How many times did you think about the activity of common dentex during the year? As spearfishers, we already know what is the best season and conditions to meet these amazing predators and try to catch one of them, but our knowledge is limited to the time that we can spend underwater. Scientists have recently satisfied our curiosity. In a recent study
published in the journal Scientific Report, a group of Spanish researchers studied the vertical movement of common dentex in the marine protected area of Medes Islands with an acoustic telemetry system. Between 2007 and 2008 a total of 12 dentex (length 42-65 cm) have been captured, anesthetized, implanted in the peritoneal cavity with a small (6.5 g) acoustic transmitter and released in the same place of capture. Each transmitter emits a unique frequency that is registered by a network of 17 underwater receivers around the marine protected area. Below you can see the average movement of the 12 dentex (c and d).
In the bottomo left part of the figure (c) the average vertical movements are related to 3 dentex that were implanted with a transmitter on July 2017, while on the bottom right (d) the movement is referred to 9 fishes implanted on February 2008. The black line represents the seasonal thermocline and the colors represent the percentage of presence of dentex at a specific depth. The first evident difference between these two groups of dentex is clearly the fact that the group on the left prefer shallower water (5-30 m) than the group on the right (20-45 m). The thermocline influences the vertical movements of the group of dentex that prefer deeper water (right), but when the thermocline sinks below 45 meters the dentex do not follow it any more. In the group on the left there is not clear effect of the thermocline on the vertical movement of dentex, however the fishes showed a preference for shallow water from April to September.
Finally in the table below you can see the summary of some parameters for each dentex. It is very interesting to notice that the dentex 33, 42 and 45 are the individuals with a higher percentage of excursions below the thermocline during the summer.
This study together with others recently published is revolutionizing the knowledge about animal behavior, in particular for aquatic animals. Part of the information reported in it was already known by spearfishers, but this kind of technology offers the opportunity to follow the same individual during a long period of time and discriminate, as we have seen in this study, between different individual strategies within the same species. What are the ecological and evolutionary reasons that induce fishes to behave so differently? This is one of the questions emerged from this interesting study, and who knows whether a future collaboration between spearfishers and science could answer some of the open questions about the ecology of dentex and even about some other species. Sspearfishers spent a great amount of hours underwater gathering a deep knowledge about ecology and ethology of many marine species. It’s paramount to set up bridges between spearfishing and science in order to contribute in the understanding of marine ecosystems.
The figures in this article have been published in the following article:
Aspillaga, E., F. Bartumeus, R. M. Starr, À. López-Sanz, C. Linares, D. Díaz, J. Garrabou, M. Zabala, and B. Hereu. 2017. Thermal stratification drives movement of a coastal apex predator. Scientific Reports 7:526.
You can download the original version of the article in pdf at the following link.