Science Friday: What is PNI about and why is it important?

In Science Friday by Nick Chambers

Another week, another Science Friday post from Wild Steelheaders United. Last week we discussed the acronyms used in hatchery literature and management plans. Now we narrow the focus a bit to understand how these acronyms are used with one another to evaluate genetic risks associated with hatchery programs — we even offer some formulas for you to geek out on.


The acronyms we reviewed last week covered a variety of individual aspects of hatchery management plans, including proportion of wild and hatchery spawners in the broodstock and the proportion of wild and hatchery fish spawning in nature. Those measures are typically incorporated into an index of hatchery risk called PNI: the proportion of natural influence.


PNI is derived by using pNOB – the proportion of natural-origin broodstock – and pHOS – the proportion of hatchery-origin spawners. The formula is:


pNOB / pNOB + pHOS


The resulting number denotes the proportion of natural influence, which is a proxy for the genetic risks posed by hatchery steelhead, or hatchery salmon or any other species of interest.


The general assumption is that a higher proportion of natural-origin fish in the hatchery broodstock and in the amount of fish spawning in nature equates to a lower level of hatchery genetic risk. The opposite is assumed to be true when the proportion of natural-origin fish is much lower.


PNI is important because it helps unify elements of hatchery programs, such as the collection of broodstock and the number of hatchery fish spawning with wild fish. The result is, in theory anyway, a more holistic approach to hatchery planning. Rather than analyzing each component separately, they are now considered within the context of one another. This improves the ability of managers to scale the program size and harvest rates in relation to the size of the wild population and the capacity of the habitat.


PNI is commonly used and referenced in recovery and hatchery management plans. In such plans, the goals tend to vary depending on the population. For example, wild steelhead recovery plans often classify populations as primary, contributing or stabilizing.  Primary populations are the strongest populations with a high likelihood of persisting into the future (i.e., a high viability). These populations have the best prospects for protection and are the foundation for recovery. Contributing populations have a lower viability and are of more moderate significance to the overall region of interest. Stabilizing populations are typically at low levels of viability and have a much higher level of uncertainty relative to recovery.


The criteria for hatchery influence often varies by type of population. Primary populations are managed more conservatively from a hatchery standpoint. Criteria tend to require higher levels of PNI (e.g., > 67%) and lower levels of pHOS (e.g., < 5%) in primary populations, while they are less restrictive in contributing and stabilizing populations. Essentially, the better off the wild population is, the more conservatively the hatchery is managed – if a hatchery is needed at all.