The occurrence of skin ulcers in wild fish is not a new phenomenon. ILVO researchers have been recording fish diseases and parasite occurrence in the southern bight of the North Sea since 1985, including recording the incidence of skin ulcers in dab (Limanda limanda). Since 2011, skin ulcers have been seen more often in dab, and fishermen have reported ulcers and wounds in sole (Solea solea) and whiting (Merlangius merlangus). Recently, the occurrence of ulcers has been attributed to pulse fishing, a technique that was introduced in the region in 2011. According to some fishermen, skin ulcers in fish are markedly more common in areas where pulse fishing is done. To test this hypothesis, scientists of ILVO and UGent analyzed existing databases for skin ulcer prevalence and microscopically examined tissue samples of ulcerated fish.
The monitoring data time series confirmed the increase in skin ulcer occurrence, especially acute ulcers, in dab since 2011. Prior to 2011, in 2000-2010, only a few ulcerated dab individuals were seen in the North Sea. The highest peak in ulcer occurrence was found in the autumn of 2013, during which 8% of the dab had acute ulcers or ulcer lesions. In May 2012 and April 2014, ulcerated fish were collected from areas subject to intense pulse fishing. These fish were then examined for bacteriology and histology. The results showed that skin ulcers were usually 1-6 cm in diameter and that they could be seen on either side of the body. Open sores were often infected with bacteria, but it remains unclear whether bacterial infection is a cause rather than a consequence of ulceration. Tissue samples of both the ulcers and the internal organs did not show evidence of any diseases, but they did indicate that the ulcers were more often subacute or chronic rather than acute.
Based on these results, no unequivocal relation could be found between ulcer prevalence and pulse fisheries. However, the absence of a cause-effect relation could not be confirmed either. Additional research is necessary to establish the primary cause of the increase in skin ulcer prevalence in fish.
Title: Mapping and researching wounds in flatfish and whiting in the North (VERWONDING)
Funding: ILVO Own Capital
Partner: Ghent University
Contact: Lisa Devriese