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.
This method utilizes length-frequency data (fish lengths) from inside and outside a Marine Protected Area (MPA) to compare the slope of the right-hand side of the log transformed age-frequency histogram from inside the no-take zone (an estimate of natural mortality (M)) to the slope of the log transformed age-frequency histogram outside the no-take zone (an estimate of total mortality (Z)). Fishing mortality (F) can then be calculated based on the difference between these two (F = Z – M).
Inputs:
- length-frequency data inside and outside MPA (preferably collected in the same manner)
- life history parameters (growth parameters)
- number of years that the MPA has been well-enforced
- information on the sizes of fish preferred by the fishery
Outputs:
- an estimate of fishing mortality (F)
Input Sensitivities, Assumptions and Caveats:
- Assumes life history data are accurate
- Assumes accuracy of individual fish length measurements
- Assumes accuracy of length-at-age relationships (Von Bertalanffy growth parameters
- Assumes correcting fitting of the curve (sensitive to estimates of MPA age, preferred fish size)
- Assumes that a MPA has been sited appropriately (including all critical habitats and large enough to protect species whole range), is well-enforced, and has been in place long enough for the population living inside the MPA to be a proxy for an un-fished population (appropriate length of time will depend on species reproductive and growth characteristics, but at least 1 generation (i.e. the lifespan of one age class of the target species) is recommended)
- Implication: May be less accurate for slower-growing species that may have had fewer recruitment events within the MPA
- Implication: May be less accurate for highly-mobile species that do not remain exclusively inside the MPA, such as snapper, tuna and mackerel
- This method depends on reliably tracking population size structure changes, thus may be less accurate with small, fast-growing species
- Appropriate F/M ratio reference points (targets and limits) are dependent on species life history characteristics, fishery selectivity, and community risk tolerances
Reference points:
- stock status based reference points for F
- TRP: M=F
- LRP: F=2M
Recommendations:
- Stakeholders decide on the appropriate target and/or limit F/M ratio (i.e., management Reference Points) based on community objectives and thresholds of risk. Higher F/M ratios (where fishing mortality is closer to, or exceeds, natural mortality) create greater risk of overfishing.
- F/M Reference Point is compared with F/M found through assessment
- Effort/ fishing mortality is adjusted through harvest control methods (e.g. catch limits, seasons, or spatial closures) based on how far apart these values are.
- Can provide a local estimate of M (natural mortality) to be used in other assessment methods that estimate fishing mortality from observation within the MPA.