Publication
Spawning Periodicity of Rio Grande Silvery Minnow During 2001
Date: 2001/01/01
Author(s): Platania S.P., Dudley R.K.
Publication: Prepared by U.S. Bureau of Reclamation, 14 p.
Abstract:
The historical Middle Rio Grande fish fauna was reflective of a Great Plains river. At least five cyprinid species that can be characterized as Great Plains river fishes formerly occurred throughout the Middle Rio Grande. Spawning by members of this reproductive guild is associated with high-flow events such as spring runoff or summer rainstorms. Upon release, eggs are about 1.6 mm in diameter but quickly swell (ca. 3.0 mm) and remain suspended in the water column during development. The last remaining member of this reproductive guild in the Rio Grande, NM is the federally endangered Rio Grande silvery minnow (Hybognathus amarus). Population monitoring studies have shown an annual decline in the number and catch rate of Rio Grande silvery minnow since 1996. Currently over 90% of the catch of Rio Grande silvery minnow is in the San Acacia Reach of the Middle Rio Grande.
Sampling for Rio Grande silvery minnow eggs was initiated 1 May 2001 and concluded 19 June 2001. The location of the collecting activities was selected so as to maximize the potential number of eggs collected and rescue eggs destined to drift into Elephant Butte Reservoir where, if hatched, larvae would be subjected to a wide array of nonnative predators. Eggs were collected three times per day (ca. 0800 hr, 1200 hr, and 1800 hr) during two-hour sets. At the end of the sample period, plastic bags containing Rio Grande silvery minnow eggs were filled with oxygen, sealed, and transported by personnel from U.S. Fish and Wildlife Service to the BioPark-Albuquerque Aquarium (BioPark).
A total of 52,853 m3 of water was sampled during the 50 day duration of this project yielding 89,542 Rio Grande silvery minnow eggs. Mean daily catch rates on dates that eggs were collected ranged from 0.08 to 2,288.68 eggs/100 m3 of water sampled. The principal spawn of Rio Grande silvery minnow, as determined by egg catch rate, occurred on 8-10 May 2001. During these three days, 88,071(98.4%) eggs were collected at a cumulative rate of 517.9 eggs/100 m3 of water sampled. Mean daily catch rate on 8 May 2001 was 2,288.7 eggs/100 m3 of water sampled but declined to 510.4 on 9 May and 176.8 on 10 May. By 11 May 2001, there were few eggs still being captured (n=55) and catch rates were only 3.4 eggs/100 m3 of water sampled. A second Rio Grande silvery minnow spawning event occurred from 17-20 May 2001. Catch rate on both 18 and 20 May 2001 was 17.5 eggs/100 m3 of water sampled but cumulatively yielded only 301 eggs. A small number of eggs continued to be collected after 20 May 2001 through the remainder of the month (n=289) with catch rates generally remaining below 10 eggs/100 m3 of water sampled (range 0.08 - 11.78 eggs /100 m3 of water sampled; mean 2.7 eggs/100 m3 of water sampled). Collecting efforts in June yielded a total of 83 Rio Grande silvery minnow eggs, all of which were taken on 13 June 2001.
Reducing the rate of downstream transport, allowing upstream passage, and salvaging eggs destined for Elephant Butte Reservoir are all options that will, to some degree, improve the current status of Rio Grande silvery minnow. While the most simple solution would appear to be collecting eggs from downstream localities and transporting them to rearing facilities, this method has only short term significance. The ability to efficiently sample even 1% of the entire volume of water that carries these reproductive propagules downstream would require a monumental effort. Future efforts should focus on reducing the deleterious effects that changes in river connectivity, flow patterns, and habitat heterogeneity have on the downstream displacement of Rio Grande silvery minnow eggs and larvae. Eliminating diversion structures would allow upstream passage of individuals to reaches from which they were displaced. Efforts to improve degraded riverine habitats could include returning the flow regime to a more historical pattern (i.e., allowing passage of large flow events) and removing or relocating structures that inhibit the lateral movement of the Rio Grande (e.g., jetty-jacks, levees, and water conveyance ditches). The long-term recovery of Rio Grande silvery minnow will depend on taking management actions that attempt to restore the natural processes of this river.