Background: Advances in ultrasonic telemetry, including the ability to accurately position a transmitter within an array of hydrophone receivers, have led to increased opportunities to investigate a broad spectrum of ecological questions in aquatic systems. The quality and efficiency of positioning a transmitter relies upon factors controlled by the researcher (for example, geometry of the receiver array) as well as environmental conditions (for example, water quality or environmental noise). While the physics of sound wave propagation are well understood, the high amount of environmental variability in and among aquatic habitats makes it difficult to predict exactly how any given ultrasonic signal will behave. To evaluate variability in system performance across different receiver arrays in diverse environments we present positional records for fixed-location tags recorded with a popular positioning array, the VEMCO Positioning System (VPS). Using these records we evaluate the relationships between system performance, measured as both horizontal positioning error and positioning efficiency, and user-controlled and environmental variables. We used generalized linear mixed models to assess performance at a coastal site, a site in a freshwater tidal estuary, and a riverine site. Results: The positioning errors were similar across sites, with median errors ranging from 1.6 to 3.3 m. In contrast, there was large variation in positioning efficiency across sites, with poor positioning efficiency in the coastal habitat (7%), possibly due to high levels of bioacoustic noise, and moderate efficiency in the river (21%) and estuary habitats (27%). Our statistical models indicate that array geometry was consistently the most important predictor of positioning performance. Environmental noise and water movement also emerged as additional predictors of performance at several sites. Conclusions: The results provide insight into VPS performance capabilities and emphasize the importance of testing array geometries. Additionally, water quality parameters should be monitored and receiver mooring designs should be carefully considered before embarking upon a telemetry study. We hope this work will guide future researchers in creating more effective designs for positioning arrays, and facilitate the collection of high quality information about movement and behavior patterns of aquatic organisms.
Densities of abalone in southern California have been dramatically reduced by overfishing and disease, leading to the collapse of some populations, and low fertilization rates may be hindering population recovery. This is a pattern typical of abalone species globally. However, movement may produce clustered distributions that promote fertilization success in broadcast spawners, such as abalone, even at low regional densities. We translocated wild, adult pink abalone Haliotis corrugata to an existing pink abalone patch to create a high-density aggregation, and then used acoustic telemetry to characterize abalone movement and monitor aggregation maintenance for a period of 14 mo in the Point Loma kelp forest near San Diego, California. Abalone showed a minimal flight response to handling that did not differ between the resident or translocated groups. Most individuals exhibited small home ranges (median area 183 m 2 ) and homing behavior consisting of regular back-and-forth movement to a single point. Nomadic movement was also observed in several individuals. Though site fidelity may help maintain aggregations, abalone density decreased, and nearest neighbor distances increased at our site to nearinitial levels after 18 mo via a combination of mortality, large movements of a few individuals, and small, incremental movements of most individuals. No coordinated movements that would suggest spawning behavior were observed. Translocation of wild abalone to produce aggregations may not result in high rates of fertilization success that promote population recovery. However, the homing behavior observed may provide more opportunities for mating than expectations based on static measures of density and aggregation state.
Quantitative biological assessment indices overcome many of the challenges faced when trying to convey the status or trends of complex biological communities with large natural variability, particularly when attempting to evaluate the impacts from human influences. In this paper, we developed a biological condition index for shallow (<30 m) rocky reefs of the Southern California Bight, evaluated its ability to distinguish healthy from stressed sites, and then applied the index by examining relative correlations with fishing and water quality as ecosystem stressors. We utilized a multivariate, predictive index based on the ratio of observed‐to‐expected taxa (O/E). O/E indices are relatively common in freshwater environments, but rarely utilized in marine systems and never before applied to rocky reefs. Based on expectations drawn from region‐wide reference reefs with the least fishing or water quality stress, the O/E index predicts expected taxa at a new site based on environmental factors such as sea surface temperature, reef area, and slope, among others. The observed taxa at that site are then compared to the predicted taxa to generate index scores; values near unity indicate intact, reference‐like communities. Overall, the accuracy of the index was high, with minimal bias, and precision exceeded the performance of an index based on null models (i.e., indices that did not account for natural gradients). The mean index score was significantly higher among reference sites than stressed sites; however, sensitivity was low, as 84% of stressed sites had scores within the range of reference sites. Ultimately, fishing pressure was more correlated with changes in index scores from the non‐reference data set than was water quality pressure. This study demonstrates that a multivariate predictive index is feasible in rocky reef assessment and illuminates additional investigative work to continue to advance index development.
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