Stock Assessments Interlock Pieces of Data to
Put Together the Whole Picture

By Patricia Smith
Fish Eye News
N.C. Division of Marine Fisheries

Developing a stock assessment can be a little like putting together a jigsaw puzzle.

“We have to use what we know about the biology of the fish to piece the information together,” said Laura Lee, senior stock assessment scientist with the N.C. Division of Marine Fisheries.

Connected, those pieces of information give an overall picture of the health of a fish species and underscore why data collection is so important to marine fisheries management.

“If a few pieces are missing, you may still be able to work the puzzle,” Lee said. “The more pieces that are missing, the more uncertainty in the picture.”

One key piece in a stock assessment is the measure of loss due to fishing. In North Carolina, this comes through commercial landings data and recreational catch estimates. This gives insight on harvest trends and on how fish stocks respond to fishing pressure, Lee said.

But harvest data can be biased because it is dependent on factors such as size limits, bag limits, trip limits and seasons, as well as fisherman preference. In order to get a true measure of a species’ population size, stock assessment scientists factor in independent data, as well.

Fishery independent data comes from biologists who go out in the field and collect samples that show the abundance and distribution of fish.

From independent data, stock assessment scientists can get a measure of recruitment — the young fish coming into a population — and the sex, age and length structure of the population.

“What happens in a population that’s getting overfished is that there isn’t enough recruitment to sustain the population,” Lee said. Overfishing can result in a shift to smaller, younger fish in the stock because fish aren’t getting as old or as big anymore.

Stock assessments also consider environmental information that is known to affect the stock, such as water temperature, salinity levels, dissolved oxygen levels, wind and currents and water quality conditions like silting or fecal coliform levels.

Stock assessment scientists enter all this collected data into a computer model, and the model calculates stock size and fishing mortality rate.

“What we’re generally interested in is the estimate of population abundance and fishing mortality in the most recent year of the assessment,” Lee said. “That’s usually what the managers need for managing.”

If overfishing is occurring, the stock assessment also calculates what reduction in harvest is needed to end overfishing.

Lee is quick to point out that what comes out of the computer is an estimate.

“Short of emptying the ocean, we have to estimate,” Lee said.

But the more data that’s available to piece the puzzle together, the better the picture will be.