Can an herbivorous fish become predator? The case of Scardinius acarnanicus (Economidis, 1991) in Lake Trichonis (Greece)
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Department of Environmental and Natural Resources Management, University of Patras, Greece
Scardinius acarnanicus Economidis, 1991 (Tseroukla in Greek) is an endemic species in freshwater ecosystems of the western Greece. In the only previous study on its feeding conducting nearly 40 years ago in the lake system of Trichonis and Lysimachia, its diet consisted solely of aquatic vegetation and phytoplankton (Iliadou, 1991). However, there have been some recent indications of its predation upon other fish species in Lake Trichonis (Tsounis, 2016). If this stands, it would be maybe the first incidence of shift in the feeding behavior of a fish species from herbivory to carnivory in such short period of time. Thus, the aim of this study was to provide some preliminary results concerning the present diet of S. acarnanicus in Lake Trichonis in comparison to the past, focusing on fish predation.
A total number of 234 S. acarnanicus specimens were taken between May to November of 2017 in Lake Trichonis, using gillnets and purse seine nets by the assistance of local fishermen. The purse seine fishing was held once a month (in May, July, August, October and November 2017) during moonless nights around lamp rafts producing green LED light for the fish attraction. In contrast, no light was used in the case of gillnets which were set late at night and were picked up early in the morning in June, July, August, September and November 2017. Gut content analysis was conducted under a stereoscope using the entire intestine of all specimens. The identification of the macrophytic vegetation was unsuccessful as the plants had been macerated by the pharyngeal teeth. Thus, the presence of this prey category along with phytoplankton is called “plant material”. In many cases it was possible to identify the fish species found in the stomach content. This prey category along with the unidentified fish bones, fins and other fish parts found in the diet is called “fish remains”. Due to the difficulty in the numerical estimation of the whole food items in most of the cases, the relative frequency of occurrence (F) for all types of prey categories was selected as the best way to present the results.
The diet of S. acarnanicus consisted of 4 food types: plant material (F = 65.4%), fish remains (F = 58.4%), terrestrial insects (F = 7.5%) and bivalve mollusks (F = 0.9%). The overall F values for the fish remains were remarkably higher in the samples taken by purse seining with the use of light (F = 73.3%), instead of those taken without light using gill nets (F = 39.4%) and these differences were statistically significant (Chi-square test, p<0.05). Large numbers of Atherina boyeri (Risso, 1810) specimens were identified in the gut contents (Fig. 1), being greater in the samples taken with light (Chi-square test, p<0.05).
The present results are in direct contrast to the only previous study on the diet of S. acarnanicus in the area (Iliadou, 1991) according to which the species was strictly herbivorous, feeding with reeds, other aquatic vegetation and phytoplankton. The present study, conducted nearly 40 years after the samplings of Iliadou (1991) back in 1977-1979, reveals a major shift of S. acarnanicus towards carnivory and especially fish predation upon the dominant fish species of Lake Trichonis, Atherina boyeri. Although the present sampling period was shorter compared to the study of Iliadou (1991), the numbers of S. acarnanicus specimens examined in the two studies (360 and 234 specimens in the older and the present study, respectively) are considered adequate to provide valid information on its diet. Moreover, the same methodology in the gut content analysis was applied in both studies. It seems reasonable, then, to assume that during the last 40 years something may have triggered the “carnivore instincts” of S. acarnanicus.
Atherina boyeri is a brackish species that was naturally introduced into Lake Trichonis from the sea via river channels in past centuries and became the dominant species in the fish community, being the main source of fishing revenue (Leonardos, 2001). Thus, neither the existence, nor the abundance of A. boyeri is new for this ecosystem and, thus, it cannot be incriminated as the prime cause of the shift of S. acarnanicus from herbivorus to carnivorus in the last 40 years. On the other hand, the fishing practices exercised in the lake have been altered during this period. Indeed, until the early 80s the fishermen were using mainly the method of bottom trawling for the fishing of A. boyeri, as well as trammel and set gillnets targeting other larger species. The specimens in the study of Iliadou (1991) were taken by these methods. In contrast, since the early 90s the method of purse seine at night with lamp rafts has been adopted by the entire fishing fleet as the most effective method for the fishing of A. boyeri and the least destructive for the ecosystem. It must be pointed, however, that the light attracts not only the target species, A. boyeri, but several other fish species among which S. acarnanicus, who approaches to take advantage and preys upon A. boyeri. This was also eye witnessed by fishermen and also by our research team members (Kehayias et al., 2018). Indeed, the greater F values of fish remains in the samples taken with light, point that S. acarnanicus was heavily preying upon this species close to the lamp rafts. It is highly probable also, that most of the unidentified fish remains belong to A. boyeri. The above suggest probably that, the use of fishing lights, and the consequent high concentration of the A. boyeri close to it, may have been one of the causes of the shift in the feeding preferences of S. acarnanicus. There is also another critical parameter concerning the on-board catch handling that could be indirectly involved to this feeding behavior. Practically, when the fishermen retrieve the purse seine nets, the catches always consist not only of A. boyeri, but also of large numbers of S. acarnanicus specimens, which are discarded alive back to the lake, due to their low demand in the market. This custom has been applied more frequently during the last decades and may have acted as “educational” for the entire S. acarnanicus population, meaning that the fish learn to prey upon A. boyeri especially close to light and at the same time suffer no harm. Indeed, the present results showed that even the smaller size class of this species was heavily preying upon A. boyeri close to light. On the other hand, the presence of fish remains in the diet of S. acarnanicus caught without light could mean that, either they had ingested specimens of A. boyeri (or perhaps other fishes) close to light and were latter captured by the gill nets, or that they accustomed to attack and capture the fishes even in the absence of light when their density is not as much as it is close to the lamp rafts. The ecological significance of the present findings is remarkable considering that none of the other species of the genus Scardinius is fish-predator and their diet consists of plankton, terrestrial insects and plant material (Kottelat and Freyhof, 2007; García-Berthou and Moreno-Amich, 2000).
In conclusion, this study reveals the shift of a former herbivorous species towards fish predation in a freshwater ecosystem. The hypothesis suggested involves the specific fishery practices exercised in Lake Trichonis, along with improper catch handling. Although there are several examples on how human behavior and practices may have been educational to animals, to our knowledge, there is no other similar case referring to the alteration of the feeding behavior of a wild fish. In order to verify this hypothesis, future investigation on the diet of S. acarnanicus in nearby lakes, which are not inhabited by A. boyeri and in which there is no purse seine fishing with light, is planning.
Figure 1. Variation of the frequency of occurrence (F%) for all prey types in the sampling months as calculated from samples taken with purse seine using light (left columns) and samples taken with gill nets without light (right columns). The frequency of occurrence (F%) for Atherina boyeri is also shown (oblique lines).
References
Froese, F., and Pauly, D. (2018). Genus Scardinius. In Fish Base. Available at: https://www.fishbase.de/identification/SpeciesList.php?genus=Scardinius
García-Berthou, E., and Moreno-Amich, R. (2000). Rudd (Scardinius erythrophthalmus) introduced to the Iberian peninsula: feeding ecology in Lake Banyoles. Hydrobiologia 436(1-3), 159–164.
Iliadou, K. (1991). Feeding of Scardinius acarnanicus Stephanidis, 1939 (Pisces: Cyprinidae) from Lakes Lysimachia and Trichonis, Greece. J. Fish Biol. 38, 669 – 680.
Kehayias, G., Bouliopoulos, D., Chiotis, N., and Koutra, P. (2016). A photovoltaic – battery – LED lamp raft design for purse seine fishery. Application in a large Mediterranean lake. Fish. Res. 177, 18 – 23.
Kottelat, M., and Freyhof, J. (2007). Handbook of European freshwater fishes. Cornol, Switzerland: Publications Kottelat.
Leonardos, I.D. (2001). Ecology and exploitation pattern of a landlocked population of sand smelt, Atherina boyeri (Risso, 1810), in Trichonis Lake (western Greece). J Appl. Ichthyol. 17, 262 – 266.
Tsounis, L. (2016). Dietary habits of Scardinius acarnanicus in lakes of western Greece: ecological and management projections. MSc Thesis. University of Patras, Greece. http://nemertes.lis.upatras.gr/jspui/handle/10889/10132
Keywords:
Scardinius acarnanicus,
Diet,
Carnivory,
Atherina boyeri,
Purse seine,
Lake Trichonis
Conference:
XVI European Congress of Ichthyology, Lausanne, Switzerland, 2 Sep - 6 Sep, 2019.
Presentation Type:
Poster
Topic:
ECOLOGY AND LIFE CYCLES
Citation:
Tsounis
L and
Kehayias
G
(2019). Can an herbivorous fish become predator? The case of Scardinius acarnanicus (Economidis, 1991) in Lake Trichonis (Greece).
Front. Mar. Sci.
Conference Abstract:
XVI European Congress of Ichthyology.
doi: 10.3389/conf.fmars.2019.07.00014
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Received:
27 May 2019;
Published Online:
14 Aug 2019.
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Correspondence:
Prof. George Kehayias, Department of Environmental and Natural Resources Management, University of Patras, Agrinio, Greece, gkechagi@upatras.gr