Fact Sheet: Overfishing and rebuilding

Currently the Council is rebuilding one groundfish stock: yelloweye rockfish.

The Magnuson-Stevens Fishery Management and Conservation Act, or MSA (the primary legislation that governs fishery management) requires that every Council end overfishing and rebuild overfished stocks. Therefore, if a Council-managed species is overfished or is being overfished, the Council must reduce catches to a level that allows the population to rebuild to a healthy size

The terms “overfishing” and “overfished” are defined in the MSA. While stocks may decline for reasons other than fishing (for example, habitat loss or environmental factors), at this point these terms are still used to indicate the status of the stock.

Stock status determinations

The amount of fishing a stock can sustain depends on its productivity. When no fishing takes place, productivity is actually lower because growth and mortality of the stock are roughly balanced.

As the size of the stock, or biomass, approaches maximum sustainable yield (designated BMSY), the stock becomes more productive because there is less competition for resources. At this point the stock actually generates more fish than are needed to replace fish that die of natural causes. Most fish populations can be fished well below their unfished biomass level (the stock size if fishing never occurred) and still be sustained and capable of returning to their unfished status. For a very productive stock, BMSY can be a small fraction of the unfished biomass.

One of the main goals of fishery management is to keep stocks around BMSY. If the population falls below that level, productivity can decrease because there aren’t as many fish reproducing. Abundance indicators are in place to help managers understand the stock’s status, or the health of the spawning population. If a population starts to fall below a certain level, these indicators can trigger a precautionary reduction in harvest (for example, annual catch limits).

If a stock is undergoing overfishing or is overfished, National Marine Fisheries Service declares a stock status change and the Council must develop measures to help ensure the stock can return to a healthy state. If a stock is declared overfished, then a rebuilding plan is required. Rebuilding plans outline measures to be taken until the stock is rebuilt.


Overfishing occurs when the level of harvest or fishing-related mortality is too high compared to the estimated population size (BMSY). The gauge to determine if a stock is subject to overfishing is called the maximum fishing mortality threshold (MFMT), and is typically described as a harvest or exploitation rate. If stock has exceeded MFMT in a given year, then it has been subject to overfishing. If overfishing continues and the population is driven too low, the stock may become overfished.


A fish stock is “overfished” when its population size (in terms of spawning biomass) falls below a certain level or threshold. The minimum stock size threshold (MSST) is commonly used to determine if a stock is overfished and is typically set at half of BMSY. In the case of salmon, MSST is the number of adult spawners associated with MSY (SMSY). Stocks can become overfished due to overfishing, but that is not always the case. In salmon management, a stock is considered overfished if the three-year geometric mean of spawning escapement is less than MSST.


After a stock has been declared overfished and the population begins to improve, the status changes from “overfished” to “rebuilding” once the stock reaches MSST, but has not yet regained BMSY or SMSY. This shows the population is making progress towards meeting the rebuilding goals. 


A stock is considered “rebuilt” when the population meets or exceeds BMSY, or, in the case of salmon, when spawning escapement exceeds the three-year geometric mean of SMSY.

Rebuilding Plans

Rebuilding plans direct the process of rebuilding overfished species. There are three important aspects to rebuilding: the abundance of the spawning stock, the time needed to rebuild the stock, and the rate of fishing that allows the stock to increase to the target level (BMSY or SMSY). A rebuilding plan prescribes the management measures and strategy that will be used to rebuild the fish stock. Often, the conditions triggering the stock’s low abundance are environmental, yet the Council must do what it can to help the stock rebuild.

Certain elements of rebuilding plans are pre-determined at the national level. For example, time limits for rebuilding are set out in the Magnuson-Stevens Act. The Council is able to choose a fishing mortality rate, the corresponding annual level of fishing, and the duration of the rebuilding plan within the statutory limits. When developing a rebuilding plan, the Council must also take into consideration the needs of the tribal, commercial, and recreational fishing interests and the economic importance of the fisheries to coastal communities.

Rebuilding Time Frame

Two boundaries, called TMIN and TMAX, are used to determine the amount of time required to rebuild a stock (the T stands for time). The minimum boundary (TMIN) is set by calculating how long the stock would take to rebuild if no fish were caught. This depends on life history traits affecting the productivity of the stock. TMAX is the maximum allowable time for stock rebuilding. It is set by Federal law, and must be no more than 10 years if the stock is capable of rebuilding that quickly. Otherwise, the statutory maximum time to rebuild (TMAX) is determined based on calculating the minimum time to rebuild (TMIN) plus one mean generation time. Mean generation time is the estimated time it takes a spawning female to be replaced by a spawning female in the next generation.

Rebuilding analyses

The Council relies on rebuilding analyses, which provide estimates of the time to rebuild under different harvest strategies.

Rebuilding analyses typically describe rebuilding in terms of probability, or the likelihood the stock will rebuild under varying time intervals. Probability is also a measure of risk. If a particular course of action is less likely to produce the desired outcome, it may be considered riskier.

The rebuilding analyses that the Council uses to determine rebuilding harvest policies estimate the probability that the overfished stock will rebuild by TMAX or any other interval under a given harvest strategy. Rebuilding probabilities inform a crucial policy choice: lower harvest levels result in a higher probability that the stock will rebuild in time, and vice versa.

The median time to rebuild in a rebuilding analysis under any given harvest rate or strategy is the year predicted to have a 50% probability that the stock will reach BMSY by this year. The Council usually uses the median time to rebuild as the target year (TTARGET) in formulating its policy recommendation, as long as it’s no greater than TMAX. By court precedent and national policy, rebuilding probabilities cannot be less than 50%.

Balancing tradeoffs

Rebuilding a stock is a tradeoff between sharply reducing catches in the short term to rebuild a stock quickly, or allowing more catch and waiting longer for a stock to rebuild.

In finding a balance between rebuilding stocks quickly and allowing some fishing opportunities during rebuilding, the Council is limited by the biology of the stock and by national policy. All other things being equal, a stock will rebuild fastest if there is no fishing at all, but this would be hard on fishing communities, and the Council is required to take community impacts into account.

Choosing a timeframe can be difficult and controversial. If managers choose a very early target date (only a few years after the lower time limit, TMIN), then almost all fishing may have to cease in order for the stock to recover. Catch limits may be too low to allow access to co-mingled, healthy target stocks. On the other hand, if managers opt for a target year far in the future (very close to the upper time limit, TMAX), it will take longer to realize the economic and ecological benefits of a rebuilt stock.