Nematodes from 361 fish from 15 species were collected between August 2010 and March 2012 at Langøya and at Torbjørnskjær in the outer Oslofjord. The study focused on the infection levels of the three most abundant nematode species, Hysterothylacium aduncum, Anisakis simplex and Contracaecum osculatum, in the three flatfish species American plaice (Hippoglossoides platessoides), witch flounder (Glyptocephalus cynoglossus) and common dab (Limanda limanda).
When all nematode and fish species were pooled the percentage of infected fish (i.e prevalence), the average number of nematodes per fish (i.e. abundance) and the average number of nematodes per infected fish (i.e. the intensity) were significantly higher at Langøya than at Torbjørnskjær. While H. aduncum and C. osculatum had higher abundances at the western site, A. simplex was more abundant at the eastern site. Among the three flatfish species, American plaice had a relatively high prevalence both at Langøya and at Torbjørnskjær (55 and 53 % respectively). Witch flounder was only infected at Langøya (59 %) and mainly with H. aduncum (53 %). Common dab had a high prevalence at Langøya (80%) and rather low at Torbjørnskjær (15 %). However, since the sample size was small for witch flounder at Torbjørnskjær (n = 4) and for common dab at Langøya (n = 5), these results are likely to be biased.
This study also supports the general view that there is a large fluctuation in the nematode populations in the outer Oslofjord. The previously observed decrease of A. simplex seems to continue. C. osculatum and H. aduncum are getting more widespread in the Oslofjord. In addition, my data contain the first recording of the flatfish nematode Cucullanus heterochrous at Langøya, suggesting that also this species is expanding in the Oslofjord.
The lengths of the nematodes were used to investigate whether the three flatfishes are suitable hosts for the three nematode species. American plaice and witch flounder are good transport and final hosts for H. aduncum. American plaice seemed to be suitable transport host for C. osculatum as well. Despite the fact that A. simplex is known to have a pelagic life-cycle with Atlantic herring (Clupea harengus) as the main transport host, my observations show that it can also use the benthic common dab as transport host in the outer Oslofjord.
The number of nematodes per witch flounder and common dab had a random distribution (Poisson distribution). However, the number of Pseudoterranova decipiens per American plaice was highly over-dispersed. It is likely that a combination of different factors, especially the feeding ecology and habitat preferences of the fish, is the cause for the observed infestation patterns. In addition, American plaice and witch flounder harboured a more diverse parasite fauna than common dab. Finally, the length measurements indicate that there may be some interspecific interaction between the nematodes.