Introduction to stock assessment

MSE compares how well different strategies for managing a fishery achieve objectives. It considers many possible realities of a stock, fishery, or ecosystem, each with different assumptions. Traditional stock assessment focuses on a single or small set of models that "best" represent a stock. It uses past data and a single set of assumptions to estimate current stock status.

MSE integrates different types of objectives from the start. This makes trade-offs between objectives more clear. Traditional stock assessment focuses on biological or conservation objectives. Managers consider social, cultural, and economic objectives after science advice is provided.

MSE provides long-term advice that accounts for uncertainty. It can project stocks into the distant future (for example, 2 or more fish generations). Advice is revisited regularly within this period. Traditional stock assessment typically provides advice based on current stock status. It often only projects a stock a few years into the future.

Updating a traditional stock assessment can take a long time. Updates may be done every 5 to 10 years or more. Once an MSE process is established, it can deliver advice faster. For example, DFO’s Sablefish MSE provides updated science advice every year.

MSE simulation requires staff with specialized technical skills. These experts must also be able to explain the results and their implications to managers.

Starting an MSE process takes more time upfront than starting a traditional stock assessment. This can be challenging when decision-making deadlines are short. Plus, traditional stock assessment often continues alongside early MSE work. This increases the short-term workload.

MSE can be time-consuming for staff with limited capacity. It often requires dedicated resources. MSE may also need more frequent updates in the early stages of development. Once an MP is chosen and applied in the real world, the MP can usually be updated with fewer resources. Re-evaluation of OMs can occur every few years as part of a multi-year cycle.

One of the most misunderstood aspects of MSE is the interpretation of expected yield from MSE simulations. MSE results will show the relative performance of different MPs. For example, if one MP, on average, could be expected to provide better yield than another MP. MSE results will not guarantee that a specific yield will be achieved in a fishery if a particular MP is followed. The actual achieved yield will depend on real world conditions and how the stock changes from one-year to the next.