The list of 57 macroparasite taxa reported in the present study (Additional file 1) comprises nearly 56% of the parasites found in G. morhua throughout its distributional range (a total of 97 species, resulting from compilation of data gathered as early as 1932 from both North West and North Eeast Atlantic, see ). It indicates a high regional richness of the metazoan parasites of cod in the North East Atlantic. These results conform to the diverse and non-selective diet of cod, its wide depth distribution and migratory behaviour. It is also possible that increased sampling effort has contributed to the high diversity of the parasite list reported here and this is supported by the seven new host records. Of these, only the gadoid specialist D. merlangi, which mainly parasitises whiting (Merlangius merlangus (L.), see ) belongs to the Arctic-Boreal category. The newly recorded helminth species mainly belong to generalist genera with a wide geographical distribution whereas the copepod Chondracanthus ornatus is typical of calionomid perciforms . The geographical distribution of the Callionymidae overlap with that of cod, thus its recovery indicates some interaction with calionomids.
However, the regional parasite faunas of cod exhibited a generally lower richness (63–65% of the total list) with a notable decrease in the Baltic Sea and Trondheimsfjord (21 and 32%, respectively). Parasites from all metazoan taxa were recorded in the present study, with eleven species present in all regions. The predominant higher taxa in the regional faunas in terms of number of species were trematodes and nematodes. However, the taxonomic structure of the faunas based on the relative abundance of the higher taxa revealed that nematodes (mostly anisakid larvae) represent the majority of all parasite individuals. This fact can be related to cod being a voracious predator, and with a long life-span, which facilitates larval accumulation).
The regional faunas exhibited differences with respect to both higher-level taxonomic structure and species-level comparisons. Generally, the fauna of the brackish-water regions [Baltic Sea (7–13.6‰) and Trondheimsfjord (10–33‰)] differed substantially from those in the high-salinity regions (Celtic, Irish and North seas and Icelandic waters, salinity range 34.2–35.4‰). Although variation in fish size between regions may have contributed to the observed variability, the much lower species richness recorded in the former two regions agrees with the lower salinity conditions that restrict the distribution and richness of the invertebrate fauna and consequently limiting the diversity of successful parasite life-cycles [87, 88]. Remarkably, none of the species characteristic of low salinity and freshwater distributions reported previously in cod (i.e. Podocotyle angulata (Dujardin, 1845), Raphidascaris acus (Bloch, 1779), Acanthocephalus lucii (Müller, 1777), Echinorhynchus salmonis (Müller, 1784), Neoechinorhynchus rutili (Müller, 1780) and Pomphorhynchus laevis (Zoega in Müller, 1776) [2, 87, 89]) was recorded in our collections from the Baltic Sea and Trondheimsfjord.
On the other hand, although both regional faunas consisted of marine parasite species, their structure differed from that of the faunas from the open water regions in: (i) the poorer numerical representation of nematodes; (ii) the absence of cestodes; (iii) the absence or low abundance of species with worldwide distribution; and (iv) the composition with respect to host specificity categories [i.e. the strong numerical domination of generalists (Baltic Sea fauna) or gadoid specialist species (Trondheimsfjord fauna)]. These differences, therefore, indicate different transmission conditions in the two low-salinity regions. This suggestion is further reinforced by the notably different structure of the faunas in the latter regions characterised by the numerical dominance of generalist acanthocephalans [mostly E. gadi (s. l.), Baltic Sea] or gadoid specialist trematodes (Lepidapedon spp., Trondheimsfjord).
The overall prevalence of 88.3% of E. gadi (s. l.) observed in the present study agrees well with the high levels of infection in cod recorded in previous studies: 71.4% in the southern Baltic Sea  and 99.4% in the Bornholm Basin of Baltic Sea . The mean intensities recorded here are similar (32.2 worms/host) to those in the latter study: 54.7 in smaller cod (21 to 30 cm body length) and 33.3 in larger cod (52 to 60 cm body length). Gammarid (Gammarus oceanicus) and caprellid (Monoporeia femorata) amphipods serve as intermediate hosts of E. gadi (s. l.) in the Baltic Sea . Whereas the high infection levels in small cod may indicate that amphipods are an important component their diet, the heavy infection of large cod (> 61 cm; normally not feeding on amphipods) was explained by a transfer of parasites from prey fish to the large cod . It is possible that both processes contribute to the infection of cod in the Baltic Sea collection since the size of the fish studied ranged from 31.4 to 89.6 cm (SL).
The dominance of trematodes in the Trondheimsfjord fauna reflects the highest infection levels of two Lepidapedon species (see comparative data in Additional file 1). Both species belong to the subfamily Lepocreadiinae of the Lepocreadiidae Odhner, 1905, which are found either in deep-sea fishes or in fishes from cold, shallow waters, most usually in gadiforms . Whereas the present data on the overall prevalence of L. elongatum (60%) agree with previous observations in cod (up to 94.3% at various stations in Danish and adjacent waters ; up to 62% in juvenile (0+) cod [48, 93–95]), L. rachion has so far been recovered at much lower prevalences in various locations in the North East Atlantic (range 3.3–20% vs 45%, see ). Bray & Gibson  listed a wider range of final hosts (mostly gadoids) in the North East Atlantic for the latter species (G. morhua, Melanogrammus aeglefinus, Merlangius merlangus, Pollachius pollachius, P. virens, Gymnacanthus tricuspis, Aspitrigla cuculus). Trondheimsfjord is characterised by a rich fish fauna (16 gadiform species including 10 species of gadoids: Gadiculus argenteus thori, G. morhua, M. aeglefinus, M. merlangus, Micromesistius poutassou, P. pollachius, P. virens, Trisopterus esmarki, T. minutus, Raniceps raninus; the latter uncommon, J.A. Sneli pers. comm.) and this may explain the higher infection levels of L. rachion in this region. It is also possible that the dominance of the two Lepidapedon species in the parasite fauna in cod from Trondheimsfjord is related to the presence of conditions enhancing completion of their life-cycles. The life-cycle of L. elongatum was elucidated by Køie . The rediae and cercariae develop in the gastropod Onoba aculeus and the metacercariae encyst in a variety of annelids; some may encyst in molluscs and echinoderms, but infections in these hosts are rare and probably short-lived . The first intermediate host of L. rachion is believed to be Nassarius reticulatus and the metacercariae are said to occur in planktonic cnidarians, ctenophores, chaetognaths and polychaetes . Sneli & Gulliksen  reported both intermediate hosts, O. aculeus and N. reticulatus, in Trondheimsfjord. However, the life-cycle of L. rachion has not apparently been completed experimentally. Bray & Gibson  considered the data on the second intermediate host puzzling, since the main final host of L. rachion, the haddock, Melanogrammus aeglefinus (L.), feeds as an adult almost entirely on benthic organisms. Nevertheless, cod studied at Trondheimsfjord were generally small-sized (SL range 16.5–48.0 cm) and it is possible that the proportion of small invertebrates in the diet of fish has contributed to the high representation of Lepidapedon spp.
Higher gadoid richness may also be associated with higher transmission rates which resulted in the dominance in the Trondheimsfjord fauna of the adult stages of two gadoid specialist nematodes, C. cirratus and C. gracilis. Final hosts of C. cirratus are Gadidae and Merluccidae, occasionally salmon, Salmo salar, see ). Although Anderson  suggests a direct infection of final host (by ingestion of free-living second-stage larvae, L2), calanoid (Acartia sp., Centropages sp., Temora sp.) and cyclopoid (Oitona similis) copepods and sand gobies, Pomatoschistus minutus, were found to serve as experimental intermediate hosts of C. cirratus . Third-stage (L3) larva of C. gracilis hatch from the egg in the intestinal tract of either the intermediate fish host (sand goby, P. minutus; experimental data) or an invertebrate transport (paratenic) host . Køie's  data, based on examination of 350 naturally infected cod (8–78 cm long), support this suggestion. She found that group 1 and older cod contained L3-stage larvae, intermediate stages and adult worms of C. cirratus, indicating that they could become infected throughout the year; however the pattern of infection suggested that cod over 20 cm long became infected mainly in summer by eating infected fish (including smaller cod). It is possible that the high infection levels with C. cirratus and C. gracilis in cod from Trondheimsfjord originate from ingestion of sand gobies which are common in the region.
One of the main results of the present study was the overall higher structural similarity of the parasite faunas in cod from Celtic, Irish and North seas and Icelandic waters, perhaps due to the similar oceanographic characteristics of these four regions. The domination of the generalist Arctic-Boreal anisakid nematodes [A. simplex (s. l.), C. osculatum (s. l.) and H. aduncum] represented a characteristic feature of the four regional faunas. A. simplex (s. l.) and C. osculatum (s. l.)] utilise marine mammal predators of cod [2, 71] as final hosts and follow a similar life history pattern.
Adult A. simplex (s.l.) have been reported in a large number of cetaceans and pinnipeds [71, 100]. Eggs passed by marine mammals embryonate to the L2-stage larvae in sea water. When ingested by marine crustaceans (e.g. euphasiids, copepods) they develop to the L3 stage. Teleosts become infected by ingesting the first intermediate hosts see  and references therein. Klimpel et al.  studied the life-cycle of A. simplex in the northern North Sea and found that one copepod and four euphasiid species served as obligatory intermediate hosts. These authors revealed an obligatory second intermediate host, Maurolicus muelleri (Sternoptychidae), and stated that piscivorous (Pollacius virens, Melanogrammus aeglefinus, Etmopterus spinax) and planktivorous and juvenile fishes (Clupea harengus, Trisopterus esmarki, Melanogrammus aeglefinus) serve as paratenic hosts of A. simplex. Although the data on the life-cycle of C. osculatum species complex are somewhat wanting  copepods may appear important as intermediate hosts [70, 102]).
Klimpel et al.  and Klöser et al.  suggested that A. simplex and C. osculatum, respectively, are able to utilise fish host species that are available in a given locality. This versatile behaviour coupled with the vagility of the final hosts, may explain the wide distribution and abundance of these species. H. aduncum possesses an even more resourceful life-cycle. Final hosts of this species are numerous predaceous teleosteans (clupeids, gadids, salmonids and others, see ). Third stage larvae develop in Acartia tonsa and harpacticoid copepods, various amphipods, isopods and mysids . The latter can also serve as second intermediate hosts . Furthermore, ctenophores, chaetognaths, polychaetes and ophiuroids which become infected by ingesting infected crustaceans, may act as obligatory intermediate hosts or paratenic (transport) hosts [104, 106].
Despite their overall structural similarity, the four faunas could be grouped in two pairs, those from Celtic Sea and Icelandic waters vs those from the Irish and North seas. It appears that the grouping with respect to the higher trematode representation in cod parasite faunas in Irish and North seas (vs Celtic Sea and Icelandic fauna) is related to the sampling locations. Thus, the fauna from deeper and ocean influenced locations in the Celtic Sea and Icelandic waters were dominated by nematodes whereas the more coastal and shallower locations (in the Irish and North seas) exhibited higher proportions of trematode individuals.
Overall, generalist parasites with Arctic-Boreal or worldwide distribution comprised the best represented group of the cod parasite fauna with respect to both richness and numerical dominance (due to the presence of anisakid nematodes). This finding supports the conclusion of Hemmingsen & MacKenzie  that cod acts as a distribution agent of generalist parasites in the North Atlantic because of its omnivorous diet, migratory behaviour and the mixture of stocks.