The Argentine anchovy Engraulis anchoita is the most abundant small pelagic fish in the southwestern Atlantic and plays a key role in transferring energy from plankton to higher trophic levels. Despite its ecological importance and long history of study, information connecting larval and juvenile stages with adult populations remains scarce. Here, we assessed growth, nutritional condition, and trophic ecology of E. anchoita from the northern stock across ontogeny, from larvae to adults, using otolith microstructure, RNA/DNA indices, and stable isotope analyses (delta C-13 and delta N-15). Daily otolith increments showed non-linear, dome-shaped growth trajectories, with reduced increment widths during winter, as reconstructed from otolith back-calculation, highlighting strong seasonal modulation of growth. RNA/DNA indices revealed two significant breakpoints at similar to 15 mm and similar to 90 mm (total length), corresponding to transitions from larval to postlarval stages and from juveniles to first maturity, respectively. Stable isotope values indicated an ontogenetic dietary shift, with a significant increase in relative trophic position between early postflexion larvae and later stages. From late postflexion onwards, juveniles and adults showed greater isotopic niche overlap and shared trophic positions, suggesting early convergence toward adult-like feeding strategies. Our integratory approach revealed that key ontogenetic thresholds in E. anchoita are reflected simultaneously in growth, condition, and trophic ecology. Furthermore, early convergence toward similar feeding strategies suggests the potential for intraspecific competition at early developmental stages, which may increase vulnerability under unfavorable environmental conditions.

The existing literature has classified Engraulis anchoita larvae as exclusively zooplanktophagous, with copepod eggs and nauplii larvae as their main prey. However, there is evidence that other plankton components that have not been identified by intestinal content analysis may be a part of their diet. The objective of this work was to explore trophic positions and main food sources of E. anchoita larvae by analyzing stable nitrogen and carbon isotopes (δ15N and δ13C) with respect to other plankton components (particulate organic material, calanoid copepods, and chaetognaths) at a fixed sampling station close to the coast of the Province of Buenos Aires, Argentina, southwest Atlantic. Samples were collected at different times of the year between the end of 2016 and the beginning of 2018. By averaging δ15N and δ13C values of all surveys, it was found that the anchovy larvae in their three development stages coincided with the same trophic position as chaetognaths, suggesting that their diets overlap. When surveys carried out in autumn were analyzed separately, trophic levels of anchovy larvae differed in a staggered manner among their stages, being situated between the positions of copepods and chaetognaths. Compared to other stages of development, anchovy larvae in the preflexion stage consumed a higher proportion of particulate organic material rather than small copepods. These findings suggest that anchovy larvae may consume a variety of food items and shift their trophic position in response to environmental conditions.

Fish larvae feeding success is crucial for adult recruitment, which affects both fisheries and ecosystem regulation processes. Engraulis anchoita is a species of great ecological and fishing relevance in the South-West Atlantic Ocean, with a wide latitudinal distribution and active spawning throughout the year and strongly associated with frontal areas. E. anchoita larvae have been described as exclusively feeding on small copepods and their early developmental stages. In this study, the diet, nutritional condition, and daily growth of E. anchoita larvae were estimated in different environmental scenarios in the South-West Atlantic Ocean. Carbon and Nitrogen stable isotopes analysis, otolith microstructure analysis, and RNA/DNA index determination were performed. The larval diet, which was evaluated based on the availability of potential prey, varied with respect to the water column stability. It was observed that the larvae tend to feed on microphytoplankton in stratified waters and on calanoid copepods in waters with a homogeneous vertical structure. The growth and nutritional condition indexes responded to ontogeny, presenting higher values at larger larval sizes. Growth indexes also responded positively to the stratified water column structure in comparison to the homogeneous one. The nutritional condition index was only explained by the size of the individuals. This study found that E. anchoita larvae had greater trophic flexibility than previously suspected, as they included microphytoplankton as the main food item under certain oceanographic conditions without negative impacts on their growth or nutritional condition.

The European green crab ( Carcinus maenas) is one of the most extensively studied decapod species worldwide, and it currently inhabits every continent except Antarctica. Its effects are observed across various spatio-temporal scales, impacting a wide range of taxa and environments. While extensive research has been conducted in the Northern Hemisphere, populations in the Southern Hemisphere (e.g., Argentina, Australia, and South Africa) have not been thoroughly investigated. This study has three main goals: (1) summarise and contextualise the invasion history of green crabs in the Southwest Atlantic since their initial detection in 2000, (2) present nine case studies identifying the potential ecological and economic impacts of green crabs on coastal ecosystems, and to highlight priority research areas, and (3) discuss appropriate management actions in response to the species' rapid spread in the region. Our findings suggest that green crabs are likely to impact foundation species along rocky shores, alter the physical characteristics of soft-bottom environments, and affect infaunal organisms in sandy shores. Most of these impacts are either occurring or expected to occur in numerous marine protected areas. We also examine green crab interactions with other key species, highlighting its dual role as both an invasive predator and prey for native species, thus serving as a novel food resource. Furthermore, we consider their effects on commercially important species, tourism, and implications for threatened native species. Finally, we recommend prioritising prevention and rapid response strategies for managing green crab invasions, emphasising the importance of early detection and prompt action to address new incursions.

Developing frameworks to identify knowledge gaps and prioritize research on diet studies in marine fish species is critical, as this knowledge is required for ecosystem-based management. We applied a framework in central Patagonia, Argentina, which faces a demand for gap assessments in diet knowledge and ecosystem management due to changes in marine fish assemblages' structure and function linked to industrial fishing and tropicalization. Our framework effectively identified gaps by examining the history of research efforts regarding spatial and temporal coverage, sampling sizes, fish life stages, and information quality of studies per species. We found critical gaps in local diet studies, particularly for mostly bony, Petromyzonti, and Myxini species, whereas elasmobranchs received the best coverage of the diet described. Most studies lack evaluation of ontogenetic diet changes and prey cumulative curves. Fixing these shortcomings improves research quality and reduces data uncertainties in community assessments. The prioritization framework identified high-priority species as those that require updated diet information, had shifted ranges, and lacked data. Our framework can be tailored to other biological traits and regions based on specific contextual needs to identify research gaps and priorities for fish assemblages impacted by global change while disseminating knowledge from diverse sources and languages.

In the context of global change, reviewing the relationships between marine fish traits and their range shifts is required to (1) identify ecological generalizations regarding the influence of traits on range shifts at a global scale and (2) investigate the rationale behind trait inclusion in models describing those relationships. We systematically searched for studies on marine fish assemblages that identified distributional shifts and analyzed the relationship between fish traits and these shifts. We reviewed 29 papers and identified 11 shift type characterizations and 41 traits, noting significant variation in measurement methods and model types used to describe their relationships. We identified global trait redundancies in the relationship between fish traits and latitudinal range shifts. These trends are related to the fishes' latitudinal range, trophic level, water column habitat, body size, size-at-settlement, growth rate, and larval swimming ability. The first four traits, along with fish bottom habitat, biogeographic affinity, diet, and thermal affinity, also showed significant relationships across four ways to characterize horizontal range shifts of fish species. The significance of these traits suggests their relevance in range shifting, regardless of the analyses conducted, biogeographic realm, and range shift type. However, trait redundancies require further consideration, mainly because some traits show opposing relationships in different studies, and important biogeographic research gaps limit global generalizations about the trait-range shift relationship. Half of the studies analyzed provided a rationale for 23 out of 41 traits. We also provide guidelines for future work to better understand the influence of traits on fish range shifts.

Maps of (baseline) δ13C and δ15N values of primary producers or consumers near the base of food webs provide crucial information for interpreting patterns in the isotopic composition of consumers that occupy higher trophic levels. In marine systems, understanding how oceanographic variables influence these values enables the creation of dynamic isoscapes across time and space, providing insights into how ecosystems function. The San Jorge Gulf (SJG) in the southwest Atlantic Ocean (45° S–47° S) is an area of particular importance, as it is located on one of the most productive continental shelves in the world, supporting large fisheries and marine mammal and seabird populations. We reconstructed spatial variation in zooplankton δ13C and δ15N values across SJG and investigated their relationship with physical and chemical oceanographic conditions. During cruises in the austral spring of 2016 and 2017, we collected medium-sized copepods whose isotopic composition integrate short-term (days to weeks) variation in oceanographic conditions recorded by phytoplankton at the base of the food web. We also collected data on water column depth, surface and bottom temperatures, water column stability, and macronutrient (nitrate, phosphate, and silicic acid) concentrations. The results revealed significant variation in both δ13C and δ15N values of up to 7-8‰ over a relatively small spatial scale (200–300 km). Copepod δ13C values were lower at the center of the SJG, showing an inverse correlation with water column stability, surface nitrate concentration, and water column depth. δ15N values showed a strong and negative relationship with surface nitrate concentration and water column stability, increasing from south to north in the SJG. δ15N values also showed a positive relationship with surface silicic acid concentration. These spatial patterns in nutrient dynamics and copepod carbon and nitrogen isotope values are interpreted in the context of the dominant northward current and temporal development of the frontal systems in the SJG.

We assessed the trophic roles of 13 medium-sized elasmobranchs in Central Patagonia (41°–47° S) by identifying their interspecific relationships and trophic overlap using stomach contents and δ15N and δ13C values. Specimens were collected from the bycatch of the Patagonian red shrimp Pleoticus muelleri trawling fishery and from the sport coastal fishing in areas without trawling. Results showed spatial differences in the diet composition of species between capture sites and from those previous diets reported for Argentina. Trophic positions assessment showed a group of top predators (TP 4) and mesopredators (TP 3.5). Three trophic guilds were identified for the chondrichthyan species recorded in the coastal and deep zones. We also found a dietary overlap among almost all elasmobranch species mainly related to the consumption of the shrimp P. muelleri. Despite not being reported previously in the diet of studied species, we consider the high consumption of this shrimp as an expected result for demersal mesopredators captured in shrimp fishing grounds. These findings underscore the importance of continually updating trophic information in a global change scenario to contribute scientific evidence to the development of adaptative management.

Ecological networks offer valuable insights into community structure, key species identification, and ecosystem management. Understanding how these networks respond to global change stressors is of increasing interest, especially along geographical gradients. This review summarizes potential stressor responses in marine food webs from the Southwest Atlantic to the Antarctic (45–78°S), encompassing areas such as San Jorge Gulf, Beagle Channel, Burdwood Bank, Scotia Sea, Potter Cove, and the Weddell Sea in Antarctica. The objectives are (1) to describe the structure of marine food webs along this latitudinal axis using a network approach; (2) to identify predominant global change-related stressors affecting each ecosystem; and (3) to summarize observed food web changes and hypothesize on stressor impacts. The effects of stressors were primarily reviewed at the species level. Alternative hypotheses for each study area were formulated considering (a) main stressors; (b) impacted parameters; (c) node-level species properties; and (d) network-level food web properties. Global warming emerges as the most common stressor among the studied areas across the latitudinal gradient, except in the Beagle Channel and Burdwood Bank, where alien species introduction and fisheries are more influential. We offer a series of alternative hypotheses on how warming may affect the food webs. This review emphasizes the benefits of using a network approach to understand and predict stressor effects in Southern Hemisphere marine ecosystems. This approach provides a holistic understanding of ecosystems, which enhances our ability to identify key species and their interactions, offering insights for ecosystem management and conservation in the face of global change stressors.

We assessed the trophic roles of 13 medium-sized elasmobranchs in central Patagonia (41°-47°S) by identifying their interspecific relationships and trophic overlap using stomach contents and δ 15 N and δ 13 C values. We also conducted a literature review of the available diet information for Argentina. Specimens were collected from the bycatch of the Patagonian red shrimp Pleoticus muelleri trawling fishery and from the sport coastal fishing in areas without trawling. Results showed spatial differences in the diet composition of species between capture sites and from those previous diets reported for Argentina. Trophic levels assessment showed a group of top predators (TP ~ 4) and mesopredators (TP ~ 3.5). Beyond these differences, there was a dietary overlap among almost all elasmobranch species mainly related to the consumption of P. muelleri , an item which had not been reported as a significant prey for several of the studied species. This consumption may be due to the change in prey supply produced by local fisheries' discards and/or to the increased abundance of crustaceans in Central Patagonia during the last decade. These findings underscore the importance of continually updating trophic information in a scenario of global change to contribute with scientific evidence to the development of adaptative management.

The green crab Carcinus maenas (L., 1758) is a highly invasive species that has altered various coastal marine ecosystems worldwide. Green crabs were first found on the Argentinian Patagonian coast in 2001 and were detected in Golfo Nuevo (Chubut, Argentina) in 2015. In this study, we described a rocky intertidal food web in the Golfo Nuevo and evaluated the effect of green crab invasion on the network. We assembled the intertidal food web from a compilation of diet studies and compared 2 scenarios: pre-invasion (without green crab) and post-invasion (with green crab). We calculated several structural network metrics to compare the scenarios. The green crab was positioned as a top predator in the intertidal food web, consuming prey from different trophic levels and exhibiting the capacity to exert top-down control. Green crab presence altered the structure and stability of the food web. Additionally, we evaluated the effect of potential competitive interactions between native species and the green crab on the food web. Possible competitive interactions increased the effect the green crab had on the structure of the food web. Our results show that green crab invasion decreased food web stability by reducing the capacity of the food web to contain small- and long-range disturbances. These results contribute to the assessment and monitoring of the possible effects of other factors of global change. The system is less stable than before green crab invasion, and the drastic changes in community composition already recorded in rocky intertidal ecosystems in Patagonia are more likely to amplify through the food web.

There are records of seven species of hagfishes in the Argentine shelf and continental slope, comprising the most diverse region for this group in the Southwestern Atlantic. Here, the authors report the occurrence of Myxine affinis and Myxine knappi in San Jorge gulf, adding two species to the ichthyofauna of central Patagonia. A specimen of M. affinis (447 mm TL; hermaphrodite) was incidentally caught by the Patagonian shrimp fishing fleet and collected by the Fishery Observer Program of Fisheries Secretariat of the Chubut Province, Argentina. Subsequently, the authors collected a second specimen of M. affinis (358 mm TL; female) and a specimen of M. knappi (303 mm TL) during the research cruises of the oceanographic vessel ARA Puerto Deseado, within the "Pampa Azul" initiative. The latter is the twelfth record for the species. These specimens are stored in the ichthyological collection of the Instituto de Investigación de Hidrobiología of the Universidad Nacional de la Patagonia San Juan Bosco, Trelew (UNPSJB-ICT), Argentina. These new records of M. affinis and M. knappi extended their known distribution by 850 km and 220 km northward, respectively. In addition, the authors reported morphometric, meristic and reproductive variables of the collected specimens.

Demersal fisheries are one of the top anthropic stressors in marine environments. In the long term, some species are more vulnerable to fishery impacts than others, which can lead to permanent changes on the food web. The trophic relationships between predator and prey constitute the food web and it represents a network of the energy channels in an ecosystem. In turn, the network structure influences ecosystem diversity and stability. The first aim of this study was to describe for the first time the food web of the San Jorge Gulf (Patagonia Argentina) with high resolution, i.e. to the species level when information is available. The San Jorge Gulf was subject to intense fisheries thus our second aim is to analyse the food web structure with and without fishery to evaluate if the bottom-trawl industrial fishery altered the network structure and stability. We used several network metrics like: mean trophic level, omnivory, modularity and quasi-sign stability. We included these metrics because they are related to stability and can be evaluated using predator diets that can weight the links between predators and prey. The network presented 165 species organized in almost five trophic levels. The inclusion of a fishery node adds 69 new trophic links. All weighted and unweighted metrics showed differences between the two networks, reflecting a decrease in stability when fishery was included in the system. Thus, our results suggested a probable change of state of the system. The observed changes in species abundances since the fishery was established, could represent the state change predicted by network analysis. Our results suggests that changes in the stability of food webs can be used to evaluate the impacts of human activity on ecosystems.

Climate change, fishing, and invasion by alien species are the drivers of global change that mainly affect Patagonian marine fish assemblages. In this chapter, we reconstructed the coastal fish assemblages and their changes in richness and composition in three areas at northern, central, and southern Patagonia between 1970 and 2020. Overall, there was an increase in fish richness (30 species) driven by an influx of species from warmer waters in northern and central Patagonia and by the invasion of alien species at central and southern Patagonia. Such a trend is consistent with the global pattern of tropicalization in temperate waters described for other regions and recent evidence of change in the sea surface temperature (SST) up to 48°S latitude. We detected only two local extinctions of skates from warm waters that disappeared from the northern area. A large number of fishes (30 species) diminished their frequency of occurrence. A total of 20 species of commercial importance, 7 of which were elasmobranchs, were the most affected by partial losses. Future research is needed to understand the functional role of the species arriving or retracting and the consequences of such changes on ecosystem functioning.

Exotic species that achieve successful invasions often show considerable dietary plasticity in resource choice, sometimes adjusting their diet to new or variable resources. This can increase species' impact in the recipient environment. In this study, we explore the diet of the invasive gray side-gilled sea slug Pleurobranchaea maculata (Quoy & Gaimard, 1832) in two environments with contrasting presence of algae (different depths) on northern Patagonian coasts, by means of stable isotope analysis. Results showed the most probable prey contribution to the diet of shallow individuals of P. maculata to be algae, whereas colonial tunicates were the preferred prey in deep environments. Therefore, we suggest that P. maculata behaves as an omnivorous species in the receiving community, because it consumes and assimilates the algae, which represents a dietary shift compared to their original feeding habits (carnivorous). This species exhibited the ability to adapt to different dietary resources by showing significant differences in niche locations, niche breadth, and exhibiting little to no overlap between different environments, which supports the previous description of P. maculata as a generalist consumer. Pleurobranchaea maculata's dietary plasticity might drive ecological consequences in the invaded community. Thus, it is important to monitor for future changes in the benthic community to develop efficient programs for the management of this invasive voracious species.

The relationship between fish functional diversity and fishing levels at which its baselines shift is important to identify the consequences of fishing in ecosystem functioning. For the first time, we implemented a trait-based approach in the Argentine Patagonian sea to identify the vulnerability and spatiotemporal changes in functional diversity of fish assemblages bycatch by a trawling fleet targeting the Argentine red shrimp Pleoticus muelleri (Spence Bate, 1888) between 2003 and 2014. We coupled seven fish trophic traits to a reconstructed fish assemblage for the study area and bycatch and evaluated changes in fish species richness and four complementary functional diversity metrics [functional richness, redundancy, dispersion, and community trait values] along with fishing intensity, temporal use, latitudinal location, and depth of fishing grounds. Resident fishes larger than 30 cm in TL, with depressed and fusiform bodies, intermediate to high trophic levels, and feeding in shallow benthic, benthodemersal, and benthopelagic areas were vulnerable to bycatch. Fish assemblages exhibited a low functional trait redundancy, likely related to species influxes in a biogeographic ecotone with tropicalisation signs. Significantly increases in fish trait richness and dispersion polewards and with depth suggested new functional roles in these grounds, matching trends in community body size, reproductive load, maximum depth, trophic level, and diet breadth. Finally, a temporal increase in fish species and functional trait removal in fishing grounds led to trait homogenisation since the first year of trawling. The identified tipping points in temperate fish functional trait diversity highlight trait-based approaches within ecosystem-based fisheries management.

Species are connected with others via trophic and non-trophic relationships; thus any effect of global change (GC) on one species may consequently impact others by changing the direction and/or the strength of biological interactions. In this chapter, we explore through study cases how GC drivers may affect the composition and structure of marine Patagonian communities by changing positive and/or negative interactions. Examples include impacts of increase in nutrient inputs, invasive species, and multiple GC drivers. Eutrophication enhances growth rates, increases biomass, changes nutritional quality, and generates shifts in species composition of primary producers, which indirectly affect consumers. In general, positive interactions (e.g., facilitation) between sessile invasive species and native ones are more frequently documented than interference processes. Multiple GC drivers may benefit some species and harm others by modifying simultaneously bottom-up and top-down processes. Finally, we point out some current research gaps and provide perspectives for future investigation in order to achieve the best ecological approach to understand how marine ecosystems as a whole are facing GC in Patagonia.KeywordsBottom-up and top-down processesCoastal areasFacilitationIndirect effectsPositive and negative interactions

The European green crab Carcinus maenas and its sister species C. aestuarii are highly invasive species causing damage to coastal ecosystems and contributing to severe economic losses worldwide. C. maenas was first detected at the Atlantic Patagonian coast in 2001. In this work, we studied the diet of the green crab in a recently invaded location in Golfo Nuevo, using three complementary techniques: direct stomach observation, metabarcoding of gut content and stable isotope analysis. Direct stomach observation and metabarcoding showed that green crabs have a broad omnivorous diet, ingesting most of the phyla present in the study area. Gut content metabarcoding allowed a detailed description of algal diversity and revealed other taxa that went unnoticed in the visual stomach analysis. Stable isotope analysis showed that the major contribution to the crabs’ diet was from the phytoplankton chain (by bivalve consumption) and not directly from algae. This study approach combining three complementary techniques also allowed us to detect some differences in the diet between sexes, which suggests that male and female crabs are not as ecologically equivalent as previously thought. Besides, we detected sequences corresponding to C. aestuarii suggesting that the green crab Patagonian population is a hybrid of both sister species. These findings are key to understanding the impacts green crabs can have on the local ecosystem.

The study compares the diet composition between juveniles and adults of the narrownose smooth-hound Mustelus schmitti in areas with and without commercial bottom trawling using stomach contents. Individuals (n = 291) were caught as by-catch by the bottom-trawl shrimp fishery and by coastal sport anglers in Central Patagonia (Southwestern Atlantic between 43 degrees and 45 degrees S). Results showed that M. schmitti is a mesocarnivorous with a mean trophic level of 3.58. The diet was based mostly on crustaceans (decapods, stomatopods, isopods and amphipods), polychaetes, fishes and cephalopods, in a decreasing order of importance. The diet varied between juveniles and adults, and between individuals captured by the trawling fishery and sport anglers. Crabs were the main prey item observed in the stomach of individuals caught by anglers while the shrimp Pleoticus muelleri was the most consumed prey by individuals caught as by-catch. Juveniles consumed more polychaetes than adults, while the latter consumed mostly shrimp. The presence of shrimp in stomach contents of M. schmitti caught by the commercial fishery is evidence of a trophic interaction between M. schmitti and the fishery. The interaction may be due to the shark and the fleet targeting the same resource, or due to the shark using the discarded shrimp as a food source.

Plastic pollution in the oceans has become a global problem, but its documentation is disparate around the world. We assess the abundance and type of microplastics in three benthic matrices: mussels, small fishes, and bottom water; in three sites nearby Puerto Madryn city (Patagonia, Argentina). Microplastics were present in the three matrices for all sites sampled. The average amounts of items observed were 1.6 and 0.3 per total wet weight in fishes and mussels, respectively, and 10.5 per liter in bottom water. Mussels and fishes presented a difference of microplastics size comparing with the surrounding bottoms waters; fishes also presented color discrimination, suggesting the necessity of more than one bioindicator to perform microplastic pollution monitoring. Moreover, small fishes had more MPs in their gastrointestinal tracts than bigger ones. The present study is the first one about the interaction between MPs and small aquatic organisms in coastal marine environments from Patagonia.