Why did we create FINDEX?
In most cases, stock assessment models are used to describe the condition or “status” of finfish populations and typically draw two conclusions:
- The stock is overfished, or is not.
- Overfishing is occurring, or is not.
In response, the NC Marine & Estuary Foundation developed FINDEX, an up-to-date index that utilizes the best available scientific data to provide a clear picture of our coastal fisheries.
How is it calculated?
FINDEX is a novel way to gauge whether a species’ abundance is trending up or down. The calculations compare the desired condition of a fish stock with its existing condition.
State and Federal Fishery Management Plans guide most of the decision-making for North Carolina’s coastal fish species. Included within most plans is a stock assessment that runs biological data for the species through various models. These models attempt to estimate current stock conditions while also predicting changes to the fishery over time under different harvest scenarios.
Most stock assessment models include Target and Threshold biological reference values established for management of each species. If the Target reference value is met, the assumption is that the stock is stable (or sustainable) and not overfished.
The Threshold reference value is a percentage of the Target value and is used to measure progress toward rebuilding. Target and Threshold biological reference values are assigned by biologists and modelers and are subjective and debatable; however, NC Marine & Estuary Foundation staff has chosen to use the values as designated in the peer-reviewed stock assessments for most FINDEX calculations.
FINDEX simply measures the gaps between either the Target or Threshold biological reference values and the most recent year’s values estimated in the stock assessment model. The most recent data used in the stock assessment model is called the “terminal-year estimate.” When more than one biological reference point is used in a stock assessment model, then gaps between multiple sets of desired conditions and existing conditions can be combined to strengthen the FINDEX metric.
The “gaps” between the Target and Terminal values are reported as ratios. For example, if the Target and Terminal values are the same, there would be no gap and the ratio would be 1.0. The multiplier on our FINDEX barometer would set the FINDEX value at 100% in this example and assign a stock status designation of “Stable.” Categories on the FINDEX barometer are measurable as they track the extent of the gap either above or below the “Stable” designation.
Biological reference points commonly used in stock assessments and the FINDEX gap assessment:
1. Fishing Mortality (F) is the rate at which fish are removed from the population due to harvest and discards (also referred to as “bycatch”). When fishing mortality rates go DOWN, this is an indication that fewer fish are being removed.
2. Spawning Stock Biomass (SSB) generally refers to the total weight of spawning females in the population; as SSB goes UP, then conceptually there would be more eggs produced, and therefore a higher abundance of fish.
3. Total Mortality (Z) is the rate at which fish are removed from the population due to both fishing mortality and natural mortality. Sources of natural mortality include predation by other fish and disease. When total mortality rates go DOWN, this is an indication that fewer fish are being removed from the population.
4. Spawning Potential Ratio (SPR) is the ratio of the reproductive potential of an average recruit in an exploited (or fished upon) population to the reproductive potential of an average recruit in an unfished population. Spawning Potential Ratio is a measure of the impact of fishing on the potential productivity of a stock. SPR calculations also include estimates of fishing and natural mortality rates for females at various ages in the stock. When SPR goes UP, then conceptually stock productivity increases.
Status categories in FINDEX (Depleted, Rebuilding, Stable, Robust, and World-Class) are measurable as they relate the terminal-year estimate to its percentage of established Target and Threshold biological reference values.
Example: “Stable” status is assigned to any stock with a FINDEX value reaching 100%. At a value of 100%, the terminal-year estimate is equal to the biological reference value used to determine overfished status. A “World-Class” designation is assigned to a population when FINDEX values exceed 200%.
Note: There is a significant time lag associated with updates to stock assessments for most of North Carolina’s fisheries. Periods between updates are usually 5 years and sometimes 10+ years. For this reason, FINDEX calculations may not accurately describe current conditions – instead, FINDEX provides the most recent scientific data available.
Our team remains hopeful that the stock assessment models will be updated annually to provide citizens and policy makers with up-to-date information to highlight the current need for improvements to our coastal fisheries.
A new species will be highlighted monthly throughout 2022 with FINDEX status provided. For those interested, FINDEX will be delivered each month to our Think Coastal subscribers.
So stay tuned…there is more to come!
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