Trophic shifts in a native predator following the introduction of a top predator in a tropical lake

Mercury levels in two freshwater fishes from Panama's two largest lakes

Mercury contamination is a global concern, yet addressing it in developing nations is particularly challenging given the scarcity of data. In 2015, along with many other countries, Panama ratified the Minamata Convention on Mercury- a global effort to tackle mercury contamination and mitigate the risks it poses to humans and the environment. However, there are currently no published data from Panama on mercury contamination in freshwater fishes nor its associated health risks to humans from fish consumption. Here, we quantify for the first time the total mercury (THg) concentrations in two predatory fish species in Panama's two largest lakes: Lake Gatun and Lake Bayano. Within Lake Gatun, the native predator Hoplias microlepis ("pejeperro") had significantly higher predicted mercury concentrations (341.77 ng/g wet weight for a fish of 25 cm in standard length) than the introduced Cichla monoculus ("sargento" or "peacock bass") (196.98 ng/g), and mercury concentrations increased with fish size for both species. H. microlepis from Lake Gatun also had significantly higher predicted THg concentrations than H. microlepis from Lake Bayano (168.37 ng/g). Mercury concentrations at the aquatic food web baseline did not differ between lakes, as shown by the primary consumer snails Pomacea sp. Additionally, using stable isotopes of nitrogen (δ15N), we found that mercury concentrations increased with trophic position in both lakes (resulting in positive and significant Trophic Magnification Slopes), suggesting mercury biomagnification. Based on these findings, we provide fish consumption guidelines for both lakes, using the U.S. EPA current reference dose for mercury exposure. These results provide critical data for Panama as it works towards meeting the objectives set under the Minamata Convention.

Carrion ecology in inland aquatic ecosystems: a systematic review

Carrion ecology, i.e. the decomposition and recycling of dead animals, has traditionally been neglected as a key process in ecosystem functioning. Similarly, despite the large threats that inland aquatic ecosystems (hereafter, aquatic ecosystems) face, the scientific literature is still largely biased towards terrestrial ecosystems. However, there has been an increasing number of studies on carrion ecology in aquatic ecosystems in the last two decades, highlighting their key role in nutrient recirculation and disease control. Thus, a global assessment of the ecological role of scavengers and carrion in aquatic ecosystems is timely. Here, we systematically reviewed scientific articles on carrion ecology in aquatic ecosystems to describe current knowledge, identify research gaps, and promote future studies that will deepen our understanding in this field. We found 206 relevant studies, which were highly biased towards North America, especially in lotic ecosystems, covering short time periods, and overlooking seasonality, a crucial factor in scavenging dynamics. Despite the low number of studies on scavenger assemblages, we recorded 55 orders of invertebrates from 179 families, with Diptera and Coleoptera being the most frequent orders. For vertebrates, we recorded 114 species from 40 families, with birds and mammals being the most common. Our results emphasise the significance of scavengers in stabilising food webs and facilitating nutrient cycling within aquatic ecosystems. Studies were strongly biased towards the assessment of the ecosystem effects of carrion, particularly of salmon carcasses in North America. The second most common research topic was the foraging ecology of vertebrates, which was mostly evaluated through sporadic observations of carrion in the diet. Articles assessing scavenger assemblages were scarce, and only a limited number of these studies evaluated carrion consumption patterns, which serve as a proxy for the role of scavengers in the ecosystem. The ecological functions performed by carrion and scavengers in aquatic ecosystems were diverse. The main ecological functions were carrion as food source and the role of scavengers in nutrient cycling, which appeared in 52.4% (N = 108) and 46.1% (N = 95) of publications, respectively. Ecosystem threats associated with carrion ecology were also identified, the most common being water eutrophication and carrion as source of pathogens (2.4%; N = 5 each). Regarding the effects of carrion on ecosystems, we found studies spanning all ecosystem components (N = 85), from soil or the water column to terrestrial vertebrates, with a particular focus on aquatic invertebrates and fish. Most of these articles found positive effects of carrion on ecosystems (e.g. higher species richness, abundance or fitness; 84.7%; N = 72), while a minority found negative effects, changes in community composition, or even no effects. Enhancing our understanding of scavengers and carrion in aquatic ecosystems is crucial to assessing their current and future roles amidst global change, mainly for water-land nutrient transport, due to changes in the amount and speed of nutrient movement, and for disease control and impact mitigation, due to the predicted increase in occurrence and magnitude of mortality events in aquatic ecosystems.

Biological invasions alter the structure of a tropical freshwater food web

Biological invasions are expected to alter food web structure, but there are limited empirical data directly comparing invaded versus uninvaded food webs, particularly in species-rich, tropical systems. We characterize for the first time the food web of Lake Gatun-a diverse and highly invaded tropical freshwater lake within the Panama Canal. We used stable isotope analysis to reconstruct the trophic structure of the fish community of Lake Gatun and to compare it to that of a minimally invaded reference lake, Lake Bayano. We found significant differences between the trophic structures of these two Neotropical lakes, notably that Lake Gatun's fish community was characterized by a longer food chain, greater isotopic diversity, a broader range of trophic positions and body sizes, and shifts in the isotopic positions of several native taxa relative to Lake Bayano. The degree of isotopic overlap between native and non-native trophic guilds in Lake Gatun was variable, with herbivores exhibiting the lowest (20%-29%) overlap and carnivores the greatest (81%-100%). Overall, our results provide some of the first empirical evidence for the ways in which multiple introduced and native species may partition isotopic space in a species-rich tropical freshwater food web.

Environmental DNA (eDNA) reveals potential for interoceanic fish invasions across the Panama Canal

Interoceanic canals can facilitate biological invasions as they connect the world's oceans and remove dispersal barriers between bioregions. As a consequence, multiple opportunities for biotic exchange arise and the resulting establishment of migrant species often causes adverse ecological and economic impacts. The Panama Canal is a key region for biotic exchange as it connects the Pacific and Atlantic Oceans in Central America. In this study, we used two complementary methods (environmental DNA (eDNA) metabarcoding and gillnetting) to survey fish communities in this unique waterway. Using COI (cytochrome oxidase subunit I) metabarcoding, we detected a total of 142 fish species, including evidence for the presence of sixteen Atlantic and eight Pacific marine fish in different freshwater sections of the Canal. Of these, nine are potentially new records. Molecular data did not capture all species caught with gillnets, but generally provided a more complete image of the known fish fauna as more small-bodied fish species were detected. Diversity indices based on eDNA surveys revealed significant differences across different sections of the Canal reflecting in part the prevailing environmental conditions. The observed increase in the presence of marine fish species in the Canal indicates a growing potential for interoceanic fish invasions. The potential ecological and evolutionary consequences of this increase in marine fishes are not only restricted to the fish fauna in the Canal as they could also impact adjacent ecosystems in the Pacific and Atlantic Oceans.

Temporal niche partitioning as a novel mechanism promoting co-existence of sympatric predators in marine systems

Niche partitioning of time, space or resources is considered the key to allowing the coexistence of competitor species, and particularly guilds of predators. However, the extent to which these processes occur in marine systems is poorly understood due to the difficulty in studying fine-scale movements and activity patterns in mobile underwater species. Here, we used acceleration data-loggers to investigate temporal partitioning in a guild of marine predators. Six species of co-occurring large coastal sharks demonstrated distinct diel patterns of activity, providing evidence of strong temporal partitioning of foraging times. This is the first instance of diel temporal niche partitioning described in a marine predator guild, and is probably driven by a combination of physiological constraints in diel timing of activity (e.g. sensory adaptations) and interference competition (hierarchical predation within the guild), which may force less dominant predators to suboptimal foraging times to avoid agonistic interactions. Temporal partitioning is often thought to be rare compared to other partitioning mechanisms, but the occurrence of temporal partitioning here and similar characteristics in many other marine ecosystems (multiple predators simultaneously present in the same space with dietary overlap) introduces the question of whether this is a common mechanism of resource division in marine systems.

Razak IH, Shahimi S, Pati S, Edinur HA, John AB, Ahmad A, Kumaran JV, Martin MB, Chong JL, Chowdhury AJK, Nelson BR. A review on introduced Cichla spp. and emerging concerns

Peacock bass (Cichla spp.) originates from the Neotropical environments of Brazil and Venezuela but, through trade and smuggling for aquarium keeping, sport fishing and aquaculture, it is now an emerging concern. Yet, less is known for Cichla spp. distribution and its ability to invade new environments. Aimed to communicate on Cichla spp. ecology, biology and introduction schemes from Scopus, Web of Science, Google Scholar and also National Centre for Biotechnology Information, this review also contains management strategies for invading fish species. While Cichla spp. can displace native fish populations, this concern is explained using ecological functions, physiological demands, direct and secondary invasion, disease tolerance and parasite spillover. Briefly, Cichla spp. has rapid embryogenesis (72 h) and matures in short periods (11-12 months), giving it an advantage to colonize new environments. With a large appetite, this true piscivore gains territorial control over water bodies by making it their feeding and nursery grounds. Perceived as an emerging concern after becoming introduced, seal-off or sport fishing were used to manage Cichla spp. but, this practice is not sustainable for the entire ecosystem. Hence, we recommend bottom-up management that involves community participation because they interact with the fish and have knowledge about their environment.