What to do when age 0 fish are mature?

[Richard Methot - NOAA Federal]

SS3 allows age 0 fish to be mature, but should it?  This can happen as a consequence of use of length-specific logistic function for maturity such that the calculated maturity extends to age 0.  The input control called #_First_Mature_Age was created to allow excluding the age 0 fish.  However, that control is overridden if you are reading age-specific egg production from a wtatage.ss file where it is possible for you to assign the age 0 value to be >0.0.

The contribution of age 0 fish to the spawning biomass can be problematic.  Fish cannot contribute to the spawning biomass that is used to create their own recruitment!  While it is not uncommon to put spawning season at month 1.0 and the settlement event for the age 0 fish also at month 1.0 there is no logic problem if those fish do not actually show up in any data until later in the year.  But the calculations go awry if the model configuration also sets age 0 fish to be mature.

I'm writing today to ask if anyone can think of a situation if which it is biologically necessary to have age 0 fish be mature without causing this internal problem.  Certainly there are situations where fast growing fish may show signs of maturity before they are 1 year old, but what matters is their maturity at the time of spawning, by which time they will be calendar age 1.  So, please let me know if there is a situation that needs age 0 maturity, else I may put in a safeguard against it accidentally happening.

Rick

[Richard Methot - NOAA Federal]

Hello SS3 users,

I am checking again to see if anyone sees a situation in which age 0 fish need to be mature at the time of spawning.  The only situation I can imagine is one in which spawning is late in the year and the young fish early the next year are forced to be age 0 at the time of settlement, even though SS3 will issue a warning telling you that fish settling in the year after spawning should be settling at age 1.  Then later that same year at the time of spawning they would still be age 0 and legitimately could be part of the spawning biomass as age 0 fish.   

What do you think?

Rick

[Mark Maunder]

If fish mature that early, you probably need to use a time step less than a year, either using a tuna-like quarter as year model or using a seasonal model.

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[Jason Cope]

My response mirrors Mark's comment. A year time-step with such early maturity may not be a good fit.

[Richard Methot - NOAA Federal]

Thanks Mark and Jason.

Even with a seasonal model one needs to be careful that the age-specific maturity*fecundity is relevant for the time of year of spawning.  If spawning is at the beginning of the year, there typically are no age 0 fish at that time of year, so it is misleading to be using a maturity*fecundity that implies that age zero fish are part of the spawning biomass.

The two warnings I am considering adding are:

if First_Mature_Age == 0 then

"First_Mature_Age read as:  0, which is unusual. Check logic of spawn_month and settlement time & age"

Also add a check against First_Mature_Age when reading wtatage.ss:

Adjustment: WTATAGE.SS has positive fecundity for year, age = 2008 0 setting to 0. because younger than First_Mature_Age