Harvest Strategies Glossary

Overview

Harvest strategies, also known as management procedures (MPs), are an efficient and effective innovation in sustainable fisheries management. Understanding the terms used to describe the many steps in the process will help fisheries managers and other stakeholders effectively engage in the development and implementation of the approach. Consistent definitions provide a universal frame of reference for discussing harvest strategies and the concepts involved. While different regions define “harvest strategy” slightly differently, they all agree on what a “management procedure” is, so this glossary will use “management procedure” going forward.

---

Italicized terms in this glossary indicate words or phrases that are defined elsewhere within the glossary.

Glossary

B

B: Biomass. The total weight of all organisms in a population or a defined part of the population.

B₀: Virgin stock biomass or unfished stock biomass. The average biomass likely to exist before fishing began or at any point in time in the absence of fishing.

B_MSY: The long-term average biomass that is achieved by fishing at a constant fishing mortality rate equal to F_MSY; in other words, the average biomass able to produce maximum sustainable yield. Since it is an average, the biomass at any particular time may differ from B_MSY because of natural variability in productivity and breeding success, though the long-term average is maintained.

C

Catch per unit effort (CPUE): The amount of catch taken per unit of fishing effort, such as the number of fish per 1,000 hooks. Often used as a proxy for stock abundance.

Closed loop simulation: The feedback loop of management strategy evaluation (MSE), which simulates the effects of candidate management procedures on a stock and fishery into the future. The steps of closed-loop simulation include an operating model, observation error model, management procedure, and implementation error model. The output of the implementation error model then feeds back into the operating model for the next management cycle.

Conditioning: The process of fitting an operating model (OM) to the available data as part of a management strategy evaluation (MSE). Conditioning the OM helps develop a set of plausible models and/or hypotheses of the stock and fishery that are consistent with the data and assumptions about uncertainty, as opposed to identifying one “best” scenario and ignoring alternative models and/or hypotheses when testing candidate management procedures.

E

Exceptional circumstances (ECs): Rare and unforeseen events that were not tested by the MSE or that the MP was not designed to manage. If monitoring indicates the MP is not meeting objectives, that can also be an EC.

Exceptional circumstances protocol (ECP): An agreement that defines what qualifies as an exceptional circumstance, specifies the frequency of EC assessments, and includes a decision tree outlining the steps to take if an EC is identified.

F

F: The instantaneous fishing mortality rate. This is the fraction of the population (or year class or other defined group) that is expected to be caught at any single point in time. The annual fishing mortality rate is calculated using the formula 1-e^-F, where “e” is the mathematical constant known as Euler’s number. For example, an F of 0.54 means that 0.417, or 41.7%, of the population is caught each year.

F_MIN: A common element of a harvest control rule that sets a minimum fishing rate to allow continued scientific data collection and to cover bycatch mortality when the stock is found to be below the biomass-based limit reference point.

F_MSY: A fishing mortality rate that, if applied constantly, would result in B_MSY and maximum sustainable yield (MSY) on average over the long term.

F_X%: A fishing mortality rate that leads to X% of the maximum spawning potential (e.g., egg production, recruits, spawners) that is obtained with no fishing.

Feedback loop: An MP’s feedback loop in the implementation phase includes a monitoring and data collection program, a method that uses the collected data to estimate stock status, and a harvest control rule (HCR) that triggers a management action based on the estimated stock status. The feedback loop is what makes an MP an adaptive management approach since it implements the action in response to changes in stock status.

H

Harvest control rule (HCR): A pre-agreed rule that sets fishing opportunities (catch limit, effort limit, etc.) based on the selected indicator(s) of stock status.

Harvest strategy: See management procedure. In most regions, the two terms are used interchangeably, but some fisheries in the Pacific consider a harvest strategy to be a more general fishery management plan, of which a management procedure may or may not be one part.

I

Implementation error model: The step in an MSE where implementation error, or deviations from the MP-prescribed catch level, are applied. These deviations include quota overages caused by illegal or unreported catch or other issues.

K

Kobe plot: A four-quadrant graphic that shows the status of a stock, the trajectory of the stock through time, or both. Stock abundance is on the horizontal axis, and fishing mortality is on the vertical axis. The axes are typically divided at B=B_MSY and F=F_MSY, respectively, and hence can graphically depict whether the stock is overfished and/ or subject to overfishing. The green quadrant on the lower right indicates a stock that is neither overfished nor subject to overfishing. The orange quadrant on the upper right indicates a stock that is not overfished but is subject to overfishing. The yellow quadrant on the lower left indicates a stock that is overfished but no longer subject to overfishing. The red quadrant on the upper left indicates a stock that is both overfished and subject to overfishing.

L

Limit reference point (LRP): A benchmark that defines an undesirable biological stock status (B_LIM) or fishing level (F_LIM) that should be avoided. To keep the stock safe, the probability of violating an LRP should be very low. However, if an LRP is violated, immediate action —such as a reduction in fishing to the F_MIN level—should be taken to return the stock or fishery to the target level.

M

M: The instantaneous natural mortality rate. The part of the total mortality rate (F+M) that is due to natural causes, including disease, predation, and starvation.

Management cycle: The time period during which the output of an MP is applied to the fishery. For example, in a 3-year management cycle, the output of the MP applies for 3 years, after which the MP is run again, and the new output is applied to the next three years.

Management objectives: Formally adopted, measurable goals for a stock and fishery, such as maintaining an abundant population or supporting high catch. They include high-level or conceptual objectives often expressed in legislation, conventions, or similar documents. When management objectives are referred to in the context of management procedures, they must also include operational objectives that are specific and measurable, with associated timelines and minimum required likelihood that they can be achieved.

Management procedure (MP): A pre-agreed framework for making fisheries management decisions, such as setting catch limits, that has been simulation-tested using MSE and designed to achieve specific management objectives. An MP typically includes a data collection program, stock status estimation method, and harvest control rule (HCR). Also known as a harvest strategy.

Management strategy evaluation (MSE): A computer simulation-based analytical framework used to evaluate the performance of alternative candidate management procedures relative to the pre-specified management objectives. Also, the process of developing MPs.

Maximum sustainable yield (MSY): The largest long-term average yield that can be taken from a stock under existing environmental conditions and a constant fishing mortality rate.

Model-based management procedure: An MP that uses an estimation method that involves a stock assessment model that combines information from several sources (fishery catch, surveys, age or size composition, etc.) to estimate a stock status indicator.

O

Observation error model: The step in an MSE where plausible levels of imprecision and bias are added to the simulated fishery dynamics to better resemble what happens in a real-world fishery system, where the available data are always imperfect.

Operating model (OM): The building blocks of MSEs. They represent the multiple plausible hypotheses for the fishery system’s state of nature, including stock biology and fishery behavior. Multiple OMs based on varying assumptions are often used within a single MSE process to test the expected performance of alternative candidate management procedures. The “reference set” of OMs are the most likely scenarios, whereas the “robustness” OMs are more extreme but still plausible scenarios.

P

Performance indicator:  A quantitative expression of a management objective used to evaluate how well the objectives are being achieved by determining the proximity of the current value of an indicator to the objective, often a reference point. For example, where high catch is an objective, a performance indicator might be the average catch level over a 10-year period. Also known as performance statistics or performance metrics.

Precautionary approach: A management philosophy that requires consideration of risk reduction in decision-making so that in the absence of full information, the decision taken results in the lowest risk to the stock.

R

Recruitment: The amount of new fish that join a defined group of fish each year—due to growth and/or migration. The defined group may be the exploited part of a population, which is described as recruitment to the fishery. The defined group may also be the whole population (fished or unfished) older than a certain age (e.g., age 1 or the age at maturity).

Reference points: Benchmarks used to compare the current status of a fishery management system with a desirable (target reference point) or undesirable (limit reference point) state. Often defined in management objectives. Reference points may or may not be used as control parameters in HCRs.

Risk: The probability of a negative outcome in a fishery, such as stock collapse, breaching the limit reference point (LRP), or unnecessarily forgoing catch. In statistical terms, it is the probability of a negative outcome multiplied by its adverse effect.

S

Science-Management Dialogue (SMD) Group: A group that provides a forum for the necessary iterative exchange among fishery scientists, managers, and other stakeholders during MSE development. It can be used for capacity development as well as decision-making on the various MP elements.

Selectivity: Measures the relative vulnerability of different age (size) classes to being caught by a specific fishing gear or fleet.

Spawning potential ratio (SPR): The lifetime contribution of spawning output (e.g., eggs) that a recruit is expected to provide under the stated fishing mortality relative to its lifetime production without fishing. Often expressed as a percentage. For example, SPR50% means that under the specified fishing mortality rate, a recruit will, on average, produce half the eggs in its lifetime that it would have produced without fishing. See FX%.

Spawning stock biomass (SSB): The total weight of the sexually mature part (i.e., adults) of a population.

Stakeholder: A person with an interest or concern in something. Stakeholders in MP processes include fishery managers, fishers, members of the seafood supply chain, and environmental representatives. Stakeholder engagement is a hallmark of MSE.

T

Target reference point (TRP): A benchmark that defines the target fishery state, both stock size (B_TARGET) and fishing level (F_TARGET), that should be achieved and maintained. Creates a buffer zone to ensure that the limit reference point (LRP) is not breached. Can be based on one or more biological, ecological, social, or economic considerations.

Threshold reference point: A benchmark that triggers a pre-defined management response to help the fishery remain close to the target reference point (TRP) and avoid breaching the limit reference point (LRP). Also known as a trigger reference point or control parameter.

Tuning: The process by which one performance standard or “tuning objective” is selected and which all candidate MPs must meet. Once that objective is achieved, the MPs’ relative performance against the other objectives can be compared. A sustainability objective is typically chosen as the tuning objective.

U

Uncertainty: Results from a lack of perfect knowledge about one or more factors that affect MSE, stock assessments, estimation of reference points, and management. Four main types of uncertainty are considered in fisheries: observation error (caused by biased data), process error (caused by natural population variability), model error (caused by incorrect assumptions or model structure), and implementation error (caused by failure to fully implement management measures).

---