Under all combinations of variables tested, there was a significant positive relationship between water current and tilt angle, although the strength and nature of this relationship varied between the type of receiver used. Current speed appeared to have lesser influence on tilt for VR2AR compared to VR4 receivers. This could be due to a number of factors such as the size, shape or weight of the receiver, with the larger surface area of the VR4 resulting in currents pushing the receiver to greater angles than VR2AR receivers. Despite the nuances identified here, the results suggest that receiver tilt may present a viable proxy for current speed in acoustic telemetry studies, in the absence of dedicated current meters. While VR4 receivers may act as a useful proxy for current flow, the greater flexibility in programming and smaller size of the VR2AR may make them better suited. The increased logging frequency afforded by the VR2AR will have a negligible impact on the battery life of the receiver.
There were some broad differences among the three VR2AR receivers in terms of the slope of the relationship, how well the regression explained the data, and the equivalency of the two variables. Specifically, VR2AR-1 appeared to display a better relationship between the two variables and showed an equivalent change in tilt in response to current speed. The two other VR2ARs tended to over-estimate current speed at lower tilt angles (similar to VR4 receivers). It is difficult to establish why this occurred, but these two receiver stations were further away from the current meter than VR2AR-1, and there were also artificial reef modules between these receivers and the meter. The modules may have somehow disrupted currents and potentially funnelled water through at greater speeds over VR2AR-2 and -3, causing higher tilt values for the same recorded speeds by the meter. These patterns suggest that differences in micro-current characteristics over short distances may be occurring, so it will be important to consider the potential influence of this over the detection range of the receiver, particularly where reef structure may be present. Furthermore, if receivers are deployed within a river or estuary, where currents can vary significantly across small spatial scales, particular care must be taken when interpreting data outputs and how these may have been influence by the position of the equipment.
When the VR2AR data were analysed at the hourly scale, there was elevated variability observed in the relationship between current speed and tilt angle. This highlights the impact of aggregating data at different scales. In the example data presented here, there was considerable variance within each hourly bin, and maximum speeds often exceed 15 cm s−1. While the average speed over an hour might be relatively low, the instantaneous recording of the tilt angle may have occurred during a short peak in current speed, thereby increasing the variability within the relationship. We cannot speculate whether such instantaneous peaks in current speed are biologically significant, but averaging tilt over longer time periods to provide a better aggregate of conditions over relevant temporal windows may provide better current estimates.
Given the broad incorporation of environmental parameters in analyses of acoustic telemetry data [12, 21, 22], there are obvious benefits for collecting multiple environmental variables to model the influence of abiotic factors on fish movement and behaviour. Despite flow being a common variable included in freshwater studies [e.g. 23–25], current data have not been widely considered in marine acoustic telemetry projects. Combining patterns in relative current activity (e.g. through a proxy such as tilt angle) with fine-scale measurements of animal activity (such as through the use of activity tags) may support novel insights into the influence of water currents on fish position, habitat, movement and behaviour, particularly in open coastal systems. Positioning systems provide some of the best fine-scale data on these variables; however, it is the quality of this fine-scale data that are most likely to suffer under “high current” conditions. Episodic events such as storms may lead to higher tilt angles due to water turbulence, which impacts tag detection rates [26, 27]. When interpreting detection patterns in relation to tilt angle as a proxy for water currents, it is important to take these effects into consideration. Although, the same caution would also need to be applied when a co-located current meter is deployed. However, the hourly detection counts of the reference tag on the VR2AR receivers indicated no decrease in rate at higher tilt angles and, therefore, current flow. Though it may have some effect, future studies can reasonably conclude that fewer detections during periods of higher tilt angles is not an simply artefact of reduced detection capacity of the receiver. Although not measured in this study, biofouling is also likely to alter the relationship between tilt angle and current flow through time, where growth on the receiver could increase its weight, surface area or both, the influence of biofouling on detection efficiency has previously been demonstrated [28], but anti-fouling coatings are effective in slowing the accumulation of biofouling.
While a relationship between receiver tilt angle and current flow was found to exist, the design of the mooring will likely influence the specific nature of this relationship. For example, changing the height and buoyancy of the float will alter the amount of current required to tilt a receiver. Therefore, some consideration prior to deployment should be given to the hydrological conditions within which the receiver will be place, and the mooring adjusted accordingly. The approach described will allow for relative current to be measured; however if more precise data are required, calibrating the gear prior to deployment would be required. This could be achieved a number of ways including placing the receiver and mooring in a location subject to variable flow, such as an estuary or river, with a co-located current meter and measure tilt angle and current under varying conditions, or run a pilot programme in situ with a current meter.