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RESEARCH PRODUCT

Parasite communities of the European Cod "Comunidades parásitas de Bacalao en aguas de Europa"

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The structure of the metazoan parasite faunas and communities in Atlantic cod, Gadus morhua L., from six NE Atlantic regions [Baltic, Celtic, Irish and North seas, Icelandic waters and Trondheimsfjord (Norway) and two fish farms in Iceland and Scotland] has been studied. 1,254 fish were sampled in the NE Atlantic during 2002-2003. Altogether 57 parasite taxa were found. The predominant groups in regional parasite faunas were the trematodes (19 species) and the nematodes (13 species). Nine parasite species were found for the first time in cod (Diclidophora merlangi, Rhipidocotyle sp., Fellodistomum sp., Steringotrema sp., Schistocephalus gasterostei, Cucullanus sp., Spinitectus sp., Acanthochondria soleae and Chondracanthus ornatus). Contracaecum osculatum, Hysterothylacium aduncum and Echinorhynchus gadi were the only parasite species found to infect farmed fish. Regional parasite faunas showed lower richness with respect to the total list (c. 65%) with a notable decrease in the Baltic Sea and Trondheimsfjord (21 and 32%, respectively). Eleven species were present in all regions: Lepidapedon elongatum, Anisakis simplex, C. osculatum, H. aduncum, H. rigidum, Pseudoterranova decipiens, Ascarophis crassicollis, Capillaria gracilis, Corynosoma semerme, C. strumosum and E. gadi. Despite of the small size of the monogenean D. merlangi recovered in the present study, the Analysis of Principal Components showed that morphologically they are more similar to D. merlangi from whiting (Merlangius merlangus, type-host), than to other congeneric species from the North Atlantic, thus supporting their assignment to D. merlangi. The aspect and smaller size of the oöcytes suggested that D. merlangi on cod could not produce viable ova, although a specimen exhibited an egg with normally sized and shaped shell. The most species rich and abundant parasite communities were observed in cod from the open water regions (Celtic, North and Irish seas and Icelandic waters). Overall, parasite infracommunities exhibited lower predictability than component communities, due to the fact that only a restricted set of the species contributing to the similarity between component communities exhibited high abundance and dominated infracommunities. The multivariate techniques applied to examine similarity patterns of component communities across regions exhibited good agreement and detected distinct compositional segregation of those in cod from the two low-salinity regions. The highest homogeneity with respect to the composition and structure of parasite communities was observed Celtic, Irish and North Sea cods. Decay of similarity with geographical distance was observed in component communities but not in regional parasite faunas, the higher homogenisation of the latter being related to the migratory behaviour of cod and the domination of generalist parasites widely distributed. The spatial compositional autocorrelation exhibited by component communities and the substantially higher rates of similarity decay compared to other marine fish systems indicate that communities in cod are strongly constrained by the spatial configuration of locations and the dispersal abilities of parasites. Nested subset analyses revealed non-random patterns of faunal/community composition with poor faunas from low-salinity regions (Baltic Sea and Trondheimsfjord) nested in the richer faunas/communities from the high-salinity open water regions. The comparison of the learning behaviour of the three classification approaches, Random Forests (RF), Linear Discriminant Analysis and Artificial Neural Networks, using the same version of the parasite community data, revealed that RF appears as the best classifier. Anisakid nematodes, C. cirratus, D. varicus, H. communis, E. gadi, and C. adunca were selected as important for RF model development. The high accuracy of the predictive models developed for the Baltic and Icelandic samples indicate that the populations of these stocks can be confidently differentiated from the other stocks studied in the NE Atlantic. These results suggest that parasite community data can be used successfully to discriminate cod populations (putative stocks) of the NE Atlantic cod using RF.