Is the polychaete Marenzelleria viridis an important food item for fish?

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1 Proceedings of the 13th Symposium of the Baltic Marine Biologists: , 1996 Is the polychaete Marenzelleria viridis an important food item for fish? H.M. Winkler University of Rostock, Department of Biology, Universitätsplatz 5, D Rostock, Germany L. Debus Institute for Baltic Sea Research Seestrasse 15, D Warnemünde, Germany ABSTRACT Investigations of a shallow brackish Baltic estuary in 1985 (southern Baltic: Darss-Zingst Bodden) recorded the new polychaete Marenzelleria viridis, distributed formerly only at the North American east coast. Since 1987, this species has been very numerous, with densities reaching levels observed previously only for Nereis diversicolor (Polychaeta) and chironomid-larvae. This research was concerned with the occurrence of this worm in the food of benthophagous, planktivorous and piscivorous fish. Marenzelleria occurred as a food item for fish at all life phases. The frequency of occurrence of this worm reaches seasonal maximum values up to 80%. This corresponds to a food biomass portion of 30%. However, the mean annual values of the biomass is below 5%. We developed a mathematical equation which solved difficulties in counting the worm, caused by its extremly high fragility. This equation relates the amount of spines with the bodylength. Piscivorous fish (Stizostedion lucioperca) feed temporarily on this polychaete. At the same time, detritus (consisting of Marenzelleria-remains) became the main food of benthophagous fish (such as Abramis brama). Planktophagous fish (such as Clupea harengus) fed on larval stages of the polychaete. INTRODUCTION Since 1985, when the North American polychaete Marenzelleria viridis first appeared in our coastal waters (Bick, Burckhardt 1989) a very fast increase of this species was observed. It developed benthos wet weights of more than 150 g m -2 and reached abundances of more than individuals m -3 in the plankton. Therefore it was expected to find it in gut of benthophagous and plankton feeding fish. Since fish feeding studies in this area have been carried out for long period already (Debus 1989, Winkler 1989, Debus and Winkler 1990, Debus 1991, Winkler 1991), it was possible to analyse which fish species feed on the immigrant, and to determine, and to determine the location and the time and quantity of feeding. The study was supported by Bundesministerium fur Forschung und Technologie, project no. 03F0027A. MATERIAL AND METHODS The study area is an oligohaiine to mesohaline estuary of the southern Baltic, the Darss-Zingster bodden Chain (Figure 1). Fish samples from the pelagial were taken from commercial catches and by a special trawl net, but fish in the shore area were caught with a beach seine. In comparison to other food items, for instance Nereis, the only remains of Marenzelleria available for counting are their bristles. A regression between the number of bristles and the length of the worms was established for estimation of the number and amount of ingested Marenzelleria.

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4 RESULTS A new food component in zooplankton feeders and benthophagous fish, Marenzelleria, has been observed by Thiel (1991) and in our studies since 1987 (Table 1). The remains of Marenzelleria were observed in 12 of the investigated 15 species. The polychaete was not found only in the gut of adult pikes, sea scorpions and lumpfish. Young and small fish, but also adult herring, feed on the Marenzelleria larvae. The only recognisable remains of Marenzelleria in bream, carp and flounder were the typical bristles. These were usually taken in with detritus. The frequency of Marenzelleria detritus in bream guts increased since 1981 to 100% (Figure 2, Table 2). The amount of Marenzelleria or its bristles also increased in fish food relative to other components (Figure 3). Roach (since 1991) and perch (since 1990) feed on detritus of Marenzelleria, but also ingest small quantities of the whole animals. Since 1989 ruff feeds on Marenzelleria. Surprisingly the polychaete was discovered (since 1991) very frequently in the food of the adult pikeperch in the Western Bodden (Saaler Bodden) (Figure 4). The frequency of Marenzelleria viridis in the food of pikeperch is highest in spring (up to 80%), but polychaetes comprise only 30% of the total wet weight of ingested food (70% of the weight is fish). In other times of the year, polychaetes comprise only 1-2% of the total weight of ingested food (98-99% of the weight is fish). Some territorial variations in the feeding behaviour of roach and pikeperch on Marenzelleria were found. Both fish species occur in all parts of the Bodden. However, roach feeds on Marenzelleria only in the eastern part and pikeperch only in the western part (Table 1). A distinct seasonal pattern in Marenzelleria consumption (Figure 5, Table 3) occurs mainly during autumn to spring. There was no Marenzelleria consumption in summer.

5 DISCUSSION The rapid increase of the Marenzelleria population has created a new food resource for fish. Bream feeds on organisms, which frequently occur in the benthos or pelagial, even if the existing food is not preferred (Debus 1989). It is thus not surprising that Marenzelleria is found in benthophagous fish, such as bream, roach, ruff, and gobies, and also in omnivores such as perch. However, typical benthos feeding bream consumed and increased amount of detritus in which polychaeta remains were observed only as bristles. Whole Marenzelleria in gut were never found. A similar situation was described previously for Russian water reservoirs, where the bream feeds on the so called grey sludge" which contains only small quantities (about 5 g ww m -2 ) of the oligochaeta (Ziteneva 1958, 1981, Ziteneva and Bakanov 1981). The only detected remains of the oligochaetes in the gut content were their bristles. Therefore, it can be assumed that the trochophora larvae, which die in large numbers during the reproduction period in autumn (Bochert 1993), enrich the sediment with organic material. Also, adult individuals die in large numbers, thus forming the detritus of Marenzelleria. Typical fish-feeders, such as pikeperch, consume tube-dwelling benthos organisms. However, it is known that when food is scarce, pikeperch will also consume untypical food organisms such as trichopterans (Winkler 1991). The composition of the stomach content of pikeperch indicates that this species consumes living worms or parts of them. Since pikeperch cannot penetrate into the sediment, the worms must be occasionally present in large amounts in the water column. Schmidt (personal communication) has observed the presence of free swimming worms. This fact was confirmed by Zettler (1993), who used emergent traps. During the night, many 50 to 70 mm long polychaetes were seen in the water. This nocturnal behaviour is due to negative phototaxis of the worm and coincides with the typical nocturnal feeding habit of pikeperch, which is based on the tape-turn lucidum. It is likely that this predatory fish swims through high concentrations of Marenzelleria and filters them. There are some reasons that can explain why bream did not feed on Marenzelleria (Figure 3). Firstly, there are differences in diurnal activities of bream and Marenzelleria. Marenzelleria is nocturnally (from dusk to dawn) present in the water column and the feeding activity of the bream occurs during twilight hours when Marenzelleria is in its tube. Secondly, during the day Marenzelleria lives in sediment layers

6 deeper than 15 cm. This idea is supported by our observations that bream feeds only on Nereis. According to Zettler (1993), Nereis is mainly concentrated in the sediment layer not deeper than 15 cm. We can thus conclude that the penetration depth of bream seems to be not more than 15 cm into the sediment.

7 The observed seasonal variations of the food composition of bream are in agreement with the biomass estimates of Marenzelleria (Figure 5). Zettler (1993) found a decrease in Marenzelleria biomass during summer both in the western and the eastern parts of the Bodden Chain. The explanation for the territorial variations in pikeperch feeding (Figure 4) on the worm may be due to differences in food supply. In the eastern part of the Bodden Chain, the abundance of Marenzelleria (Zettler 1993) is lower, but fish suitable for food are more plentiful. There are insufficient investigations on Marenzelleria as fish food in Europe. Studies by Essink and Kleef (1991) on young plaice and flounder described the consumption of Marenzelleria heads. Adult flounders we examined contained the same food as bream: detritus containing bristles of Marenzelleria. This leads to the conclusion that adult flounders presumably have different feeding behaviour then young specimens. CONCLUSIONS AND FURTHER STUDIES -Since 1987 Marenzelleria viridis occurred in the food of fish. -Marenzelleria viridis is consumed as larve by planktivorous fish. -Sessile worms are observed in the food of some predatory fish species like perch and pikeperch. -Typical benthivorous fish feed surprisingly not on whole worms but only on detritus containing Marenzelleria bristles. -There are local differences and seasonal patterns in the feeding on Marenzelleria. -In order to understand the importance of Marenzelleria, in comparison to traditional food items, it is necessary to estimate the assimilation rate of the polychaete. -A special problem is to determine the nutritive value of Marenzelleria detritus. REFERENCES Bick, A., Burckhardt R., (1989). Erstnachweis von Marenzelleria viridis (Polychaeta, Spionidae) für den Ostseeraum, mit einem Bestmmungschluessel der Spioniden der Ostsee. Mitt. Zool. Mus. Berl. 65, 2: Bochert, R. (1993). Untersuchungen zur Reprodukton und Larval-entwicklung. Diplomarbeit, Universitaet Rostock (unpublished) Debus, L. (1989). Food composition of bream and roach from shallow, brackish coastal waters of the southern Baltic proper, with comments on possible diet overlap. Rapp.P., V. Reun. Cons. int. Explor. Mer 190: Debus, L. (1989). Veraenderungen der Ernaehrung benthophager Fische (Blei, Karpfen und Kaulbarsch) als Resultat eines grobraeumigen Besatzexperimentes mit Karpfen. Fischerei-Forschung, Rostock 29, 4: Debus, L. Winkler, H. (1990). Nahrungsuntersuchungen an benthophagen Nutzfischen unter Berucksichtigung des Nahrungsangebotes. Fischerei-Forschung, Rostock, 28, 2: Essink, K., Kleef, H.L. (1991). Distribution and life cycle of the North American apionid polychaete Marenzelleria viridis (Verrill, 1873) in the Ems eastuary. Netherlands Journal of Aquatic ecology, 27(2-4): Thiel, R. (1991). Untersuchungen zur Okologie der Jung- und Kleinfischgemaeinschaften in einem Bodengewässer der sudlichen Ostsee. Dissertation A, Universitat Rostock, (unpublished) Winkler, H.M. (1989). The role of predators in fish communities in shallow coastal waters of the Southeast Baltic. Rapp.P., v. Reun. Cons. int. Explor. Mer, 190: Winkler, H. M. (1991) Der Zander (Stizostedion lucioperca) in den Ostseerandgewassern, Bestandssituation und Bedeutung der Nahrungsbasis. Fischerei-Forschung, Rostock 29, 3: Ziteneva, T.S. (1958). On the feeding of bream in the Rybinsk reservoir. Tr. biologiceskoj stancii "Borok"3: (in Russian) Ziteneva, T.S. (1981). Consumption of Oligochaeta by bream Abramis brama orientalis Berg in different biotopes of the Rybinsk reservoir. Voprosy ichtiologii. vol. 21 (2(127)): (in Ruissian) Ziteneva, T.S., Bakanov, A.I. (1981). Evalution of the food reserve and feeding of the older age groups of bream in Volga compartment of the Rybinsk reservoir. Trudy Instituta biologii vnutrennich vod, 47(50): (in Russian) Zettler, M. (1993). Untersuchungen zur Biologie und Oekologie von Marenzelleria viridis (Polychaete, Spionidae) in der Darss- Zingster boddenkette. Diplomarbeit Universitat Rostock (unpublished)