Recent Data on Nematode Infestation of Anchovy (Engraulis encrasicolus) on the Romanian Black Sea Coast

Abstract

Marine fish populations can be affected by various infectious and parasitic diseases. The species investigated during 2021–2023 along the Romanian coast was European anchovy [Engraulis encrasicolus (Linnaeus, 1758)], a small pelagic fish with both economic and ecological value. Four species of endoparasites (nematodes) were identified, namely: Hysterothylacium sp. Ward & Magath, 1917, Contracaecum sp. Railliet & Henry, 1912, Porrocaecum sp. Railet & Henry, 1912, and Anisakis sp. Dujardin, 1845. Nematode worms were found as larvae and infested the abdominal cavity, both in the free state and enclosed in internal organs. The parameters analyzed included abundance, prevalence, and average intensity. Abundance results indicated 14 parasites/fish in length classes ranging from 11–12 cm. A prevalence of 100% was reported in the anchovy population sampled from several stations. A high level of infestation intensity was recorded in 11 cm long specimens (15 nematode worms/host). In this study, the accumulation of nematode worms was observed in large mature specimens, with a negative impact on the host organism through the presence of internal lesions, slowing of growth rate, appetite reduction, and disturbance of the reproductive process. The potential risks to anchovy stock status, threats to human health, and control measures are also tackled.

1. Introduction

Anchovy [Engraulis encrasicolus (Linnaeus, 1758)] is a common, pelagic fish of great commercial importance, found throughout the Black Sea basin apparently as a single stock [1], forming dense shoals, and approaching the shore from April to September [2]. On the Romanian Black Sea coast, the most important agglomerations occur in the southern part of the continental shelf [2]. When the water cools in autumn, adult anchovies migrate to the open sea, dispersing in the mass of water and descending to depths of 60–70 m to overwinter. It spawns between June and September, both offshore and near the coast. Individuals reach maturity at the age of one year and reproduce two or three times in their lifetime. At the Romanian coast, large amounts of anchovies are fished and are consumed fresh, salted, or canned [2]. Anchovies are also ecologically important because their large biomass serves as a link in coastal food webs, transporting energy from plankton and small organisms to larger fish, seabirds, and marine mammals. As mid-trophic forage fish, they have a significant impact on the health of higher trophic levels [3].

Several converging factors put populations of small pelagic fishes, such as anchovies, at risk, from habitat destruction, ocean acidification, coastal pollution, overfishing, to parasite infestation [3]. Parasitic diseases of marine fish are caused by parasites of animal origin—protozoa, worms, and crustaceans. The sources of disease can be diseased fish, fish carrying parasites, or the corpses of infested fish. Invasions are spread during the migration of fish populations in search of food and the spawning period, through the intermediate hosts of the different parasites, through the habitat, and through direct contact between fish. Parasites are an important limiting factor in the development of fish, and even if parasites do not always trigger mortality, through profound morphological and physiological changes, they can cause great damage to fish populations, particularly by decreasing the resistance of the fish body to various environmental factors, delays in growth, and reduced prolificacy [4,5,6]. There is a close link between parasites and hosts, based mainly on the fact that parasites lead to increased mortality of hosts. However, there are situations where a balance is obtained; this balance depends on the intensity of parasitism, host immunity to different parasites, and the general living conditions of the host [7]. In addition, there is increasing evidence that, at the individual level, these effects can influence host–parasite dynamics down to the population level [8,9].

Moreover, climate change, whether naturally occurring or caused by anthropogenic factors, can alter the parasite–host balance, leading to disease and mortality in fish. The influence of parasites on the organism of infested hosts can be very different from one specimen to another, and manifests through the appearance of lesions due to mechanical action, the removal of nutrients from the intestine of the parasitized hosts, the intoxication of infected organisms, and/or the inoculation of other pathogens. As a result of the action of parasites on hosts, affected fish populations may suffer the following changes: delay of growth rate and decreased appetite, destruction of infested organs and tissues, poisoning, and ultimately death of the fish [10].

The impact and evolution of parasitic diseases depend, on the one hand, on the parasite (species and degree of adaptation to parasitism, invasion, and life cycle) and, on the other hand, on the host (ecology and ethology of infested fish species and predisposition to parasitic diseases). In recent years, research on the parasite fauna of marine fish has become particularly important. Moreover, parasites are used as biological markers of fish and important indicators for the study of fish movements, migrations, and natural stock assessment, as they are one of the causes of the decline of some fish populations [11]. Although parasitism is common in industrially fished populations, parasitic diseases only occur when environmental conditions allow parasites to multiply and host aquatic ecosystems support parasite transmission and carrier or intermediate host resistance. The various models for studying the influence of parasites on hosts have started from the diversity and abundance of parasites, which are largely determined by the characteristics of the host population or its habitat [12].

Several scientific papers examine factors that generate different dispersal patterns in the distribution of parasites within host populations [13]. Of particular importance in the distribution of parasites in fish populations are the fundamentals of the growth and reduction in parasite populations and the influence of different types of demographic processes. Simulation experiments, based on probability models of the growth and reduction in host populations, are used to study the dynamics of parasites, focusing on the role played by parasites in inducing mortality in hosts [14,15,16].

Nematodes are regular components of marine ecosystems. Studies on the ecology of parasite communities are often descriptive, with a focus on models for determining the quantity of parasites that infect the host population rather than the mechanisms underlying parasite–host interactions. These interactions that occur in the host organism are critical in determining the impact and the paths of transmission [17].

In the Black Sea, the life cycle of members of the Superfamily Ascaridoidea in their intermediate and definitive hosts is not clearly defined [6]. In general, it is known that these parasites use almost all living marine organisms, from small invertebrates to mammals, as hosts in different stages of their life [17]. On the north-western Black Sea shelf, pollution rates are known to be higher due to anthropogenic discharges and nutrient inputs along the rivers. This physically measured information does not show how it can affect the ecology of the ecosystem [18]. Consequently, the prevalence of parasites may be one of the important parameters to demonstrate this. For example, eutrophication, thermal effluents, and oil spills are documented as critical pollution parameters that increase nematode abundance [10].

This paper assesses the nematode infestation degree of the anchovy population from the Romanian coast by determining the impact on the body of the anchovy by the presence of nematodes enclosed in organs or free in the abdominal cavity. Nematode worms were identified in E. encrasicolus specimens analyzed during 2021–2023 as the predominant parasites in fish caught from the pound nets located from north to south along the Romanian continental shelf. Moreover, the potential risks to anchovy stock status, possible threats to human health, and control measures are also considered.

2. Materials and Methods

2.1. Fish Sampling

The determination of the degree of parasitism was carried out by analyzing 698 anchovy specimens collected from bi-monthly samples taken from the catch of pound nets from fishing grounds located along the Romanian Black Sea coast between Vadu and Vama Veche (Figure 1). The specimens were randomly selected from samples of about 1–2 kg. Parasitological analyses were performed for 10–20 fish/sample.

The pound net is a large fishing gear that is installed at depths of 5–12 m and pound net fishing is the traditional way anchovy is caught at the Romanian coast. In larger nets, the concentration enclosure and the restraint (catch chamber) of the fishing target are installed parallel to the shore, reaching a length of 70 m. The role of guiding the fish belongs to the 300–500 m long wings (leaders) made of mesh, located perpendicular to the shore direction [19].

2.2. Parasitosis Identification Analyses

Macroscopic and microscopic examinations of the fish were performed to identify the parasites and the reactions they can cause in the hosts [20]. The identification scheme is visually represented in Figure 2 below. Parasites were visually identified according to their specific characteristics, related to size, external appearance, shape, color, and internal structure, determined by macroscopic and microscopic examinations, according to reference identification guidelines [21].

The macroscopic examination was carried out to identify possible parasites and the changes produced by them on the hosts. It was done with the naked eye and with a magnifying glass, observing the body surface, the eyes, and the gills of the fish. Each organ was then observed separately to highlight possible necrotic areas, cysts, large parasites, color changes of the affected organs, and other visible modifications. Fish were sectioned on the abdomen using scissors, carefully, so as not to damage the internal organs, and the gill cover was subsequently removed [21,22].

The microscopic examination of fresh/frozen biological material is the most important step in diagnosing parasitosis [6]. Samples for microscopic examination were taken directly from fresh or thawed fish (in the case of frozen samples) and consisted of:

• Scraped preparations—material scraped with a scalpel from the surface of the area to be investigated (usually gills, tegument, intestinal mucosa). The scrapings were then placed on a slide in a drop of water, over which a top slide was attached, and immediately examined under a microscope.
• Squashed preparations—consisting of small portions of tissues and organs, squashed between the slides so that the film formed became translucent and as thin as possible and potential pathogens could be visualized.

Parasitological analyses were carried on a large number of specimens (698), and the parameters analyzed included [6,11]:

• ▪ abundance—the average number of parasites/total fish analyzed—infested and non-infested;
• ▪ prevalence—the percentage of infested fish of the total analyzed sample;
• ▪ average intensity—the average number of parasites/infested host.

All investigations were performed for each fish in the relevant length class to get as close as possible to the reality of parasite dominance and the effects of infestation on fish in relation to their size and age.

In order to assess the fattening rate of anchovy in relation to parasite infestation, the state of fattening was estimated as follows: lean fish (no trace of fat on the intestine), slightly fat (a fat cord of one mm wide, visible along the intestine), fat (the fat cord is wider but does not surround completely the intestine), and very fat (digestive tube completely covered with fat) [23].

Finally, a Zeiss Axio Imagea A1 microscope equipped with a camera was used to detect the parasites.

2.3. Statistical Analysis and Map Creation

Excel 2016 and PRIMER v. 7.0 software were used for data processing and statistical analysis [24]. Shade plots with no square-root transformed data were elaborated (figures in Section 3.2). The distribution maps were obtained using ArcMap 10.6.1 (Tom Sawyer Software, Berkeley, CA, USA) (Figure 1, Supplementary Figures S1–S3).

3. Results

During 2021–2023, 698 specimens of E. encrasicolus, with length classes ranging from 8 cm to 14 cm, were analyzed. The corresponding age of these lengths was 1:1+–4: 4+ years [25]; specimens aged between 1:1+ years and 2:2+ years were predominant in the total population analyzed. The investigated anchovy samples showed a higher level of parasitism (expressed by the share of parasitized specimens of the total analyzed in the Eforie Sud (80%), Vama Veche and Năvodari (75%) areas, compared to the other studied zones (Figure 3).

3.1. Nematode Worms Identified

The nematodes identified in the bodies of the fish analyzed during this study were: Hysterothylacium sp. Ward & Magath, 1917, Contracaecum sp. Railliet & Henry, 1912, Porrocaecum sp., Railet & Henry, 1912, and Anisakis sp. Dujardin, 1845.

In agreement with literature, Hysterothylacium sp. in this study was identified in the body cavity of fish, both enclosed in organs [26,27,28,29] and free [29,30,31,32]. Contracaecum sp. was found in the body cavity, on the internal organs of fish. Infestation occurs through ingestion by fish of intermediate hosts of the parasite or transmission of the parasite from one fish to another by parasitized large fish to small fish [13,21]. Porrocaecum sp. larvae in this study were mostly identified on the liver. Anisakis sp. larvae were located in the body cavity of fish, immobile, encapsulated in the mesentery, liver, pyloric appendix, intestine, gonads, and mobile, migrating from one organ to another. Anisakis larvae were often present together with Contracaecum sp. and Porocaecum sp. larvae, parasitizing internal organs, particularly the liver.

3.2. Average Parasite Intensity and Prevalence

Nematodes in E. encrasicolus were identified in a free state in the abdominal cavity, intestine, and encapsulated in the liver of the fish. Larvae of Porocaecum sp. were found in the musculature near the abdominal cavity, migrating into different organs of the fish (gonads), and encapsulated in the liver; larvae of the nematode worm Anisakis sp. were found in the immobile state encapsulated in the liver, pyloric appendages, and mobile, migrating into different organs of the body, musculature, gonads.

During the study period, the highest value of the average parasitism intensity with the nematode worm Porrocaecum sp. (12 parasites/host) was reported in 2023 for anchovy specimens collected from the pound net located in the Eforie Sud area. The degree of parasitism with Contracaecum sp. recorded values of up to 8 parasites/host in Eforie Sud station in 2021 and Năvodari in 2023. The nematode Hysterothylacium sp. was reported both in the northern and southern parts of the Romanian coast, with an average parasitism intensity of 1–8 parasites/host. The average parasitism intensity of the nematode Anisakis sp. increased from 3 parasites/host in 2021 (Vama Veche) to 5 parasites/host in 2023 (Năvodari) (Figure 4).

The prevalence of nematode worm species identified in E. encrasicolus between 2021 and 2023 varied according to the sampling area, the most significant variation being recorded for Hysterothylacium sp., from 80% at Vama Veche to 5% at Vadu in 2022. Hysterothylacium sp. has not been reported in E. encrasicolus samples during the whole period studied in the Costinești station (Figure 5). The nematode worm Porrocaecum sp. recorded the highest value of prevalence in 2022 (75%) in the Năvodari station. Contracaecum sp. recorded values of 60–65% in the Năvodari and Eforie Sud stations in 2021 and 2023. It is worth mentioning that Contracacecum sp. was not identified in the Costinești and Agigea stations in 2021. Anisakis sp. showed an increasing trend in prevalence values from 10% in 2021 to 55% in 2023 in the Năvodari station (Figure 5).

3.3. Yearly Screening of Nematode Infestation

E. encrasicolus caught in 2021, with a total length between 8 and 9 cm, showed an abundance of 3–5 parasites/fish. Length classes 11–12 cm registered abundance values of 8–14 parasites/fish. In 2021 the highest abundance value was recorded in the Năvodari station (12 cm) and the lowest in the Costinești station (8 cm) (

Table 1. Abundance of nematode worms in E. encrasicolus during 2021–2023 (number of nematode worms/fish length class/station).

Period/Length Class	Sampling Station
	Vama Veche	Costinești	Eforie Sud	Agigea	Năvodari	Vadu
2021	Abundance (number of parasites/analyzed fish)
8 cm	2.6	1.6	3.6	2.8	3	2
9 cm	3.2	2.1	5	2.4	5	2.2
10 cm	5	5	5.4	3.8	8.8	6
11 cm	8.8	7.6	8.1	9.2	11.1	10.5
12 cm	11.1	9	12.1	9	13.8	12.6
13 cm	9.1	7	10.4	5.8	9.8	9.3
14 cm	8.8	6.4	8.3	4.4	8.1	7.6
2022	Abundance (number of parasites/analyzed fish)
8 cm	3.1	2.8	3.3	3	3.2	3.5
9 cm	3.3	3.8	3.7	3.3	5.1	4.2
10 cm	7.3	9	7.9	6.9	9.1	7.5
11 cm	12.1	8.8	12.6	7.2	11.6	10.8
12 cm	12.4	9.6	13.9	10.1	12.8	12.1
13 cm	11.4	9.1	13.2	10.2	11.9	11.1
14 cm	9.8	7.8	9.2	8.8	7.8	8.7
2023	Abundance (number of parasites/analyzed fish)
8 cm	2.5	1.8	3.8	2.6	2.4	3.9
9 cm	3.3	2.6	5.4	2.9	4.4	5
10 cm	6.7	5.5	7.8	8.8	8.5	9
11 cm	9.8	7.7	12.9	7.3	12.8	10.3
12 cm	11.3	9.8	12.1	9.7	13	11.1
13 cm	8.9	7.9	8.5	10.1	9	8
14 cm	0	0	0	0	0	0

).

The E. encrasicolus samples assessed in 2022 from the six areas of the Romanian coast recorded values of nematode abundance that varied from one station to another, depending on the size of the fish but also on their immunity. As the fish are caught by pound (trap) nets, this fishing gear may induce a state of stress in the fish specimens caught, thus weakening them. The highest values were found in the 11 cm length classes, 12 parasites/fish, and in the 12 cm length classes, reaching 14 parasites/fish. Smaller fish recorded abundance values of 3–5 parasites/fish (Table 1).

The anchovy individuals surveyed in 2023 showed the highest abundance values for fish in the 11 cm and 12 cm length classes, with values ranging from 8–13 parasites/fish. The bodies of the infested fish showed lesions in the affected organs (liver, intestine), a lack of food in the stomach, and a thin fat layer, being extremely lean. The lowest abundance values were recorded in the 8 cm and 9 cm length classes, 2–4 parasites/fish (Table 1).

Larger E. encrasicolus reached the maximum level of infestation intensity. Thus, anchovies in the 11 cm length class recorded an average infestation intensity of 15 nematode worms/host (Eforie Sud). In the other 12 cm long specimens, the number of parasites/hosts ranged from 10 to 18. The smaller-sized anchovies with a total length between 8 and 10 cm showed an average parasitism intensity of 3–12 parasites/host. The most affected anchovy individuals were those fished in the areas of Vama Veche, Eforie Sud, and Năvodari (

Table 2. Average parasitism intensity of E. encrasicolus with nematode worms during 2021–2023 (number of nematode worms/fish length class/station).

Period/Length Class	Sampling Station
	Vama Veche	Costinești	Eforie Sud	Agigea	Năvodari	Vadu
2021	Average intensity (number of parasites/infested hosts)
8 cm	4.6	4.1	6.5	3.1	6	4.5
9 cm	8.6	5.2	8.8	3.6	8.5	6.2
10 cm	11	8.6	12.6	7.4	10.8	10.4
11 cm	13.8	11.3	18.1	11.8	14.4	12.8
12 cm	14.9	10.1	17.2	10.6	13.8	10.8
13 cm	12.6	9.6	13.7	9.6	10.8	10.4
14 cm	9	8.4	9.3	8.8	8.2	9.1
2022	Average intensity (number of parasites/infested hosts)
8 cm	5	4.2	5.6	3.4	6.5	5.4
9 cm	7.5	4.4	8.5	4.2	6.8	5.8
10 cm	10.9	7.7	9.8	7.4	8.5	10.8
11 cm	13.2	11.1	15.8	8.9	13.8	11.2
12 cm	14.6	12.8	14.9	10.4	14.1	13.8
13 cm	12.3	9.8	13.3	9.8	8.8	11.8
14 cm	9.7	8.6	11.2	5	8.6	7.7
2023	Average intensity (number of parasites/infested hosts)
8 cm	5	4	6.4	3.6	6.8	5.4
9 cm	5.4	4.4	7.6	4	8	5.8
10 cm	10.4	8.8	11.5	8.4	11.9	10.8
11 cm	10.4	10.2	14.3	11.4	13.4	12.3
12 cm	11.8	9.6	12.1	13.1	14	14.1
13 cm	11.2	9.4	10.8	9.2	10.2	13.2
14 cm	0	0	0	0	0	0

).

The average number of parasites/infested fish (average parasitism intensity) showed in 2022 higher values for E. encrasicolus of 11–12 cm in length (14–15 parasites/host) in the areas of Vama Veche, Eforie Sud, Năvodari, and Vadu. Lower values of 4–6 parasites/host were recorded for smaller specimens (8–9 cm) in all the six fishing areas surveyed (Table 2).

The average intensity of nematode parasitism was highest in anchovy specimens collected from the Vadu, Năvodari, and Eforie Sud stations for the 11 cm and 12 cm length classes. The maximum value of the mean parasitism intensity was 15 parasites/host (11 cm), and the minimum value recorded was 4–5 parasites/host (8 cm) (Table 2).

The prevalence in anchovy showed a high level of parasitism in 2021 for length classes between 10 and 13 cm in all six stations surveyed. The anchovy fished at the pound nets in the marine areas of Vama Veche, Eforie Sud, and Năvodari showed prevalence values of 100% for the 12 cm length class. The total parasitism of the anchovy specimens analyzed indicated a slower fattening rate, with a small amount of fat identified during dissection. The lowest prevalence values were recorded in the 8 cm, 9 cm, and 13 cm length classes at Agigea, with values ranging from 15–45% of the total fish studied (

Table 3. Prevalence of nematode worms in E. encrasicolus during 2021–2023 (number of nematode worms/fish length class/station).

Period/Length Class	Sampling Stations
	Vama Veche	Costinești	Eforie Sud	Agigea	Năvodari	Vadu
2021	Prevalence (percentage of infested fish)
8 cm	54.9	42	65	23.3	45	40
9 cm	70	55	80	45	55	70
10 cm	78	50	85	50	80	85
11 cm	85.8	80	88	68.8	90	90
12 cm	100	65	100	70	100	80
13 cm	88	35	85	65	80	70
14 cm	37	20	55	15	45	30
2022	Prevalence (percentage of infested fish)
8 cm	62.6	34.6	63.5	55	60.3	56.2
9 cm	74.5	50	77	48	61	54
10 cm	85	59	83.3	42	70	60
11 cm	95.6	77.3	93.7	63.3	100	90.4
12 cm	100	73	92.6	81	90	90
13 cm	70	71.3	67.2	67	57.5	42.5
14 cm	43.3	63	36	54	42	33
2023	Prevalence (percentage of infested fish)
8 cm	62.6	34.6	70	55	60.3	56.2
9 cm	74.5	50	77	48	71	54
10 cm	85	59	83.3	42	70	60
11 cm	100	77.3	93.7	63.3	100	90.4
12 cm	100	73	100	81	90	90
13 cm	70	71.3	100	67	57.5	42.5
14 cm	0	0	0	0	0	0

).

In 2022, the prevalence reached the value of 100% parasitized fish in the Năvodari marine area for the 11 cm length class and at Vama Veche for the 12 cm length class, and 94% at Eforie Sud for the 11 cm length class. The lowest prevalence value was recorded in all stations for the 13 cm length class anchovy. In the other areas, the prevalence reached maximum values between 50 and 86%. The maximum percentage of fish infested with nematodes (100% of all fish analyzed) showed the presence of parasites, particularly in larval form, in the abdominal cavity, mostly in the free state and less enclosed in the internal organs, which they can destroy mainly by their mechanical action (Table 3).

In 2023, the prevalence of anchovy showed a high degree of parasitism in all six stations of the Romanian coast. The anchovy fished in Vama Veche, Eforie Sud, and Năvodari showed prevalence values of 100% for the 11 and 12 cm length classes. The high parasitism of the anchovy specimens analyzed revealed the presence of lesions in the internal organs, as well as liver hemorrhage for a small number of fish. The lowest prevalence values were reported in the 8 cm and 9 cm length classes at Costinești, with values ranging from 35–50% of the total fish studied (Table 3).

4. Discussion

The occurrence of diseases in the natural environment can be accentuated under the conditions in which the pollution of marine waters with wastewater and domestic and industrial waters increases. Often this type of contamination leads to losses in ichthyofauna, especially as a result of asphyxiation and poisoning. A series of natural and anthropogenic factors act on fish populations, with effects highlighted by the reduction in abundance [33]. In wild populations, it is difficult to isolate and quantify the effects of these factors on the size of commercially fished stocks, such as destruction by predators, lack of food, or disease. In recent years, the idea that some animal parasites can act as severe pathogens, causing direct mortality or increasing vulnerability to other stressors, environmental or biotic, is increasingly supported [15]. The ecology and behavior of the host, but also the interdependence between host–parasite and host–ecosystem interactions, are responsible for the parasitic variability of fish according to species, population, and region, as well as for the divergence of parasitism in the same fish stock throughout the year [33].

In this study, during 2021–2023, four species of nematode worms were identified, belonging to the Family Anisakidae (Nematoda: Ascaridoidea), particularly to the genera Anisakis and Contracaecum. These parasites are present in a wide range of aquatic organisms and with indirect cycles in the aquatic environment, mainly in the marine environment [34]. Parasites of the genus Hysterothylacium are among the most widespread nematode worms in the marine environment and have been found in many fish species [26]. For Hysterothylacium sp. at least one intermediate host, a crustacean, is required for transmission to fish. Infested copepods are a way of infecting fish [27]. Porrocaecum sp. parasitize in the body cavity, on the liver, intestine, and gonads of fish, where they cause lesions, inflammation, degeneration, atrophy, slowing of the growth rate of fish, and, in severe invasions, mortality [13,21]. The development cycle is similar to that of the nematode Contracaecum sp., the larvae having a mechanical action on the affected organs, with degeneration, atrophy, and slowing of the growth rate, and in the case of very severe infestation, it can cause the death of the infested fish [6,13,21].

This research revealed, in a small number of parasitized anchovy individuals, some specific changes produced by the intensity of parasitism with nematode worms higher than 40 parasites/host as being relatively severe, consisting of bleeding lesions at the level of the digestive tube, which were observed when performing dissections. If the number of fish with such intensities of parasitism had been representative and considering that the effects of parasitism can be lethal, stock sizes would have been seriously damaged. Sublethal effects include muscle degeneration, liver dysfunction, nutritional disorders, heart disease, nervous system damage, reduced reproductive performance or mechanical interference with spawning, weight loss, and whole-body damage [35,36,37,38,39].

The samples for this study were collected during the anchovy spawning period, from May to September, and we found that while the specimens with a low number of parasites were not weakened, those with a high degree of parasitism showed signs of energy depletion, as specimens were very lean, with almost no fat observed during dissection. Parasites can reduce the body weight of hosts and cause a decrease in their mobility. A high degree of parasite infestation of anchovies affects their long winter migration, as they do not have sufficient food resources (lipids) to migrate [40]. Research into the effect of stress due to the presence of nematode larvae on fish has shown that they do not have a significant impact on the fish but can generate effects on locomotion [41].

In recent years, the marine ichthyofauna from the Romanian coast has suffered great changes through the reduction in stocks, catches, and the reduction and increasingly rare appearance in samples of some fish species, such as the anchovy [42]. Following these observations, it was considered that the research approach regarding marine fish parasitofauna is opportune, with the idea of obtaining data that could contribute to the explanation of these phenomena. As such, for the Romanian coast, research and publications have been produced regarding the influence of parasites on the host organism by analyzing the degree of parasitism of fish species of commercial interest, finding that anchovy is one of the species most affected by nematode worms. In these studies, mechanical damage and tonic influence due to delayed growth rate and decreased appetite, up to severe damage, destruction of infested organs and tissues, intoxication, and death of the fish were described [4,5,15,43,44].

The results of this research filled gaps with new knowledge on the average prevalence and distribution of parasitic diseases in E. encrasicolus. The mean anchovy prevalence during the study period recorded high values in three stations (Năvodari, Eforie Sud, and Vama Veche). Infested fish from these areas showed numerous inflammations in the intestine and liver. The lowest degree of parasitism was recorded in 2021, while the anchovies caught in 2023 showed a higher degree of infestation. The anchovy samples analyzed from the Agigea area showed the lowest nematode worm infestation during the whole period covered (Supplementary Figures S1–S3). In this study, it was observed that the demographic structure of the host population affects the level of sampling. From the data obtained and analyzed, a high level of parasitism was evident for larger fishes aged 1:1+ years and 2:2+ years, and smaller anchovy specimens (8–9 cm) showed a lower prevalence (<40%), which was also noted in other previous publications covering the Black Sea and not only [4,15,45,46,47,48].

Fish populations are subject to many natural and anthropogenic factors that reduce their abundance [3]. Environmental factors, feedstocks, diseases, pollution, overfishing, and other biological elements such as parasite infections are some of the important parameters that affect the health status of fish, causing an economic loss for the fish market as well as serious risks for human health [49,50].

Nematodes are a common presence in anchovy fish populations fished on the Romanian coast, and these parasites could have a potential zoonotic risk, parasitizing mostly in the larval stage. In Romania, as in other countries, people are traditional consumers of smoked or marinated fish, usually eaten whole. In these conditions, the marketed fish can be infected with nematodes ingested by those who consume it, and the worms can end up in the digestive tract of humans. By digesting fish meat, there is a risk that nematode larvae develop in the human body, becoming adult worms, which, through their spoliating, toxic, inoculating action, can cause serious illnesses to people [13,50]. Not only live worms, but also dead worms can cause allergic reactions, sometimes with serious consequences, including anaphylactic shock [51,52]. Worldwide, especially Anisakid species represent a health and economic concern because they are responsible for zoonosis transmitted by fish to humans known as anisakidosis, but also for economic losses caused in the fishing industry [36,53,54].

In our study, an increase in anchovy infestation with Anisakis sp., a nematode worm that can cause ichthyozoonosis, was observed. This parasite causes eosinophilic granuloma in the human alimentary tract when raw or improperly cooked fish infested with live larvae are ingested [55]. Studies have shown that the nematode worm Anisakis sp. can be found in the most emblematic Mediterranean dish—anchovies in vinegar [56]. Although European Union regulations require restaurants to freeze fish that is eaten raw, people are still at risk of infection with Anisakis sp. from anchovies in vinegar if they did not freeze the fish for at least 24 h at −20 °C [49]. Anisakiosis poses a risk to human health through intestinal worm infection from the consumption of poorly processed fish and allergic reactions to chemicals left by the worms in fish meat [52,57,58]. For all wild fish from the marine or freshwater environment, the risk of containing any viable parasite affecting human health must be considered if these products are to be consumed raw or almost raw [36]. Consequently, the most feasible control strategy is applying thermal treatment to the fish in order to avoid nematode contamination with all its subsequent effects on human health.

Spanish researchers have shown that parasites (Anisakis sp. and Hysterothylacium sp.) are present more frequently and with higher parasitism intensity in anchovies caught on the coasts of the southeast Atlantic and northeast Mediterranean, which is why they urge consumers to freeze or cook the fish before consumption [56,59]. The presence of Anisakis sp. larvae in anchovy has been documented mainly in the Mediterranean Sea [60,61,62] and such research must clearly be undertaken on the Romanian coast as well, aiming to estimate the potential threats to human health at regional Black Sea level. In order to accurately identify the potentially hazardous nematode species, state-of-the-art molecular analyses must be undertaken [63,64], thus widening the research on parasite influence both on fish stocks themselves and consumer health.

5. Conclusions

This research aimed at assessing the potential risks posed by nematode parasites to the wild anchovy population at the Romanian Black Sea coast and, by extension, human health, focusing on identifying the emerging threats and potential control strategies. The parameters analyzed on abundance, average parasite intensity, and nematode prevalence in anchovy showed variations within the six areas sampled. The maximum abundance of 14 parasites/fish was recorded in length classes ranging from 11–12 cm. The maximum prevalence of 100% was reported in the anchovy population in Năvodari, Eforie Sud, and Vama Veche. An accumulation of nematode worms was found in mature and large specimens with a negative impact on the host organism through the presence of internal lesions, slowing of the growth rate, reduced feeding appetite, and disturbances of the reproductive process. Following the results obtained in this study, further molecular and histological investigations on the impact of nematode worm infestations on anchovy, which is an important fish for human consumption, are highly needed. The results of our study can be useful not only for ecological implications but also because of the potential effects on human health; larger specimens offer consumers important nutritional benefits, but also possible health risks (higher parasitism levels). The most feasible control strategy we have identified is applying thermal treatment to the fish in order to avoid nematode contamination with all its subsequent effects on human health. The relationships between parasitism level impact and stock status identified in this study indicate a correlation between these parameters, yet the cause–effect relationships are to be further explored. Therefore, future research on anchovy health, feeding, stock status, and fish reproduction, as well as the potential threats to human health, should be undertaken.