A Scientist's Life: Colleen Petrik

Biological oceanographer seeks to help society understand marine-life safe limits of fishing, greenhouse gas emissions

Colleen Petrik is an assistant professor of biological oceanography at Scripps Institution of Oceanography at UC San Diego. She received her bachelor’s degree in marine science and biology from the University of Miami in 2005. She received her PhD from the MIT/Woods Hole Oceanographic Institution Joint Program in Biological Oceanography in 2011. After stints at several universities, most recently Texas A&M, she joined Scripps Oceanography in 2021.


explorations now (en): What do you do for a living?

Colleen Petrik (CP):  I would say I have two jobs. One is to study how climate affects marine ecosystems and the other is to train the next generation of scientists.

When I started as a graduate student, I was trying to understand how climate variability affects the survival of fishes in their early life stages. When fishes are eggs and baby larvae, I studied how year-to-year differences can impact whether or not you have a lot of baby fish that survive and go on to become adults in that fish population or what happens when there are bad conditions, so I wanted to know what are the differences between the good years and the bad years? I was looking at interannual climate variability, then it became much more apparent that climate change was rapidly happening. 

Now I study the trends from anthropogenic greenhouse gas emissions and how climate change affects marine ecosystems. One of the interesting things you find is how complicated relationships are in the ocean when you study biology. They're not simple, linear relationships. They're nonlinear relationships because ecosystems are not simple food chains. They're complex food webs. That can lead to all these interactions that might dampen the effect of climate change, or it might amplify it. Climate change is definitely going to have both winners and losers, but it could really restructure ecosystems.

My hope is that my research contributes to better decision making by people. It could be decisions on how many fishes to harvest from the ocean, or how many greenhouse gases we can emit into our atmosphere. For both of these decisions, it'll help create a more sustainable planet.

en: What are some of the main questions those in your field are trying to answer?

CP: One of the most surprising questions in my field is how many fish are in the sea? We really do not know. The estimates can span several orders of magnitude. It’s pretty crazy that we don't have any real certainty on how many fishes are there because we need to know how many fishes are there to understand how many we can catch. 

The next question is what drives the amount of fish in the sea? How much fish biomass could the environment support in different parts of the ocean? Thinking about that purely from a physics energetics perspective, you start to build in the complexity of the ecosystems and how fish are different sizes. Is it all bottom-up from the physical environment and food resources, working their way up the food chain to the fish? Or, how much of an impact do humans have with their fishing practices?

Then there's kind of a third big question: What role do fish serve in removing carbon dioxide from the atmosphere? We almost never think about that. We think about the plankton that are creating carbon dioxide or removing carbon dioxide and then keeping it in their bodies and sinking to the deep sea floor and storing it. That's something we call the biological pump, but animals also serve a similar role. They migrate, so they can move carbon in any form from the surface to the deep. Then they also create waste matter that sinks to the deep ocean. We are trying to figure out how much of the waste that fish produce gets stored and tell how important that process is – but first, we need to know how many fish there are in the sea.

en: What are the tools you use in your research?

CP: I use computers and different computational resources to do my research by creating simulation models. I create a mini ecosystem on a computer that translates important processes into equations. And then the computer actually evaluates those equations so that we're able to have a laboratory to make manipulations and test hypotheses in a way that's not possible in the real world.

en: What are you working on right now?

CP:  Instead of thinking about long-term climate change, like what our world is going to look like in 2050 or 2100, I'm thinking about short-term climate projections. You could think of these similar to a weather forecast. It’s trying to come up with a picture of what the ecosystem is going to look like in six months or one or two years down the road. Those are the timescales that are really important for fisheries management or any kind of decision making that happens in reference to fisheries, living marine resources and marine protected areas, where we might be thinking five to 10 years down the road. 

Like weather models, we create a huge ensemble of different scenarios and those create this forecast about what is the most probable future.  What I'm working on now is trying to study how skillful those forecasts are. Are they actually capturing what happens in the environment and how useful will they be to decision making?

en: What got you into this field in the first place?

CP: I guess the reason I wanted to become a biological oceanographer was the experience I had growing up going to the Baltimore Aquarium regularly, as well as going to Ocean City, Maryland, on the eastern shore of Maryland. So I just, you know, had a lot of experience with the ocean by swimming in it, though, I don't think I could ever see anything besides maybe jellyfish from time to time, but then going to the aquarium was when I got to see all the animals and learn more about them. That drove my passion for marine life. And as a young kid, I was really drawn to marine mammals, but once I got to college, I learned not everybody gets to work with marine mammals. That's OK, though, because my interests shifted to the really, really small things that live in the ocean: plankton. They just seem so small that they wouldn't have the ability to control anything. The currents can just sweep them anywhere they want, but plankton have all these incredible adaptations to live in that 3-D environment. That was just fascinating to me and that’s how I was led to biological oceanography.

en: Why did you want to come to Scripps Oceanography?

CP: I wanted to come to Scripps because it's really just a world-renowned institution for oceanography, that it has just an incredible wealth of talent in terms of just really smart people and that's not just the faculty, but the students as well. So knowing that I'm going to work with some of the best and the brightest people was really enticing.






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