Pelagic Fish Assemblages Research Articles

Distribution of pelagic and demersal fish assemblages by video prospection in the archipelago of Fernando de Noronha

This study presents an approach that complements previously identified trends in the distribution of acoustic biomass of demersal and pelagic fishes in Fernando de Noronha. Using underwater video as the main method of analysis, patterns of relative fish abundance were evaluated as indicators of biomass, richness, and trophic levels of demersal and pelagic assemblages. Thus, depth stratum, Marine Protected Area (MPA) including the National Marine Park and the Sustainable Fishing Zone, and oceanographic/abiotic factors (e.g., winds, currents, and substrate complexity) were considered. The results confirm that complex substrates and shallow areas (0–20 m) are characterised by greater abundance and species richness. However, it is important to note the limitation of video in detecting fish in deeper areas. Wind exposure also positively influences the abundance and diversity of demersal fish, demonstrating the impact of the "island effect" on species distribution. The research also highlights the relevance of the MPA in Fernando de Noronha for both biodiversity conservation and the sustainability of fishing activities. However, the analysis of trophic levels revealed an increased prevalence of higher trophic levels outside the MPA, mainly due to the abundance of the species Melichthys niger. The presence of complex substrates and lower trophic competition may explain this observation. Therefore, the study provides important insights for the planning of MPAs and the need to include the spatial distribution of higher trophic level species for marine conservation.

Trophic niche isotope analysis of the fish assemblages in a subtropical river, Southern China

AbstractFreshwater fishery resources in Chinese rivers have been markedly impaired as the results of overfishing, damming, and watershed development. However, little is known about trophic ecology of fish assemblages in the rivers of China. In this study, fishes were collected from an upstream, midstream, and downstream site of the Moyangjiang River, Southern China, to investigate trophic niche structure using stable isotope analysis. We calculated several trophic niche metrics using δ13C and δ15N ratios of the fish assemblages at each study site. Results showed no significant differences in the δ13C ratios between the pelagic and benthic fish assemblages at each study site. This suggests a homogenous dietary base shared by all fish at each site, likely as the result of continuous water column mixing in this shallow and fast‐moving river. The upstream fish assemblage had the greatest species abundance and trophic diversity. The fish assemblage at the midstream site displayed the smallest trophic niche space (total area of the δ13C and δ15N bi‐plot space) and trophic diversity. The midstream site also showed the most depleted 13C, suggesting a more degraded habitat compared with the other two study sites. The fish assemblages at the downstream site displayed the greatest basal resource diversity, largest CR, and trophic niche space and trophic diversity. However, the fish assemblage at the downstream site also displayed the lowest species abundance, redundancy, and evenness of trophic spacing. These findings suggest that the trophic niche of the fish assemblage at the downstream site, which received resources from upstream flow, experienced greater watershed development and greater in situ production, demonstrating positive and negative impacts by anthropogenic activities. Future studies should also gather information on fish production, watershed development, and water quality, to aid in the interpretation of the stable isotope analysis of fish trophic niches.

First records of two large pelagic fishes in the Red Sea: wahoo (Acanthocybium solandri) and striped marlin (Kajikia audax)

AbstractThis report provides the first confirmed identifications of wahoo (Acanthocybium solandri) and striped marlin (Kajikia audax) in the Red Sea, expanding the known ranges of these species into the basin. Potential mechanisms responsible for the lack of regional documentation of the two species are further discussed. These findings illustrate the need for systematic biodiversity surveys of pelagic fish assemblages in the Red Sea.

Hydroacoustics and concurrent experimental trawling reveal extreme annual variation in the density of 0+ pikeperch in late summer

The density and mean size of 0 + pikeperch (Sander lucioperca) were studied in late summer in 2004–2017 in clay-turbid and eutrophic Lake Tuusulanjärvi in southern Finland. Hydroacoustics and simultaneous experimental trawling were used to estimate the density of 0 + pikeperch. In some autumns, 0 + pikeperch was the most abundant species in the pelagic fish assemblage. However, the annual amplitude of pikeperch density was extremely high (1,300 - 19,900 individuals ha−1). The density and size variations of 0 + pikeperch were analysed in relation to air temperature sums and degree days. Unexpectedly, the density of 0 + pikeperch showed no correlation with the air temperature sums or degree days, but both the mean length and weight of 0 + pikeperch correlated positively with these temperature variables. The highest correlation coefficients between mean size and temperature were found with temperature sums over 10 °C and degree days over 5 °C. The correlation between density and condition of 0 + pikeperch was negative but non-significant. The high density of 0 + pikeperch in some autumns suggests that pikeperch may play a central role in the pelagic food web in eutrophic lakes – not only as a predator of planktivores but also by its own planktivory.

Influence of anticyclonic, warm-core eddies on mesopelagic fish assemblages in the Northwest Atlantic Ocean

Variability in mesopelagic fish assemblages of the deep-scattering layer were explored in relation to mesoscale anticyclonic (warm-core) eddies in the western North Atlantic Ocean. Exploratory fishing with a mid-water herring trawl was combined with oceanographic sampling aboard the RV Celtic Explorer in the Spring of 2015 and 2016 to characterize mesopelagic fish assemblages along transects through anticyclonic eddies identified using real time satellite-derived sea level anomaly. Sampling yielded a total of 6087 fish specimens, representing at least 109 species across 38 families. The families Myctophidae and Stomiidae were the most speciose taxa sampled and comprised >50% of total catch abundance in each year. Species richness and diversity (H’) were significantly higher in sets occurring inside relative to outside the eddy structures. Community analyses indicate significant differences in daytime assemblage structure between both depth groups (shallow 200–400 m versus deep 400–700 m) and eddy groups (inside versus outside), with ordination discrimination of 5 pelagic fish assemblages related to eddy groups. Anticyclonic eddies also harbored more juveniles and rare species, including species with more southerly distributions within the Gulf Stream. Our results indicate anticyclonic eddies may play an important role in structuring fish assemblages and the dispersion of mesopelagic species in the Northwest Atlantic.

What happened in the mid-1990s? The coupled ocean-atmosphere processes behind climate-induced ecosystem changes in the Northeast Atlantic and the Mediterranean

Northeast Atlantic marine ecosystems such as the Bay of Biscay, Celtic Sea, English Channel, Subpolar Gyre region, Icelandic waters and North Sea as well as the Mediterranean Sea show concomitant ‘regime shift’-like changes around the mid-1990s, which involved all biota of the pelagial: phytoplankton, zooplankton, pelagic fish assemblages, demersal fish assemblages and top predators. These shifts were caused by complex ocean-atmosphere interactions initiating large-scale changes in the strength and direction of the current systems, that move water masses around the North Atlantic, and involved the North Atlantic Oscillation (NAO), the Atlantic Meridional Overturning Circulation (AMOC), and the subpolar gyre (SPG). The contractions and expansions of the SPG and fluctuations of the Atlantic Multidecadal Oscillation (AMO) play a key role in these complex processes. Small pelagic fish population trends were the sentinels of these changes in the mid-1990s in the ecosystems under investigation.

Small pelagic fish assemblages in relation to environmental regimes in the Central Mediterranean

The complex interactions between surface circulation and coastal and continental shelf morphology, as well as the presence of temperature and salinity gradients, make the Central Mediterranean an area characterized by high degree of ecosystem patchiness. The well-known effect of the physical environment in shaping community structure leads us to hypothesize the presence of differences in the pelagic fish community organization. Based on the analysis of specific environmental factors, nine sectors were identified and possible differences in the relative proportions of some key pelagic species were assessed by analysing the acoustic data collected during two surveys carried out in the summer period. The obtained results evidenced a clear separation between the northern (Strait of Sicily) and southern parts (Tunisian waters) of the study area, highlighting the strong effect of temperature on the proportions of some species. Furthermore, even if most of the variability in fish assemblages was due to the differences between the northern and southern parts, specific spatially nested differences were identified, highlighting a kind of hierarchical structure in the pelagic habitat, definitely allowing fish species to find their optimum in a high variable ecosystem, thus improving the system’s resilience.

Long-term Changes in the Fish Assemblage in Sandybeach Lake, Northern Ontario, Following the Introduction of Rainbow Smelt (<i>Osmerus mordax</i>)

Rainbow Smelt (Osmerus mordax, Mitchill, 1814) has been widely introduced into lakes across North America and are reported to have an adverse effect on native fishes. In this study, we investigated the long-term changes to the pelagic fish assemblage in a northwestern Ontario lake (Sandybeach Lake) after Rainbow Smelt were introduced in the 1980s. We repeated an earlier summer, pelagic gill-netting survey of Sandybeach Lake and a nearby reference lake (Little Vermillion Lake) that does not contain Rainbow Smelt. Fishes throughout the water column were sampled with overnight sets of 5.2-m-deep, multi-mesh horizontal gill-nets. Compared with 1990 pelagic sampling, native fishes were significantly less abundant, less diverse, and largely absent from the deeper parts of Sandybeach Lake in 2012. Cisco (Coregonus artedi, Lesueur, 1818), Emerald Shiner (Notropis atherinoides Rafinesque, 1818), and Lake Whitefish (Coregonus clupeaformis, Mitchill, 1814) were notably absent in 2012. In contrast, Rainbow Smelt remained abundant at all depths sampled. A small number of ciscoes — including Shortjaw Cisco (Coregonus zenithicus (Jordan and Evermann, 1909)) — and Lake Whitefish were captured from benthic gill-nets. Based on the lack of ciscoes in pelagic gill-nets, the very old ages (mean 30 years) of the few individuals captured in benthic gill-nets, and a severely skewed sex ratio, it is highly likely that ciscoes will be extirpated from Sandybeach Lake. Although the composition of the pelagic fish assemblage differed between years, Cisco was still abundant in Little Vermillion Lake with the presence of young individuals indicating ongoing recruitment.

Vertical segregation of age-0 and age-1+ polar cod (Boreogadus saida) over the annual cycle in the Canadian Beaufort Sea

The offshore marine ecosystem of the Canadian Beaufort Sea faces the double pressure of climate change and industrialization. Polar cod (Boreogadus saida) is a pivotal forage species in this ecosystem, accounting for 95 % of the pelagic fish assemblage. Its vertical distribution over the annual cycle remains poorly documented. Hydroacoustic records from 2006 to 2012 were analysed to test the hypothesis that age-0 polar cod segregate vertically from larger congeners. Trawls and ichthyoplankton nets validated the acoustic signal. Fish length, weight, and biomass were estimated from new regressions of target strength and weight on standard length. Polar cod were vertically segregated by size in all months, with small age-0 juveniles in the epipelagic (<100 m) layer and larger age-1+ deeper in the water column. From December to March, the biomass of age-1+ peaked in a mesopelagic layer between 200 and 400 m. With increasing irradiance from April to July, the mesopelagic layer deepened and extended to 600 m. Starting in July, age-0 polar cod formed an epipelagic scattering layer that persisted until November. From September onward, age-0 left the epipelagic layer to join small age-1+ in the upper mesopelagic layer. Low biomass in the mesopelagic layer from February to September likely resulted from large polar cod settling on the seafloor to avoid diving marine mammals. Longer ice-free seasons, warmer sea-surface temperatures, or an oil spill at the surface would likely impact epipelagic age-0, while mesopelagic age-1+ would be vulnerable to an eventual oil plume spreading over and above the seafloor.

Trophic structure and food resources of epipelagic and mesopelagic fishes in the North Pacific Subtropical Gyre ecosystem inferred from nitrogen isotopic compositions

AbstractWe used bulk tissue δ13C and δ15N values and δ15N values of individual amino acids (AA) to characterize the trophic structure of a pelagic fish assemblage from the North Pacific Subtropical Gyre (NPSG) ecosystem. We focus on energy flow between fishes inhabiting distinct epipelagic, mesopelagic, and upper bathypelagic habitats and on predatory fish foraging across and within these depth habitats. Trophic positions (TPs) estimated from a combination of trophic and source AA δ15N values (TPTr‐Src) spanned a narrow range of 0.7 TP for 10 species of large fishes, including tunas, billfishes, and gempylids (TPTr‐Src 4.3‐5.0). Similarly, 13 species of small micronekton fishes encompassed a range of 1.2 TP (TPTr‐Src 2.6‐3.8). The δ15N values of three source AAs were found to increase with increasing depth of capture across the 13 micronekton fish species (δ15NPhe range = 6.6‰; δ15NGly range = 13.4‰; δ15NSer range = 13.6‰), indicating that some species from epipelagic, mesopelagic, and upper bathypelagic communities access distinct food resources, such as suspended particles. These isotopic depth trends are consistent with previous observations in particulate organic matter and zooplankton from the NPSG, providing new evidence that large pelagic and micronekton fishes access a food web fueled by particles formed in surface waters but that are highly modified by microbes as they slowly settle to remote depths. On the contrary, no significant relationships between the δ15N values of source AAs and habitat depth were observed in the large predator fish group, of which many species move and forage across large depth gradients.

Warming shelf seas drive the subtropicalization of European pelagic fish communities.

Pelagic fishes are among the most ecologically and economically important fish species in European seas. In principle, these pelagic fishes have potential to demonstrate rapid abundance and distribution shifts in response to climatic variability due to their high adult motility, planktonic larval stages, and low dependence on benthic habitat for food or shelter during their life histories. Here, we provide evidence of substantial climate-driven changes to the structure of pelagic fish communities in European shelf seas. We investigated the patterns of species-level change using catch records from 57,870 fisheries-independent survey trawls from across European continental shelf region between 1965 and 2012. We analysed changes in the distribution and rate of occurrence of the six most common species, and observed a strong subtropicalization of the North Sea and Baltic Sea assemblages. These areas have shifted away from cold-water assemblages typically characterized by Atlantic herring and European sprat from the 1960s to 1980s, to warmer-water assemblages including Atlantic mackerel, Atlantic horse mackerel, European pilchard and European anchovy from the 1990s onwards. We next investigated if warming sea temperatures have forced these changes using temporally comprehensive data from the North Sea region. Our models indicated the primary driver of change in these species has been sea surface temperatures in all cases. Together, these analyses highlight how individual species responses have combined to result in a dramatic subtropicalization of the pelagic fish assemblage of the European continental shelf.

Artificial marine habitats favour a single fish species on a long-term scale: the dominance of &lt;em&gt;Boops boops&lt;/em&gt; around off-shore fish cages

Off-shore fish cages are new artificial habitats that can affect pelagic fish assemblages and constitute an important food source for wild fish assemblages. This aggregation has noticeable ecological consequences in cage areas in impoverished ecosystems such as those in the Canary archipelago (NE Atlantic Ocean). However, this new habitat could be dominated by a single species, reducing its positive ecological effects. Wild fish assemblages associated with an off-shore fish lease on the northeastern coast of Tenerife (Canary Islands) were sampled for six years. Fish assemblage structure beneath fish cages and at controls ( > 500 m from cages) differed significantly between locations, with 13 times greater abundance at cage locations. These differences were mainly explained by the dominance of bogue ( Boops boops ) around fish cages. This trend was consistent in the long-term throughout the study period (2004-2009), affecting local fisheries. The presence of fish cages significantly altered wild fish assemblages in the study area, enhancing mainly biomass and abundance of one species, bogue, and causing shifts in species composition.

Effects of a spatial closure on highly mobile fish species: an assessment using pelagic stereo-BRUVs

The effects of a spatial area closure on pelagic fish assemblages within the Houtman Abrolhos Islands were assessed using mid-water pelagic stereo-BRUVs. The spatial area closure within the Easter group of the Houtman Abrolhos Islands was found to have no significant effect on the species composition and relative abundance of pelagic fish assemblages. The most abundant demersal target species recorded was pink snapper (Chrysophrys auratus) and, individuals measured within the spatial closure were significantly larger than those sampled in the area open to fishing. This spatial area closure is of moderate size (22.3km2), but the spatial management of highly mobile species may require larger area closures than those for reef-associated species. The monitoring of pelagic species both in large and small spatial area closures is required in order to better understand how mobile species respond to this management strategy. Some species were only recorded in relatively low numbers using pelagic stereo-BRUVs. Moreover, some of the highly mobile pelagic fish species, such as tunas, mackerel and some shark species have proven difficult to measure, as these species were observed furthest from the camera systems. While pelagic stereo-BRUVs are an effective fishery-independent approach to monitor spatial fishing closures, improving the power of replicates by pooling individual deployments, and increasing the attraction rate of pelagic fish to the stereo-cameras, will enhance their performance in future studies.

Development and validation of a mid-water baited stereo-video technique for investigating pelagic fish assemblages

Understanding the abundance, demographics and composition of pelagic fish communities has historically relied on fisheries catch data or destructive fishery-independent methods. Here, we test and validate the use of a pelagic stereo-Baited Remote Underwater Video system (BRUVs) as a non-destructive, fishery-independent approach to study pelagic fish assemblages. We investigated whether differences in the vertical composition of fish assemblages could be detected with pelagic stereo-BRUVs by sampling at different depths in the water column. The effects of soak time and replication on the precision and cost of sampling were explored to allow for the optimization and standardization of future pelagic stereo-BRUVs studies. Pelagic stereo-BRUVs effectively identified 43 fish taxa from 18 different families in the mid-water, 5 and 20m below the surface, in the Ningaloo Marine Park (Western Australia). The fish assemblages sampled at the two mid-water depths were significantly different demonstrating that this method could be used to investigate the vertical distribution and diel migration patterns of both pelagic and demersal fishes. Precision estimates under different sampling regimes showed that a soak time of 120min and a sample size of at least 8 replicates per treatment would be optimal for sampling using pelagic stereo-BRUVs in tropical or warm-temperate areas. In order to account for the spatial and temporal variability of the system and to facilitate future comparisons across studies using this method, we encourage maximizing replication given the resources available while standardizing the soak time. Pelagic stereo-BRUVs may provide a useful, non-destructive method to improve our understanding on the ecology and behavior of fishes in pelagic ecosystems.

Assessing pelagic fish populations: The application of demersal video techniques to the mid-water environment

In the open ocean, the movements and habitat use of large mobile predators are driven by dynamic interactions between biological and physical variables and complex predator–prey relationships. Understanding the spatial and temporal distributions of pelagic fishes and sharks is a critical component of conservation and fisheries management. Here, we report on a novel non-extractive method for the study of pelagic wildlife, based on baited stereo-camera rigs. The mid-water rigs were derived from existing methodology commonly used in demersal fish surveys. We present new data from 66 moored deployments in Shark Bay, Western Australia (26°10′S, 113°06E) in seabed depths of up to 60 m as a demonstration of the rigs’ ability to resolve spatial variability in pelagic fish and shark assemblages, and to make accurate stereo-measurements of animal lengths. We observed 248 pelagic fishes and sharks from 27 species and 10 families and were able to distinguish between assemblages based nominally on location. We make some general recommendations on optimal deployment protocols and sampling effort regimes, based upon species accumulation rates and times of MaxN (maximum number of individuals of a given species in a single video frame). Regression analyses between high quality and low quality stereo-measurements of fish fork-lengths and range were highly significant, indicating that body lengths and distance estimates were consistent even when stereo-measurements were deemed of low quality. Mid-water stereo-video camera rigs represent an efficient tool for the rapid and non-extractive monitoring of pelagic fish and shark populations, with particular relevance for application in no-take marine protected areas.

Pelagic fish and zooplankton species assemblages in relation to water mass characteristics in the northern Bering and southeast Chukchi seas

This research explores the distributions and community composition of pelagic species in the sub-Arctic and Arctic waters of the northern Bering and central and southern Chukchi seas during September 2007 by linking pelagic zooplankton and fish assemblages to water masses. Juvenile saffron cod (Eleginus gracilis), polar cod (Boreogadus saida), and shorthorn sculpin (Myoxocephalus scorpius) were most abundant in warm, low salinity Alaska Coastal Water (ACW) of the central Chukchi Sea, characterized by low chlorophyll, low nutrients, and small zooplankton taxa. Adult Pacific herring (Clupea pallasii) were more abundant in the less stratified Bering Strait waters and in the colder, saltier Bering Shelf Water of the northern Bering and southern Chukchi seas, characterized by high chlorophyll, high nutrients, and larger zooplankton taxa. Juvenile pink (Oncorhynchus gorbuscha) and chum (O. keta) salmon were most abundant in the less stratified ACW in the central Chukchi Sea and Bering Strait. Abundances of large zooplankton were dominated by copepods (Eucalanus bungii, Calanus glacialis/marshallae, Metridia pacifica) followed by euphausiids (juvenile Thysanoessa raschii and unidentified taxa), whereas small zooplankton were dominated by bivalve larvae and copepods (Centropages abdominalis, Oithona similis, Pseudocalanus sp.). Pelagic community composition was related to environmental factors, with highest correlations between bottom salinity and large zooplankton taxa, and latitude and fish species. These data were collected in a year with strong northward retreat of summer sea ice and therefore provide a baseline for assessing the effects of future climate warming on pelagic ecosystems in sub-Arctic and Arctic regions.

The role of ecological strategies in the colonization success of pelagic fish in a large tropical reservoir (Petit-Saut Reservoir, French Guiana)

Although many studies have been made on fish community changes in reservoirs, the diversity of situations means that general models are still difficult to construct. In order to be useful to managers, the information gathered in these studies must cover the regional taxonomic peculiarities. The inclusion of ecological strategies allows the detection of general patterns of fish community change. In the present study, multivariate analyses and non-parametric tests were used in order to detect relationships between ecological traits of 39 fish species and pelagic fish assemblages. We used gillnet captures of fish made in the Sinnamary River before filling of the Petit-Saut Reservoir and in the reservoir 8 years after closure of the dam and published data on fish species diet, life history parameters and morphology. Significant relationships were detected between fish assemblages and every ecological trait, which would have allowed the forecast of the type of species able to colonize the Petit-Saut Reservoir. Provided that the general characteristics of a river before damming are taken into account, it seems that feeding strategies can be used to predict future fish population and species changes on a general basis. Conversely, the lack of published data prevents definitive conclusions from being drawn concerning life-history and swimming strategies.

Geographic Patterns of Fishes and Jellyfish in Puget Sound Surface Waters

AbstractWe explored patterns of small pelagic fish assemblages and biomass of gelatinous zooplankton (jellyfish) in surface waters across four oceanographic subbasins of greater Puget Sound. Our study is the first to collect data documenting biomass of small pelagic fishes and jellyfish throughout Puget Sound; sampling was conducted opportunistically as part of a juvenile salmon survey of daytime monthly surface trawls at 52 sites during May–August 2003. Biomass composition differed spatially and temporally, but spatial differences were more distinct. Fish dominated in the two northern basins of Puget Sound, whereas jellyfish dominated in the two southern basins. Absolute and relative abundance of jellyfish, hatchery Chinook salmon Oncorhynchus tshawytscha, and chum salmon O. keta decreased with increasing latitude, whereas the absolute and relative abundance of most fish species and the average fish species richness increased with latitude. The abiotic factors with the strongest relationship to biomass composition were latitude, water clarity, and sampling date. Further study is needed to understand the spatial and temporal heterogeneity in the taxonomic composition we observed in Puget Sound surface waters, especially as they relate to natural and anthropogenic influences.Received March 24, 2010; accepted December 23, 2011

Spatial distribution of pelagic fish off Adélie and George V Land, East Antarctica in the austral summer 2008

Pelagic fish assemblages and community structure were examined along longitudinal and meridian transects off Adélie and George V Land, East Antarctica, in the austral summer 2008. Fish were sampled with an RMT 8 net principally from six discrete depth layers (0–50–100–200–500–1000–2000 m) in the oceanic zone and from three depth layers (0–50–100–200 m) over the continental shelf zone. A total of 20,281 individuals from 27 species were collected. Pleuragramma antarcticum was the most dominant species by number (18,710 inds), followed by Chionodraco hamatus (768), Trematomus newnesi (375), Cyclothone microdon (101), Electrona antarctica (92), Bathylagus antarcticus (51) and Notolepis coatsi (54). Cluster analysis revealed that the fish community was clearly divided at the Antarctic Slope Front into separate oceanic and shelf assemblages, being dominated by mesopelagic fish and notothenioids, respectively. The Southern Boundary of Antarctic Circumpolar Current likely restricted a more northern distribution of notothenioids in the upper 200 m. Mesopelagic fish dominated the large biomass below 500 m and notothenioids dominated that in the upper 100 m. It is considered that mesopelagic fish in the oceanic zone would unlikely be eaten by seabirds because no distinctive diel vertical migration to the surface layer was observed. In the neritic zone, notothenioids ( C. hamatus, T. newnesi and P. antarcticum) possibly play an important role as prey items for flying seabirds, penguins and other notothenioids fish especially in the shallow depth stratum (0–100 m).

Temporal and spatial variability of pelagic wild fish assemblages around Atlantic bluefin tuna Thunnus thynnus farms in the eastern Adriatic Sea

The abundance and size structure of wild fishes aggregated around the sea-cages of two commercial Thunnus thynnus farms, including control locations, were assessed and compared over a 1 year period. The T. thynnus farms were located in the eastern Adriatic Sea, offshore of the islands of Ugljan and Brač. Fish assemblages were evaluated through visual census using scuba at 2 month intervals at two sites within each farm. The data suggest that wild fish assemblages at the study sites differed greatly; 20 species occurred at the Ugljan farm and 17 at the Brač farm, while only seven species were observed at the control locations. The abundance and diversity of wild fish assemblages were greater at the farms in comparison to control locations. The most abundant families were Sparidae and Belonidae (>80% of aggregated fishes). At both farms, the abundance and diversity of wild fishes were highest during summer, while diversity was lowest in winter and was mainly characterized by schools of bogue Boops boops and garfish Belone belone. Variability was also detected in spatial assemblages between farms; B. boops and B. belone were the most abundant species for the overall study at the Brač farm, while B. belone and saddled bream Oblada melanura were the most abundant at the Ugljan farm. The settlement also played a significant role in farm-associated fish assemblages, as both juveniles and advanced juveniles were common residents at farms. The majority of species which settled at the farms belonged to the sparids. Results indicate that aggregations of wild fishes at T. thynnus farms are persistent year-round, though the assemblage compositions and size structures of dominant species vary in respect to location and season.