Masking a fish's detection of environmental stimuli: application to improving downstream migration at river infrastructure

Abstract

According to Signal Detection Theory, the ability to detect a stimulus (discriminability, d') is inversely related to the magnitude of internal and external noise. In this study, downstream moving brown trout Salmo trutta were used to investigate whether external hydrodynamic noise (in this case turbulence) could mask a signal that induced an unwanted response, such as rejecting accelerating velocity gradients commonly encountered at entrances to fish bypass channels. S. trutta behaviour was quantified in the absence (control) or presence of an accelerating velocity gradient created by an unconstricted or constricted channel, respectively, under two levels (low and high) of background turbulent kinetic energy (hydrodynamic noise). Experiments were conducted in an indoor recirculating flume in the dark and a range of passage metrics were quantified. Under the control condition, most (ca. 91%) S. trutta passed, usually oriented downstream (67%), with minimal delay (median 0.13 min). In comparison, fewer S. trutta (ca. 43%) passed under constricted conditions, they tended to orient facing into the flow (ca. 64%) and delay was greater (median > 20 min). When viewed from a coarse-scale perspective, discriminability of the velocity gradient was lower when turbulence was high suggesting masking of the signal occurred. However, the resulting increase in the percentage of fish that passed, decrease in time to pass and reduction in the distance at which S. trutta reacted (switched orientation) was subtle and non-significant. Despite the mixed results obtained, the use of masking to manipulate an animal's perception of environmental stimuli as a fisheries management tool is conceptually valid and the results of this experiment present a useful stepping stone for future research.

Keywords: brown trout; bypass entrance; fish behaviour; sensory ecology; signal detection theory; velocity gradient.

Figure 1

(a) Top and (b) side‐view schematic of the experimental area with velocity gradient present under low levels of hydrodynamic noise (TL, treatment low). (), The entrance and exit of the release chamber; (), direction of bulk flow; (), the flow modification location, the start of the observation zone, and the point of maximum constriction

Figure 2

Colour intensity plots of velocity (m s⁻¹) in the observation zone under the four treatments created in the absence (a, b) and presence (c, d) of a constriction under low (a, c) or high (b, d) turbulent kinetic energy. The behavioural response of Salmo trutta was quantified under the four treatments. (), Delineation of the extent of the velocity gradient (acceleration zone)

Figure 3

Colour intensity plots of turbulent kinetic energy (TKE – J m⁻³) in the observation zone under the four treatments created in the absence (a, b) and presence (c, d) of a constriction under low (a, c) or high (b, d) hydrodynamic noise. The behavioural response of Salmo trutta was quantified under the four treatments

Figure 4

Flow velocity along the central longitudinal axis of the flume, measured from the point of maximum constriction (or equivalent under control conditions), under the four treatments in which the behavioural response of brown trout Salmo trutta was quantified. CH: velocity gradient absent, high hydrodynamic noise; CL: velocity gradient absent, low hydrodynamic noise; TH: Velocity gradient present, high hydrodynamic noise; TL: velocity gradient present, low hydrodynamic noise. () TH, () TL, () CL, and () CH

Figure 5

The four potential signal‐response outcomes that may occur in the presence or absence of a specific environmental stimulus. In this study, the signal was the velocity gradient and the response was downstream passage (coarse‐scale assessment) or displaying avoidance behaviour (fine‐scale assessment) during a trial

Figure 6

Mean (±95% CI) percentage of all Salmo trutta that passed downstream when velocity gradient was absent with low () hydrodynamic noise (CL, n = 22), velocity gradient was absent with high () hydrodynamic noise (CH, n = 21), velocity gradient was present with low hydrodynamic noise (TL, n = 21) and velocity gradient was present with high hydrodynamic noise (TH, n = 21)

Figure 7

(a) Cumulative probability of passing downstream against time for Salmo trutta in the presence (, ) and absence (, ) of a channel constriction under low (, ) and high (, ) turbulent kinetic energy (TKE). (b) Cumulative probability of passing downstream against time for S. trutta that entered the observational zone facing upstream (positive approach rheotaxis, ) and downstream (negative approach rheotaxis, ); aggregated data for constricted and control treatments. X, instances of right‐censored data

Figure 8

Mean (±95% CI) response distance at which Salmo trutta responded (rejection or orientation switch) to a velocity gradient created by a channel constriction under low () and high () turbulent kinetic energy

Figure 9

Location of initial (a) orientation switch and (b) rejection by Salmo trutta when the velocity gradient was present under low () and high () turbulent kinetic energy superimposed onto colour intensity plots of flow velocity (U). Arrow head indicates head position of S. trutta

Figure 10

Receiver‐operating characteristics plot of hit rate against false‐alarm rate for coarse ( , ) and fine ( , ) scale assessment of the behavioural responses of Salmo trutta to a velocity gradient under low ( , ) and high ( , ) turbulent kinetic energy. Reference discriminability (d’ = 0, 1, 2, 3; ) and response criterion (c = −1, −0.5, 0, 0.5, 1; − – –). Increases in d’ represent greater signal discriminability and increases in c represent greater bias towards responding