribution, five of which did so across management areas. A cross‐species analysis suggested that shifts in areas of suitable thermal habitat, and density‐dependent use of these areas, are at least partly responsible for the observed changes. These findings challenge the current use of relative stability to allocate quotas.

Predicted climate change effects on European perch (Perca fluviatilis L.) - A case study from the Curonian Lagoon, south-eastern Baltic

Climate change is likely to increasingly impact estuarine fish populations. Changes in water temperature or salinity can have deleterious effects on fish growth and behaviour. A decrease in the abundance of freshwater fish in the northern areas of the Curonian Lagoon has been attributed to increased salinity of inflowing water from the Baltic Sea. Thus, this study investigated the effects of possible changes in environmental salinity and temperature on the growth and behaviour of perch (Perca fluviatilis L.). Laboratory experiments revealed that brackish water (salinity 3 and 6) has neither a negative nor a positive effect on growth rates among perch young-of-the-year, when compared to freshwater (salinity 0). In contrast, results from behavioural experiments demonstrated that perch prefer to remain in brackish water (salinity 6) when given a choice between brackish and freshwater, and in warm water (18 °C) when given a choice between warm and cold water (12 °C). For this reason, the temperature rather than the salinity of inflowing colder brackish water from the Baltic Sea will be the most likely driver of short-term changes in fish distribution in the Curonian Lagoon.

Harvesting geo-spatial data on coastal fish assemblages through coordinated citizen science

In response to repeated complaints from recreational and commercial coastal fishermen about declining fishing opportunities in inner Danish waters, focus was directed to inshore fish stocks. However, without data targeting inshore areas, it was not possible to investigate potential changes in fish distribution or abundances, or their causes. As a first step, a voluntary catch registration system was initiated in 2002, in collaboration with locally organized recreational fishermen. Using citizen science as a methodology, scientists and the fishermen developed a protocol for data collection, which the fishermen then implemented. The aim was to establish regular monitoring of fish catches from gill net and fyke net fisheries in coastal waters around Denmark in order to provide data that could inform management. After three years, during which time recreational fishermen could use their own gear and fish where they normally fished, the data was evaluated. As a result, the fishing method was switched in 2005 to fixed gears and fixed positions, to enable comparison between areas, years and season. The project has been very successful in recruiting highly motivated fishermen, who register their entire catch regularly. The time-series of data spans more than a decade and covers over 16,000 instances of fishing. The data from this project are now being used to create coastal fish indicators for managers to assess environmental status at a regional scale. Here we present an analysis of a subset of the data on one species, the European flounder (Platichthys flesus), to illustrate how the spatial and seasonal coverage can be utilized further for investigation of coastal ecosystems and to inform management.

Ocean futures: Exploring stakeholders' perceptions of adaptive capacity to changing marine environments in Northern Norway

Globally, the projected increase in sea surface temperature is expected to facilitate large changes in the marine food web, affect coastal communities, marine ecosystem goods and services, global fisheries, tourism, aquaculture and others. This ongoing change takes place on different time scales, and coincides with increasing global demand for seafood, a growing middle class, and expectations of future need for food security from the marine sector. This article contributes to advances in climate change research by exploring, within the context of the results of the viewExposed program for assessing Integrated Vulnerabilities in different municipalities in Norway, to what extent stakeholders in different industries are willing to accept, or expect to be able to adapt to, these new realities. This was investigated through stakeholder driven workshops with stakeholders from maritime industries, commercial fisheries, tourism and aquaculture in Norway using the conceptual modeling tools of systems thinking and Bayesian Belief Networks. Results showed that commercial fishers are worried about quota allocations with changes in fish distribution. The aquaculture industry was worried about allocations of new areas for industry expansion in contested areas in Northern Norway, and the tourism industry was worried about changes in communications (transport) in light of a changing climate. Only the commercial fishers perceived that their adaptive capacity was low in light of these changes, especially the small-scale coastal fishers. This was because they expected the combination between increased power of the aquaculture industry and new species requiring expensive quotas would be too critical.

Physiological Basis of Climate Change Impacts on North American Inland Fishes

Global climate change is altering freshwater ecosystems and affecting fish populations and communities. Underpinning changes in fish distribution and assemblage‐level responses to climate change are individual‐level physiological constraints. In this review, we synthesize the mechanistic effects of climate change on neuroendocrine, cardiorespiratory, immune, osmoregulatory, and reproductive systems of freshwater and diadromous fishes. Observed climate change effects on physiological systems are varied and numerous, including exceedance of critical thermal tolerances, decreased cardiorespiratory performance, compromised immune function, and altered patterns of individual reproductive investment. However, effects vary widely among and within species because of species, population, and even sex‐specific differences in sensitivity and resilience and because of habitat‐specific variation in the magnitude of climate‐related environmental change. Research on the interactive effects of climate change with other environmental stressors across a broader range of fish diversity is needed to further our understanding of climate change effects on fish physiology.

Seasonal and diel changes in fish distribution in a tropical hydropower plant tailrace: evidence from hydroacoustic and gillnet sampling

AbstractHydropower plant operation can lead to direct impacts on fish near the dam, causing injuries and death. In Brazil, it is more common to observe these impacts in the tailrace. To understand temporal and spatial variations in fish abundance in the tailrace, to assist plant operation management, surveys were performed in Três Marias Hydropower Plant using hydroacoustics and multimesh gillnets. Data were collected in the tailrace backwater over 1 year, during 18 field trips, each covering a 24‐h cycle with six sampling intervals. A greater abundance of fish was found in the tailrace during the wet season, mainly explained by total discharge. More fish were detected by hydroacoustics and gill nets during night sampling. Siluriformes, which are known to exhibit nocturnal habits, were the predominant species in both dry and wet seasons. Thus, damaging operational procedures for fish should be planned in the dry season, low‐abundance period.

Changes in the fish species composition in the coastal zones of the Kuroshio Current and China Coastal Current during periods of climate change: Observations from the set-net fishery (1993–2011)

Changes in fish distribution and migration patterns have occurred in mid- and high-latitude oceans worldwide in response to climate change. Since the 1980s, the sea surface temperature (SST) of the southern East China Sea has increased significantly, particularly in winter. The mechanisms behind these changes in migratory fish assemblages are difficult to elucidate from general capture fisheries databases. This study collected a long-term data set of set-net catches, reported from the remote Tung-Ao Bay in northeastern Taiwan to analyze catch composition. Both the total number of species and the Shannon–Wiener index (H′) showed an increasing trend, while the cumulative percentages of the top 10 captured fish species decreased annually. These results indicated that in the coastal zone at the front of Kuroshio Current (KC) and China Coastal Current (CCC), increased SST caused fluctuations in the presence of cold-water and warm-water fishes and in the timing of fishing seasons. Additionally, results based on multidimensional scaling (MDS) and cluster analyses showed that the study period could be divided into two clusters, 1993–1997 and 1998–2011, with an 80% similarity value. The boundary of these clusters was consistent with changes in SST. A species composition change analysis of these clusters showed that clustering was associated with changes in the intensities of the CCC and KC, especially in winter seasons. A northward expansion of low-latitude fish species was observed in Tung-Ao Bay, similar to expansion of high-latitude fish species into Polar region.

The impact of environmental degradation on reproduction of the black-chinned tilapia Sarotherodon melanotheron from various coastal marine, estuarine and freshwater habitats

There is growing evidence that climate change has greatly altered environmental conditions in many aquatic ecosystems over the last decades, leading to changes in fish distribution and life history traits. Recent works conducted in Senegalese and Gambian coastal marine, estuarine and freshwater ecosystems have shown important changes in the intensity, frequency and breeding timing of wild population of Sarotherodon melanotheron in response to changes in salinity regimes. In addition to salinity, this study investigates the potential influences of other environmental factors that have received less attention on the reproduction of S. melanotheron from three different aquatic ecosystems in Senegal. The results demonstrate that day-length and temperature affect sexual maturity in both males and females from Guiers Lake and Hann Bay, but no such effects were found in upstream of the Saloum Estuary, where the spawning activity seems to be under the synergetic control of rainfall and salinity that apparently predominate over all the other ambient factors. This study demonstrated for the first time that aside from photoperiod and temperature, rainfall also influences fish reproduction in Hann Bay probably through its effects on water quality. Furthermore, our results confirm previous findings that changes in salinity regimes resulting from seasonal variations in precipitation and evaporation are the ultimate causes of temporal changes in the reproductive activity of the species in this estuary.

Fish behaviour in response to tidal variability and internal waves over a shelf sea bank

The availability of fish to top mobile predators such as seabirds and fishermen and to fisheries surveys depends on their schooling behaviour. In temperate coastal ecosystems, tide is the main driver of ecosystem dynamics. To investigate the mechanisms linking fish schooling behaviour to bio-physical oceanography in a tidal ecosystem, we combined novel fine scale survey data of fish behaviour using fisheries acoustics with co-located oceanographic data from contrasting topographical locations. The schooling behaviour of pelagic fish was examined in relation to bio-physical oceanography by carrying out repeated surveys of the same locations over the daily tidal cycle at spring and neap tides over a small bank (Jones Bank) and a nearby flat region. Different fish species were found over the bank and in the flat region 14km to the south-east, and fish schooling behaviour varied similarly. Results showed that the bank played an important role in influencing fish distribution and behaviour, with shallow pelagic schools concentrated over the bank and closer to the surface at times of high internal wave activity. Behaviour of schools identified close to the bottom was partially influenced by oceanographic variables that drove internal waves over the bank slopes, but were most strongly influenced by the spring–neap cycle. These deep schools were larger, closer to the bottom and less dense at neap tides. Identifying some the of the bio-physical variables which drive changes in fish distribution and behaviour will help quantify fish catchability and assist in the design and interpretation of better targeted surveys for top predators and prey fish species.

Summer stream temperature metrics for predicting brook trout (Salvelinus fontinalis) distribution in streams

We developed a methodology to predict brook trout (Salvelinus fontinalis) distribution using summer temperature metrics as predictor variables. Our analysis used long-term fish and hourly water temperature data from the Dog River, Vermont (USA). Commonly used metrics (e.g., mean, maximum, maximum 7-day maximum) tend to smooth the data so information on temperature variation is lost. Therefore, we developed a new set of metrics (called event metrics) to capture temperature variation by describing the frequency, area, duration, and magnitude of events that exceeded a user-defined temperature threshold. We used 16, 18, 20, and 22°C. We built linear discriminant models and tested and compared the event metrics against the commonly used metrics. Correct classification of the observations was 66% with event metrics and 87% with commonly used metrics. However, combined event and commonly used metrics correctly classified 92%. Of the four individual temperature thresholds, it was difficult to assess which threshold had the “best” accuracy. The 16°C threshold had slightly fewer misclassifications; however, the 20°C threshold had the fewest extreme misclassifications. Our method leveraged the volumes of existing long-term data and provided a simple, systematic, and adaptable framework for monitoring changes in fish distribution, specifically in the case of irregular, extreme temperature events.

Niche segregation in two closely related species of stickleback along a physiological axis: explaining multidecadal changes in fish distribution from iron-induced respiratory impairment.

Acute exposure to iron can be lethal to fish, but long-term sublethal impacts of iron require further study. Here we investigated whether the spatial and temporal distribution (1967–2004) of two closely related species of stickleback matched the spatial distribution of iron concentrations in the groundwater. We used the ‘Northern Peel region’, a historically iron-rich peat landscape in The Netherlands as a case study. This allowed us to test the hypothesis that niche segregation in two closely related species of stickleback occurred along a physiological axis. Patterns in stickleback occurrence were strongly associated with spatial patterns in iron concentrations before 1979: iron-rich grid cells were avoided by three-spined stickleback (Gasterosteus aculeatus, Linnaeus 1758) and preferred by nine-spined stickleback (Pungitius pungitius, [Linnaeus, 1758]). After 1979, the separation between both sticklebacks became weaker, corresponding to a decreased influence of local groundwater on stream water quality. The way both species changed their distribution in the field provides a strong indication that they differ in their susceptibility to iron-rich conditions. These observed differences correspond with differences in their respiration physiology, tolerance of poor oxygen conditions and overall life-history strategy documented in the literature. Our results exemplify how species can partition niche along a non-structural niche axis, such as sublethal iron-rich conditions. Other fish species may similarly segregate along concentration gradients in iron, while sublethal concentrations of other metals such as copper may similarly impact fish via respiratory impairment and reduced aerobic scope.Electronic supplementary materialThe online version of this article (doi:10.1007/s10452-012-9395-y) contains supplementary material, which is available to authorized users.

Evidence of Young Humpback Chub Overwintering in the Mainstem Colorado River, Marble Canyon, Arizona, USA~!2009-10-07~!2010-01-25~!2010-04-30~!

Global climate change models predict that the southwestern United States will become warmer and drier, re- sulting in decreased reservoir volumes, increased water temperatures, and changes in fish distributions. Such conditions may also result from drought and water extraction. Our observations of humpback chub Gila cypha allowed us to test pre- dicted impacts of increased water temperatures released from Glen Canyon Dam on this federally listed endangered cyp- rinid endemic to the Colorado River Basin. We modeled the potential that the young humpback chub we captured in 2006 and 2007 were hatched and reared up to 50 km upstream from the Little Colorado River, the natal origin of the majority of the humpback chub population below Glen Canyon Dam. Larval and adult humpback chub have been observed by others in our study reach in the post-dam era. Humpback chub assumed to be juveniles have been observed in our study area, but their ages were not confirmed. Our observations of humpback chub were coincident with not only increased water tem- peratures as a result of regional drought, but also decreasing rainbow trout population numbers and some constraints on dam release fluctuations. One or all of these factors could have contributed to increased survivorship of young humpback chub. The most parsimonious explanation for our observations was that the largest young humpback chub captured at our study site took advantage of warmer water temperatures to grow to one and two years of age. Continued favorable habitat conditions in this reach would likely support recruitment to adulthood and reproduction. Managers wish to increase the likelihood that a self-sustaining mainstem Colorado River population of humpback chub is established, distinct from the larger Little Colorado River population. Our results indicate that a separate population may be successfully established.