The Colorado River in Grand Canyon is highly regulated, with hypolimnetic releases that are generally unfavorable for endemic native fishes. However, both long-term drought and changes in dam operations have led to changes in river conditions, including the addition of approximately 125 km of riverine environment due to the contraction of Lake Mead. Through sampling of small-bodied fish, we were able to describe the Grand Canyon fish community and define the current native fish distribution from near Bright Angel Creek downstream to Pearce Ferry. Beginning in 2014 and continuing through 2018, we sampled the fish community via seining and documented a fish community that was dominated (>95%) by native fish through approximately 300 km of river. Nonnative species that were once commonly captured, such as Red Shiner Cyprinella lutrensis, Common Carp Cyprinus carpio, and Channel Catfish Ictalurus punctatus, were rarely encountered in Grand Canyon during this more recent sampling, which makes the Colorado River in Grand Canyon National Park a rare contemporary example of native fish populations regaining dominance over invasive fishes in the desert southwest.

The Colorado River in the Grand Canyon holds the largest remaining population of Humpback Chub Cila cypha, an endangered fish endemic to the Colorado River basin. Early surveys in the 1990s found that most Humpback Chub occupied the Little Colorado River and nearby areas in the mainstem Colorado River and were uncommon in the western Grand Canyon (below Kanab Creek, at river kilometer [rkm] 257.2). From 1939 to 2002, the Colorado River was typically inundated by Lake Mead to rkm 407 (at full pool). Since 2000, Lake Mead water levels have declined, and the current inflow is located almost 100 km downstream at rkm 503.1. Thus, the Arizona Game and Fish Department (AGFD) extended its system-wide monitoring downstream to Pearce Ferry rapid (rkm 478.6) in 2011. Electrofishing is relatively inefficient at capturing humpback Chub, but a small number of captures from 2011 to 2015 in the western Grand Canyon suggested that the Humpback Chub may have expanded its range downstream. Subsequently, hoop nets were added to AGFD system-wide monitoring in 2016 to better describe Humpback Chub distribution. The distribution of Humpback Chub observed in 2016 and 2017 differed from previous descriptions; we documented relatively large numbers of Humpback Chub, including age-0 fish, in the western Grand Canyon and observed the highest catches downstream of Diamond Creek (rkm 389.1). This suggests that a recent range expansion has occurred and that Humpback Chub in the western Grand Canyon do not utilize the Little Colorado River for reproduction and might therefore constitute a new subpopulation.

During the last century, populations of the endangered Razorback Sucker (Xyrauchen texanus) have declined in the Colorado River Basin. Dramatic changes in habitat resulting from altered flow regimes and the presence of nonnative fishes were likely major factors contributing to this decline. Since 1996, studies in Lake Mead have resulted in the discovery of 4 areas where wild Razorback Suckers were spawning; all of these locations have tributary or wastewater inflows. Since the Lake Mead Razorback Sucker population has persisted near these prominent inflow areas, we hypothesize that complex inflow areas with vegetative cover and turbidity provide important habitat for the species. In 2011, Razorback Sucker investigations at the Lake Powell-Colorado River inflow and the Lake Powell-San Juan River inflow were initiated and, similar to Lake Mead, we found large numbers of Razorback Suckers using both of these inflow areas. Multiple age classes of Razorback Suckers were found in Lake Powell, along with spawning activity and Razorback Sucker larvae, but wild recruitment was more difficult to document due to the large numbers of fish stocked upstream into the Colorado and San Juan Rivers that may have moved into the reservoir. Since Razorback Suckers are present at the prominent inflow areas of Lakes Mead and Powell, we compared reservoir-specific research and monitoring data to highlight the potential for Razorback Sucker recruitment and to promote the importance of these areas for species conservation and recovery. Our research suggests that inflow areas in Lakes Mead and Powell may provide the criteria necessary for Razorback Suckers to reproduce, grow, and persist.

From 1944 to 1990, only 10 razorback suckers (Xyrauchen texanus) were captured in the Colorado River within the Grand Canyon, Arizona. Except for a single larval razorback sucker captured in 1998 near Havasu Creek, the species was not found during fish surveys conducted between 1990 and 2011. Biologists confirmed the presence and spawning of razorback sucker within the Grand Canyon in 2014 and 2015 when conducting sampling for small-bodied (age 0+) and larval fishes. Although no juvenile or adult razorback suckers were found, larvae were captured indicating that razorback sucker are using historical Colorado River habitats within the Grand Canyon.

Biased perspectives of fisheries researchers may hinder scientific progress and effective management if limiting factors controlling productivity go unrecognized. We investigated whether river and lake researchers used different approaches when studying salmonid production and whether any differences were ecologically supported. We assessed 564 peer-reviewed papers published between 1966 and 2012 that studied salmonid production or surrogate variables (e. g., abundance, growth, biomass, population) and classified them into five major predictor variable categories: physical habitat, fertility (i. e., nutrients, bottom-up), biotic, temperature, and pollution. The review demonstrated that river researchers primarily analyzed physical habitat (65% of studies) and lake researchers primarily analyzed fertility (45%) and biotic (51%) variables. Nevertheless, understudied variables were often statistically significant predictors of production for lake and river systems and, combined with other evidence, suggests that unjustified a priori assumptions may dictate the choice of independent variables studied. Broader consideration of potential limiting factors on fish production, greater research effort on understudied genera, and increased publication in broadly scoped journals would likely promote integration between lentic and lotic perspectives and improve fisheries management.

AimNon-native reptiles are often detrimental to native communities and ecosystems and can be extremely difficult to manage once established. Thus, there is considerable interest in predicting the likelihood of establishment of non-native reptiles. We assessed three hypotheses describing possible factors contributing to the successful establishment of introduced reptiles in an effort to better identify potential invaders.LocationWorldwide.MethodsUsing a global invasion database (1307 introductions of 398 species) and Bayesian generalized linear mixed models, we tested the relative importance of event-level (e.g. propagule pressure), location-level (e.g. climate matching) and species-level (e.g. parthenogenesis) factors in reptile establishment success.ResultsThe factors that positively influenced establishment success included: (i) Event-level: longer time since initial introduction, greater number of introduction events and intentional introductions; (ii) Location-level: smaller differences in latitude between native and introduced ranges and the presence of native congeners in the introduced range; and (iii) Species-level: smaller body size, herbivores, larger native range size, parthenogenesis and high fecundity.Main conclusionsWe found that location-level factors were most important in describing reptile establishment success, followed by event- and species-level factors, respectively. This pattern matches closely with what others have found in a variety of vertebrate taxa. However, the importance of species traits may be underestimated considering the insufficient knowledge of reptile life history within introduced ranges. Importantly, individual variables from all three hypotheses contributed to global reptile establishment. Managers should be especially cognizant of small herbivorous and fecund reptiles that are frequently introduced into areas with a strong climate match to their native range. Further, parthenogenesis greatly facilitated establishment, indicating that obligate parthenogenetic species may become ubiquitous through modern globalized trade.