Quarterly Fishery Surveys - Salton Sea [ds428]

In the spring of 2003, California Department of Fish and Game (CDFG) personnel began quarterly sampling of Salton Sea fish at fourteen stations around the sea, as the basis of a long term monitoring program. To allow comparison of current and future monitoring efforts by CDFG to past results, the protocol was adapted from those previously used by researchers at the Salton Sea. Each quarter, if conditions allow, this protocol will produce about 816 net-hours of sampling. To date data collection was started in the spring of 2003, continuing quarterly. Data collection is ongoing as of 2008. Two seasons were missed due to unavailability of launch sites: Fall 2007 and Winter 2007. *Note: This dataset should be viewed with the 'Quarterly Water Quality Surveys - Salton Sea [ds429]' dataset. Methods: The 11 sampling sites comprise three broad habitat types: pelagic (3 sites), near-shore (8 sites), and estuarine (3 sites). The pelagic sites are in the approximate middles of the north basin, south basin and inter-basin areas of the Sea. The near-shore sites are spaced widely apart, four each, near the west and east shores, to capture as much breadth of habitat as possible. The estuarine sites are in the body of the Sea, close enough to the mouths of the New, Alamo, and Whitewater Rivers, to be under the influence of their outflows. Sampling takes place during each of the putative seasons, as follows: spring- April and May; summer- July and August; fall- October and November; winter- January and February. We attempt to compress the total sampling period into as few days as possible, to the extent that the weather, equipment maintenance, and personnel scheduling constraints allow. Nets are typically set at one or two sites in the morning, and hauled in after approximately 24 hours. The exact number of hours set is recorded for each net, to the nearest quarter-hour. Fish are sampled by deploying multi-panel monofilament gill nets with 6 X 30 foot panels of 0.5, 1, 2, 3, and 4 inch mesh. Two nets are set at all sites at the water's surface. The nets are set far enough apart to allow room for maneuvering a boat during setting and retrieval, usually 100-200 meters. The nets at near-shore and estuarine sites are set in 2.5 to 4.5 meters of water, typically 200-300 meters from the shore. Two additional nets are set at the bottom of water column at the three pelagic sites. The conditions fish experience at the bottom in deep water is different enough from the surface water, in dissolved oxygen, light, food availability and temperature, that this can be considered a discrete habitat, and thus we sample it as though it were a separate site. At the time of each set and retrieval, water depth, water temperature, conductivity, salinity, and dissolved oxygen are measured and recorded. When nets are pulled in the following day, all fish are removed and immediately stored on ice. Data are collected from these fish as soon as possible, almost always the same day they are hauled in. All fish are identified to species level and counted. For the four sport fish in the Salton Sea, (tilapia, Gulf croaker, orangemouth corvina and sargo) weights, lengths (fork length), sex, physical condition, and reproductive status are recorded. Fish above five pounds are weighed to the nearest ounce. Fish below five pounds are weighed to the nearest half ounce. Lengths of fish under 50 centimeters are recorded to the nearest millimeter. Lengths of fish over 50 centimeters are recorded to the nearest centimeter. The sex of adult fish is determined by dissection. A sample of at least ten fish of each species is also dissected to determine physical condition and breeding status. Changes to Protocol after Year One: For previous researchers, deep water habitats provided some low level of productivity for the fisheries, and were important habitat components to sample. During the first year of sampling, however, the three deep water sites (north basin, south basin and inter-basin) were completely unproductive, a costly element of our efforts, and the least probable site for fish use, given the severe reduction in population size which was discovered in 2003. We therefore eliminated sampling at the three deep water sites, which reduced our efforts by 288 net-hours, to a quarterly total effort of 528 net-hours. These sites were left in the protocol, since they will likely provide useful information about population trends and habitat use, should the fisheries rebound to levels which allow robust comparisons among these and the other sampling sites. Footnote: There was a marked change in the weather immediately after our first sampling run on October 13, 2004 resulting in a 6º C drop in water temperature. After seeing how far the number of fish sampled on October 13, 2004 was out of the range of results at subsequent sites, we felt this drop in water temperatures justified resampling the two October 13, 2004 sites. The results were added to the total to compute a CPUE. We originally selected the months of October and November to enclose a putative Fall season, following the convention of Costa-Pierce and Riedel’s study at the Salton Sea in 1999-2000. This approach assumes a biologically significant change in water temperatures between sampling seasons. We have become more hesitant about assuming that the seasons at the Salton Sea fall within their assigned calendar months, and are so clearly discrete. As we saw during this Fall 2004 sampling period, water temperature changes within a sampling period may be as great or greater than those between sampling periods.