Blog posts from recent graduates: PhD studies of Ignasi Arranz

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We are glad to announce that we start a new series of blog posts where we will post PhD studies of early career scientists. They will explain the findings from their research and possible applications. If you want to prepare a summary post about your studies, we would be happy to share.

The first one of this series comes from Ignasi Arranz.

Happy reading!

About Ignasi :

I started my PhD at the University of Vic-Central University of Catalonia (Barcelona, Spain) focusing on the size structure of fish assemblages in European lakes. During my thesis, I gave several talks in international and national conferences as well as disseminated my research to the local community and students by organizing several seminars and teaching classes. I successfully defended my Ph.D. in July 2017 with several publications in high impact scientific journals. I am currently working as a postdoctoral researcher at University of Concordia (Montreal, Canada) with the supervision of Professor Pedro R. Peres-Neto.

Title of the thesis: The body size structure of lake fish and its response to biotic interactions and environmental variation


Body size is perhaps the easiest metric to quantify in an organism but with a powerful background in ecology. It can be considered as a predictor of the basal metabolism and used at different spatial scales, group of organisms and levels of organization regardless species identity. This is for a relevance importance since functional traits are considered relevant to predict species loss or habitat fragmentation triggered by the ongoing global change. At a larger biological organization, the body size structure represents the negative relationship between the numerical abundance of organisms and their individual size. The parameters derived from the size structure can estimate community and population properties such as trophic transfer of energy, total biomass productivity or diversity of functional roles in aquatic and terrestrial ecosystems.

The research conducted in my Ph.D. focused on the study of body size structure (i.e. the relationship between organisms’ body size and abundance) in lake-fish systems at different ecological (both fish populations and communities) and spatial scales (at regional and continental scales). The general objectives of the thesis were twofold: 1) to understand the trophic interactions in fish communities and populations as well as the effect of environmental and anthropogenic factors in lake systems; and 2) to expand the focus on fish-size structure and integrate other functional and taxonomic approaches aimed at increasing our current knowledge about trophic interactions among lake-fishes. This was done by using one of the largest dataset in freshwater fish that encompassed large-scale biotic and abiotic gradients in European lakes (Fig. 1).

picture1Figure 1. Map of the geographic position of the total lakes and reservoirs included in the thesis. Blue squares are the reservoirs from the reports of the CHE (Confederación Hidrográfica del Ebro) of the Iberian Peninsula. Yellow circles are the lakes from the European lake database. Green rhombuses are the lakes from Turkey. Red triangles are the lakes from Greenland. Note that the triangles and circles denominating the position of the lakes in Greenland, Germany, France and Sweden are overlapped.

In general, the fish size structure responded more strongly and consistently to density-dependent effects than the environmental variables and anthropogenic pressures. That is, in situations with high fish densities, disproportionally more number of small fish than large fish occupy small size classes resulting in steeper slopes, narrower size diversity and smaller mean body size for most fish populations. From a fisheries management point of view, this is quite relevant because we can control fish-size and productivity of small fish by controlling total abundance.

One of the main goals of the second part of thesis was to explore the relationship between the size diversity and species diversity. The relationship can determine the degree of overlap in size among species and thereby the overlap in niches in a fish community across different spatial scales (both continental and regional scale). The main result suggested that species diversity is not a strong surrogate of size diversity in European lake fish communities, particularly in cold regions where only few species occur but large size ranges are found.

The PhD thesis concluded that fish size structure responded to trophic interactions as well as to environmental variables and anthropogenic pressures. Moreover, the fish size structure approach coupled with other functional and taxonomic approaches allowed detecting the importance of anthropogenic effects on fish community or population structure; as well as forecasting their vulnerability to environmental change with relevant implications for conservation ecology and fisheries management. Within the context of the Water Framework Directive, the fish size structure may be used as a bioindicator for the management of freshwater ecosystems, after controlling for trophic interactions and natural variation that may significantly modify the size structure.


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