Length-Based Spawning Potential Ratio (SPR)

The spawning potential ratio is based on the concept that enough fish have to survive to spawn and replenish the stock at a sustainable level in order for the population to remain viable. It is a measure of current egg production relative to maximum possible production at un-fished levels. SPR is used to assess a fishery for recruitment overfishing. Recruitment overfishing occurs when a fishing mortality is so high that fish are captured before they reach maturity and have the ability to spawn. In general, it is assumed that egg production is proportional to the weight of a species. For this reason, information about the size and weight structure of a stock is particularly helpful in determining the amount of spawning potential retained. Spawning Potential Ratio (SPR) is defined as the percent of unfished spawning potential retained under a given harvest policy. As the fishing mortality increases, the number of fish surviving to a time when they can produce eggs decreases, and so the total egg production decreases. This fished egg production is always expressed as a percentage of the total expected eggs produced under unfished conditions.

This method estimates the fraction of unfished reproductive potential that a fished stock may be theoretically capable of producing by calculating egg production from each length class sampled in the catch, at a given level of fishing mortality.

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:

• Spawning Potential Ratio (the ratio of current reproductive capacity to maximum potential reproductive capacity of an unfished population) at different levels of fishing mortality.

Input Sensitivities, Assumptions and Caveats:

• Assumes accuracy of individual fish length measurements
• Sensitive to representativeness of the length data
• Assumes accuracy of life history information, particularly growth and maturity parameter
• Natural mortality rate (M)
• 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:

• Uses stock status-based reference points to estimate sustainable yield and maintain F.
• TRP, for fast growing targets: SPR₂₀; slow growing targets: SPR₄₀

Recommendations:

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