Parasite diversity in plaice (<i>Pleuronectes platessa</i>): potential tool for stock identification in Icelandic waters?

Understanding the stock structure of a commercial species is essential for sustainable management. Failure to do so can lead to the depletion of regional sub-populations, erosion of genetic diversity, and ecosystem services loss. Plaice, <i>Pleuronectes platessa</i>, is a commercially exploited species inhabiting the continental shelf around Iceland. Despite a tagging study providing support for strong spawning site and feeding ground fidelity, and otolith microstructure analysis revealing local population structure, plaice is managed as a single stock in Icelandic waters. Here, we describe and quantify the parasite fauna of plaice and assess the potential of parasites as biological tags for stock identification of plaice in Icelandic waters. A total of 82 plaice were sampled from different geographical locations (north and south) and seasons (summer and winter) in Iceland. Our sampling identified 11 parasites, five of which are new parasite records for plaice in Icelandic waters: the trematodes <i>Zoogonoides viviparus</i> (adults) and <i>Rhipidocotyle</i> sp. (metacercariae), and the nematodes <i>Contracaecum osculatum</i> (larvae), <i>Dichelyne</i> sp. (adults), and <i>Hysterothylacium aduncum</i> (larvae and adults). Additionally, we recovered metacercariae of the trematode genus <i>Apatemon</i>, which has not been recorded previously from plaice. Two parasites were identified as potential biological tags for stock identification - namely, the nematode <i>Anisakis simplex</i> and the trematode <i>Z. viviparus.</i> Our findings support a complex stock structure for plaice in Icelandic waters and the need for an integrative strategy to stock identification to provide fine spatial scale data required to inform fisheries managers.