Experimental Evidence of Threat-Sensitive Collective Avoidance Responses in a Large Wild-Caught Herring School

Aggregation is commonly thought to improve animals' security. Within aquatic ecosystems, group-living prey can learn about immediate threats using cues perceived directly from predators, or from collective behaviours, for example, by reacting to the escape behaviours of companions. Combining cues from different modalities may improve the accuracy of prey antipredatory decisions. In this study, we explored the sensory modalities that mediate collective antipredatory responses of herring (Clupea harengus) when in a large school (approximately 60 000 individuals). By conducting a simulated predator encounter experiment in a semi-controlled environment (a sea cage), we tested the hypothesis that the collective responses of herring are threat-sensitive. We investigated whether cues from potential threats obtained visually or from the perception of water displacement, used independently or in an additive way, affected the strength of the collective avoidance reactions. We modified the sensory nature of the simulated threat by exposing the herring to 4 predator models differing in shape and transparency. The collective vertical avoidance response was observed and quantified using active acoustics. The combination of sensory cues elicited the strongest avoidance reactions, suggesting that collective antipredator responses in herring are mediated by the sensory modalities involved during threat detection in an additive fashion. Thus, this study provides evidence for magnitude-graded threat responses in a large school of wild-caught herring which is consistent with the “threat-sensitive hypothesis”

Learning process promoted by sensemaking and trust: a study related to unexpected events

The objective of this study is to understand the interrelations among the micro processes which comprise sensemaking (WEICK, 1995) and the construction of trust (LUHMANN, 1996) in unexpected events. Unexpected events transcend routine and planning orders. However, when they happen, they demand from the organizations solutions as efficient and effectual as those programmed events. The methodology consists of the analysis of six semi-structured interviews and Conversation Analysis (CA) of the main extracts of the conversation between the agents and the supervisor from the beginning of event until its conclusion (recorded in real time). As empirical evidence, an unexpected event at the Operations Center of the Brazilian Electrical System (COS) of an important Brazilian state, on September 24 and 25, 2009 is investigated. The conclusion is that there is an interrelationship between micro processes of sensemaking, considered as ambiguity, experience and interactions, and the micro processes of trust, considered as complexity, familiarity and social behavior

Comparison of model results for two prolate spheroids.

<p>Length-normalised target strength (<i>nTS</i>) of a prolate spheroid with <i>ka</i>  = 12 and <i>kb</i>  = 5 (upper curves) and <i>ka</i>  = 12 and <i>kb</i>  = 1 (lower curves) as a function of angle off broadside from the prolate-spheroid-modal-series (PSMS), Kirchhoff-approximation (KA), Kirchhoff-ray-mode (KRM), and finite element (FE) models, where <i>a</i> is the semi-major axis and <i>b</i> the semi-minor axis of the prolate spheroid. The shaded ellipses show the relative shapes of the prolate spheroids.</p

Target strength error for the Kirchhoff-approximation model.

<p>Difference in tilt-averaged (mean of 0° and standard deviation of 10°) length-normalised target strength between the Kirchhoff-approximation and prolate-spheroid-modal-series models as a function of <i>ka</i> and <i>kb</i> (negative values indicate that the Kirchhoff-approximation model result is less than the prolate-spheroid-modal-series model). The thick solid line is drawn at <i>ka</i> = <i>kb</i>.</p

Comparison of the Kirchhoff-approximation and finite element models to exact solutions.

<p>Length-normalised target strength (<i>nTS</i>) of a 25 mm diameter pressure release sphere immersed in water as a function of <i>ka</i>, calculated using the Kirchhoff approximation model (open circles), Kirchhoff integral (dashed line), finite element model (filled circles) and series solution (solid line).</p

Target strength error from the Kirchhoff-approximation and Kirchhoff-ray-mode models at two tilt angles.

<p>Difference in length-normalised target strength for the Kirchhoff-approximation (KA) and Kirchhoff-ray-mode (KRM) and models compared to the prolate-spheroid-modal-series model as a function of <i>ka</i> and <i>kb</i> at a tilt angle of 0° (panel A: KA, panel C: KRM) and 10° (panel B: KA, panel D: KRM). Negative values indicate that the model result is less than the prolate-spheroid-modal-series model result. The thick solid line is drawn at <i>ka</i> = <i>kb</i>.</p

Comparison of prolate-spheroid-modal-series model to published results.

<p>Length-normalised target strength (<i>nTS</i>) of a pressure release prolate spheroid with aspect ratio (width/length) of 0.15 at broadside as a function of <i>ka</i> from the prolate-spheroid-modal-series model (solid line) and digitised from Figure 3 of Furusawa <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0064055#pone.0064055-Furusawa1" target="_blank">[10]</a> (open circles). A similar comparison for end-on backscatter is also shown (dashed line from the prolate-spheroid-modal-series model and closed circles from Figure 3 of Furusawa <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0064055#pone.0064055-Furusawa1" target="_blank">[10]</a>).</p

Maximum depth of the collective responses (mean ± SEM) to the different predator models showing magnitude-graded threats responses.

<p>Stronger vertical avoidance responses were observed when the herring school was exposed to the black-coloured predator-shaped model, which is more visible and produces a stronger hydrodynamic stimulus, compare to when the herring school exposed to the flat and/or transparent models and to the control.</p