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.2003年春季，加利福尼亚鱼类与狩猎局（California Department of Fish and Game, CDFG）工作人员启动索尔顿湖（Salton Sea）周边14个站点的季度鱼类采样工作，作为长期监测计划的基础。为便于该局当前及未来的监测工作与历史数据对比，本次采样方案改编自此前索尔顿湖研究人员使用的作业规程。若现场条件允许，每季度可完成约816网时（net-hours）的采样作业。截至2008年，该项目自2003年春季启动以来持续按季度开展数据采集，目前仍在进行中。其中2007年秋季与冬季两个季度因采样作业场地无法使用而暂停采样。*注：本数据集需结合「索尔顿湖季度水质调查[ds429]」数据集一同使用。 采样方法：本次布设的11个站点涵盖三大类生境类型：敞水带（pelagic，3个站点）、近岸带（near-shore，8个站点）与河口带（estuarine，3个站点）。敞水带站点分别位于索尔顿湖北盆地、南盆地及盆间区域的大致中心位置。近岸带站点沿东西两岸各布设4个，分布间距较大，以尽可能覆盖更广泛的生境范围。河口带站点则紧邻纽河（New River）、阿拉莫河（Alamo River）与怀特沃特河（Whitewater River）的河口区域，受河流径流影响显著。 采样覆盖所有推定季节，具体时间安排为：春季（4月至5月）、夏季（7月至8月）、秋季（10月至11月）、冬季（1月至2月）。采样团队尽可能将总采样周期压缩至最少天数，具体执行以天气状况、设备维护需求及人员排班约束为准。通常于晨间在1至2个站点布设网具，约24小时后起网。每一张网具的布设时长精确至1/4小时，并单独记录具体时长。 鱼类采样采用多面板单丝刺网（multi-panel monofilament gill nets），网片规格为6×30英尺，网目尺寸涵盖0.5、1、2、3及4英寸。所有站点的表层水体均布设2张网具，网具间距需足够大以保障布设与回收作业时船只的操控空间，通常为100至200米。近岸带与河口带站点的网具布设于水深2.5至4.5米的水域，通常离岸200至300米。此外，3个敞水带站点的底层水体额外布设2张网具。深水底栖生境与表层水体在溶解氧、光照、食物资源及水温等核心环境参数上存在显著差异，可视为独立生境，因此将其作为单独站点开展采样。 在每次网具布设与回收作业时，均需测定并记录水深、水温、电导率、盐度及溶解氧含量。次日起网后，立即将所有渔获物移走并置于冰上保存。采样数据会尽快完成整理，通常在起网当日完成。所有鱼类均需鉴定至物种水平并计数。针对索尔顿湖内的4种游钓鱼类——罗非鱼（tilapia）、海湾石首鱼（Gulf croaker）、橙口鳘（orangemouth corvina）与sargo，需记录其体重、叉长（fork length）、性别、生理状态与繁殖状况。体重测量方面，5磅以上的鱼类精确至盎司，5磅以下的精确至半盎司。体长测量方面，50厘米以下的鱼类精确至毫米，50厘米以上的精确至厘米。成鱼的性别通过解剖确定。每个物种至少选取10尾样本进行解剖，以确定其生理状态与繁殖状态。 首年度采样后的方案变更：此前的研究认为深水生境对渔业具有一定生产力，是重要的采样生境组成部分。但在首个采样年度中，3个深水站点（北盆地、南盆地及盆间区域）完全无渔获，不仅造成了采样资源的浪费，且结合2003年发现的鱼类种群规模大幅缩减的情况，这些站点成为鱼类利用可能性最低的区域。因此采样团队取消了3个深水站点的采样作业，将季度总采样努力量减少288网时，降至季度总528网时。不过这些站点仍保留在采样方案中，因为若未来渔业种群恢复至可开展稳健对比研究的水平，这些站点的数据将有助于了解种群趋势与生境利用情况。 脚注：2004年10月13日首次采样后，当地天气发生显著变化，导致水体温度下降6℃。鉴于当日采样的渔获量与后续站点的结果偏差极大，采样团队认为此次水温下降有必要对2004年10月13日的2个站点进行补采。补采结果被纳入总数据以计算单位捕捞努力量渔获量（CPUE）。我们最初参考1999-2000年Costa-Pierce与Riedel在索尔顿湖的研究惯例，选取10月至11月作为秋季采样时段，该方案假设不同采样季节间水温存在生物学意义上的显著变化。但目前我们对索尔顿湖的季节划分是否严格符合对应公历月份、且各季节界限分明的假设愈发存疑。正如2004年秋季采样期间所观察到的，单次采样周期内的水温变化幅度可能等于甚至超过不同采样周期间的水温变化。