Structure Of Larval Assemblages Research Articles

Seasonal patterns in the structure of the fish larvae assemblages in the lower Amazon River

Abstract This study assessed the phenology and functional structure of parental stocks of fish larvae, and evaluated how the species modulate their reproductive activities as a function of natural hydrological regimes in the lower Amazon River, Brazil. Larvae were collected using conical plankton nets monthly over two hydrological cycles between January and December 2013, and between May 2017 and April 2018. The collections were conducted around an alluvial island located near the municipality of Santarém, Pará, Brazil. After identification, individuals were classified into four functional groups (e.g., life‐history strategy, feeding habits, type of migration, and swimming‐performance/microhabitat‐use in the water column) based on species traits. The relationships between seasonal hydrology, environmental variables and larval assemblage structure were evaluated using multivariate analyses. A total of 31,347 fish larvae were collected, from nine taxonomic orders, 26 families, 40 genera and 53 species. The larval assemblages showed highly variable taxonomic and functional composition between different phases of the hydrological cycle (rising water, high water, receding water and low water) and were strongly influenced by environmental variables. Fish reproductive phenology in the lower Amazon is complex and distributed among the hydrological phases. The different ecological characteristics allow the species to withstand specific environmental conditions during each phase. Changes in the structure of larval assemblages indicate that hydrological variability is a key ecological factor influencing the reproductive dynamics of fish and reflects adaptive strategies used to reduce interspecific competition for space and food, maximising recruitment. The variation of fish reproduction across the hydrological cycle that we observed is a result of different ecological characteristics enabling species and functional groups to adapt to environmental conditions of each phase.

Use of fragmented reservoir habitats by larval fish assemblages across years with contrasting hydrological conditions

Sedimentation in aging reservoirs generally reduces fish habitat quality and quantity, often isolating coves and pools from the main body of the reservoir. In some cases, however, habitat fragments created by sediment deposition in the river-reservoir interface zone support high fish diversity, and they could potentially replicate features of nursery habitats in natural river floodplains if they are regularly connected to the greater river-reservoir. We examined the structure of larval fish assemblages in fragmented coves and pools within the transitional zone of an aging reservoir (Lake Texoma) over 2 years with contrasting hydrological conditions (dry year with low connectivity versus wet year with high connectivity to the main body of the reservoir). We found that larval assemblage structure varied spatially across the two river arms of the reservoir and temporally across the dry and wet year. In both years, assemblages were dominated by Dorosoma species and Menidia beryllina, taxa considered habitat generalists. The abundance of other taxa, particularly Lepomis, Pomoxis, and Morone species, increased with greater habitat connectivity in the wet year. Many taxa considered dependent upon riverine or floodplain habitats were collected during the wet year at sites where they had not been collected in the dry, low-connectivity year. Our results suggest that these fragmented habitats can provide nursery habitats for a variety of fish taxa, but that the structure of larval assemblages using them varies widely based on hydrological connectivity.

Seasonality, floods and droughts structure larval fish assemblages in prairie rivers

AbstractPrairie stream fish assemblages experience seasonal shifts in habitat conditions, often coupled with powerful disturbance regimes, where systems fluctuate widely between drought and deluge. These abiotic drivers, more often than biotic factors, structure fish assemblages. Scouring floods may reset stream communities to early successional seres, and droughts may progressively eliminate species from communities. Therefore, fish assemblage structure resilience depends on recolonisation and reproductive success following a disturbance. Many plains fishes have life history traits that facilitate the wide dispersal and immediate contribution of offspring following disturbance events. However, descriptions of temporal and spatial patterns in larval assemblage structure in prairie streams are scarce, and few studies evaluate the effect of disturbances on reproduction. The objectives of this research were to (i) study the timing and contribution of native prairie stream fishes to drifting larval fish assemblages in three eastern South Dakota basins (Vermillion, James and Big Sioux) and (ii) describe larval fish responses to floods in the James River. We sampled drifting larval fish diversity during the spawning season from 2010 to 2012 throughout each basin. We also examined the effect of flooding on larval fish assemblage structure using data collected near the confluence of the James and Missouri rivers during baseflow (2003, 2004 and 2012) and flood conditions (2010). Subtle variability in spawning traits of prairie fishes and an apparent seasonal progression of larval fish taxa that secondarily respond to disturbance regimes suggests reproductive efforts are adapted to maximise the potential for successful recruitment in these severe systems.

Different key roles of mesoscale oceanographic structures and ocean bathymetry in shaping larval fish distribution pattern: A case study in Sicilian waters in summer 2009

Fish larvae data collected in year 2009 were used to examine the effects of particular environmental conditions on the structure of larval assemblages in two oligotrophic Mediterranean areas (the Southern Tyrrhenian Sea and the Strait of Sicily). For this purpose, relationships with environmental variables (temperature, salinity and fluorescence), zooplankton biomass, water circulation and bathymetry are discussed. Hydrodynamic conditions resulted very differently between two study areas. The Southern Tyrrhenian Sea was characterized by moderate shallow circulation compared to the Strait of Sicily. In this framework, distribution pattern of larval density in the Tyrrhenian Sea was mainly driven by bathymetry, due to spawning behavior of adult fish. There, results defined four assemblages: two coastal assemblages dominated by pelagic and demersal families and two oceanic assemblages dominated by mesopelagic species more abundant in western offshore and less abundant in eastern offshore. The assemblage variations in the western side was related to the presence of an anti-cyclonic gyre in the northern side of the Gulf of Palermo, while in the eastern side the effect of circulation was not very strong and the environmental conditions rather than the dispersal of species determined the larval fish communities structure. Otherwise in the Strait of Sicily the currents were the main factor governing the concentration and the assemblage structure. In fact, the distribution of larvae was largely consistent with the branch of the Atlantic Ionian Stream (AIS). Moreover, very complex oceanographic structures (two cyclonic circulations in the western part of the study area and one anti-cyclonic circulation in the eastern part) caused the formation of uncommon spatial distribution of larval fish assemblages, only partially linked to bathymetry of the study area. Typically coastal larvae (pelagic families: Engraulidae and Clupeidae) were mostly concentrated in the offshore areas and off Capo Passero, where the presence of a thermo-haline front maintained their position in an area with favourable conditions for larval fish feeding and growth.

Relative importance of water column vs zooplankton variables in the determination of late-stage larval fish assemblage structure in coastal waters of a coral reef lagoon

SUMMARY: The relationship between fish larvae and their zooplanktonic prey has not been fully explored for late-stage larvae of coral-reef fish in lagoonal environments. However, compared to most temperate taxa, these larvae are characterized by strong sensory and swimming abilities, which may influence their feeding behaviour in the water column. The present study aims to determine the relative importance of the water column and zooplankton variables for the structure of pre-settlement larval fish assemblages within a single season in three bays of the coral reef lagoon of New Caledonia, southwest Pacific. The structure of larval assemblages was found to be explained better by water column variables in two out of the three bays examined. Zooplankton variables only played a role in one bay out of the three, probably due to the lower variability in the water column variables. Moreover, the relationship between total larval fish abundance and zooplankton density was not significant in any of the three bays. These results suggest that the relationship between late-stage coral-reef fish larvae and their prey: 1) is difficult to detect at small spatial and temporal scales, 2) is probably complex and non-linear, 3) depends on environmental conditions, and 4) probably varies between fish taxa.