| Popis: |
Several studies have attempted to compare subtidal animal population estimates obtained in a variety of ways using SCUBA diving and have reported a lot of variation between the estimates obtained. This study investigated individually scale-, tidal-, equipment- and observer-induced variation through analysis of animal population density indices obtained using a number of techniques based on SCUBA diver visual survey. The study was based on shallow subtidal sites on the west coast of Scotland including natural rocky slopes and recently deployed artificial reef sites. Groups of species investigated varied in levels of size, abundance, mobility and detectability. Variability in estimated densities between different sizes of sampling unit, state of tide, type of SCUBA equipment used (open circuit versus rebreather) and observation technique was found to be considerable. A decreasing asymptotic relationship with quadrat size was evident in the majority of cases for estimated density, coefficient of variation and number of samples required to attain a given precision. However, the size of the sampling unit employed had to be large enough to encompass the variability in spatial distribution of the target organism. Improved precision for estimates of cryptic species was obtained from relatively small sampling units, which focused the observer’s attention onto a specific area. Underwater photography as a population estimation technique produced data comparable with that from visual surveys but was limited by the size of sampling unit. The use of remotely deployed infra-red video demonstrated that certain animal species and life-stages actively avoid divers and were largely absent from the diverbased surveys. Although larger quadrats generated more precise estimates, the longer sampling times involved reduced their overall efficiency for most groups surveyed; optimum sampling unit sizes are suggested for particular animal groups. |
