Stratification and detection range
Through modeling the proportion of transmissions received per day at four different distances, ambient ultrasonic noise and temperature stratification emerged as the dominant variables controlling detectability at a given distance. Within this shelf environment, representative of the larger US Mid-Atlantic Bight, temperature stratification arose as more influential than noise, and the best model selected both effects and their interaction. The Mid-Atlantic Bight is typified by strong seasonality in temperature stratification and wind-driven noise events, and daily detection efficiencies were driven by their joint dynamics.
The best model predictions were within 19% of observed values, resulting in a window of 30–70% detectability when the true frequency of detection is 50%. Although the model closely explained detectability during Cold-Pool-absent periods, the most unexplained variance occurred during Cold-Pool-present periods, suggesting that the interaction between noise and temperature stratification is unstable or that further terms could explain detection probability during times of temperature stratification. Detectability > 90% at 800 m was not infrequent during Cold-Pool-present periods, occurring in 3.4 and 33.5% of days in the nearshore and mid-shelf sites, respectively. As a result, the model is likely unable to accurately predict detecatbility at greater distances -- an effect of this can be seen in the model-imposed upper limit of D50 near ~ 1100 m during the period of Cold Pool presence. An improved design would include test distances centered on either side of the expected D50, as well as one beyond the range limit [24].
Temperature stratification has been reported to decrease [2, 25–27] or cause little-to-no change [23, 28, 29] in the detectability of acoustic telemetry transmissions; however, thermoclines on the SMAB shelf are much stronger and shallower than those reported in other detection range studies that incorporated the effect of temperature stratification (Table 2). In an analysis of detection probability conducted in the same area of the SMAB, Oliver et al. [29] found density stratification to have a significant negative effect on detection distance. However, this played a minor role in detectability as compared to other variables related to noise generation and stratification was only present for 24% of their study period, leading the authors to suggest that further exploration is needed. Klinard et al. [2], who used the ΔT index of stratification utilized in the present study, reported that temperature stratification played a minimal-to-negative role in detection distance within their array. The authors note that the array was not deployed during high-stratification months and that further investigation could show otherwise.
Table 2
Reported influence of a thermocline on detectability. Gradient, depth, and respective placement of transmitters and receivers across the thermocline were reported or calculated from reference figures.
| | Thermocline |
Reference | System | Gradient (°C/m) | Depth (m) | Transmitter-receiver side | Effect on detectability |
Secor et al. [21] | Southern Mid-Atlantic Bight, USA | 1–2 | 8–15 | Same (below) | + |
Oliver et al. [29] | Southern Mid-Atlantic Bight, USA | | 15–20 | Mix | - |
Jossart et al. [30] | Caribbean, USVI | 0.2 | 30–40 | Within | - |
Singh et al. [31] | Kromme Bay, South Africa | 1 | 12–14 | Mix | - |
Huveneers et al. [26] | New South Wales, Australia | 0.44 | > 20 | Mix | - |
Cagua et al. [32] | Red Sea, Saudi Arabia | > 0.04 | < 37 | Mix | - |
Gjelland and Hedger [28] | Lake Skrukkebukta, Norway | | 10–21 | Opposite | - |
Klinard et al. [2] | Lake Ontario | > 0.08 | 11 | Mix | - |
Studies that report a negative effect of thermocline presence often have transmitters or receivers within, or on different sides of, the thermocline. In these instances, detection requires signal transmission across a steep change in density, which can result in acoustic power loss [15]. In a study by Huveneers et al. [26], a transmitter was placed at mid-depth, with a suite of receivers suspended in the water column. Inspection of the partial effects of their model reveals that although a significant negative trend is apparent, there were instances when a significant positive effect can be noted -- namely when the thermocline was shallow, placing transmitters and receivers far from a rapid change in density. This is not always the case however, as Singh et al. [31] explicitly investigated detectability on the same side of the thermocline, and found a negative effect. Further, Loher et al. [7] also observed rapid increases in detection probability attributed to destratification. In the present study, transmissions did not have to cross the thermocline as both transmitters and receivers were positioned near the sea bottom. As it is clear that detection probability is much reduced when crossing the thermocline, this study is likely more applicable to demersal species that spend the majority of time below the thermocline, such as Atlantic sturgeon, Atlantic cod, or black sea bass, than pelagic species, such as striped bass.
Though attributed to storms here, reduction in detectability could be attributed to other masking factors such as an increased presence of boat traffic or tagged fish. Increased summer boating and commercial shipping activity has been shown to substantially increase sonic noise within in the SMAB (10–800 Hz, [33]), to the degree that it can mask acoustic communication [34]. Shipping and boating traffic create noise dominating frequencies up to 40 kHz, but can span to 100 kHz range and overlap with 69 kHz transmitters [12, 35]. Although increased shipping should create noise which can mask transmission, we recorded the opposite: a decrease in masking ambient noise at 69 kHz in the stratified summer months. The presence of fish tagged with 69 kHz transmitters could also result in reduced detection probability due to code collision [6]. At these sites, the dominant tagged species were Atlantic sturgeon and striped bass; though these species are most-abundant during periods of low detectability, pulses of fish recorded in a co-occuring study did not overlap with the observed change points in detectability [36].
The distance when transmissions have an effectively random chance of detection, estimated as D50, is a common metric to investigate detection range. D50 is intuitive when conceptualizing logistic acoustic power loss due to spreading over distance and simple to extract from nonlinear least squares [26], LOESS smoothing [37], generalized linear [38], and generalized additive models [39]. It also provides an answer to the question most investigators actually want answered: how far away could I have reliably heard this transmitter? D50 is often lower in marine than freshwater systems, ranging from 100–800 m in marine systems and exceeding 1000 m in freshwater [2, 6]. In marine systems, most reported range tests have been conducted in subtropical reef systems, where stratification is not as extreme and topology and biogenic ultrasonic noise are much more influential [40]; D50 in these systems is frequently less than 400 m. D50 in temperate marine systems, however, has been less-frequently investigated. In deep water systems off the Pacific coast of Alaska, Loher et al. [7] measured D50 greater than 1200 m. On a shallow shelf system off the Atlantic coast of Georgia, Mathies et al. [14] attained D50 greater than 280 m. Here, we report values for the shallow shelf of the Mid-Atlantic Bight straddling those reported in temperate climates, ranging from 0–1100 m with means of 550 m during non-stratified and 875 m during stratified periods.
The exact relationship between ultrasonic sound propagation and an increase in detectability associated with temperature stratification is beyond the scope of this study, but likely lies within optic theory’s Snell’s Law [41]. As transmissions within a cold bottom layer propagate toward the surface, they come in contact with a rapid decrease in density associated with the thermocline. Using seasonal salinity differences [42] and observed temperatures, this corresponds to an increase in the speed of sound ranging from 4–6 m/s at the D50 change point (2 °C ΔT) to 69 m/s at the maximum recorded value of ΔT in this study (17.4 °C ΔT) [43]. The transition from comparatively-slower sound speed in the bottom layer to fast sound speed in the surface layer bends transmission rays back toward the bottom, and it is likely that some rays at shallow angles of incidence would be reflected back to the bottom layer. As there was minimal change in detectability beyond 4.6 °C ΔT at a given distance and noise level, there may be no functional effect of the strength stratification beyond this point, corresponding to a difference in the speed of sound greater than 14 m/s.