Note: For the purposes of the below discussion, and throughout the FISHE website, the term “Marine Protected Area (MPA)” refers explicitly to an area that has been fully closed off to fishing and other marine uses. Sometimes these areas are called “No-Take Zones” or have other, more specific names, while “MPA” in some places may refer to a less strict form of area-based management (i.e., a “mixed-use area,” or an area where just some forms of fishing are prohibited). However, the following method, and all FISHE methods that rely on “MPAs,” will only work if the area in question is fully closed to all fishing activities. Furthermore, for this method to work, the MPA must also be designed based on scientific guidance, well-enforced, and old enough to have allowed species within its bounds to recover to “unfished” levels.
No-take marine protected areas (MPAs) and other well-enforced, fishing-restricted areas provide excellent reference baselines against which to evaluate the status of stocks that are fished. This is because they provide empirical information on the unfished density and length structure of the stock, rather than estimates. Fish densities (measured in kg/ha) inside and outside the MPA can be estimated from the results of fishing or in-the-water visual surveys. The MPA Density Ratio (fished/unfished fish density) can then be calculated to serve as an indicator of stock status. Harvest control rules based on measured fish densities can be generated directly, or the results of the analysis can be used in conjunction with PSA results to prioritize stocks for further assessment in order to set catch limits and other management measures.
When using marine protected areas as a reference baseline, it is important that both fished and unfished areas contain similar habitats and the reference area has been effectively managed (i.e., truly excluding fishing pressure completely) for a long enough period of time for fish stocks to exhibit the characteristics of an unfished population (the necessary length of time for this to happen will depend on the growth rate of the species and other local characteristics). Make sure the same survey methods are used in the fished and unfished areas.
Use the calculator in the Workbook to determine the density ratio of your fishery.
Inputs:
- Common Life History Parameters
- Fish density inside and outside of a well-managed no-take area
Outputs:
- Ratio of fished to unfished density as proxy measure of depletion
Input Sensitivities, Assumptions and Caveats:
- Assumes that a fully-functioning and well-enforced MPA has been sited appropriately with representative habitat inside and outside of the MPA, and has been in place long enough for the population living inside the MPA to be a proxy for an un-fished population.
- Implication: May be less accurate for highly-mobile species that do not remain exclusively inside the MPA such as snapper, tuna and mackerel.
- Assumes life history data are accurate
- Time trends in this data can be difficult to interpret if densities inside the MPA are changing rapidly.
- Habitat quality and productivity are similar inside and outside of the no-take area and the sampled fishing area(s)
- Survey data in fished and unfished data are taken with the same methods and are unbiased
- Fish density in the no-take area is representative of unfished density (i.e., no-take area is well-designed, with representative habitats inside and outside of the managed area; well-enforced; and old enough for fish populations to have equilibrated to no-take conditions)
- Consistent monitoring program
Reference points:
- Proxy reference points for stock status, since MSY reference points cannot be calculated using this method
- For most finfish species: TRP, above 60% (fished/ reserve); no restrictions
- For most finfish species: LRP, between 20 % and 60 % (fished/ reserve), reduce length of open season; below 20% (fished/ reserve) - close all year
Recommendations:
- Effort controls (e.g., season length) or catch can be adjusted in response to changes in MPA density ratio to adjust fishing mortality aimed at moving the MPA density ratio toward targets and away from limits
- For data on fish density inside and outside of MPAs that are not paired:
- Calculate average density across fished sites (density.fished)
- Calculate average density across reserve sites (density.reserve)
- DR=density.fished/density.reserve
- For paired sites inside and outside of MPAs
- Calculate density.fished/density.reserve for each pair and take the mean
- May need to divide by the density ratio observed during the year the MPA was established to account for differences in habitat between fished and reserve sites.
- Combined with methods that estimate F or a reference value for F, effort can be adjusted through harvest control methods (e.g. catch limits, seasons, or spatial closures) based on how far the MPA density ratio is from the proxy reference points