Rebuilding Depleted Fish Stocks
To be considered sustainable, a population of fish must produce enough young to replace the fish that are lost each year due to natural causes and to fishing. However, when the rate of fishing exceeds the stock’s capacity to regenerate, the stock can become overfished. Rebuilding strategies help reduce fishing pressure on stocks and are designed to allow the stocks to grow and recover. This chart plots the trajectory, from 1980 to 2010, of 55 stocks that were declared overfished since 1997.
As rebuilding plans are implemented, the rate of fishing slows, allowing stocks to rebound. However, in some cases fish stocks have not regenerated as quickly as plans project, and other stocks are still subject to excessive fishing pressure. This is due to factors such as overestimates of stock size, leading to catch limits that are set too high, and challenges associated with establishing reliable estimates of MSY. Even when fishing is reduced, the rate at which rebuilding occurs also depends on ecological and environmental conditions that aren’t under fisheries management control.
What Are B/BMSY and F/FMSY?
Fishery managers use a concept called Maximum Sustainable Yield (MSY)—the maximum, long-term yield that can be sustainably harvested by fishing—to set annual catch limits and rebuilding targets for stock size. The fish population size (biomass) that generates MSY is called the Biomass MSY (BMSY). The fishing rate (fishing mortality) that maintains the average population size at BMSY is called the Fishing Mortality MSY (FMSY). Each stock has its own BMSY and FMSY based on the life history and population dynamics of that species. To put all 55 stocks in the same frame of reference, this chart is based on the ratios of B/BMSY and F/FMSY.
Stocks above the horizontal line on the chart where F/FMSY is equal to 1.0 are subject to overfishing—that is, fish are being caught too quickly. In most cases, stocks to the left of the vertical line where B/BMSY is equal to 0.5 are considered overfished populations; for some stocks the overfished threshold is closer to 1.0 BMSY. This chart depicts recent analyses that indicate that some of the fish stocks were not actually overfished at the time they were classified as such—a finding that reveals how their perceived status can change as more data become available and assessment methods change over time.
©2013 National Academy of Sciences. All rights reserved.
The content of this data visualization is drawn from the National Research Council report, Evaluating the Effectiveness of Fish Stock Rebuilding Plans in the United States, which can be viewed, downloaded, or ordered in print at http://www.nap.edu/catalog.php?record_id=18488.