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The Problem
Populations of the desert pupfish (Cyprinodon macularius) have declined dramatically over the past few decades, culminating in its current status as an endangered species. Desert pupfish originally inhabited the lower Colorado and Gila rivers in Arizona and Baja California, the Salton Sea and its tributaries in California, and the Sonoyta River in northern Sonora, Mexico. Today, the last remaining natural habitats of this species include San Sebastian Marsh and San Felipe Creek on the southwestern edge of the Salton Sea, and Salt Creek on the northern shore. Transplanted stocks of desert pupfish also inhabit ponds at Palm Canyon, the Anza-Borrego Desert State Park, the Living Desert Museum in Palm Desert, the Coachella Valley Preserve, and Oasis Springs near the Salton Sea. A recent field survey by the California Department of Fish and Game also uncovered many heretofore undocumented populations of desert pupfish in irrigation canals and drains flowing to the Salton Sea. The decline of desert pupfish populations has been attributed to loss of habitat and unfavorable changes in environmental conditions caused mostly by construction and operation of dams on major rivers, capping of springs, and flooding of springs and marshes during formation of the Salton Sea. The introduction of western mosquitofish (Gambusia affinis), sailfin molly (Poecilia latipinna), and tilapia (Tilapia spp.)--which possibly compete for food and space, and prey on eggs and juveniles--may have further depleted natural populations of pupfish. Elevated concentrations of selenium and agricultural pesticide residues occur in water, sediment, and aquatic biota from these canals and drains, and could adversely affect reproductive success and survival of pupfish, and potentiate the effects of competition and predation.
Objectives The 5-year research goals of the Fisheries and Aquatic Resources Program are to understand relationships between and among aquatic species habitats and provide science for restoring and maintaining declining species and their required habitat. This study will compare relative abundance, population structure, and spawning success of desert pupfish from natural (reference) and manmade or disturbed habitats in the Salton Sea basin. In addition, this study will determine the importance of fish predation as a source of mortality in desert pupfish populations. Finally, this study will attempt to relate abundance, population structure, and spawning success of desert pupfish to physicochemical conditions and to abundance of other fishes (potential predators and competitors). Methodology 1. Sampling sites. Sampling sites were established in one natural (reference) and four manmade habitats within the Salton Sea Basin. 2. Sampling times. Sampling trips (5-7 days in duration) were scheduled at seasonal (once every three months) intervals. In addition, one preliminary trip occurred prior to initiation of the study to finalize the selection of sampling sites and evaluate the suitability of various gear for collecting fish samples. 3. Fish collections. At all sampling sites, fish were collected with baited and unbaited minnow traps. Relative abundance of all fishes was determined from catch-per-effort. Immediately upon collection, desert pupfish was hand-sorted from the catch and retained in buckets of water for
With some exceptions, other fish were identified, counted, and promptly released. Representative samples of cohabiting fishes were preserved in 10% formalin for analysis of gut contents to determine if they consumed desert pupfish. The gravimetric method was used to quantify gut contents. When unidentifiable fish species were encountered, voucher samples were preserved in 10% formalin for later identification with taxonomic keys. 4. Environmental measurements. At each sampling site, the following physicochemical variables were measured in-situ as grab samples with a Hydrolab Datasonde 4 multiprobe logger immediately prior to each seasonal collection of fish: water depth, water temperature, dissolved oxygen, pH, specific conductance, and turbidity. Standard methods were used to verify measurements made with the Hydrolab Datasonde 4 multiprobe loggers. In addition, grab samples
5. Statistical analyses. Raw data were stored as Microsoft Excel files on a microcomputer, and will be summarized with SAS, SAS Graph, and other appropriate statistical and graphical software. The level of significance for rejecting null hypotheses of statistical tests is 0.05. Highlights and Key Findings During August 1999-September 2000, several field sampling trips were made to Avenue 81 Drain, Avenue 84 Drain, County Line Drain, and Salt Creek (a natural tributary flowing into the Salton Sea). Judging from minnow trap catches, the most numerous species consisted of western mosquitofish, porthole livebearer (Poeciliopsis gracilis), sailfin molly (Poecilia latipinna), shortfin molly (Poecilia mexicana), redbelly tilapia (Tilapia zilli), and mozambique tilapia (Sarotherodon mossambica). Other common species included longjaw mudsuckers (Gillichthys mirabilis) and desert pupfish. Preliminary analysis of minnow-trap data indicated that fish species assemblages did not differ between upstream and downstream sites on the drains and Salt Creek. Fieldwork resumed in April 2001 and continued at monthly intervals until December 2001. Preliminary results indicated that desert pupfish were more abundant in Salt Creek (a natural waterway) than in several agricultural drains discharging into the Salton Sea. During 2000, 159 pupfish were caught in Salt Creek but only 59 pupfish were caught in agricultural drains. Moreover, pupfish populations were dominated by juveniles in Salt Creek (20 adults and 138 juveniles were captured) whereas, in the drains, adults and juveniles occurred in roughly equal proportions (26 adults and 33 juveniles were captured). During 2001, however, only 38 pupfish were caught in Salt Creek whereas 62 pupfish were caught in three drains. The relatively low numbers of pupfish captured in Salt Creek during 2001 coincided with a decrease in stream flow that was most pronounced in late summer (along with near anoxic conditions and very high salinity). According to multiple regression analysis, pupfish abundance (measured as catch per unit effort) in the study area was positively associated with abundance of mollies, water temperature, percent cover, and discharge, and negatively associated with current velocity and sediment particle
Where Are We Headed In 2003 Field work on this study has been completed. During FY-03, gut contents of cohabiting fish species (but not pupfish) will be analyzed, and all data will be entered into computer spreadsheets. The data will be subjected to statistical analysis and interpretation. We will also prepare a draft final report. Project Contact Michael Saiki Email: michael_saiki@usgs.gov Publications |
