link stratification in apportioning page

docs/theory/03_stratification.md

@@ -1,6 +1,7 @@
 # Stratification to obtain biomass density
 
 
+(stratification)=
 ## Stratification
 In practice, the acoustic quantities and biological estimates discussed in previous sections can vary depending on geospatial variation of the biological aggregations themselves. For example, the size and age of fish can vary depending on the survey location, as well as the sex of the fish. Therefore, the acoustic measurements and biological samples are typically stratified to account for these variations, and the biomass density is a function of the stratum (which depends on the geospatial locations) and sex, i.e.
 

docs/theory/04_apportioning.md

@@ -58,7 +58,7 @@ A superscript is used to denote the set of fish samples:
 - $B^k$ is the biomass in transect interval $k$
 
 ```{attention}
-The sex, length, and age distributions of fish vary across strata. Therefore, the specific strata is implicit in the description below. Most quantities can be annotated with an additional superscript $i$ to indicate the stratum the sample or quantity belongs to. Exceptions arise when the quantity is specific for a transect interval ($k$) or a haul ($k$), since a transect interval or a haul only belongs to one stratum.
+The sex, length, and age distributions of fish vary across strata (see [](stratification)). Therefore, the specific strata is implicit in the description below. Most quantities can be annotated with an additional superscript $i$ to indicate the stratum the sample or quantity belongs to. Exceptions arise when the quantity is specific for a transect interval ($k$) or a haul ($k$), since a transect interval or a haul only belongs to one stratum.
 ```