Yield Per Recruit (YPR)

In the term “yield-per-recruit (YPR),” yield refers to the expected weight of fish caught by the fishery in a given season, and recruits are the youngest fish to be caught by the fishery each season. The concept behind this method is that it is possible to maximize the fishery’s yield by catching fish of an “optimal” age.  YPR examines the potential for “growth overfishing” (fishing too hard on the juvenile members of a stock, resulting in low recruitment to the fishery) under a given fishing strategy, and it can help to identify what the optimal fishing rate is that would lead to the maximum yield per recruit, providing an understanding of how fishery selectivity and fishing mortality will affect yield. YPR models can be age or length based depending on the information available (i.e., whether the user has the ability to accurately age the fish or determine the growth rate of a fish).

This method estimates the harvest rate that maximizes cohort biomass under equilibrium conditions by estimating the number of fish surviving to each subsequent age class (given both natural and fishing mortality), as well the length (or weight) of an individual fish at each age. The number of fish surviving to each age class will decrease continually (as some fish die at each age), while the size of each individual surviving fish will increase.

Inputs:

• Length-frequency data from a fished population
• Gear selectivity
• Common Life history parameters (fecundity, von Bertalanffy (k), natural mortality (M), age-at-maturity, length at age relationships)
• Weight to length parameters (W_a, W_b)
• Fecundity at age parameters (f_a, f_b)

Outputs:

• Yield-per-recruit analyses, reflecting schedules of mortality and weight at age in the catch, in the context of growth overfishing

Input Sensitivities, Assumptions and Caveats:

• Assumes constant natural mortality (M) over time
• Assumes constant recruitment to the population.
• Assumes accuracy of individual fish length measurements
• Sensitive to representativeness of the length/weight data
• Assumes accuracy of life history information, particularly growth and maturity parameter
• Dependent on reliably tracking changes in population size structure – may be less accurate for small, fast-growing species
• Fishery is in equilibrium and conditions are relatively stable (environmental conditions, fishing pressure, stock status, etc.)
• Less accurate if fishing pressures has been changing dramatically year to year

Reference points:

• F_max, the (fully-recruited) fishing mortality rate which produces the maximum yield per recruit.
• F0.1, the fishing mortality rate corresponding to 10% of the slope of the yield-per-recruit curve at the origin. The F0.1 reference point was conceptualized as a biologically precautionary target relative to F_max: at F0.1, catch-per-unit-effort is not reduced substantially, but the fishing mortality rate is lower than F_max.

Recommendations:

• Estimate F using another method such as Catch Curve or LBAR, and then identify YPR at that F level from the YPR-F output curve.
• Adjust fishing mortality through harvest control measures (e.g. catch limits, seasons, or spatial closures) based on the difference between estimated YPR and the YPR Reference Point (often 40% for finfish species).