I am not completly sure about what you want to do. but if you want to understand how freshwater discharge impact the fish communities and if the both data are multivariated (=several measured parameters for water discharge and sevearal species measure), you can maybe use a coinertia analysis. But I am not sure that you have several parameters for the freshwater descharge. If you have univariate measures for descharge I would do as you propose but I do not have any idea how to make you classes.

I hope it help a bit :D

"Fresher water" (< 5 ppt) -changes the biota in a brackish 14 - 25 ppt salinity estuarine complex...

Some fish species sensing the salinity changes will either 1. flee as a response to salinity drop or 2. regulate the osmotic changes to their bodies by adaptations... Some produce mucus - some find containment in hollows or burrows.. some will withdraw to any place that will allow them to regulate the osmotic changes to slow down their bodies (metabolism) as they adapt.

Categories would be 1. Species that flee the area for higher salinity 2. Species that remain and adapt to the changes through internal regulation... 3. Species that succumb and die off...

One would study the behavioral aspects of fish species through direct observation - as a close watch upon the chemistry - Salinity ..changes...

There is a big difference between regulation of osmotic stress and adaptation.

some fish can regulate some species needs time to make needed adaptions.

I hope this helps...

I consult with estuarine fisheries on some issues.. because we deal with tropical conditions which includes torrential rainfall... severe osmotic changes limit species selection to those species that use their osmoregulatory response naturally and not adapt to the stress out of dire need until they find a higher salinity environment...

Estuarine Complex environments have a plethora of variables... but being an old school person .. i would go with discharge levels - salinity changes - and CPUE - per fish species but tie that data into behavior... You can gain more insight through behavioral changes via salinity changes... Species behavior is valuable data to a fishery...

Hope this will help...

Nature will reveal all of it's secrets if you take time to observe..>

Sg Wn

One thing that the USGS typically does with fall-line freshwater discharge values is to use quantiles to assess high and low flow values. Several investigators I know have adopted this approach as a means to categorize periods of high and low flow to assist in the analysis of with Chesapeake Bay Program water quality data. See the following:

https://www.nature.com/articles/s41598-019-43036-6.pdf

You say you have average discharge levels. I believe for this approach you would need to figure out the quantiles and therefore would need the daily values for your estuary.

You should be able to find something suitable here if what you are using isn't the same thing and you already used the dailies to make your averages.

https://waterdata.usgs.gov/nwis/rt

That should get you your annual groupings for Primer. Good luck!