Environmental and Physical Factors Affecting the Diversity and Distribution of the Ichthyoplankton in an "Inverse Estuary" , the Sine Saloum (Senegal)

Abstract

Estuarine mangrove ecosystems are considered essential nursery grounds and feeding areas for the early life stages of many fish species, often including commercially important ones. Particularly, climate change is expected to affect and have a substantial impact on mangrove estuaries, through processes including changes in precipitation, increased temperature, and changing patterns of ocean and estuarine circulation. As a result, the inversion of the salinity gradient in several estuaries throughout the dry tropics is either underway or can be expected in the near future. Such modifications of these important estuarine environments are cause of concern because spawning and nursery grounds of fishes have requisite environmental and physical attributes. Thus, there is a strong scientific consensus that related pressing contemporary research questions regarding the early life history of fishes in these transformed estuaries should be addressed. Located in Senegal, West Africa, the Sine Saloum system is representative of estuaries where the salinity gradient has been inverted due to climatic changes in the region. Given the high overall salinity and the resulting mangrove degradation that is taking place there, its potential role (compared to a classic estuarya ) as a recruitment and nursery area for fish larvae is far to be clear. Thus, the Sine Saloum estuary is a natural and excellent choice to study how these climatic environmental transformations are affecting the ichthyoplankton community. The aim of this thesis is to gain knowledge on the environmental and physical factors affecting the Sine Saloum ichthyoplankton diversity and distribution. The survival of fish larvae is known to be influenced by complex interactions between environmental changes and tropho- and hydrodynamic processes. Consequently, I first analysed the spatial and seasonal distribution of the fish larval assemblages related to environmental parameters. Second, a field experiment measuring simultaneously vertical current profiles and larval transport were conducted to investigate the effectiveness of larval fishes in regulating transport in and out of the estuary. Lastly, stable isotope analysis was used to evaluate the contribution of the sea surface microlayer (SML) to the diet of larval and juvenile African halfbeaks (Hyporamphus picarti), one of the dominant species in the system. The summarised key findings of the thesis are: A total of 41 taxa representing 24 families and 34 genera were sampled in the estuary, which is lower than that of other tropical estuaries, providing evidences that high salinity environment may harbour a less diverse ichthyoplankton fauna. Additionally, the distributional pattern of fish larvae revealed that the total abundance and the richness in the estuary decreased from the lower to the upstream areas with salinity and water temperature as the variables that best explained the spatial and temporal differences observed. Larval fish assemblages also showed a clear vertical structure corresponding to three distinct water strata.The circulation at the entrance of the Sine Saloum estuary was characterized by the existence of a longitudinal gravitational circulation with vertical shear and net near-surface inflow into the estuarine system. This is of critical importance in the context of fish recruitment because it offers a natural path into (resp. out of) the estuarine system for organisms that would be able to maintain themselves in the upper (resp. lower) part of the water column. The distribution of the fish larvae taxa that were examined revealed depth range preferences that did not change in time, independent from tide conditions, and were consistent with the use of these pathways. From a behavioural perspective, this mechanism can be viewed as simpler than selective tidal stream transport in that it does not require the organisms to synchronise their vertical migrations with the phase of the tidal currents. By obtaining statistically distinct A 13C and A 15N isotopic signatures for the SML, its presence at the entrance of the Sine Saloum estuary was confirmed. The organisms contained in the SML presented an important food source for H. picarti larvae and juveniles, contributing to more than 70 % of their diets. These results underline the importance of the SML and the role of this estuary as a spawning and nursery habitat for H. picarti.