Pathologies represent a relevant bottleneck for aquaculture development and must be taken into account in the selection of potential species for diversification. Determining pathogens affecting new and traditional cultured species as well as their potential impacts on fish health becomes essential for optimising the development and viability of fish productions. Among pathogens, monoxenous metazoans, such as platyhelminth monogeneans, have a marked influence on fish health, especially in aquaculture where they cause significant economic losses in the main fish-farming industries. Population dynamics in host-parasite systems are modulated by environmental conditions. Therefore, effective management of monogenean infections requires a good understanding of the host-parasite relationship as well as the abiotic factors affecting the balance between both organisms. Integrating biological and applied aspects into routine management procedures becomes essential to minimize the infection risks in fish farming facilities and design accurate strategies under changing environments. The present thesis aims to identify the parasite fauna of Mediterranean sciaenid fishes with potential for species diversification in aquaculture and compile information of key biological aspects for the management of potentially hazardous monogeneans in the gilthead sea bream and meagre cultures, especially under the seasonal environmental conditions occurring in the Western Mediterranean Sea. Altogether, results of this PhD study have led to the following conclusions: The parasite fauna of Sciaena umbra, Umbrina canariensis and Umbrina cirrosa in the Western Mediterranean Sea includes thirty-four parasite taxa, with twenty metazoan parasites being identified in each sciaenid host. The parasitological analysis allows detecting new parasite records together with previously reported species; 9 new host records and 11 previously reported parasites are registered in S. umbra while 13 new host records and 7 previously reported species are found in U. canariensis as well as U. cirrosa. One new locality record was also found in U. canariensis. Four parasite taxa constitute new host records for the three sciaenid species (Lecithocladium excisum, Lernaeolophus sultanus, Rhipidicotyle sp. and Tetraphyllidea gen. sp.) while four previously reported species are also shared by these hosts (Caligus cf. dakari, Diplectaninae gen. spp., Lernanthropus gisleri, Stephanostumum bicoronatum). Among parasite species first reported in these sciaenid hosts, none is common in S. umbra, while 4 species are common in U. canariensis (Calceostomella inermis, Contracaecum sp., Hysterothylacium fabri, S. cesticillus), and 3 in U. cirrosa (Rhipidocotyle sp., Tetraphyllidea gen. sp. and Zoogonus sp.). Among previously reported parasites, 5 taxa are common in S. umbra (Calceostomella inermis, Diplectaninae gen. spp., Gnatia vorax, Hysterothylacium fabri and Siphoderina aloysiae), 3 in U. canariensis (Anoiktostoma elongatum, Diplectaninae gen. spp and Dycheline elongatus) and 5 in U. cirrosa (C. inermis, Diplectaninae gen. spp., D. abbreviates and Lernanthropus gisleri, S. bicoronatum). Most parasite species constituting the parasite fauna of the Mediterranean sciaenids are generalist. Only 1 sciaenid specific parasite species was included in the common parasite fauna of S. umbra (S. aloysiae), 1 genus-specific (A. elongatum) and 1 species-specific parasite (D. elongatus) in U. canariensis and 1 sciaenid-specific parasite (L. gisleri) and 1 species-specific parasite (D. abbreviatus) in U. cirrosa. The majority of the common species parasitizing S. umbra, U. canariensis and U. cirrosa showed seasonal differences in prevalence over 10% while mean intensity differences between seasons were comparatively lower and showed by fewer species. Sciaena umbra: seasonal differences between prevalences were equal or below 25% in three of the four common species while higher in S. aloysiae (30%), which also was the only species showing differences higher than 5 specimens/fish of mean intensity (10 specimens more in summer). Umbrina canariensis: seasonal differences between prevalences were equal or below 25% in four of the six common species while higher in C. inermis (30%) and S. cesticillus (92.6%). Seasonal differences between intensities were usually low (<5 specimens/fish) except for S. cesticillus (summer-autumn> winter-spring) and A. elongatum (winter-autumn > spring-sum-mer), differing in more than 35 specimen/ fish between seasons. Umbrina cirrosa: seasonal differences between prevalences were above 25% in all the species except for S. bi-coronatum (16.9%). Seasonal differences between intensities were usually low (<5 speci-mens/fish) except for Tetraphyllidea gen. sp. differing up to 7.9 parasites/fish from winter-spring to summer-autumn samplings. The fact of sharing hosts suggests a higher risk of parasite transmission. Sciaena umbra, U. canariensis and U. cirrosa share five parasite taxa that are categorized as common for at least one host: two monogeneans, Calceostomella inermis, Diplectaninae gen. spp.; two copepods. Caligus cf. dakari, L. gisleri; and one trematode, S. bicoronatum. These parasites are also A. regius the only sciaenid species in intensive productions in the Mediterranean Sea. Regarding Diplectaninae gen. spp., two species are shared between the three wild sciaenids in the Mediterranean and one is shared between the two Umbrina spp. The two parasite species shared by wild sciaenids are also found in A. regius. Only four out of these five taxa represent a potential risk for fish in aquaculture, the two monogeneans and the two copepods because they present direct life-cycles and pathological damages have been associated with these or phylogenetically closely-related taxa. Monogenean diplectanids represent the most prevalent parasite taxa in Mediterranean sciaenids. Morphological and molecular data show taxonomic incongruences in species composition and host specificity. Based on the phylogenetic position and genetic divergences for the 28S rDNA region, diplectanids from sciaenid hosts constitute two different genera from Diplectanum and five putative species with low host-specificity. By contrast, intraspecific analysis of the ITS region and mitochondrial data reveals a noticeable genetic structure by host species among subpopulations of the generalist Diplectaninae gen. sp. 1.2. Particularly, distinct haplotypes are dominant for each wild sciaenid (S. umbra and U. cirrosa), but both host species share haplotypes with the cultured meagre (A. regius). The population variability of Diplectaninae gen. spp. should be, therefore, considered for estimating infection risks among sympatric hosts as different cross-infection potential is expected among sciaenids. The diagnosis of the monogenean species S. pancerii should be updated from new morphological data on parasite development and adult size variability. Morphological ranges of relevant taxonomic features such as parasite body size, haptor length, the smallest clamps size, intrauterine eggs size, testes number and clamp pair number are widened. Additionally, the clamp pair number of the specimens should be considered for species diagnosis due to significant size increments in parasite body, haptor the largest clamp, testes and the germanium length occur after parasite maturity. This monogenean exhibits a mixed attachment strategy between both families by incorporating a high number of clamps at early developmental stages and developing after parasite maturity the haptoral asymmetry. Regarding the gilthead seabream, assessment of the infection risks of the monogenean microcotylid S. chrysophrii in fish farms requires a good understanding of the influence of environmental variations on the transmission and life-cycle of this monogenean. Analysis of the environmental effects on infective stages of S. chrysophrii reveals that decreases in pH slightly reduced developmental and survival times as well as hatching rates, although larval emergence remains above 50% at both pH levels (7.0 and 7.9). Photoperiod mainly affects the hatching moment of S. chrysophrii eggs; darkness regulates the circadian larval emer-gence and synchronises most of the hatching with the first hours of the night. By contrast, salinity effects on developmental, survival and hatching parameters of S. chrysophrii are considered inconsistent and negligible. Water temperatures have a huge influence on the life-history stages of S. chrysophrii by affecting the duration of egg incubation and hatching, larval survival, swimming behaviour and maturity periods of this monogenean. Biological periods are generally shorter as temperature increases, but the developmental versatility and larval emergence of S. chrysophrii are restrained to a specific range of temperatures. The minimum incubation period and time to maturity are coincident at temperatures above 20ºC, suggesting that endogenous factors could limit thermal effects on oncomiracidial development. The incubation period does not decrease below 5 days whereas this monogenean provides a new parasite generation in one month at temperatures above 22°C, over a month at 18°C (42 days) and almost two months at 14°C (51 days). Moreover, transmission successes of S. chrysophrii are maximised at particular thermal ranges; hatching success is the highest at temperatures between 14ºC and 22ºC, infection success at 18ºC and 22ºC and survivor to maturity at 22ºC. Altogether, transmission ratios reveal an optimal thermal range between 18ºC and 22ºC whereas the minimum records of hatching success (below 10%) at 10ºC and 30ºC point out that lower and upper thermal margins should be closer to these temperatures. The range of environmental tolerance of S. chrysophrii exceeds by far the increases of 2.5ºC on temperature and 0.5 ppt in salinity as well as the decreases of 0.5 units in water pH estimated for the 2100 scenario in the western Mediterranean region. Based on the effects of the abiotic factors on S. chrysophrii and the gilthead sea bream, this host-parasite system will be little affected by the short-term variations predicted for temperature, pH and salinity under the climate change projections. Temperature and larval age affect the swimming behaviour of S. chrysophrii. The vertical swimming capability is reduced as oncomiracidia age as well as temperature increases; oncomiracidia swam slower in the deepest sections after 24h from emergence, especially at 26ºC. Duration of swimming patterns and swimming speed decreases at increasing temperatures. Based on these results, active dispersion and host-finding processes of S. chrysophrii are compromised shortly after the larval emergence and are particularly critical at higher water temperatures, at which transmission mostly relies on chance encounters with potential hosts. Swimming variations with age and temperature are not so critical for parasite transmission in aquaculture, where the high fish enhance the host encounter. Based on temperature effects, two main strategies that may enhance host-parasite coordination are suggested for infective stages of S. chrysophrii; in spring the increasing temperatures promote the fast development and early hatching of S. chrysophrii which would favour parasite transmission among grouped gilthead sea breams. The decreasing temperatures in autumn result in longer development, hatching and survival of S. chrysophrii, which would increase host-finding probabilities in more dispersed fish. Transmission chances are also improved by light conditions since the nocturnal hatching of S. chrysophrii coincides with gilthead sea bream resting and low presence of predators on the seabed. The life-cycle of S. chrysophrii is successfully completed at any of the seasonal water temperatures occurring in the Western Mediterranean. However, the shortest life-cycle duration together with the maximization of the transmission success at 22ºC, show this temperature as the most hazardous for S. chrysophrii infections in gilthead sea bream cultures, although infection risk can be of concern from 18ºC. Knowledge about the thermal influence on S. chrysophrii should be integrated into management strategies to control infections of this monogenean in gilthead sea bream aquaculture. Avoiding reinfection processes require a secondary treatment after the first treatment application. Based on the effects of water temperature on the incubation period, a moderate treatment against larvae should be applied 8 days after the main application at 26°C, 9 days at 22°C, 11 days at 18°C and 14 days at 14°C. Data on parasite maturity allow to extend these periods up to 14 days after the first application at 26°C, 21 days at 22°C, 28 days at 18°C and 35 days at 14°C, although a more severe treatment could be required to treat nearly mature specimens. The accurate combination of the chemical treatments and prophy-lactic measures would provide at least three months with low parasite loads. Considering the generation times and transmission success per temperature, one management method per season would be required for controlling infections, except for spring when two treatments are recommended. Routine monitoring on parasite loads allows a better adjustment of the recommended treatment strategies to in situ conditions of each aquaculture facility. Estimates of parasite loads of S. chrysophrii can be accurately obtained from partial gill analyses. Microhabitat preferences of S. chrysophrii are determined from the differential distribution of the parasites among the holobranches as well as the longitudinal and transversal sections of the holobranch. Location preferences differ among post-larval, juvenile and adult stages of S. chrysophrii. Parasitological analysis should be accordingly designed: parasite numbers of adult stages can be inferred from analysis of the dorsal sections or the external gill arches whereas examination of the medial and dorsal sections of the three outermost gill arches allow for accurate estimates of the infection levels of the different parasitic stages. Size variations of S. chrysophrii should be considered to avoid diagnosis problems in routine parasitological analyses. The monogenean S. chrysophrii continues growing after maturity but at a slower rate. Both, parasite age and host size were positively and significantly correlated with parasite body size. Moreover, significant variations in the number, as well as the size of the clamps, are also associated with host features. Late mature stages (>60 clamp pair numbers) are exclusively found on fish more than 18.3 cm long (standard length), whereas mid-anterior and posteriormost clamps were generally larger in larger gilthead sea breams. The posteriormost and mid-anterior clamps achieve their maximum dimensions in early post-larval and mature stages, respectively, thus suggesting that host features modulate parasite size from the initial stages of parasite development. By contrast, gill location and parasite abundance are of minor relevance on intraspecific size variations of mature stages of S. chrysophrii.