Dr. John Mohan
Department of Marine Biology
I am a marine fish ecologist with interest in characterizing fish life histories, migration patterns, population connectivity, food web interactions, and investigating how environmental variation influences population dynamics. To address questions in fish ecology, I utilize natural chemical and electronic tags and integrate these complimentary techniques. Natural tags include calcified structures of fish that grow incrementally (like tree rings) including ear-stones (otoliths) and scales. As calcified structures in fish grow sequentially, they incorporate dissolved elements from the water and thus reflect the habitat and environmental conditions a fish lived in. Soft tissues of fish, such as muscle and liver, assimilate carbon and nitrogen isotopes from the diet, and thus can reflect feeding ecology. Combining multiple natural tags is a powerful approach to relate fish movement and habitat use to feeding ecology, which provides important information for resource managers of fish populations and habitats.
During my dissertation research at the University of Texas Marine Science Institute, I developed and applied a novel elemental tracer in fish ear stones to quantify hypoxic (low-oxygen) stress and the consequences for feeding interactions in the northern Gulf of Mexico. My current post-doctoral research integrates natural and electronic tags in large predators such as bluefin tuna and pelagic sharks, to link fish movement to trophic dynamics and oceanographic processes in the Pacific Ocean and Gulf of Mexico. The most exciting research I am currently working on is applying natural tags in cartilaginous sharks, which no dot have ear stones. Shark vertebrae cartilage is highly mineralized and grows similarly to fish otoliths, accreting rings that record time, age, and growth rates. This research is finding the mineralized vertebrae cartilage of three shark species exhibit elemental patterns that reflect age and environmental upwelling history. Applying natural tags in sharks has the potential to drastically increase our understanding of shark life histories and population connectivity that can help to inform management and conservation efforts.
In my free time, I really enjoy skateboarding, surfing, disc golf, hiking, camping and fishing. I feel very fortunate to live on the coast and get to see the ocean everyday and appreciate spending time at the beach.
* Photo of Me holding spotted seatrout in Matagorda Bay TX that we tagged with acoustic telemetry tag to study habitat use, and sampled a scale for stable isotope analysis to link habitat use with feeding ecology (Tinhan et al. 2018)
** Photo of Me holding large white shark vertebrae that was sampled for trace elements and stable isotopes to characterize habitat use and feeding ecology.
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