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Scientists determine most precise method for ageing Gulf menhaden

By: Melissa Schneider / Published: Apr 20,  2018

Changing the way state resource agencies include Gulf menhaden survey data and age-at-length information could help improve stock assessments, according to a research team at The University of Southern Mississippi’s Gulf Coast Research Laboratory.

A Mississippi-Alabama Sea Grant-funded study at The University of Southern Mississippi’s Gulf Coast Research Laboratory has shown that state resource agencies could save time and effort while doubling the precision of their age estimates if they use certain sampling methods.

The study set out to fill data gaps so Gulf menhaden stock assessment models could provide a robust understanding of how the population responds to harvest. Stock assessments are used to determine how much of a species can be harvested while maintaining sustainable levels, and they serve as a barometer of population health.

Gulf menhaden (colloquially known as the pogy) is the second largest fishery by weight in the United States. It is considered a keystone species in the ecosystem, which means many other species depend on it for prey. It is used for fish oil supplements, fish meal and other products.

Gulf menhaden are arranged and labeled before scientists extract their otoliths.
Gulf menhaden are arranged and labeled before scientists extract their otoliths.

Scientists analyze menhaden ageing methods in lab

Over the past two years, Assistant Professor Robert Leaf and members of his lab have analyzed Gulf menhaden samples that state resource agencies collected using scientific methods in Alabama, Mississippi, Louisiana and Texas.

Currently, the state resource agencies catch Gulf menhaden in their surveys, but the length and age composition data from the surveys are not included in the stock assessment model. The only age composition data used in the model comes from sampling commercial landings. This is a potential source of bias, and one that Leaf’s group is addressing.

Their approach was to find the age of the fish by counting rings on scales and otoliths (ear bones).

After retrieving the fish from the state agencies, personnel in Leaf’s lab determined the age of about 400 sub-samples using three techniques: reading scales, reading polished otoliths (that were sanded) and reading whole otoliths.

Under transmitted light, a fish scale shows light and dark rings. Scientists in Robert Leaf’s lab aged this scale at 2 years old.
Under transmitted light, a fish scale shows light and dark rings. Scientists in Robert Leaf’s lab aged this scale at 2 years old.

A whole otolith (ear bone) is pictured from the dorsal, ventral and side views.
A whole otolith (ear bone) is pictured from the dorsal, ventral and side views.

Scientists found that reading polished otoliths gave them the most precise results when ageing Gulf menhaden. A pair including a dark and a transparent ring represents one year. Scientists in Leaf’s lab aged this otolith at 3 years old.
Scientists found that reading polished otoliths gave them the most precise results when ageing Gulf menhaden. A pair including a dark and a transparent ring represents one year. Scientists in Leaf’s lab aged this otolith at 3 years old.

They found that reading the scales, the traditional way menhaden are aged, resulted in low precision of age estimates. Different scientists reading the same set of scales had only 42-percent agreement in their age determinations. When reading whole otoliths, they had 59-percent agreement. And, when using polished otoliths, agreement was the best at 80 percent.

The scientists also found that it took about 13 minutes to process and age fish using the polished otoliths while it took about 14 minutes to process and age fish

using scales. Reading whole otoliths took the least amount of time: an estimated 9 minutes.
 

Gulf menhaden ageing models: Precision and effort
Reading method Agreement  Estimated time to process 
and age a fish
Fish scales 42% 14 minutes
Whole Otoliths  59% 9 minutes
Polished Otoliths 80%  13 minutes

These findings suggest that resource agencies could nearly double the precision of their age estimates and spend less time doing it.

“Our objective was really about finding efficiencies,” Leaf said. “We wanted to make better use of data that resource management agencies were already collecting that could be included in the assessment model.”

Scientist sees potential use of findings

Leaf expects that these findings will change the way the four state agencies determine the age and length composition of their samples.

The study could lead to the inclusion of much more menhaden data in the primary age-structured stock assessment models. Fishery-independent age data are not used in the assessment, but Leaf believes this study is wrapping up at the right time to be included in the future.

The SouthEast Data, Assessment, and Review (SEDAR), a cooperative process used to conduct stock assessments in NOAA Fisheries’ Southeast Region, will hold a series of workshops this summer to re-evaluate the data and the models.

“We have data available, thanks to this project, of the age structure of the Gulfwide population,” Leaf said. “So, by understanding this age structure and putting accurate numbers into the model, we’ll be able to make better inferences of how resilient the population is to harvest.”

The federal government regulates Gulf menhaden stocks in coordination with the Gulf States Marine Fisheries Commission.

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