Cold Stun: Mathematical Models Aim to Advance Fisheries Management
To fortify the future of North Carolina’s fishing industry, NC State University doctoral student Johnna Brooks is using biomathematics to quantify the effect of winter cold stun events on spotted seatrout in North Carolina estuaries.
Spotted seatrout inhabiting North Carolina waters are at the northern edge of their geographic range, preferring warm estuarine regions in the South Atlantic and Gulf of Mexico. Low winter temperatures can be fatal to spotted seatrout, leading to cold stun events. A significant cold stun event with large-scale fish mortality can be detrimental to spotted seatrout populations, which in turn negatively impacts recreational and commercial fishing.
According to the N.C. Division of Marine Fisheries (NCDMF), a significant cold stun event is characterized by eight consecutive days at 5 C (41 F) or a 24-hour period at 3 C (37.4 F). In response to a significant cold stun event, the NCDMF can temporarily close spotted seatrout fishing to allow the population to recover.
North Carolina experienced unusually frigid weather in January 2025, leading to the first significant cold stun event since 2018. Hundreds of spotted seatrout floating belly-up on partially frozen waterways were seen along the coast.
The full impact of a cold stun event cannot be verified by what is seen on the water’s surface. “Natural mortality is rarely observable, making it hard to estimate,” Brooks explains. “But it’s a fundamental parameter for stock assessment.”
Brooks is pursuing her Ph.D. in biomathematics with a concentration in fisheries ecology, co-advised by Professor Jeffrey Buckel and Associate Professor Jie Cao of the Department of Applied Ecology. Using a Vector Autoregressive Spatio-Temporal Model (VAST), Brooks quantified the seasonal variation in the abundance and distribution of spotted seatrout by creating maps that illustrate fish density before and after winters from 2003–2019.
Changes in population density and abundance were then compared to temperature data collected from the Beaufort Inlet. She found that natural mortality occurs at a seasonal scale, especially in severely cold winters when water temperatures drop below 7 C (44.6 F).
“Cold stun events are significantly affecting spotted seatrout at the population level,” Brooks says. “It’s not just one creek that has significant fish loss. For some of the cold stun events, they happened at a very large scale and affected most of the area in our study.”
By quantifying the severity of past cold stun events, Brooks’ research could potentially assist the NCDMF in how it responds to future events. Her model provides a way to predict abundance loss in relation to a temperature metric, which could aid fish management decisions such as fisheries closures.
Brooks’ research recognizes the need to incorporate and estimate seasonally varying natural mortality into the stock assessment model. This project could further aid fisheries management. It is also personally significant to her.
“My undergraduate degree was pure math with a minor in statistics,” Brooks says. “My graduate degree is biomathematics and I’m getting another minor in statistics. But I also grew up on the coast [of North Carolina] so fish is my subject matter of choice. I’m combining my two favorite things; math and fish.”
As she continues her graduate research, Brooks is reminded of a George E. P. Box quote popular with statisticians; ‘All models are wrong, but some are useful’. Brooks knows the model she develops will not be perfect, but hopes it will aid fisheries management practices and be improved with each iteration.
This post was originally published in College of Agriculture and Life Sciences News.
