Reducing the number of individuals to monitor shoaling fish systems - Application of the Shannon entropy to construct a biological warning system model
Journal article, Peer reviewed
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Original versionFrontiers in Physiology. 2018, 9 1-13. 10.3389/fphys.2018.00493
The present study aims at identifying the lowest number of ﬁsh (European seabass) that could be used for monitoring and/or experimental purposes in small-scale ﬁsh facilities by quantifying the effect that the number of individuals has on the Shannon entropy (SE) of the trajectory followed by the shoal’s centroid. Two different experiments were performed: (i) one starting with 50 ﬁsh and decreasing to 25, 13, and 1 ﬁsh, and (ii) a second experiment starting with one ﬁsh, adding one new ﬁsh per day during 5 days, ending up with ﬁve ﬁsh in the tank. The ﬁsh were recorded for 1h daily, during which time a stochastic event (a hit in the tank) was introduced. The SE values were calculated from the images corresponding to three arbitrary basal (shoaling) periods of 3.5 min prior to the event, and to the 3.5 min period immediately after the event (schooling response). Taking both experiments together, the coefﬁcient of variation (CV) of the SE among measurements was largest for one ﬁsh systems (CV 37.12 and 17.94% for the daily average basal and response SE, respectively) and decreased concomitantly with the number of ﬁsh (CV 8.6–10% for the basal SE of 2 to 5 ﬁsh systems and 5.86, 2.69, and 2.31% for the basal SE of 13, 25, and 50 ﬁsh, respectively). The SE of the systems kept a power relationship with the number of ﬁsh (basal: R2 = 0.93 and response: R2 = 0.92). Thus, 5–13 individuals should be the lowest number for a compromise between acceptable variability (<10%) in the data and reduction in the number of ﬁsh. We believe this to be the ﬁrst scientiﬁc work made to estimate the minimum number of individuals to be used in subsequent experimental (including behavioral) studies using shoaling ﬁsh species that reaches a compromise between the reduction in number demanded by animal welfare guidelines and a low variability in the ﬁsh system’s response.