SUMMARYThe effect of turbulent eddy diameter, vorticity and orientation on the 2min critical swimming speed and stability of creek chub (Semotilus atromaculatus) is reported. Turbulent eddies were visualized and their properties were quantified using particle image velocimetry (PIV). Flow fields with an increasing range in eddy diameter were created by inserting cylinder arrays upstream from the swimming test section. Eddy vorticity increased with increasing velocity. Two orientations of eddies, eddies spinning about a vertical axis and eddies spinning about a horizontal (wall-to-wall) axis, were investigated. Stability challenges were not observed until the largest (95th percentile) eddy diameters reached 76% of the fish body total length. Under these conditions fish were observed to spin in an orientation consistent with the rotational axis of the large eddies and translate downstream. These losses in postural control were termed 'spills'. Spills were 230% more frequent and lasted 24% longer in turbulent flow fields dominated by horizontal eddies than by vertical eddies of the same diameter. The onset of spills coincided with a 10% and 22% reduction in critical swimming speed in turbulent flows dominated by large vertical and horizontal eddies, respectively. These observations confirm predictions by Pavlov et al., Cada and Odeh, Lupandin, and Liao that the eddy diameter, vorticity and orientation play an important role in the swimming capacity of fishes.

The current understanding of the effects of turbulence on the swimming performance of fish is primarily derived from laboratory experiments under pressurised flow swim tunnels and open-channel flow facilities. These studies have produced valuable information on the swimming mechanics and behaviour of fish in turbulent flow. However, laboratory studies have limited representation of the flows fish experience in nature. The flow structure in rivers is imparted primarily by the highly heterogeneous nonuniform bed, and the flow is generally much more complex than in laboratory experiments. The goal of the current work is to direct future laboratory and field studies to adopt a common framework that will shape the integration of both approaches. This article outlines four characteristics of turbulent flow, which we suggest should be evaluated when generalising results from fish turbulent studies in both the laboratory and the field. The framework is based on four turbulence characteristics that are summarised under the acronym IPOS: intensity, periodicity, orientation and scale. Figure 3. (A) Mean longitudinal velocity profiles. (B) Longitudinal turbulence intensity profiles. The dashed horizontal line indicates z = 0.5 m (borrowed with permission from Neary and Sale, 2010).

The physical habitat requirements of cover, depth, and current speed for brown trout Salmo trutta are associated with high shear zones in stream flows, which in turn result in high turbulence. Observations were made on current speeds and turbulence intensity (TI) in a sand-bed trout stream. Exemplary transects showed that current speeds ranged from 0 to 60 cm/s and that TI ranged from 0 to 0.7. Turbulence intensity was inversely related to current speed. Brown trout were usually found in the lower 5 cm of the stream, where shear forces result in high turbulence. Locations occupied by brown trout had lower TI than similar locations without brown trout but higher TI than is typical of an average stream.

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There are 104 hydroelectric facilities proposed to be installed in the watersheds that feed the Pantanal, a vast floodplain wetland located mostly in Brazil. The Pantanal is host to 23 long‐distance migratory fish species that ascend upland tributaries to spawn. A Geographic Information System was used to predict the impact of hydroelectric dams on potential migration routes for these species. Both anthropogenic (hydroelectric dams) and natural barriers were included in the analysis. Natural barriers were identified by river slope. Critical river slopes of 10 and 25%, above which fish were predicted to be incapable of ascending, were modeled as natural barriers. Based on this model, we show that between 2 and 14% of rivers in the Pantanal watershed are naturally blocked to fish migration. An additional 5 to 9% of rivers are currently blocked due to 35 existing hydroelectric facilities. If all proposed dams are built, the area flooded by new reservoirs will triple and the river kilometers blocked will double, blocking 25 to 32% of the river system to fish migration. The Taquari and Cuiabá River sub‐basins will be the most impacted, each having more than 70% of their rivers blocked. The impact of individual proposed facilities on the loss of migration routes is related to their proximity to existing barriers. Fourteen of the proposed dams are upstream from existing barriers and will therefore not further restrict long‐distance fish migration routes while proposed dams are predicted to close an additional 11,000 to 12,000 km of river channels.

Tropical river basins have experienced dramatically increased hydropower development over the last 20 years. These alterations have the potential to cause changes in hydrologic and ecologic systems. One heavily impacted system is the Upper Paraguay River Basin, which feeds the Pantanal wetland. The Pantanal is a Ramsar Heritage site and is one of the world's largest freshwater wetlands. Over the past 20 years, the number of hydropower facilities in the Upper Paraguay River Basin has more than doubled. This paper uses the Indicators of Hydrologic Alteration (IHA) method to assess the impact of 24 of these dams on the hydrologic regime over 20 years (10 years before and 10 years after dam installation) and proposes a method to disentangle the effects of dams from other drivers of hydrologic change using undammed “control” rivers. While most of these dams are small, run-of-the-river systems, each dam significantly altered at least one of the 33 hydrologic indicators assessed. Across all studied dams, 88 of the 256 calculated indicators changed significantly, causing changes of 5–40%, compared to undammed reaches. These changes were most common in indicators that quantify the frequency and duration of high and low pulses, along with those for the rate and frequency of hydrologic changes. Importantly, the flow regime in several undammed reaches also showed significant alterations, likely due to climate and land-use changes, supporting the need for measurements in representative control systems when attributing causes to observed change. Basin-wide hydrologic changes (in both dammed and undammed rivers) have the potential to fundamentally alter the hydrology, sediment patterns, and ecosystem of the Pantanal wetland. The proposed refinement of the IHA methods reveals crucial differences between dam-induced alteration and those assigned to other drivers of change; these need to be better understood for more efficient management of current hydropower plants or the implementation of future dams.

A portable underwater particle image velocimetry (PIV) device has been developed, tested and demonstrated. The underwater PIV uses a 532 nm battery-powered 90 mW continuous laser. The laser beam is pulsed via a camera-synchronized chopper wheel. Images were recorded using a 1 megapixel black and white 10-bit CCD battery-powered camera controlled via a PCMCIA frame grabber card connected to a laptop computer. The system was validated against a standard laboratory PIV for average velocities up to 15 cm s −1 downstream from a 1.6 cm circular cylinder. The average vorticities calculated between the two systems were similar with a maximum difference of 3.6%. The average velocities were also similar with the largest difference occurring at the slowest flow recorded (difference of 0.5 cm s −1), resulting in a 9.4% difference. The maximum eddy size was comparable between the two systems with an average error of 4%. The system was field tested in the Huron River, Michigan downstream from a 1.2 cm diameter submerged limb. Mean velocities and standard deviations were comparable to acoustic Doppler velocimeter data. This paper presents the first published subsurface PIV data from a fluvial environment, demonstrating potential applications for a number of ecological and geomorphological studies.