ABSTRACT
Environmental changes or stressors can result in the development of diseases. Through regular fish disease surveys in the Belgian part of the North Sea, attention was drawn to a sudden increase of skin ulceration prevalence between 2011 and 2014 in common dab (Limanda limanda). Information on prevalence, ulceration, bacteriology, fish-related (e.g., length, age, and sex) and (spatial and temporal) environmental factors, and fishing intensity were gathered. This detailed investigation was framed within a long-term monitoring program, executed every spring–autumn from 2000 to present. Ulcerations were observed in 1.3% of fish (n=3,999). Spatial and temporal differences were evident, and highest prevalence was found in summer. Vibrio was the dominant cultivated bacterial genus present in the lesions. Skin ulcerations appeared to be correlated with length and body condition of the fish, as well as with temperature and pH of the seawater and fishing vessel density. Our research suggested the involvement of multiple factors in the development of skin ulcerations in common dab and endorsed the effects of changing environment and human influence on the marine ecosystem through activities such as fishing.
INTRODUCTION
The Belgian part of the North Sea (BNS), is influenced by human activities such as shipping and fisheries, and climate change is affecting temperature and pH of the seawater. Environmental changes can result in functional alterations in biochemistry and physiology of the blood and tissues of fish, resulting in pathologic changes and causing diseases (Sindermann 1990; Giltrap et al. 2017). Fish diseases can, therefore, be relevant as bio-markers of environmental quality and health (Snieszko 1974; Giltrap et al. 2017). International standardized monitoring surveys were performed in various regions in the North Sea, guided by the International Council for the Exploration of the Sea (ICES). In the BNS, monitoring of fish diseases has been carried out since 1985 (Devriese et al. 2015). Through these regular fish disease surveys, attention was drawn to a sudden increase of skin ulceration prevalence in 2011 (Devriese et al. 2015). A similar increase was observed in the German Bight (ICES 2012) and, in cod (Gadus spp.), populations in the Polish economic exclusive zone of the Baltic Sea (ICES 2012). This sudden change in disease prevalence preoccupied the scientific and sea-guarding communities (Devriese et al. 2015) and triggered research on the associations between the prevalence and possible environmental and anthropogenic risk factors.
Skin ulcerations are defined as lesions where epidermis and basement membranes are missing, resulting in exposure of the dermis and muscle (Wiklund and Bylund 1993). On the basis of previous reports and observational data, flatfish are more vulnerable to the development of these lesions than are round fish (Möller 1981; Wiklund and Bylund 1993). We focused on common dab (Limanda limanda) because it is a common flatfish and is included in the fish disease monitoring.
Inferences on the cause of these skin ulcerations are complex, and a multifactorial etiology is suspected. Presumably, fish-related characteristics, environmental conditions, and pathogenic agents play a direct or indirect role in the development of skin ulcerations. In 2015, two bacteria, Vibrio tapetis and Aeromonas salmonicida, were isolated from active skin ulcerations in dab in the BNS (Vercauteren et al. 2018). Each bacterium is separately able to cause these ulcerations experimentally, but skin abrasions seemed to be a major contributing factor in the development of skin ulcerations, indicating the multifactorial causality (Vercauteren et al. 2019, 2020).
To grasp the complexity of the etiology of skin ulceration, a multidisciplinary survey was conducted to quantify the disease's geographic spread, prevalence rates, and involvement of bacteria. Associations between the environmental and anthropogenic parameters and the prevalence of skin ulcerations were studied on the basis of data collected in the BNS during surveys every 2 mo from 2016 until 2019 (six campaigns per year).
MATERIALS AND METHODS
Study area and sampling sites
The study was carried out in the BNS (3,457 km2), part of the Southern North Sea (Marineregions 2019). Eight sampling sites were chosen (Fig. 1; commonly used names for the locations are provided in Supplementary Material S1). The sampling sites were monitored every 2 mo between 2016 and 2019 on a ship equipped with a beam trawl (3 m; mesh size 22 mm) and a conductivity-temperature-depth (CTD) probe (Seabird 19plusV2, Sea Bird Electronics, Bellevue, Washington, USA). Fish were caught by a trawl towed approximately 20 min (3–4 knots; 3.7–5.6 km/hr) at each site during each survey. Sampling was carried out in accordance with the approved guidelines and legislation in force with regard to animal welfare.
Map of the survey locations (A–H) in the Belgian part of the North Sea that were monitored during this research. The outline of the Belgian part of the North Sea is provided together with the 3, 12, and 24-nautical mile (nm) zones.
Map of the survey locations (A–H) in the Belgian part of the North Sea that were monitored during this research. The outline of the Belgian part of the North Sea is provided together with the 3, 12, and 24-nautical mile (nm) zones.
Fish surveys—Disease prevalence
The total catch was sorted and dabs were subjected to further investigation. Each dab was examined thoroughly for the presence of externally visible diseases. Fish with skeletal deformities, hyper- or hypopigmentation, healed lesions, and papilloma-like lesions were excluded for further analyses, but information regarding these abnormalities is included in Supplementary Material S2.
Fish with skin ulcerations were investigated thoroughly. The ulcerations were recognized as clearly delineated hemorrhagic lesions with a rim of cutaneous petechial hemorrhages surrounding the lesion, in agreement with the guidelines compiled by ICES (Bucke et al. 1996) and with our previous research (Vercauteren et al. 2018). The prevalence of the disease was calculated as the number of fish with skin ulcerations divided by the total number of dab caught at each sampling site. Catch per unit effort (i.e., the number of dabs caught per hour of trawl time) for each sampling site and each survey was used as a proxy for population density (Mellergaard and Nielsen 1995).
Examination of ulcerations
On board, fish with ulcerations were euthanatized with an overdose of benzocaine (ethyl 4-aminobenzoate, final concentration 200 mg/L of seawater; Sigma Aldrich, Overijse, Belgium) or tricaine methanesulfonate (MS-222, 500 mg/L, Sigma Aldrich, Overijse) and subjected to an examination. The characteristics of the ulcerations were noted: location on the body, stage (acute or healing), and intensity (i.e., the number of ulcerations per fish). A swab from an active edge of the ulcer was taken for bacteriologic examination. Sagittal otoliths were collected to determine the age (ICES 2016). On board, a full postmortem examination was performed immediately after euthanasia to inspect the internal organs on macroscopically visible abnormalities. From photographs, the area (cm2) of ulceration was measured by scientific image software (ImageJ, version 1.4).
Bacteriologic examination
Bacteriologic swabs of the lesions of 48 fish were immediately inoculated on marine agar (Scharlau Microbiology, Barcelona, Spain), Shieh agar supplemented with 1.5% sodium chloride (Shieh and Maclean 1975; Shieh 1980; Song et al. 1988), and Columbia agar with 5% sheep blood (blood agar PB5039A, Oxoid, Hampshire, UK). In the lab, agar plates were incubated for 1 wk at 16±1 C, isolates were purified, and pure cultures were frozen. Cultures with more than five colony types (without a clear dominant type) were not further analyzed. Frozen samples were thawed later and plated on their original medium type. After 24 hr, colonies were identified with the matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) technique by the direct transfer method. Each isolate was spotted on a steel target plate in triplicate, air dried, covered with 1 µL alphacyano-4-hydroxycinnamic acid matrix (Bruker Daltonics, Bremen, Germany), and processed with an Autoflex III Smartbeam MALDI-TOF MS (Bruker Daltonics) and commercial software (flexControl 1.4, version 3.4, Bruker Daltonics). The spectra were analyzed by MBT Compass version 4.1 (Bruker Daltonics) that included a reference database (including V. tapetis and A. salmonicida). A log10 score between 1.7 and 2.0 represented identification at species level at low confidence, and a value ≥2.0 represented identification at high confidence.
Fish-related factors
Total length (L), weight (W), and sex were determined for each dab caught. The Fulton body condition factor was calculated from the weight and length (K=100(W/L3); Fulton 1904). Because of practical restrictions, fish smaller than 10 cm (n=1,071) were not weighed separately, although they were individually inspected for the presence of diseases. These fish were excluded from further analyses.
Environmental factors
On board ship, seawater temperature, depth, turbidity, oxygen saturation, and salinity of the water were measured just above the seabed with the conductivity-temperature-depth probe. The pH of the water collected at the seabed was measured (Hanna Instruments, Temse, Belgium).
Data on anthropogenic activities on each sampling location was extracted from Kustportaal (2019). The unsmoothed Atlantic Multidecadal Oscillation index, representing a monthly index of the North Atlantic sea surface temperatures, was downloaded (NOAA-PSL 2019). Data on the degree of pollution (concentration of heavy metals and polychlorinated biphenyl in the sediment; granular fraction <63 µm) was assessed by using the five defined pollution cluster zones in the BNS (Lagring et al. 2018). Distance from the shore was calculated perpendicular to the shore from the coordinates of the sampling points. Information regarding substrate on the seabed was downloaded from the online database of the European Marine Observation and Data Network (EMODnet)–Seabed Habitats (EMODnet 2019a). Mean shipping density (various types of vessels) and mean fishing vessel density (hr/km2 per month) were downloaded from the EMODnet Human Activities data portal (EMODnet 2019b). Both fishing vessel density in the month of the survey and 1 mo before were used.
Data analysis—Determination of risk factors linked to ulceration development
Risk factors linked to ulceration development were determined by a linear regression model (LRM). Collinearity between variables was tested beforehand by graphic interpretation and the Pearson correlation coefficient. If collinearity was observed, only one of the variables (the one considered to be biologically the most relevant) was introduced in the model. Collinearity was observed between length and weight, length and age, depth and distance to shore, season and seawater temperature, and season and fishing intensity.
Stepwise model selection by LRM was performed with forward selection by adding the factor with lowest P value in each step. The model was considered to be final when the addition of extra factors did not result in improvement of the model (according to Akaike information criterion) or significance of newly added factors. The presence or absence of ulcerations was used as the response variable. Fish-related and environmental factors that were used as explanatory variables are listed in Table 1. A mixed logistic regression model was tested with sampling location as the random intercept; however, this test resulted in a singular fit of the data. Despite the overfitting, the results of the mixed logistic regression model and LRM were similar. All statistical analyses were performed by RStudio (Boston, Massachusetts, USA).
RESULTS
Catch data
A total of 3,999 dab (>10 cm) were caught. Fish (with and without ulceration) had an average length of 17.2±3.5 cm with a maximum size of 31 cm. The average weight was 55.4±36.5 g. The average body condition (K) of the fish was 0.99±0.26, and the fish were between 1 and 7 yr old. In 3,819 (95%) dabs, sex was determined; most fish were females (59%). Sex ratios were mainly similar (40% male and 60% female), with exceptions at sites A, F, and G. Except for site G, females always dominated. Female fish had an average length of 17.8±3.7 cm and male fish of 16.3±2.9 cm.
Overall ulceration prevalence
Of the 3,999 examined fish (>10 cm), 52 fish (1.3%) had a skin ulcer. Maximum ulceration prevalence observed per location per sampling when sufficient (>50) fish were caught was 16%. Skin ulcerations (1.3%) were the second most prevalent disorder, the most prevalent were hyper- or hypopigmentation disorders (see Supplementary Material S2).
Examination of ulcerations
Of the 52 fish with ulcerations, 27% showed signs of healing, recognized by a white, raised edge around the lesion and little to no hemorrhage in the surrounding tissue. Even though they were healing, the centers of the lesions were still ulcerated. No distinction between active and healing ulcerations is made in the analysis in this study.
Lesion surface area ranged between 0.03 cm2 and 1.8 cm2 with a mean size of 0.4±0.5 cm2. The highest number of ulcerations per fish was five, which was observed in two fish. In the majority of fish (75%), only one ulcer was present. Ulcerations were mainly located on the nonpigmented side (65%) and to a lesser extent on the pigmented side (33%). The same distribution was found in males and females, with slightly different ratios. A total of 52% of the ulcerations were found on the ventral side (independent of pigmented or nonpigmented side). No consistent abnormalities in the internal organs were observed in fish with skin ulcerations.
Bacteriologic examination
Bacteriologic results indicated that Vibrio was the most commonly cultivated bacterial genus in the skin ulcerations, present in 81% of fish with skin ulcerations. Additional identified genera were Bacillus (14%), Psychrobacter (14%), and Actinobacteria (10%). Other genera were present in less than 5% of the ulcerations. Among the Vibrio, Vibrio alginolyticus was the most prevalent (27%), with V. tapetis (23%), Vibrio tasmaniensis (18%), Vibrio gigantis (13%), and Vibrio harveyi (4%) also isolated. Although all those bacterial species were found in a polyculture, they appeared to be dominant. The three most prevalent species were each found once in a pure culture. An Aeromonas sp. was identified in one lesion with a low confidence score for species identification.
Fish-related differences in ulceration prevalence
Mean standard lengths were 20.5±2.9 cm for fish with ulcerations and 17.2±3.5 cm for healthy fish. The highest prevalence of skin ulcerations was found in fish larger than 25 cm, and no ulcerations were found in fish smaller than 15 cm (Table 2). The weight of fish (in the same length class; 15–28 cm) with and without ulcerations was 77.9±33.4 g and 66.7±33.6 g, respectively. Fish with ulcerations had a body condition factor of 0.9±0.1. Mean body condition of healthy fish (from the same length class; 15–28 cm) was 1.0±0.2. Male and female fish showed skin ulceration prevalences of 1.3% and 1.4%, respectively. Male fish had on average 1.3±0.9 ulcerations per fish and females 1.5±0.9 ulcerations per fish. Age was determined for 45 fish with skin ulcerations and control fish (n=35). Fish with skin ulcerations were on average 3.5±1.2 yr old, and healthy fish were 2.9±1.0 yr old.
Number of common dab (Limanda limanda) from the Belgian part of the North Sea with and without skin ulcerations in each length class, on which basis, prevalence was calculated as the ratio of the number of fish with ulcerations divided by the number of fish without an ulceration in each length class.

Spatial and temporal differences in ulceration prevalence
Ulceration prevalence of fish from different sampling locations varied between 0.6% and 2.8%. Stations C (2.1%) and E (2.8%) showed the highest prevalences of skin ulcerations (Fig. 2). The highest prevalence of skin ulcerations was found in summer with 3% of the fish affected. In spring, 1.4% of the fish showed skin ulcerations. In winter prevalence was 0.7%, and in fall it was 0.4%. This pattern was roughly similar in all years except in 2016, with more ulcers in winter. A summary table of the spatial and temporal variability in environmental factors is available in Supplementary Material S3.
Spatial distribution in skin ulceration prevalence of the eight locations (A–H) in the Belgian part of the North Sea that were sampled on the 4-yr monitoring survey.
Spatial distribution in skin ulceration prevalence of the eight locations (A–H) in the Belgian part of the North Sea that were sampled on the 4-yr monitoring survey.
Determination of risk factors linked to ulceration development
Analyses on possible risk factors for the development of skin ulcerations were only made with fish larger than 15 cm, because no ulcerations were observed in smaller fish. The prevalence of ulcerations was, according to the LRM, significantly associated with length (P<0.001) and body condition (P<0.001) of the fish (Table 3). Sex was not significantly associated with the presence of a skin ulceration (P=0.705). Most of the environmental parameters did not show an association with skin ulcerations (P>0.050). Both seawater temperature (P<0.001) and pH (P=0.031) showed a significant positive correlation with the presence of skin ulceration (Table 3). Fishing vessel density during the month of the survey showed both spatial differences and seasonal variations and was positively associated with skin ulcerations (P=0.017; Table 3). Fishing density 1 mo before the survey did not affect ulceration development nor did the general shipping density (various types of vessels).
DISCUSSION
The health status of the North Sea has improved since the 1980s (Lang et al. 2017). Nevertheless, sudden and climate-driven changes, new stressful stimuli, and emerging contaminants may be resulting in increases in the prevalence of skin ulceration (Fig. 3; Mellergaard and Nielsen 1997; Devriese et al. 2015). Formulating inferences on causality of changes is not straightforward, and the presumed multifactorial etiology of skin ulcerations acts as a complicating factor. A multidisciplinary, 4-yr-long survey was needed to get insights into the correlation between risk factors and the presence of skin ulcerations. In our study, variations in prevalence were observed and could be explained by fish-related characteristics, environmental factors, and fishery activity, confirming the presumed multifactorial etiology of skin ulcerations. We believe that the results we obtained are relevant for all marine ecosystems.
Evolution of average disease prevalence (calculated as the number of fish with skin ulcerations divided by the total number of fish caught) of skin ulcerations in common dab (Limanda limanda) in the Belgian part of the North Sea over time. Data collected during the standardized two-yearly (SP=spring; AU=autumn) monitoring survey by the Flanders Research Institute for Agriculture, Fisheries and Food with research vessel Belgica. The numbers above each bar represent the number of fish caught. ND=prevalence not determined; survey was cancelled.
Evolution of average disease prevalence (calculated as the number of fish with skin ulcerations divided by the total number of fish caught) of skin ulcerations in common dab (Limanda limanda) in the Belgian part of the North Sea over time. Data collected during the standardized two-yearly (SP=spring; AU=autumn) monitoring survey by the Flanders Research Institute for Agriculture, Fisheries and Food with research vessel Belgica. The numbers above each bar represent the number of fish caught. ND=prevalence not determined; survey was cancelled.
The prevalence of ulceration found in this study (approximately 1.3%) was in line with the values (1.9%) of the long-term, external fish diseases monitoring of the same period (autumn; Fig. 3). Both were lower than the skin ulceration prevalences (5%) observed in 2011–14, a period that seemed to be an exception on the normal pattern. Nevertheless, the current prevalence is still slightly higher than observed before 2011 (1.0%), which needs further attention and stresses the importance of executing long-term monitoring for fish diseases.
Fish-related characteristics
This study confirmed the choice of investigating skin ulceration for regular monitoring purposes (Bucke et al. 1996) on dabs greater than 15 cm, because no fish smaller than 15 cm showed skin ulceration. The length of the fish were positively correlated with the presence of skin ulcerations (Table 2), suggesting that larger fish were more likely to develop ulcerations, in agreement with previously reported results in dabs (Mellergaard and Nielsen 1995; Mellergaard and Nielsen 1997; Vethaak et al. 2009) and flounder (Platichthys flesus; Wiklund and Bylund 1993). Wiklund and Bylund (1993) noticed that the length of the smallest flounder found with a skin ulcer correlated with the length of sexual maturity, presumably related to behavioral and anatomical changes. Similarly, the chance of dabs developing an ulcer might increase after sexual maturity, which occurs at a length of about 14 cm in males and 11 cm in females (Rijnsdorp et al. 1992). However, testing this hypothesis would require a more extensive study because it cannot explain the observation that the prevalence of ulceration increases with length.
Skin ulcerations were negatively correlated with body condition, reflecting the nutritional stage of the fish. The association between body condition and skin ulcerations has not been consistently reported in fish. Mellergaard and Nielsen (1995) reported no correlation between skin ulcerations and body condition, and Mellergaard and Nielsen (1997) reported a negative correlation, as did Vethaak et al. (2011). This negative correlation is merely an observation, and the cause of this correlation is not clear. Fish with lower body condition might be more susceptible to diseases. However, the presence of an ulcer can cause a depression of the body condition, for example, by impeding the osmotic balance (Mellergaard and Nielsen 1997). Further research is needed.
We did not find sex to be associated with the development of skin ulcerations. There is no consensus on the association between sex and the development of skin ulcerations (Mellergaard and Nielsen 1995, 1997; Dethlefsen et al. 2000). Sex-related differences were mostly linked to differences in size (Mellergaard and Nielsen 1997) or migratory patterns and in growth and physiology (Dethlefsen et al. 2000).
Ulcerations were more commonly observed on the nonpigmented side of the studied dabs. Possible differences in susceptibility to ulcer development of the pigmented and nonpigmented sides of flatfish have been a point of interest of researchers without consensus (Vethaak 1992; Wiklund 1994). Vethaak (2013) reported higher incidences of skin ulcerations on the nonpigmented side in wild-caught flounder. Wiklund and Bylund (1993) reported a sex-related distribution of ulcerations on pigmented and nonpigmented sides of flounder. Possible morphologic differences might explain this difference (Failde et al. 2014), but we did not observe such in histologic sections of dab skin. Functional differences between both sides might also explain differences because pathogens can reside in the sediment, thereby exposing the nonpigmented side to higher bacterial concentrations. However, the exact reasons for this difference remains obscure.
Bacteriologic research
On the basis of bacteriologic examination, Vibrio spp. seem to be important pathogens regularly isolated from these lesions. Not only V. tapetis, previously described as an important factor in the development of skin ulceration (Vercauteren et al. 2018, 2019), V. alginolyticus seemed abundantly present. Outbreaks of vibriosis caused by V. alginolyticus have affected several marine aquaculture systems, such as sea bass (Dicentrarchus labrax; Khalil et al. 2019), seabream (Sparus aurata; Zorrilla et al. 2003; Khalil et al. 2019), and turbot (Scophthalmus maximus; Austin et al. 1993). Clinical signs linked with infection include hemorrhagic ulceration (Austin and Austin 2012; Khalil et al. 2019). Vibrio spp. have increasingly been reported in the North Sea, wherein their geographic spread could be correlated with biotic and abiotic factors such as seawater temperature, salinity, and phytoplankton composition (Oberbeckmann et al. 2012). Importantly, sampling and analytical methods are inherently somewhat limited. Bacterial species that cannot be cultivated on agar plats or species that are slow growing and thus easily outcompeted will not be detected. Furthermore, other possible contributors, such as viruses and fungal pathogens, will also not be detected. Molecular methods could offer an all-encompassing view on contributors in skin ulceration development.
Environmental characteristics
Temporal variation in disease prevalence was observed, with the highest prevalence mostly observed in the spring and summer, in contrast with studies that report peaks mainly in autumn (Wiklund and Bylund 1993; Devriese et al. 2015). The temporal pattern in our study appeared to be mainly driven by seasonal changes in seawater temperature and pH, with increases in both factors resulting in a higher chance of developing skin ulcerations. We assumed this to be an indirect effect that can act both on the pathogens involved and on the susceptibility of the fish to fight infection. Correlation between skin ulceration development and rising seawater temperature have been recently described in various species after El Niño events (Lamb et al. 2018). Both seawater temperature and pH are expected to be changing slowly and might have a more seasonal effect (Mellergaard and Nielsen 1997). Oxygen depletion affects the development of epidermal papilloma and lymphocystis, but not the development of skin ulcerations (Mellergaard and Nielsen 1995, 1997), as observed in our study. On the basis of our data, pollution was not directly associated with the development of skin ulcerations. Vethaak et al. (2011) pinpoint a role of pollution in the development of skin ulcerations in flounder. Changes in bioavailability or biotransformation, or changes in redistribution of these substances, might explain the observed effects (Vethaak et al. 2011).
The methods we used did not allow measurement of sudden environmental changes (changes that occurred between two monitoring campaigns). Because of the absence of associations between an environmental factor and the presence of skin ulcerations, we cannot exclude an important influence of short-term fluctuations. For salinity, such fluctuations were described to be important in skin ulceration development (Mellergaard and Nielsen 1997; Vethaak 2013). Additional regular measurements of environmental parameters could possibly include these sudden changes in the model. Our short-term study revealed important environmental risk factors to be considered for long-term monitoring in any area (e.g., long-term pH data are not available).
Fishing intensity
One important result of this study was the association between fishing vessel density and the presence of skin ulcerations. The fishing vessel density map was based on the Automatic Identification System and not on the Vessel Monitoring System, which might result in an underestimation of vessel density but is considered a good proxy (Natale et al. 2015; ICES 2019). Both locations with the higher prevalence of skin ulcerations (areas C and E in Fig. 2) are core fishing grounds for offshore fisheries (van der Reijden et al. 2018). Previous research in the Skagerrak region has also hypothesized that fishing intensity can be linked with the development of skin ulcerations (Mellergaard and Nielsen 1997). The hypothesis is that fish can be injured during the fishing process (Lüdemann 1993; Davis and Ottmar 2006). Such injuries might develop into ulcerations after the fish are released. The link with fishing intensity might also explain the effect of the length of the fish, because fish in the length range between 18 cm and 23 cm might be able to escape the net of commercial trawls or be discarded (Miller and Verkempynck 2016). Both can result in abrasion of the skin, which can in further stages lead to skin ulceration development. The necessity of skin abrasion for the development of skin ulcerations has been shown in experimental trials (Vercauteren et al. 2019). However, one location (area A in Fig. 2) has a high density of fishing vessels in spring but does not show increased skin ulceration prevalence, indicating that fishing intensity is only part of the cause. Gear type might also influence ulceration because location A is in an area where only shrimp boats may fish. Our finding of a significant correlation between fishing intensity and ulceration development is unique and contributes to understanding the multifactorial etiology of skin ulcerations. An analysis of fishing intensity in the Belgian part of the North Sea between 2011 and 2014 might provide some insights on what might have changed.
The results of this research pointed toward possible multiple risk factors involved in the etiology of skin ulcerations in dab in the BNS, including fish-related (length and condition), temporal, and spatial (temperature, pH, and fishing intensity) factors. Future laboratory experiments will help identify causalities of the identified risk factors.
ACKNOWLEDGMENTS
The research was funded by the European Fisheries Fund (EVF project VIS/15/A03/DIV), the Flemish Government, and the Research Foundation–Flanders (FWO). This work makes use of resources, facilities and services provided by Ghent University and Flanders Marine Institute as part of the Belgian contribution to the European Marine Biological Resource Centre–European Research Infrastructure Consortium (EMBRC-ERIC). Flanders Marine Institute (VLIZ), the Research Institute of Agriculture, Fisheries and Food (ILVO), and the crew of the research vessel Simon Stevin are gratefully acknowledged for help during the surveys. All coworkers of Ghent University and VLIZ that helped on board research vessel Simon Stevin are gratefully acknowledged. We thank Bavo De Witte, Kevin Vanhalst, and coworkers of ILVO for the work regarding the fish disease monitoring. We acknowledge the ILVO for the analysis of the otoliths. Gert Everaert and Pascal Hublutzel (VLIZ) are thanked for their help in the initial data analysis. The authors thank Serge Verbanck for the MALDI-TOF MS analysis on the bacterial isolates. The MALDI-TOF mass spectrometer was financed by the Research Foundation Flanders (FWO-Vlaanderen) as Hercules project G0H2516N (AUGE/15/05).
SUPPLEMENTARY MATERIAL
Supplementary materials for this article are online at http://dx.doi.org/10.7589/JWD-D-20-00088. Also the data is available in the data repository and can be found online at https://doi.org/10.14284/410.