Abstract
A significant increase in the infection level of Baltic cod Gadus morhua with the anisakid nematode larvae Contracaecum osculatum and Pseudoterranova decipiens has been recorded during recent years due to the expanding local population of grey seals Halichoerus grypus, which act as final hosts for these parasites. Here, we report from an investigation of 368 cod (total length [TL] 6-49 cm; caught in ICES Subdivision 25) that the infection level of juvenile cod (TL 6-30 cm) with larvae of C. osculatum and P. decipiens is absent or very low, whereas it increases drastically in larger cod (TL 31-48 cm). A third nematode Hysterothylacium aduncum was rarely found. The study indicates that the prey animals for large cod act as transport hosts for the parasite larvae. Analyses of stomach contents of cod caught in the same area (2007-2014) showed that small benthic organisms (including polychaetes Harmothoë sarsi) are preferred food items by small cod, the isopod Saduria entomon is taken by all size classes, and sprat Sprattus sprattus are common prey items for cod larger than 30 cm. Parasitological investigations (microscopic and molecular analyses) of H. sarsi (100 specimens) and S. entomon (40 specimens) did not reveal infection in these invertebrates, but 11.6% of sprat (265 specimens examined) was shown to be infected with 1-8 C. osculatum third stage larvae per fish. Analyses of sprat stomach contents confirmed that copepods and cladocerans are the main food items of sprat. These observations suggest that the C. osculatum life cycle in the Baltic Sea includes grey seals as final hosts, sprat as the first transport host and cod as second transport host. It may be speculated that sprat obtain infection by feeding on copepods and/or cladocerans, which could serve as the first intermediate hosts. One cannot exclude the possibility that the size-dependent C. osculatum infection of cod may contribute (indirectly or directly) to the differential mortality of larger cod (>38 cm) compared to smaller cod (<30 cm) recently recorded in the Baltic cod population.
Highlights
Baltic cod is a stationary local strain of the Atlantic cod Gadus morhua inhabiting the Baltic Sea, a semienclosed brackish water area connected to the North Sea through the narrow Danish straits
A few fish (TL 21−30 cm) harboured a low infection (1 to 2 parasites per fish) but only with Contracaecum osculatum (Table 1, Fig. 1). The occurrence of both C. osculatum and Pseudoterranova decipiens increased with host size from 31 to 49 cm, with regard to the first species
No worm infections were detected in Harmothoë sarsi and Saduria entomon by any method, but sprat was found infected with C. osculatum (11.6%, 1−8 parasites per fish, mean no. of parasites per infected fish = 1.6) and H. aduncum (0.4%, mean no. of parasites per fish = 1.0)
Summary
Baltic cod is a stationary local strain of the Atlantic cod Gadus morhua inhabiting the Baltic Sea, a semienclosed brackish water area connected to the North Sea through the narrow Danish straits. (Perdiguero-Alonso et al 2008, Buchmann & Kania 2012, Haarder et al 2014, Mehrdana et al 2014, Nadolna & Podolska 2014) This development has been noted during a period with a marked increase of the grey seal Halichoerus grypus population (Haarder et al 2014). These pinnipeds are final hosts for both C. osculatum and P. decipiens (Marcogliese et al 1996, McClelland 2002, Mattiucci & Nascetti 2008, Skrzypczak et al 2014, Lunneryd et al 2015). We conducted a basic parasitological investigation of different size classes (TL 6−49 cm) of Baltic cod caught by the research vessel ‘DANA’ from 2013 to 2015 in order to elucidate any sizedependent infection with larval nematodes. Samples of main prey organisms identified were caught during the same research cruises and subsequently examined for presence of parasite larvae
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