Unraveling the Mysteries of Fish StocksIn Stewarding the Ecosystem
Fish stocks are the fundamental unit of fisheries management. Through regular assessment, managers can monitor the overall health of these groups of fish and define appropriate fishing limits.
But one primary question is sometimes glossed over—what is a stock?
Stocks are typically defined by geographic boundaries. All fish caught within a stock area are counted as part of that group; however, we are increasingly finding that the picture is much more complex.
Multiple populations of the same species, which can exhibit unique behavioral or physical traits, often mingle together and move freely across the invisible boundaries we have defined for them.
If we manage species according to stock boundaries that do not reflect naturally occurring populations, this diversity of traits can erode over time. And much like a diverse financial stock portfolio, diversity of a fish population increases its resilience and buffers it against potential losses due to changing conditions.
Our fisheries ecologist, Lisa Kerr, focuses on the dual challenge of identifying natural populations and examining the implications of how we manage them as stocks.
In New England, stock boundaries are a series of tidy boxes that were drawn in the 1970s. Our collective knowledge about the population structure of species in the Gulf of Maine has evolved greatly since that time, but the information we’ve learned can be hard to integrate into existing management schemes.
While we now know that the management boundaries of some species are not representative of the natural populations, the next question is, does it really matter?
Lisa is developing models of cod and Atlantic bluefin tuna that incorporate all the information we have about the unique populations that make up the resource, as well as their movement. Through these models, she is helping fisheries managers examine their current approaches and understand their implications on the populations over time.
With Atlantic bluefin tuna, for instance, the stock boundary is drawn down the middle of the Atlantic Ocean and fish are managed as two distinct stocks with the assumption that fish don’t cross the boundary. A fish caught in the west, came from the western population, and vice versa.
However, we now know that these fish migrate across the Atlantic. Additionally, the eastern stock is an order of magnitude greater in number than western stock. So, even a small amount of mixing across the stock boundary can drastically misrepresent the abundance of the western stock.
Before managers can begin to operationalize information about natural populations, scientists must both tease the groups apart and discern their identifying characteristics. Even if managers know there are different populations groups, they can’t manage them as such if there is no way to tell where a fish caught at sea came from.
This year, Lisa helped to write the book on that challenge. “Stock Identification Methods” has long been the go-to textbook describing the suite of research tools that scientists use to distinguish fish stocks, such as genetics, body shape, tagging, and chemical signatures in their inner ear bones. Lisa co-edited the latest edition and authored several chapters on identification and modeling methods.
Her influential work also led to her appointment this year as Chair of the Stock Identification Working Group for International Council for the Exploration of the Sea (ICES)—the fisheries management organization for the European Union. In this role, she advises the Council as they evaluate their management policies and their impact on the long-term health of fish populations.