Thirty-Year-Old Paradigm about Unpalatable Perch Egg Strands Disclaimed by the Freshwater Top-Predator, the European Catfish (Silurus glanis)

RESEARCH ARTICLE Thirty-Year-Old Paradigm about Unpalatable Perch Egg Strands Disclaimed by the Freshwater Top-Predator, the European Catfish (Silurus glanis) Lukáš Vejřı́k1,2☯*, Ivana Vejřı́ková1☯, Luboš Kočvara1‡, Zuzana Sajdlová1‡, Son Chung Hoang The1‡, Marek Šmejkal1‡, Jiřı́ Peterka1‡, Martin Čech1☯ a1111111111 a1111111111 a1111111111 a1111111111 a1111111111 OPEN ACCESS Citation: Vejřı́k L, Vejřı́ková I, Kočvara L, Sajdlová Z, Hoang The SC, Šmejkal M, et al. (2017) ThirtyYear-Old Paradigm about Unpalatable Perch Egg Strands Disclaimed by the Freshwater TopPredator, the European Catfish (Silurus glanis). PLoS ONE 12(1): e0169000. doi:10.1371/journal. pone.0169000 Editor: Vyacheslav Yurchenko, University of Ostrava, CZECH REPUBLIC Received: October 25, 2016 Accepted: December 11, 2016 1 Biology Centre of the Czech Academy of Sciences, Institute of Hydrobiology, Na Sádkách 7, České Budějovice, Czech Republic, 2 Faculty of Science, University of South Bohemia in České Budějovice, Branišovská 31, České Budějovice, Czech Republic ☯ These authors contributed equally to this work. ‡ These authors also contributed equally to this work. * Abstract So far, perch egg strands have been considered unpalatable biological material. However, we repeatedly found egg strands of European perch (Perca fluviatilis) in the diet of European catfish (Silurus glanis) caught by longlines in Milada and Most Lakes, Czech Republic. The finding proves that perch egg strands compose a standard food source for this large freshwater predatory fish. It extends the present knowledge on catfish foraging plasticity, showing it as an even more opportunistic feeder. Utilization of perch egg strands broadens the catfish diet niche width and represents an advantage against other fish predators. Comparison of datasets from extensive gillnet and SCUBA diver sampling campaigns gave the evidence that at least in localities where food sources are limited, multilevel predation by catfish may have an important impact on the perch population. Published: January 6, 2017 Copyright: © 2017 Vejřı́k et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: All relevant data are within the paper and its Supporting Information files. Funding: The study was supported by projects No. CZ.1.07/2.3.00/20.0204 (CEKOPOT) of the Ministry of Education, Youth and Sports, No. 7F14316 of the Norwegian Financial Mechanism 2009–2014 under contract number MSMT-28477/2014, No. 206/09/P266 of the Czech Science Foundation, No. 677039 (CLIMEFISH) of the European Union’s Introduction A study published 31 years ago by Newsome and Tompkins [1] described perch (Perca spp.) egg strands as repellent matter for predators. Observation and testing of six fish and four invertebrate species proved that perch egg strands are an undesirable food source for them. One year later, Diamond and Wakefield [2] published a topical study referring to the utilization of perch egg strands by three species of caddisfly larvae (Trichoptera) and two species of flatworms (Tricladida). The authors pointed out that these represent only a small fraction of aquatic invertebrates and predation pressure exerted on perch is irrelevant. The paradigm of perch egg strands as unpalatable biological matter has since been widely accepted by researchers all over the world [3–9]. European catfish (Silurus glanis), one of the world biggest freshwater fish, is a typical opportunist with a wide diet niche. It has successfully spread worldwide accompanied by human PLOS ONE | DOI:10.1371/journal.pone.0169000 January 6, 2017 1/9 European Catfish Eat Perch Eggs Horizon 2020 research and innovation programme, and No. 04-151/2016/P of the Grant Agency of University of South Bohemia. Competing Interests: The authors have declared that no competing interests exist. activity [10–14]. Recent studies demonstrated that European catfish uses atypical food sources including sources not originating from freshwater but marine or terrestrial ecosystems [15,16]. Although the European catfish is a top predator, only a few studies regarding its diet have been carried out, and most of them are recent (for review see [12]). The main reason is the poor capture success by standard ichthyological methods [17]. In the present study, we prove that perch (Perca fluviatilis) egg strands are a part of the European catfish diet. Further, we discuss the potential impact of European catfish on perch populations as a result of multilevel predation. Materials and Methods Study site The study was conducted in two water bodies created after aquatic restorations of mining pits, Milada and Most Lakes, Czech Republic. The oligo- to mesotrophic Milada Lake has an area of 250 ha, volume of 36×106 m3 and maximum depth of 25 m (Fig 1). Aquatic restoration lasted from 2001 to 2011. Northern pike (Esox lucius) was introduced in 2005 (789 individuals, mean weight 0.3 kg) and European catfish in 2007 (316 individuals, mean weight 1.2 kg), both for biomanipulation purposes. The oligotrophic Most Lake has an area of 310 ha, volume of 70×106 m3 and maximum depth of 75 m (Fig 1). Aquatic restoration lasted from 2008 to 2014. Northern pike (2332 individuals, mean weight 1.1 kg) and European catfish (694 individuals, mean weight 3.7 kg) were both introduced in 2011, 2012 and 2013. In both lakes, all catfish individuals were individually tagged with a passive integrated transponder tag (PIT-tag, Oregon RFID, fullduplex, length 12 mm, diameter 2.15 mm, weight 0.11 g, 11784/11785 compatible). Fish sampling and stomach content analysis European catfish and northern pike from both lakes were caught by longlines in early May 2014 and 2015 during a 4-day-and-night-long campaign (see Fig 2 for illustrative description). Animal treatment (including method of longlines and stomach content analysis) was performed in accordance with the guidelines from the Experimental Animal Welfare Commission under the Ministry of Agriculture of the Czech Republic (Ref. No. CZ 01679) and with permission of Palivový kombinát Ústı́, státnı́ podnik, owner of the study sites. The work was approved by the Ethics Committee of the Czech Academy of Sciences. The field study did not involve endangered or protected species. The main line was 60 m long with three main buoys situated at both ends and in the middle of the line. Anchoring ropes, 3.5–7 m long, with weights (32 kg each) fixing the main line in place were tied to the buoys. Auxiliary buoys were situated every 5 m between the main buoys with a hanging 2.5 m long snood made of two parts i) 2-m long fishing-line with maximum load of 50 kg and ii) more durable 0.5 m long fishing-line with maximum load of 100 kg with a swivel between the two parts to preventing twisting. (...truncated)