Glenn F. Black
MATERIALS AND METHODS
Fish populations were sampled with minnow traps made of 0.25-cm wire mesh. Traps were 41 cm long, 23 cm in diameter, and had a 3.8-cm opening at each end of the trap. Traps were baited with canned cat food that had several holes punched in the top of the can and set for approximately 24 h before they were pulled from the water and checked for catch.
3/More recent sampling has produced what appears to be another species of African cichlid, possibly S. aurea, in the Salton Sea. Identification is pending.
The number of trap sites selected within each irrigation drain, etc., was determined by the diversity in physical habitat types encountered within that specific area. For example, within some irrigation drains only two sites were selected for sampling because there was little habitat diversity (substrates, water depth, water clarity) and types of aquatic vegetation present), while another drain was sampled at 10 sites due to the variety of habitat types found (Appendix 1). A total of 65 minnow traps were set in 18 irrigation drains, 8 traps placed in 2 shoreline pools, 14 traps set in 3 permanent tributaries to the Salton Sea, and 19 traps placed in 13 areas of the Salton Sea proper. Sampling effort was maintained at these levels in each of the quarterly surveys.
The data recorded at each trap site included the following: (1) specific location, (2) date and time trap was set and pulled, (3) water depth, (4) number of various fish species captured, and (5) observations of substrate type and aquatic vegetation.
Dissolved oxygen, temperature, water clarity, and salinity were measured as time permitted at selected sites in irrigation drains, shoreline pools, and tributaries. Dissolved oxygen was measured in parts per million with a YSI (model 5lA) meter, water clarity was measured in JTU's with a Hach turbidity meter, and salinity was measured in parts per thousand with a Myron L-DS meter.
The same materials and methods were used in the two surveys of San Felipe Creek except that water quality measurements were not taken. However, in the first survey 18 minnow traps were set along a 6.4-km section of the Creek and in the second survey 8 traps were set along a 3.3-km section of the Creek not sampled in the first survey.
Agricultural irrigation drains were sampled on both the northern and southern ends of the Salton Sea (Figure 2). It was felt these drains would contain representative fish populations. They varied in length from 55 to 4,000 m and in width from 1 to 5 m (Appendix 1). Water depth, clarity, salinity, and dissolved oxygen varied greatly between and within individual drains (Appendices 1 and 2).
Shoreline pools sampled during the quarterly surveys were located on the southern and eastern portions of the Salton Sea (Figure 2). These pools were areas of standing Salton Sea water partially separated from the Salton Sea proper by bars of either barnacles, or sand built by wave action. They were irregular in shape, ranging from 25 to 1,000 m in length, 5 to 50 m in width, and 5 to 61 cm in depth (Appendix 3). Because they were partly isolated from the main body of the Salton Sea and relatively shallow, the salinity of these pools sometimes was two to three times higher than the Sea proper (Barlow 1958b). Daily water temperatures could fluctuate as much as 15 to 20ūC in shallow pools during the summer (Barlow 1958a) and reach as high as 43ūC (pers. observ.), the reported critical thermal maximum for desert pupfish (Low and Heath 1969). The shoreline pools lacked rooted aquatic vegetation, but algae were present. Terrestrial vascular plants were also present, having been inundated by the rising level, of the Salton Sea. The substrate of the shoreline pools consisted of silt and decaying organic matter. Five shoreline poo1s were sampled during the spring survey, however, water evaporation during the summer and fall resulted in the temporary loss of three shoreline pools.
Figure 2. Areas sampled during desert pupfish surveys
The permanent tributaries sampled in the quarterly surveys were located in the northern, northeastern, and eastern portions of the Salton Sea (Figure 2). With the exception of Whitefield Creek, only a small portion near the mouths was sampled. These tributaries varied in relation to water depth, clarity, and salinity (Appendix 2). The amount of surface flow, type of substrate, and kinds of aquatic vegetation varied between tributaries. Whitefield Creek and Salt Creek are natural drainages with natural flows while the Whitewater River consists primarily of irrigation runoff in the areas sampled (Figure 2).
San Felipe Creek, the intermittent tributary sampled in two separate surveys, is located at the southwestern end of the Salton Sea (Figure 2). Springs provide a year round flow for the first 7.2 kin, but the remainder of the Creek (8 km) is intermittent, connecting to the Salton Sea only during and after periods of heavy rainfall. The normal surface flow is minimal and the total dissolved solids range from 8,000 to 10,000 ppm (Brown 1923). The substrate varies from sand-silt to quicksand.
The areas of the Salton Sea proper sampled in the quarterly surveys were at the northern and northeastern portions of the Salton Sea (Figure 2). Traps were set near the mouths of the irrigation drains and off the sand and barnacle bars associated with the shoreline pools. The Salton Sea lacked rooted aquatic plants but terrestrial vascular plants were present in the nearshore areas due to inundation from the rising level of the Salton Sea. The substrate in these areas was either densely packed barnacles, or soft mud. The salinity was that of Salton Sea water which varied, depending upon the time of year, from 36 to 38.