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Fish Passage in Floodplains 

Scientific research can inform the best designs and strategies to help fish move past migration barriers, such as dams, weirs, and road crossings. That was the take-home message of the Lao National Workshop on Fish Passage that FISHBIO recently attended in Vientiane, Lao People's Democratic Republic. Experts from Lao PDR, Australia, Brazil, France, and the United States shared experiences addressing fish passage in their respective countries. Speakers stressed that fish passageways in Lao PDR need to be designed specifically for the native species of the region, which may require different approaches than those that have worked elsewhere. A particular challenge to fish passage design in a region with as much biodiversity as the Mekong is accommodating the wide variation in sizes and swimming abilities of fish species that range from 3 cm to 3 m in length.


The conference focused primarily on floodplain barriers, which block fish from moving laterally off the main river into the tributaries and wetlands of the watershed. The first day of the workshop focused on helping fish traveling upstream, which may be breeding adults trying to migrate to their spawning grounds, or juveniles accessing nursery habitat. Researchers presented methods for prioritizing existing structures that need modification for fish passage -- a necessary exercise, since they identified nearly 2,500 potential barriers in a single river catchment (Xe Chamopone). Such barriers can be roads, irrigation structures, or natural obstructions like logs. Scientists also described an experiment to test various fish passage designs at a floodplain regulator in the Pak Peung wetland of Central Lao PDR (Baumgartner 2012). Unlike salmon, which can leap over barriers 2 m high, Mekong fish in the 3-60 cm size range could generally clear a step that is 10 cm tall (4 in), the researchers found during laboratory studies. Based on these findings, they constructed a fishway with 43 sequential 10-cm steps, which stretched 150 m (492 ft) in length. Because fish passageways can't be too steep, the taller a barrier is, the longer a fishway needs to be to clear it, and hence the more expensive it is to construct and maintain.


The second day of the conference focused on creating passage for downstream migrating fish, which can be either adults or juveniles travelling from floodplains to the mainstem river. Several researchers discussed the potentially hazardous hydraulic conditions fish may encounter when passing through structures, including pressure changes, shear between two shifting masses of water, and collision with hard surfaces. Researchers indicated that structures requiring water and fish to pass underneath a barrier (such as a sluice gate) proved more damaging to fish than those allowing fish to pass over the top of a structure (such as a fixed crest weir). Scientists also used a pressure chamber to demonstrate the barotrauma, or physical injury from changes in pressure, that fish experience when passing through turbines and weirs: as the pressure drops, the gasses inside a fish expand. Participants discussed plans to build fish swimming and testing facilities at the National University of Laos so more fish passage experiments can be conducted on a greater variety of local species. The workshop highlighted many successes in helping fish move across floodplain barriers, which are generally 6 m (20 ft.) or shorter. A number of additional factors will need to be considered when designing and evaluating fish passage at larger barriers.

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Strange sightings 

Recently, some local visitors stopped by our Oakdale office to let us know they had seen a deer near Yosemite National Park that did not look like any of the other deer in the area. Luckily, one of our fisheries biologists used to be wildlife naturalist, and was able to identify the species as a fallow deer (Dama dama). The very next day, a UC Davis student came by our office and showed us a picture of a fallow deer he had snapped near Oakhurst, CA (shown above).


The fallow deer  is an ungulate belonging to the Cervid family that is native to western Eurasia, but was introduced to California in the late 1800s. As an introduced species, Fallow deer can have a disruptive effect on native tule elk and black-tailed deer. They compete for the same food source and can potentially transmit paratuberculosis to native ungulate species.


Fallow deer most commonly have a chestnut-colored coat with white mottles in the summer, and an unspotted coat in the winter. They also have a light colored area around the tail that is outlined in black; however, other color variations do occur. Fallow bucks (males) typical stand between 32-38 inches at shoulder height, and can range in weight from 130 to 220 pounds.  


Currently, the largest known population of fallow deer in California is at the Point Reyes...  Read more> 

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