Mean length (LBAR) uses fishery-dependent or fishery-independent length frequency data to estimate fishing pressure using the method described in Ault et al. (2005). Fishery-dependent data primarily samples the fish that have recruited to a fishery or length and therefore life stages that are select by the gear used in the fishery. Whereas, fishery-independent data can also sample the same length distribution as fishery dependent data, if using the same fishing gear or it can be a more complete representation of the fish that have recruit to the area, including most if not all length classes. It will depend on how the data is collected. Either way, when using length data, the user needs to consider the season the data is collect, if fishing gear is used, and how selective the gear is to various length-classes, and if a species is cryptic or not observable, when using and interpreting length-based methods.
In this method, an estimate of fishing mortality is calculated using the minimum (LC), maximum (L∞), and average length reported in the catch, together with estimates of the von Bertalanffy growth parameter k and natural mortality (which can be estimated from k). In addition, the first length at full selectivity is uses to estimate fishing mortality. Together, assuming that FMSY equals natural mortality, this method allows the user to calculate F/FMSY. The control rule of LBAR then adjusts fishing pressure according to the distance of current F/FMSY from the target F/FMSY.
Inputs:
- Fishery-dependent or fishery-independent length-frequency data of fished population
- Theoretical maximum length (L∞)
- Theoretical length at age zero (T0)
- Length at maturity (LM)
- Natural morality rate (M)
- von Bertalanffy growth parameter (k)
- Average length reported in the catch
- First length at full selectivity to the fishery
Outputs:
- An estimate of fishing mortality (F)
Input Sensitivities, Assumptions and Caveats:
- Assumes accuracy of individual fish length measurements
- Sensitive to representativeness of sampling
- Assumes life history parameters, including natural mortality and growth parameters, are known and accurate
- Length is related to age throughout life (i.e., growth is indeterminate – the species just keeps growing longer and longer as it ages until it dies)
- Recruitment is constant (i.e., juveniles are becoming adults at about the same rate each year) – this is a simplifying assumption that probably does not hold for any species
- Mortality is constant – another simplifying assumption that probably does not hold for any species
- Natural mortality (M) is known (this is often not the case)
- FMSY = natural mortality (M)
- System is at equilibrium
- This method depends on reliably tracking population size structure changes, thus may be less accurate for small, fast-growing species
- This method is less reliable when mean fish length is very low
Reference points:
- TRP: M=F
- LRP: F=2M
Recommendations:
- Stakeholders set target and/or limit F/M ratio (i.e., management Reference Points) based on community objectives and thresholds of risk
- 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