Science researcher finds fish do not age like mammals and birds
The international collaboration links more than 40 years of measurements from individual animals in the wild with state-of-the-art modern scientific techniques.
“Aging is associated with a decline in biological performance, a process known as senescence,” said Dr. Craig Purchase, a professor with the Department of Biology in the Faculty of Science. “It is easily observed in humans and our domesticated animals, but it may not be universal.”
Dr. Purchase says theory predicts senescence to be prominent in species which stop growing at adulthood, such as mammals and birds.
But those which continue to grow as adults, such as some trees, fish and reptiles, may escape it.
That theory has been difficult to test – until now.
In 2017 Dr. Purchase was contacted by long-time friend Dr. Michael Rennie. The Lakehead University professor works at the International Institute for Sustainable Development’s experimental lakes area in northwestern Ontario.
“Mike said during the monitoring of some of these lakes, they’ve caught the same lake trout every year for decades and collected information on them. He wanted to know if there was something we could do with the data to compare ages.”
Working with Dr. Jason Treberg (PhD’06), a Memorial alumnus and associate professor of biological sciences, and Michael Gaudry at the University of Manitoba, Drs. Purchase and Rennie planned a cross-sectional study on young and older males.
Dr. Purchase travelled to the ELA site for the 2017 fall spawning season, accompanied by then-research assistant Anna Rooke and technician Mohammed Fahmy, to collect semen samples and determine sperm quality of the fish.
Later, Drs. Michael Morrissey and Lizy Mittell from the University of St. Andrews in Scotland used historical catch records to model the survival probability of fish as they aged.
Combining the different sources of data produced a picture of changes (or lack of changes) in adult mortality, body condition, fertility and cellular function with age.
Their findings were published in a paper by the Proceedings of the Royal Society B on Jan. 5, 2022.
Theories on aging
Dr. Purchase says, theoretically, evolution should remove issues that cause senescence in some fish, because the consequence of not removing them is greater than in mammals.
If, like a mammal, older adults can’t produce more offspring than young adults, there is no great evolutionary incentive to maintain the body.
In contrast to mammals, by continuing to grow after adulthood, fish that live to be old tend to be big, and can produce more offspring in a given year, says Dr. Purchase. Evolution should reward that and enable the body to maintain function — but in many species, such as small fish in environments with high predation, the odds of living to old age are poor given predators are going to eventually kill you.
“There isn’t much point in protecting your body in that case, and investing more into reproduction while you are young pays off. However, in low predation environments, individuals can live to be old, and therefore evolution should enable them to perform well at old age, to take advantage of being big.”
Testing in the wild
However, there’s a problem in trying to test this idea in the wild.
“As fish get bigger, they have a better ability to catch other things to eat, so often large and small adult fish of the same species eat different things,” explained Dr. Purchase. “That can cause biases when trying to compare young and old adults.
“To solve this problem, we exploited a little quirk,” he continued. “The studied lakes are very small, with a simplified food web, so young adult trout eat the same things as the 40-year-olds. Despite being a species with the potential for indeterminate growth, in these particular populations, the trout maturation size and maximum size is very close together.”
Evolutionary theory predicts little senescence when fertility increases with size, but maturation and maximum size is similar.
“It’s really a function of how evolution has shaped us depending on how we grow.”
“These fish, with their restricted growth, had only slight declines in body condition and increases in mortality across decades of age, and their reproductive performance was maintained,” said Dr. Purchase.
“We think in different lakes, with something different to eat that would allow them to keep growing, they will show reversed or negative senescence, which means their performance would actually increase as they age.”
Mitigating human aging
So, what does that mean for humans who want to stop or slow the ravages of time?
Dr. Purchase says the first step is to understand senescence — what happens, why it happens and how it happens.
He says our view of aging is biased towards the things that are like ourselves; we think what happens to us as we get older is the same for everything. But theory predicts that is not always the case.
“It’s really a function of how evolution has shaped us depending on how we grow. Realizing that, one can think about what causes aging in the first place and what can be done to mitigate it. There are species that don’t have this problem.”