Ekau, Werner

Preferred name

Ekau, Werner

Official Name

Ekau, Werner

GND

1255663707

Gerade angezeigt 1 - 5 von 5

Some of the metrics are blocked by your
consent settings
Unravelling the reproductive tactics of a tropical clupeid fish (Ethmalosa fimbriata, Bowdich 1825) against the backdrop of climate change
(2018-05-29)
Döring, Julian
;
Hagen, Wilhelm
;
Ekau, Werner
;
Ekau, Werner
Marine fishes employ specialized reproductive tactics in response to hydrographic fluctuations in their spawning habitats. Unprecedented environmental conditions induced by climate change will challenge these behavioral and physiological adaptations. Taking into account the key drivers of stock productivity, this work aimed at assessing the reproductive adaptations of a clupeid fish species, Ethmalosa fimbriata, towards hydrographic alterations in its spawning habitats: the upwelling area at the Senegalese South Coast and the inverse hypersaline Sine Saloum estuary. Obtained results show that E. fimbriata's is adapted towards increasing its reproductive investment at high water temperatures (26 - 30AdegreeC) and at salinities (42 - 51), which by far exceed marine conditions, in an effort to maximize recruitment success. Examination of oocyte fatty acid profiles revealed that oocytes spawned inside the estuary under these high temperature/high salinity conditions were likely to develop normally. Investigation of spawning energetics and otolith microchemistry evidenced distinct stock spawning components, which are characterized by delimited home ranges and specific productivity. Because of this complex stock structure and by employing auspicious reproductive tactics the species is so far likely to benefit from the severe impacts of climate change on its spawning habitat. The adaptations described herein potentially allow for outcompeting other pelagic fish species with lower adaptive potential. All in all, high plasticity in reproductive traits combined with high fecundity and small generation times in clupeid fishes such as E. fimbriata may lead to an enhanced fitness during rapid environmental changes.
Dissertation
796 549
Some of the metrics are blocked by your
consent settings
Environmental and Physical Factors Affecting the Diversity and Distribution of the Ichthyoplankton in an "Inverse Estuary" , the Sine Saloum (Senegal)
(2018-08-24)
Sloterdijk, Hans
;
Ekau, Werner
;
Saint-Paul, Ulrich
;
Ekau, Werner
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.
Dissertation
359 149
Some of the metrics are blocked by your
consent settings
Early life history traits of coastal pelagic fishes in the northern Benguela Current ecosystem off Namibia
(2013-08-29)
Geist, Simon Joscha
;
Ekau, Werner
;
Saint-Paul, Ulrich
;
Ekau, Werner
Several traits of larval stages of the three most important coastal pelagics species in the northern Benguela Current upwelling ecosystem (NBCE)- Sardinops sagax, Engraulis encrasicolus and Trachurus capensis were investigated. The study is based on samples from four consecutive research cruises conducted in the NBCE region during 2008 to 2011. All species showed maximum larval densities within the upper 50 m of the water column over the continental shelf during austral summer. Larvae fed mainly on planktonic crustaceans occupying a similar trophic level. T. capensis larvae and juveniles showed the highest degree of eurythermy and tolerated exposure to hypoxic conditions.They were characterised as voracious feeders with a high plasticity of prey items leading to a high feeding success. In contrast, nutritional condition of S. sagax larvae was affected by larval densities. In conclusion, T. capensis seems to be best adapted to present conditions in the NBCE, characterised by frequent intrusions of warm water, shoaling of hypoxic zones and a shift towards smaller copepods in the zooplankton community, probably leading to their higher recruitment success.
Dissertation
286 168
Some of the metrics are blocked by your
consent settings
Impact of microplastic on early life-history stages of seabream in the Atlantic Ocean
(2021-12-21)
Müller, Carolin
;
Erzini, Karim
;
Ekau, Werner
;
Ekau, Werner
Plastic litter has been entering the marine realm ever since the mass production of synthetic polymers began in the 1950’s, thereby potentially complementing and amplifying the effects of persistent stressors like ocean warming, acidification, overexploitation or habitat degradation. Despite the fact that first reports on plastic ingestion by marine fish date back to the 1970’s and that considerable scientific and public concern has been raised recently over the potential detrimental effects of micro-sized (i.e., < 5 mm) plastic items, research on the explanatory variables for uptake as well as on the physiological effects of exposure and ingestion is still at an early stage, impeding a thorough evaluation of the actual implications for living marine resources. The present thesis thus investigates the effects of microplastic exposure and uptake on growth, condition, and survival of early life-history stages of fish. It provides a critical analysis of the current state of the art on the extent of microplastic ingestion by marine fish, along with an examination of the effects and explanatory variables for plastic uptake based on a literature review of 90 in-situ studies published between 1972 and 2019 (Müller 2021). This review simultaneously lays the groundwork for an integrated research design, combining an in-situ (Müller et al. subm.) and an in-vivo (Müller et al. 2020) study, to determine the impacts of and driving factors for MP ingestion by nursery-dependent, omnivorous juveniles of a commercially and recreationally important fish species, the white seabream Diplodus sargus (Linnaeus, 1758) in the Ria Formosa lagoon, Portugal. The results obtained highlight the fact that larval and juvenile fish are underrepresented in scientific initiatives investigating the extent of microplastic pollution. Furthermore, the outcomes of the in-situ and in-vivo part of the thesis challenge the existing hypotheses that omnivores in general and early life-history stages of fish in particular are prone to elevated microplastic ingestion rates. Moreover, the notion of an inevitable deterioration of physiological condition upon uptake of synthetic polymers was refuted. The findings emphasise the need for a holistic integration of field and laboratory studies to determine species- and life-stage specific driving factors for and potential detrimental effects of plastic ingestion to enable a sound evaluation of the ecological and economic risk arising for coastal fish stock and fisheries.
Dissertation
336 424
Some of the metrics are blocked by your
consent settings
The organic flight Recorder : Lifetime connectivity and habitat reconstructions with fish otolith amino acids
(2018-02-05)
Vane, Kim
;
Krumme, Uwe
;
Larsen, Thomas
;
Ekau, Werner
With increasing disturbances to fish habitats, it is becoming essential to understand how fish species use and move between different habitats during their lifetime. This will lead to a better comprehension of how environmental changes will impact the population and how to mitigate these effects. The proteinaceous matrix of fish otoliths could provide a direct connection to the biological environment and facilitates the extraction of lifetime resource utilization, trophic position, nitrogen dynamics, and migration. With compound specific isotope analysis of delta13C and delta15N values in amino acids (CSIA-AA) of Cynoscion acoupa otoliths, this thesis is among the first to identify how this technique can be applied to long-term biogenic carbonate records such as fish otoliths in a technical aspect and the use of CSIA-AA principles for identifying lifetime resource utilization and trophic position.
Dissertation
405 224

Ekau, Werner

Filter

Einstellungen

Sortieren nach

Ergebnisse pro Seite