Effects of Introduced Round Goby (Neogobius Melanostomus) on Diet Composition and Growth of Zander (Sander Lucioperca), a Main Predator in European Brackish Waters
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Aquatic Invasions (2016) Volume 11, Issue 2: 167–178 DOI: http://dx.doi.org/10.3391/ai.2016.11.2.06 Open Access © 2016 The Author(s). Journal compilation © 2016 REABIC Research Article Effects of introduced round goby (Neogobius melanostomus) on diet composition and growth of zander (Sander lucioperca), a main predator in European brackish waters 1,2, 2 1 Mattias Hempel *, Rüdiger Neukamm and Ralf Thiel 1University of Hamburg, Centre of Natural History, Zoological Museum, Martin-Luther-King-Platz 3, D-20146 Hamburg, Germany 2Landessportfischerverband Schleswig-Holstein e.V., Papenkamp 52, D-24114 Kiel, Germany *Corresponding author E-mail: [email protected] Received: 11 May 2015 / Accepted: 16 December 2015 / Published online: 19 January 2016 Handling editor: Kit Magellan Abstract We inspected diet and growth of zander Sander lucioperca, a main predator in North European brackish waters, from the Kiel Canal, northern Germany, to analyse effects of the introduced round goby Neogobius melanostomus as possible prey. Round goby represented the most important fish prey taxon in the zander diet in 2011–2013 with 21.8% of relative importance index ( RI) over all inspected specimens. In particular zander of total length ≥200 mm fed heavily on round goby, at least in the warmer quarters (2nd and 3 rd) of the years under study. RI for these quarters was ranging from 10.4% to 52.1% for predator specimens of 200–399 mm and from 45.6% to 79.7% of RI for specimens of ≥400 mm. In the colder quarters of the inspected years (1st and 4 th) , zander ≥400 mm showed a high amount of cannibalism, with RI of young-of-the-year zander as prey ranging from 23.9% to 38.9%. The asymptotic total length (L ∞) of the zander, calculated by the von Bertalanffy growth equation, was 937 mm. In comparison with data from 1995/1996, before the round goby invasion in 2006, zander from our study showed considerable differences in diet composition. RI of the prey organisms herring and Pomatoschistus spp. decreased from 26% to 13.4% and 14.2% to 4.4%, respectively. RI of young-of-the-year zander as prey for larger zander increased from 1.2% to 7.7% and round goby occurred as newly established prey taxon. Zander also showed faster growth and a better condition factor. We assume that this development has the potential to enhance the situation of the local fisheries that are strongly related to the catch of zander. In addition, abundance control of round goby seems to result from the high predator abundance. Key words: Gobiidae, invasive species, diet shift, predator-prey relationships, Kiel Canal Introduction on the invaded ecosystems and its inhabitants. Kornis et al. (2012) summarize that round goby The zander Sander lucioperca (Linnaeus, 1758) is competes with native species by resource competition, an important top predator (in terms of ecosystems spawning interference and displacement to sub- and fisheries) of many brackish water ecosystems optimal habitats. In addition, the round goby in northern Europe (Lehtonen et al. 1996; Gröger seems to have a strong predation impact on et al. 2007). Total catch from European inland invertebrate communities (e.g. Kuhns and Berg fisheries reported to FAO was 17,892 t in 1999 1999; Lederer et al. 2008). (FAO 2014). The non-indigenous round goby In contrast to the above mentioned impacts on Neogobius melanostomus (Pallas, 1814) has been native fish species of similar trophic level and introduced into many ecosystems in the northern invertebrates, non-indigenous round goby provide hemisphere and continues to spread (Kornis et al. an important food resource for predatory fishes 2012; Kalchhauser et al. 2013). It is assumed (e.g. Steinhart et al. 2004a; Almqvist et al. 2010; that round goby became established in the eastern Reyjol et al. 2010; Taraborelli et al. 2010; Pothoven parts of the Kiel Canal in 2006 (Neukamm 2009a). and Madenjian 2013; Rakauskas et al. 2013). It has been abundant in the studied middle region The newly available food can enhance growth of the Canal since 2009 (see result section). and have positive effects e.g. on survival and The establishment of the non-indigenous goby reproduction. The effect of incorporating N. is described as having strong negative impacts melanostomus in diets seems to differ strongly 167 M. Hempel et al. between species and locations (see Crane et al. by water exchange with the River Elbe at the 2015). Predator species in North America from western end and with the Baltic Sea at the the same genus as the studied zander are found eastern end. Throughout the investigated period to consume high proportions of round goby (2011–2013), mean salinity near the western end (Reyjol et al. 2010; Crane et al. 2015). There are of the canal was 2.7 and 10.4 near the eastern also descriptions of important changes in prey end. The study site (54°19′19″ N; 9°42′49″ E) composition following round goby invasion for was situated close to the city of Rendsburg fish eating birds and snakes (e.g. Jakubas 2004; (Figure 1), where mean salinity was ~7. The fish King et al. 2008). fauna of the Kiel Canal comprises numerous Round goby is not only a potential prey for limnic, euryhaline and marine taxa. In 1995/1996, predatory fish species but can also act as a 46 fish species were recorded in the Kiel Canal predator on fish eggs and larvae (e.g. Steinhart et by Kafemann (2000), whereby abundance of some al. 2004b). Kornis et al. (2012) gave multiple fish species in the canal was highly correlated examples for species suffering from egg depredation with the seasons. and concluded that benthic eggs of any species were probably vulnerable to predation by round Sampling gobies. Although many predators of round goby have Sampling was performed in collaboration with a been identified, no study has examined the resident commercial fishery. The net (opening effects of round goby on zander diet and growth. width 14 m, 10 mm mesh size in the cod end) To fill this gap in knowledge, we investigated was trawled along the bottom with a speed of these aspects in the framework of a case study in ~2.5 knots, mostly in areas with depth >10 m the brackish Kiel Canal. Supported by reports of (verified by echo sounding). Bottom material in local fishermen, we hypothesized that the zander this depth was characterised mostly by sand and in the Kiel Canal showed faster growth due to mud (verified by bottom grab sampling). In 2011 predation on round goby than before the round and 2012, monthly sampling from June to goby invasion. To test this hypothesis, we compared October took place with 10 km trawling distance the results of our current research from the years in total (5 hauls, each 1–3 km in length) in the 2011–2013 with data from 1995/1996 (Kafemann framework of a monitoring program on European and Thiel 1998; Kafemann 2000). These analyses eel Anguilla anguilla (Linnaeus, 1758). That were complemented by the inspection of the program has taken place annually since 2007 development of biomasses of zander, round goby (compare Neukamm 2009b). Biomass data from and other dominant fish species in the study area the years 2008–2010, later presented in the in the years 2008–2013. results section, have been assessed within the above mentioned monitoring program as well. Zander specimens for the months November to Materials and methods March of the years 2011–2013 were provided by fishermen for the same area. The specimens were Study site caught using the same fishing technique and the The Kiel Canal in northern Germany is the most same net. Thus, an influence of different sampling commonly used artificial waterway worldwide techniques on the food composition in the (Heitmann et al. 2013). In 2012, about 35,000 digestive tract can be ruled out. Fish were frozen vessels transporting 104 million tonnes of cargo and kept at -20°C until analysis. In large passed the Kiel Canal (WSD Nord 2013). It was specimens with total length (TL) >350 mm, the built in the late 19th century and connects the digestive tracts and complete heads were frozen North Sea with the western Baltic Sea over a after determining weight (0.1 g precision) and length of about 100 km. Maximum depth of the TL (to the nearest mm) of the whole individuals. Kiel Canal is 11 m and width varies between 162 m and 240 m (Kafemann et al. 2000). The Biomass calculation shoreline of the Kiel Canal is mostly lined with stone packaging to avoid damage due to shipping Biomass (B) of zander, round goby and other traffic. The stone packaging represents a common important fish species in the region under study way to ensure stability of shorelines, not only in was calculated as kg ha-1 according to the artificial waters but also in large streams and equation: B = (BT / AT) × 10,000, where BT was lakes. The canal shows a salinity gradient, caused the determined biomass of a particular species in 168 Effects of round goby on diet composition and growth of zander Figure 1. Location of the study site. the trawled area in kg and AT was the size of the digestion were counted and their relevant structures trawled area in m2. For biomass calculations, (bones or body parts; e.g. otoliths, dentary bones, only results from the trawling procedures from vertebrae, maxillary bones) were measured for June to October of the years 2011 and 2012 were subsequent back calculation of ingested biomass. considered. That was because exact biomass was Back calculation of biomass was performed measured only in the monitoring program described using published regression equations between above and not during conventional fisheries.