I'm not so sure. Big brain not the same as 'intelligent'.

A story: every year a Japanese dog food company goes to a certain island lagoon where dolphins migrate to reproduce. They net and kill thousands and take them away.

Each year some dolphins escape, inevitably. They swim around for a year, and then the next year come back (with many other dolphins) to the lagoon, and likely get netted and killed.

Didn't they remember what happened? Didn't they communicate it to other dolphins? Don't they care?

I have to conclude either

   Dolphins are not smart enough to remember from year to year,

  or Dolphins can't communicate with other dolphins in a sophisticated enough way to say "danger! evil here!" which is pretty basic,

 or Dolphins are dicks and don't care if other dolphins are killed,

 or heck, maybe Dolphins, like Klingons, enjoy the challenge? "Today is a good day to die!"

I conclude that, in none of these scenarios, is there much wrong with killing dolphins for dog food. Not in a 'killing our sentient brothers!' way.

Many humans get killed every year in driving accidents. Every year humans continue to drive. Didn’t they remember what happened? Didn’t they communicate it to other humans? Don’t they care?

If you spend some time, I bet you can think of lots of reasons a creature might return to the same place. Many humans engage in self defeating behavior, should they be made into dog food, or should we empathise with them?

Without communicating with others to warn them of any danger. So, no dolphin cares about that? If they don't care, should we?

I think its pretty cut-and-dried, dolphins are nowhere near as bright as humans.

But does that mean they are not intelligent sentient beings?

Dolphins do communicate with each other, they use vocal communication and body language. Our failure to understand it doesn't invalidate its existence.

They form lifelong familial bonds. They have friends and enemies and form complex social structures.

They play regularly and have advanced hunting tactics that require a high degree of coordination.

They even take recreational drugs.

I have spent a significant amount of time in the water with dolphins throughout my life, and I have developed relationships with dolphins I regularly see that are at the very least as complex as the one you have with the barrista that makes your coffee every day.

If the cutoff for intelligence is human intelligence, which human is the standard?

My point is brightness or intelligence shouldn’t be the measuring stick for compassion for living things.

Right. But that goes for everything? When are we required to get concerned? Just mammals? All the way to bugs?

There's lots of grey area, and it all ends up being personal preference, but I think if one makes it a goal to always empathize it's a great starting point.

Oh yes, the slaughter of dolphins is tragic and wrong. Not because they are our 'brothers', but because they have value as part of the whole ecosystem.

I'm pretty sure our grandkids will have much worse things to learn about our time.