Tests here suggest that shrouded receivers can be an effective tool for improved edge detection in acoustic telemetry studies. Only transmitters on the unshrouded side of the receiver could be detected reliably at a >50 % detection rate [recommended by VEMCO, 15] or even a much more generous detection rate of 15 %. It is recognized that interpretation of detections and selection of appropriate cut-off values will depend on the particular setting, research questions, and behavior of the organism of interest and may differ from the 20 % threshold estimated here [1, 16]. This will require a customized set of decision rules to be established to determine the probability that an organism is on one side of a boundary or the other (e.g., ).
The direction of shroud placement, listening into versus out of an MPA, and other receiver placements will depend on the objectives of a particular study. It may be better to more conclusively detect when fish leave an MPA and are exposed to fishing than knowing that they remain safe in a given protected area. Additional omni-directional receivers could be deployed inside and outside the MPA boundary to provide a more complete understanding of directional movements (Fig. 3). This would also help with interpretation of low detection rates that could either be due to fish being present close by the shrouded side of the hemi-directional receiver or far away but on the unshrouded side. Although it doubles the number of receivers needed, for some applications it may also be useful to place two hemi-directional receivers on a single mooring but facing in opposite directions to determine on which side of a boundary line that an organism is located. The approach is not only useful for MPA boundary evaluation. For example, shrouded receivers can be set along linear habitat boundaries such as hardbottom/softbottom or reef/seagrass interfaces to more conclusively detect which habitat a tagged fish is utilizing. The approach may also be useful in other constrained settings where it is not desirable to detect fish presence throughout the entire circumference of an omni-directional receivers’ detection range. More detailed monitoring of arrival or departure from small landscape features such as artificial reefs, spawning sites, or boat channels may also be enhanced with this technique.
It is important that shrouds be consistently shaped and snug against the transducer. Any deviations in shroud geometry should be well under the wavelength of the transmitters (69 kHz in this case, or ~21.7 cm). This will avoid irregularities in constructive or destructive interference with incoming signals on the unshrouded side of the receiver (VEMCO pers. com.). Depending on the material used to construct the shroud, an acoustically absorbent coating or randomized scattering texture may also be useful to minimize any irregular lobes or nulls in directional sensitivity.
Of course the shroud is not suitable for all applications. The approach doesn’t yield position estimates, it merely provides hemispheric presence/absence for a better estimate of which side of a receiver an organism may be positioned. For detailed location and habitat utilization information, other systems are required [7–9]. Those existing approaches enable fine-scale tracking of fish position to within a few meters by deploying many receivers in high density with overlapping detection range. However, this reduces spatial coverage of a study since so many receivers must be placed in a confined area, yields much extraneous information (i.e., constant position) for applications where only boundary-crossing data is of interest, and can be cost prohibitive due to computational requirements and the large number and density of receivers that are needed.
Our analyses represent a composite value of performance for 11 receivers in a real landscape. The natural variability in landscape features present in the study area contributed to the variance in our results (Fig. 3). Two range-test sites in the unshrouded NW quadrant of Fig. 5a were within the detection range of receivers (160 and 156 m, respectively) but did not record any transmissions. This side of the shroud was of course identical to the other side that did have detections at similar angles and distances. We therefore looked beyond the shroud, at habitats surrounding each test location for an explanation. One of those test sites lacking detections was the shallowest and most complex reef setting in our study and the other had patch reefs nearby but unfortunately no detailed habitat information is available between the receiver and range-test site to further diagnose potential landscape interference at the second site. Therefore, although we displayed maximum detection range as a single, composite value of ~180 m due to the relatively homogeneous environment in which most receivers were deployed, it should be recognized that detection range on the unshrouded side of individual receivers will certainly vary depending on their particular setting in the landscape [4, 13, 15, 19].
Our experiments were encouraging, that the shroud blocks most signal detection from one hemisphere, but do not replace the need to conduct robust range tests at each receiver site [15, 16]. In addition, use of sentinel tags is also advisable to evaluate the long-term influence of variations in environmental noise on detection rate in most settings. Further tests are also needed to: (1) evaluate shroud performance on tagged fish now that controlled field tests have yielded positive results, (2) test the approach on data loggers available from other manufacturers, and (3) refine and evaluate other shroud materials, coatings, and designs.