To help ensure that Virginia's James River continues as a exceptional resource for sport fishing, Washington and Lee University biologist Robert Humston and undergraduates in his lab are using the chemical fingerprint found in the otoliths, or ear stones, of smallmouth bass.
During the past two years, Humston's team has been collecting smallmouth bass (SMBs) from the James and its tributaries to determine the movement ecology of the fish in order to manage and develop the fishing opportunities. He wants to know not only where a fish was born — a tributary or the main stem — but also how often it has moved between the rivers during its life.
"The practical implications have to do with conserving smallmouth bass as a resource," said Humston. "They are an incredibly popular sport fish with significant economic value to the state and to local communities along the rivers in which they are found. The vast majority of sport anglers are targeting bass (either smallmouth or largemouth, collectively called black bass) when they fish in freshwaters. In Virginia, nearly a third of this sport fishing is by anglers visiting from other states, hence their economic input includes substantial travel expenditures."
A recent report from the U.S. Fish and Wildlife Service concluded that fishing by both in-state and out-of-state residents brought $1.1 billion into the Virginia economy in 2011.
The James is a highly popular SMB fishery in Virginia, added Humston, noting that some would call it a "flagship" fishery because it is managed for trophy fish using a "protected slot limit." That means that if a fish is between 14 and 22 inches in length, it must be released. Per day, anglers can keep up to five smaller fish and one trophy fish over 22 inches.
"This method allows medium-sized fish a better chance to grow to large trophy fish, while still preserving good reproductive potential in the population and allowing anglers to take the more abundant small fish for the dinner table," Humston said. "But all the tributaries of the James are managed differently. There are no size restrictions on these rivers, only daily limits of five fish. This means that if these valuable, protected fish in the James routinely move into the tributaries during the year — perhaps annual spawning migrations in the spring, for example — then they are vulnerable to capture and harvest, and this could undermine management of the trophy fishery in the James."
That is why understanding both how often and why the fish move between rivers is important, Humston said.
Most studies of smallmouth bass have used traditional methods such as marking and then recapturing them to track movement. The best information comes from lakes rather than rivers. Humston said that recent research with advanced radio telemetry is demonstrating that SMBs move great distances under certain conditions, and that they may move extensively among the network of tributaries in a river basin.
Because such large-scale movements are hard to capture with traditional methods, Humston and his students are examining the otoliths, which operate as a natural tag of the fish’s past river residence. The otolith accumulates calcium carbonate and trace metals from the surrounding water and serves as a tiny chronological recording device of ambient conditions, much like the information that tree rings capture.
"Provided the chemical fingerprint of a river is consistent and doesn't vary between seasons or between years and is consistently captured as the fish grows, we can use the otoliths to reconstruct the movement history of these fish by analyzing the chemical composition of the otolith at different points along its axis of growth," Humston explained.
After originally attempting to examine smallmouth bass from the Maury River, Humston determined that the chemical fingerprint of the Maury varies by season and by year. That meant it was possible to determine reliably only in which river the fish was born. He eventually discovered that the tributaries in the Blue Ridge and Piedmont regions were much different from the James than the Maury. Using strontium isotopes, he can indeed determine a fish's birthplace and its movement patterns.
Humston credits one of his former student researchers, Sasha Doss, of the Class of 2012, with a breakthrough at the end of her summer project in 2012.
"Sasha knew that we can determine a fish's age based on the number of rings in its otolith, and she had also read that you can use the distance between rings as an indicator of how much the fish grew in any given year," said Humston. "So she asked whether or not we could couple this retrospective analysis of growth history with our reconstruction of movement history — that is, could we simultaneously look at where a fish was and how much it grew over the course of the year."
Doss and Dave Dennis, a 2011 graduate, did preliminary work to lay the foundations for the analysis. Then current junior Matthew Moore spent last year developing a different methodology for measuring otolith size that provides faster analysis and better correlations with body size.
"One interesting thing we observed was that one particularly popular measure of otolith size — the medial radius — was poorly correlated to growth in the first year," said Humston. He and the student researchers — Doss, Dennis, Moore and senior Caroline Wass — have just submitted a paper on that topic.
Other W&L students who have been working on the research are seniors Forrest Behne, Julie Sorenson and Mark Faubion and junior Garrett Muckleroy.
"This is the critical foundation to what we intend to describe in a subsequent paper that will explore to what extent both movement behavior and river residency influence growth in the smallmouth bass," said Humston.
Jeffery G. Hanna
Executive Director of Communications and Public Affairs