Inter-annual changes in fish communities of a tropical bay in southeastern Brazil: What can be inferred from anthropogenic activities?

Environmental DNA highlights the influence of salinity and agricultural run-off on coastal fish assemblages in the Great Barrier Reef region

Agricultural run-off in Australia's Mackay-Whitsunday region is a major source of nutrient and pesticide pollution to coastal and inshore ecosystems of the Great Barrier Reef. While the effects of run-off are well documented for the region's coral and seagrass habitats, the ecological impacts on estuaries, the direct recipients of run-off, are less known. This is particularly true for fish communities, which are shaped by the physico-chemical properties of coastal waterways that vary greatly in tropical regions. To address this knowledge gap, we used environmental DNA (eDNA) metabarcoding to examine fish assemblages at four locations (three estuaries and a harbour) subjected to varying levels of agricultural run-off during a wet and dry season. Pesticide and nutrient concentrations were markedly elevated during the sampled wet season with the influx of freshwater and agricultural run-off. Fish taxa richness significantly decreased in all three estuaries (F = 164.73, P = <0.001), along with pronounced changes in community composition (F = 46.68, P = 0.001) associated with environmental variables (largely salinity: 27.48% contribution to total variance). In contrast, the nearby Mackay Harbour exhibited a far more stable community structure, with no marked changes in fish assemblages observed between the sampled seasons. Among the four sampled locations, variation in fish community composition was more pronounced within the wet season (F = 2.5, P = 0.001). Notably, variation in the wet season was significantly correlated with agricultural contaminants (phosphorus: 6.25%, pesticides: 5.22%) alongside environmental variables (salinity: 5.61%, DOC: 5.57%). Historically contaminated and relatively unimpacted estuaries each demonstrated distinct fish communities, reflecting their associated catchment use. Our findings emphasise that while seasonal effects play a key role in shaping the community structure of fish in this region, agricultural contaminants are also important contributors in estuarine systems.

Aliphatic and polycyclic aromatic hydrocarbons in surface sediments as a tool for the assessment of the contamination status of mangrove forests in Rio de Janeiro (Brazil)

The distribution of aliphatic and polycyclic aromatic hydrocarbons (PAHs) in surface sediments from mangrove forests of the Rio de Janeiro State was investigated. Ten sampling stations were selected in the mangroves of Sepetiba Bay and the Jacarepaguá Lagoon Complex (JLC), which are areas affected by multiple human activities. The total aliphatic hydrocarbons concentrations showed marked variation between samples (27-407 μg g^-1), mostly related to the total organic carbon contents. The total PAHs concentration ranged between 38 and 792 ng g^-1. Diagnostic indices and statistical analysis showed that the mangrove forests can be divided into three groups: the western portion of Sepetiba Bay with the lowest level of contamination; the inner portion of the bay with the most intense presence of local sources of contamination, especially of pyrolytic character; and the JLC with a greater accumulation of hydrocarbons, mainly derived from petroleum combustion, resultant from the intense urbanization.

Functional stability despite anthropogenic influences on the ichthyofauna of a tropical bay

Classifying species into groups based on taxonomic relationship and functions are objective approaches to evaluate environmental and anthropogenic influences on coastal fishes. We evaluated temporal (1993-1995 and 2012-2015) changes in the taxonomic and functional indices in three zones of a tropical bay heavily impacted over the recent decades. We tested the hypothesis that both indices decrease over time as result of the environmental degradation. A decrease in the taxonomic richness and abundance was observed mainly of the inner zone. The functional structure remained relatively stable, but the functional originality decreased significantly between the two periods. This functional loss is of particular concern because the loss of species with unique treats may generate a series of ecosystem damage. This information tells us that the use of functional indices is essential to complement taxonomic assessments and to detect a more detailed understanding of the real dimension of biodiversity loss in impacted environments.

Molecular characterization of nearshore baitfish populations in Bermuda to inform management

Small-bodied marine fishes play an important role in the food web, feeding both larger fishes and seabirds. Often referred to as baitfishes, they concentrate seasonally in coastal areas in large, often heterospecific assemblages that are targeted by both commercial and recreational fishers. Given apparent declines in at least some of Bermuda’s baitfish species over the past 40 years, it is useful to determine the species composition of baitfish assemblages, and how it varies among sites, in order to inform management. Using genetic barcoding of the Cytochrome c oxidase 1 gene (COI), we confirm species identity, assess intraspecific genetic diversity locally, and determine rates of broader genetic connectivity for baitfish assemblages in Bermuda. Species analyzed included Hypoatherina harringtonensis, Anchoa choerostoma, Jenkinsia lamprotaenia, Harengula humeralis, Opisthonema oglinum and Sardinella aurita. Species identification based on molecular barcoding revealed some misidentification of individuals based solely on gross morphological characteristics, with an error rate of 11%, validating the usefulness of this approach. Interestingly, sequence results for the endemic Bermuda anchovy, A. choerostoma, were within 1% similarity to the more broadly distributed big-eye anchovy, A. lamprotaenia, and thus additional analyses are warranted to evaluate the genetic basis for endemism. Estimates of genetic diversity within and among baitfish assemblages in Bermuda were high, indicating high rates of local connectivity among sites for all species. As such, management should consider Bermuda’s baitfish species as single, highly mixed populations. However, with the exception of H. humeralis and the endemic A. choerostoma, significant genetic differentiation and population structure were found when comparing Bermuda’s baitfish populations with those from other regions, suggesting limited gene flow between other regions and Bermuda for these species. Limited regional connectivity has implications for management, as strong genetic divergence suggests that populations in Bermuda are predominantly self-seeding and thus not likely to be replenished from distant populations. These results therefore support precautionary management of baitfish species in Bermuda.

Leave forever or return home? The case of the whitemouth croaker Micropogonias furnieri in coastal systems of southeastern Brazil indicated by otolith microchemistry

Movements of coastal fish species between estuarine systems (rearing grounds) and adjacent platforms (spawning grounds) are complex and can be influenced by multiple biotic and abiotic factors associated with the life cycle of each species. Understanding fish movement patterns between these habitats is crucial for sustainable management of these resources. We aimed to identify movement patterns of the whitemouth croaker Micropogonias furnieri, an important fishery resource along the southeastern Brazilian coast, through characterization of elemental concentration profiles in otoliths. Signal intensity ratios of Sr⁸⁶:Ca⁴³ and Ba¹³⁸:Ca⁴³, measured by a Laser ablation inductively coupled to a plasma mass spectrometry (LA-ICPMS), were analysed. Juveniles and adults fish from three semi-enclosed systems (two bays and one coastal lagoon) and adults from nine adjacent coastal areas were examined. Three main behavioural patterns were identified: 1) Marine Migrant, with a unique and definitive movement from the estuarine area towards the adjacent platform when adult; 2) Estuarine Visitor, with movements from the estuarine area towards the adjacent platform when adult but also performing excursions back to the estuarine areas and back again to the adjacent platform; and 3) Nearshore Resident, with occurrence in the adjacent coastal areas that are influenced by estuarine systems. Estuarine areas with large sea connections (e.g., bays) favour more frequent fish entrances and exits compared with coastal lagoons that have a narrow sea connection. The high eurioic condition and capacity related to adapting to different environmental conditions were detected in this study for M. furnieri, a likely strategy to maintain variation in life histories and successfully adapt to coastal marine systems.