Can someone please explain this?!? Video included...

BTW regarding the OP. Maybe they separate them Male and Female. Just a thought

Making sure they are getting only Herring and not smelt/shad/perch/baby stripers 

Mt brother will swim for it if you give him half a bucket.

I saw that pic of him in the water.

Also, with all the water it takes to wash out all the slime in the cooler/bucket, why not line the cooler/bucket with a large trash bag?!

Mt brother will swim for it if you give him half a bucket.

I saw that pic of him in the water.

Also, with all the water it takes to wash out all the slime in the cooler/bucket, why not line the cooler/bucket with a large trash bag?!

Bingo!  You Da Man, Howard!!!!!!

Making sure they are getting only Herring and not smelt/shad/perch/baby stripers 

I think it's this.  Sometimes there are baby salmon or other no-take species in there.

Why do some of the herring guys on the piers dump their nets next to their cooler just so their friend can pick the herring up one by one and toss them in the cooler, rather than emptying the net directly into the cooler?

I've seen this several times and it always baffles me.
-Andrew

https://www.youtube.com/watch?v=FUQeQS_WMNc&feature=youtu.be

I always go straight into the bucket or cooler unless there is debris or other species, but even then it's easier to sort in a container then having to gather a bunch of bait flipping around.
Castnets are great at catching bait, but they also catch lots of other species as bycatch.  I have caught salmon smolts several times over the years.  I always try to get them safely released as quick as possible.