Article published in Plos One magazine with support from CENBAM shows aspects about the trophic and reproductive ecology of Australian fish.

The article "Temporal Uncoupling between Energy Acquisition and Allocation to Reproduction in a Herbivorous-Detritivorous Fish was published in March in Plos One magazine that deals with aspects related to the trophic and reproductive ecology of the Australian migratory fish "mullet" (Liza alata). The publication was headed by Francisco Villamarín, doctoral student of the INPA Ecology Program and who received a grant from CNPq to participate in the Science without Borders Program through an institutional fee from CENBAM to visit the Australian Rivers Institute at Griffith University.

 

"When I started in 2012 my doctorate in INPA's Ecology program, I didn't imagine that one of the chapters of my thesis would be developed in Australia and focused on a species of fish. My doctoral research project seeks to answer questions about trophic ecology of aquatic ecosystems : What are the main energy resources supporting large ectothermic predators such as Amazonian crocodilians?

 

To track the route that energy takes through food chains, the researchers analyze the natural abundance of chemical markers present in biological tissues. The technique is called stable isotope analysis. The stable isotopes are analogous to "chemical fingerprints" and allow to trace the primary sources of energy that support the different organisms on the planet.

 

In order to get better training to develop my doctoral project, I realized that I should meet researchers and get involved in projects that use the same techniques to answer similar questions, even if I was studying other organisms in different environments. Through an institutional quota from CENBAM, in 2013 I received a grant from CNPq through the Science without Borders Program to visit the Australian Rivers Institute at Griffith University, in Australia. In this way, I became involved in the institute's projects by collecting and analyzing data on the components of the food webs of the floodplains of Kakadu National Park, located in the Northern Territory of Australia. Collections were focused on groups such as primary producers, invertebrates and fish.

 

Within the fish collected in Kakadu, a species related to the mullets that inhabit the Brazilian coast attracted a lot of attention. This fish is called "mullet" (Liza alata) and feeds mainly on algae and organic matter from the lakes where it lives, but mainly on the temporarily flooded forests of Northern Australia. A very important characteristic of these fish is that, in the study area, they go down the river to spawn in the estuaries of the Indian Ocean. This happens during the beginning of the full season, when the lakes connect with the main rivers and the mullet can reach the estuaries.

 

In the lakes where the mullets live, food resources are very limited during the dry season, but when the river begins to fill, the fish generally spread within the igapós and benefit from the abundant resources of the flooded forest. When mullet females enter igapós to feed, they do not have enough time to develop eggs and migrate to spawn in estuaries before the lakes become isolated during the next drought. On the other hand, they need large energy reserves to be able to develop their eggs and go out to spawn as soon as they have access to the sea. But, if resources are so limited within the lakes during the dry season, where and when do they obtain the energy that would sustain such demand?

 

Thus, to understand where the energy that supports the synthesis of eggs in mullets comes from, we use the technique of stable isotope analysis. The idea is that it is possible to discover the proportions in which the primary resources originating in different environments form part of the tissues of these fish. The time it takes for the isotopes to be incorporated into the fish's body depends a lot on the type of tissue analyzed. For example, the liver is a very active tissue and shows the isotopic sign of the food a few days or weeks after eating, while the muscles take on the order of weeks to months to assimilate that sign. If the food that sustains reproductive growth comes from flooded forests and is obtained during or prior to the full season, then the reproductive tissue (the gonads) would show an isotopic signal more like the muscle. On the contrary, the gonads would present the isotopic signal more similar to the liver if the energy is obtained in the lakes during the same time they are being synthesized.

In addition to stable isotopes, we use a molecular technique capable of quantifying instantaneous growth in different fish tissues, the RNA: DNA ratio. In this way, we were able to identify the moment when these tissues were being synthesized.

 

We found that, although the mullets are in better bodily condition during the flood season, most of the allocation for breeding happens during the drought, which is when they are thinner. Evidence suggests that when they invest energy to reproduce, they do not grow. The analysis of stable isotopes and RNA: DNA showed that the gonads are synthesized during the dry season, using the energy stored in the form of fat months in advance, coming from the flooded forests during the full season. In combination, the information found suggests that the mullets present a marked temporal separation between ingestion, energy stock and subsequent allocation for reproduction.

 

The results found allowed us to infer about the importance of connectivity and the natural hydrological dynamics between different water bodies to maintain the populations of these migratory fish ".