As you can tell from the title, this is a post about how selectivity is interesting. Its essentially a response to an objection I often hear from ecologists, who sometimes comment that stock assessment is not engaging. It is admittedly preliminary, besides being one sided and poorly researched (but you knew that already didn’t you?)
The background is as follows. Stock assessment models approximate the demographics and dynamics of fish (and shellfish) populations. They often use age-structured demographics, which are well-accepted in ecology too (ever heard of the Euler-Lotka equation?). However, age-structure means that the modeler must, explicitly or implicitly, declare whether fishing occurs with equal or unequal intensity for different ages. How fishing intensity varies over ages (and perhaps sizes) is often called selectivity
Now, selectivity is complicated because it includes many different processes. Fishing gear (and targeting behaviors) will select for certain sizes/ages of fish, and this can be measured experimentally and results can be included in a model. However, the shape of selectivity also depends upon how fishing is allocated spatially (as illustrated by Dave Sampson, among others, here). Essentially, if fishing occurs in two areas, aggregate selectivity is the weighted average of selectivity in each area (weighted by the proportion of the population in each area). If fishing intensity is greater in one area than another, aggregate selectivity will more resemble the area with lower fishing for old fish (because the survivors are generally in the area with little fishing). Hence, spatial allocation of fishing can cause some wacky shapes for selectivity, which doesn’t resemble selectivity in either area individually.
For this latter reason, selectivity is generally estimated within a stock assessment model, rather than being informed by prior information (like gear experiments). It may therefore seem to be a “nuisance parameter” — some effect that must be estimated, but has little direct interest to any particular researcher. And its notoriously hard to argue from first principles about what selectivity should look like for a given species and fishery.
Despite this, I think its wrong to suppose that selectivity is an uninteresting “nuisance” parameter. Instead, I propose that these issues with selectivity will also occur for other age-structured effects, i.e., size-based predation. We already know that fishes are gape-limited (see here for a recent example), and will generally predate individuals within a certain size range, which shifts as an individual grows. So the intensity of predation follows a function that depends upon the age of the prey. Given different predator densities in different areas, aggregate predation may not resemble the age-based function for predation in either area individually. In this case, experimental measures of how predation varies among prey ages will not be appropriate to represent aggregate predation.
I don’t claim to have a good and general answer for how to account for this effect when modeling age-specific predation. However, I think that stock assessment approaches to selectivity will provide some ideas — and hope to post about the topic more after I’ve had time to ruminate…