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From satellite to microscope: how remote data helped me study fish eggs

Illustration of a blonde woman in a white lab coat, looking through a microscope on a desk. Behind her is a dark sky filled with stars and a satellite is shining a light on her. Her desk is on top of a globe and there is a Brazilian flag indicating she is in this country

Illustration by Joana Ho

Many people do not understand why I, a biologist with a masters and doctorate in oceanography, went to do a post-doctorate at the National Institute for Space Research (INPE), in São José dos Campos - SP. The understanding is made even more difficult by the fact that I have been working with fish since graduation. My husband often jokes that I went to work at the National Institute for Space Fisheries, or that I'm hunting the constellation Pisces.

What many people don't realize is that satellites are powerful tools in oceanographic studies, providing data that can be used in various studies. We even created a blogpost about the “reason for being” of satellite oceanography.

Ok Jana, I've already reread the post above and understand that some satellites provide data for estimating important variables such as surface temperature, chlorophyll-a concentration, wave height, and surface wind field, among other factors… But where do the fish come in?

To make the connection between satellites and fish eggs, I'll need to go back to the very beginning of my PhD. The main objective of my research project was to evaluate the long-term fluctuations in the abundance and distribution of eggs and larvae of Engraulis anchoita, heavily fished in Argentina and Uruguay, which for this text I will simply call anchovies. With this, I sought to understand the oceanographic factors that caused these fluctuations, where the spawning sites were located, and to provide information that may be used in future management of the species. Anchovy is not fished commercially in Brazil, but there are studies on the feasibility of starting to fish it commercially in the southern region of the country.

Anchovy eggs observed through a stereomicroscope. You can even see the fish embryos developing in several eggs.

The anchovy egg samples that I analyzed were collected in different years between 1970 and 2010, across the South Brazil Bight (PCSE), which extends from Cabo Frio, in the state of Rio de Janeiro, to Cabo de Santa Marta, in Santa Catarina. To identify the anchovy eggs, they were measured (I previously described the method we developed to identify eggs more quickly in another post). And it was during these measurements that we noticed that the eggs in the south of the study area were larger than the ones in the north. We also noticed that the eggs sampled during the winter were larger than those in the summer. And as curious scientists, we asked ourselves: why?

We know from previous studies that:

  1. The anchovy is widely distributed over the southwest Atlantic continental shelf, from Vitória, in Brazil (20°S) to the Gulf of São Jorge, in Argentina (48°S);

  2. Its population is divided into three stocks: the Patagonian (48-41°S, occurs only in Argentina), the Bonaerense (41-27°S, occurs in Argentina, Uruguay and southern Brazil) and the South Brazil Bight (PCSE, 27°-20°S, Brazilian stock only);

  3. The size of the body and eggs of individuals from the Bonaerense stock are larger than those of the PCSE.

Based on this information, we formulated a hypothesis to explain why the size of anchovy eggs was larger in winter than in summer: it would be possible that the larger individuals of the Bonaerense stock were migrating north to spawn during the winter. To confirm this hypothesis we took satellite data, i.e. remotely generated data, and drew horizontal distribution maps of surface temperature and sea surface chlorophyll-a concentration for the Southwest Atlantic Ocean during the summer and winter of 2001 and 2002. With this, we sought to visualize a possible flow of water that could guide the migration of anchovy adults.

What we noticed, through the images obtained (see the figure below), was that the flow of water from Pluma do Rio de la Plata (on the border of Argentina and Uruguay) goes towards the north only during the winter. As the anchovy really likes in the region where the water from the Rio da Prata meets the sea, we believe that this flow can serve as a guide for the migration of individuals from the Bonaerense population to spawn in the central and northern region of the PCSE, during the cold season. This indicates that our hypothesis may be correct, and that the eggs found in the winter in the PCSE may indeed belong to the Bonaerense population

Horizontal distribution of surface temperature and chlorophyll-a concentration during the summer and winter of 2001. The black line highlights the region where anchovy eggs were sampled. Note that the remote data made it possible to analyze a much broader area than the sampled one. In winter it is possible to see a flow of cold water with a high concentration of chlorophyll-a to the north.

Of course, there are other factors included and discussed in the study. So, for those who are curious and want to know more about the subject, I share the link to the article published in 2017 in the Fishery Bulletin, a scientific journal (). Here, I just wanted to bring an example of how satellites can even help in the study of fish eggs. Science is really fascinating, isn't it?!

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