Project Leader

John Mohan

John Mohan

Who I am

Growing up in eastern Pennsylvania, I developed an obsession for fly fishing and fly tying at an early age. I would fly fish for trout in my local streams and enjoyed observing fishes in their natural habitat. I would catch flying insects and flip over rocks to study what bugs the fishes might be eating to replicate them with hooks and feathers. Without knowing it at the time, through my fly-fishing hobby I was becoming a fish ecologist. Throughout college, my interest in fisheries ecology grew and I felt it was the career path for me. After years of catching many small native brook trout in freshwater, I got the desire to catch larger saltwater fishes. My college roommate had a boat on the New Jersey coast, so we started taking fishing trips in saltwater. Once I’d caught my first large striped bass, I was hooked.
My fascination for migratory striped bass led me to graduate school. Striped bass migrate along the US east coast but move into freshwater rivers to spawn. The juvenile striped bass then feed and grow in brackish water habitats before they mature and move into the ocean. How can we determine what ‘nursery’ habitats are most important to support the growth and survival of small fishes? It turns out that fishes possess a biomineral ear stone or ‘otolith’ that serves as a ‘black box’ flight recorder, incorporating information over the entire life of the fish. Fishes use the otolith organ for hearing and balance, but as a structure it is perhaps even more important for fish scientists. As a fish grows, the otolith accretes mineral layers, much like tree rings, so the layers are useful for determining the age of the fish. They incorporate dissolved ions from the water the fish lives in, so if the fish moves between freshwater (low ions) and saltwater (high ions) that is reflected in the elements contained in the otolith. The combination of multiple elements contained in otoliths can serve as a ‘fingerprint’ or ‘signature’ of the habitat a fish occupied over its lifetime.

Where I work

After finishing my PhD on fish otolith chemistry at the University of Texas, I started a post-doc at Texas A&M University. I work on a variety of different fisheries projects that involve highly migratory species, like bluefin tuna and pelagic sharks. To study fish movement ecology, I use complementary tools like natural tags (otolith chemistry) and electronic satellite tags to gain a better understanding of population connectivity and broad-scale movement patterns. Highly migratory species cross national boundaries and are vulnerable to international fisheries. Therefore, understanding the movement dynamics of migratory fishes is essential for effective management and conservation. In addition, accurately estimating the age of fishes is important, because age reflects growth rate, and growth rate determines overall population productivity, which has consequences for fisheries exploitation.


What I do

Sharks and rays do not possess otoliths; in fact, they do not even have true bones! Luckily for researchers, the vertebral cartilage of sharks is highly mineralised and grows in layers, like otoliths. Scientists are currently debating whether counting the rings in shark vertebrae reflects age. Some species, like the shortfin mako, deposit two rings per year as juveniles, but then shift to one ring per year as adults, at least in the Pacific Ocean. Is that growth pattern common to all shortfin mako populations around the world? This project will examine the chemistry of shortfin mako vertebrae samples to attempt to validate age patterns in all ocean basins, to support the management and conservation of endangered mako populations.

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