Introduction to stock assessment
Unit 3.3: Management Strategy Evaluation
One way to deal with uncertainty in fisheries science is through Management Strategy Evaluation (MSE).
Definition:
Management Strategy Evaluation (MSE) uses computer simulation to test different strategies for managing a fishery. It compares how well each strategy achieves management objectives.
The strategies evaluated in MSE are known as management procedures (MP). Each MP defines:
how stock status is assessed or indicators are calculated
what monitoring data are required for those assessments or calculations
what management actions can be taken based on stock status or indicators, such as:
catch, size, or fishing effort limits
gear restrictions
fishing seasons
fishery closures
Example: Management Procedures
DFO's Sablefish MSE defined MPs that estimated stock trends using a tuned Schaefer state-space production model. The monitoring data required for the model were total landed catch and 3 abundance indices. A pre-defined harvest control rule was applied to the estimated stock trends to get a catch limit. A harvest control rule (HCR) defines what action to take depending on stock status. HCRs can include changes to limits (catch, fishing effort, size) or establishment of restrictions (seasons, area closures, gear type). Catch limit increases were only recommended if catch exceeded the previous year's Total Allowable Catch by 200 tonnes. There were no constraints on catch limit decreases.
Two field technicians on a Sablefish survey. Photo credit: DFO Pacific Sablefish Survey Team.
MSE tests MPs across a range of possible future scenarios. This identifies which are most likely to succeed even with uncertainty. Several MPs can be evaluated in a single MSE process. For example, DFO's Pollock MSE evaluated 7 MPs. DFO's Sablefish MSE evaluated 17 MPs.
MSE evaluates how well each MP achieves management objectives. Objectives are set by fishery managers, often with input from:
Objectives can be set for a single fish stock or an entire fishery. They often include reference points . They can focus on:
conservation goals
economic outcomes
social and cultural values
species biology
Measurable objectives define:
a desired outcome
the timeframe to achieve it
the probability of success
Performance metrics measure how well an MP achieves objectives and allow different MPs to be compared to each other. Performance metrics often include minimum thresholds that an MP must pass to be acceptable.
Example: Management objectives and performance metrics
Objective: Maintain a 95% probability that spawning stock biomass (SSB) stays above the Limit Reference Point (LRP) over 20 years. Performance metric: Measure SSB each year of the MSE simulation. SSB must remain above the LRP in at least 19 years.
Objective: Maintain a 75% probability that SSB in 10 years is above the current SSB. Performance metric: Measure SSB at year 10 of the MSE simulation. SSB must be above the current SSB in 75% of simulation repetitions.
Objective: Maintain average annual catch variability below 25% over 3 fish generations. Performance metric: Measure catch each year of the MSE simulation. The difference in catch between years must stay below 25%.
Fishery managers usually set several objectives for a fish stock or fishery. Different MPs achieve some objectives better than others. Managers decide in advance what risks they are willing to tolerate if certain objectives are not achieved.
Example: Competing objectives
Trade-offs are a key consideration in an MSE process. Achieving some of the objective(s) may require compromises in meeting other objective(s). A commonly encountered trade-off in an MSE is between the conservation performance and catch/stability performance of an MP. Improving performance in one objective may correspond with declining performance in the other objective. For example, an MP that results in high average catches in the short term (strong catch/stability performance) may increase the risk of stock depletion over time (poor conservation performance).
In DFO's Pollock MSE , managers identified:
2 conservation objectives
1 catch objective
1 stability objective
Managers prioritized conservation objectives over catch and stability. They agreed in advance that it was acceptable to not achieve the catch and stability objectives in order to achieve the conservation objectives.
Atlantic Pollock. Photo credit: DFO MAR Ecosystem Survey.
