My short answer is this:
For wild fish, not overfished, not endangered species.
For wild fish, minimum bycatch, or minimize damage to the ocean.
For farm-raised fish, a minimum environment to where the fish is raised.
(this is my simplified version, NOT AN OFFICIAL definition.)
Long answer here.
"Does sustainable fish taste better?"
On attendee asked me during our sushi class.
"No. It's not about the taste. It's about how we catch, not overfish and farm-raised in a way that is kind to the environment," I replied.
After my answer, I realized I did not fully understand the exact definition of what sustainable fish was.
One of the reasons was because I never intended to use only sustainable fish to begin with. It just happened, well, sort of. That was my excuse.
Around 2009, My chef friend introduced me to Monterey Fish Market in San Francisco Pier 33. (The name "Monterey" came from Monterey Street in Berkeley, where their original retail store was located.) That was when I learned about the Seafood Watch guide by Monterey Bay Aquarium. Paul Johnson, the founder of Monterey Fish was a board member of the Seafood Watch Guide.
When I asked Paul about his involvement in the Sustainable Fish, this is what he told me.
"When we started, the only fish we bought was local fish. It so turned out that the best fish were caught by hook & line by small local fishermen. So, later one, ten-fifteen years later, everyone jumped on the bandwagon and said, "Oh, local hook and line fish is good and sustainable, " but that was the way we always did. We were slow in expressing the fact that they were Sustainable because that was all we ever bought. We thought that was the only way." (his full interview is here)
So, now I knew about sustainable fish, it made me curious. I knew in Japan, locally hook & line and locally caught fish were considered as premium, so it made me a sense to start using sustainable fish for Breakthrough Sushi.
I looked at the copy of the Seafood Watch Guide and looked at it. It was clear which fish to use based on sustainability, but then, I couldn't figure out why some fish were sustainable while others weren't. For example, Tuna caught in the Indian ocean was not sustainable but was sustainable if it came from the Pacific Ocean?
When using the Seafood Watch guide, there are a few things you need to know about fish. They are:
Whether the fish is wild or farm-raised
If wild, where it was caught
If wild, how it was caught
Then you look at the guide and determine its sustainability.
It's all tricky, right?. To make things more complicated, there are other organizations like Ocean Wise, WWF has a sustainable seafood guide, NOAA Fisheries has an explanation for their definition of what sustainable fish is.
Marine Stewardship Council has this.
From stocks with healthy populations;
With minimal impact on the marine environment;
In an area with effective, responsive, and responsible management.
To a consumer, I understand this can be confusing.
So, after going through many organization's websites, I came up with a simpler version of what makes fish sustainable.
Now, this is NOT AN OFFICIAL definition.
Sustainable fish is:
For wild fish, not endangered species.
For wild fish, minimum bycatch, or minimize damage to the ocean.
For farm-raised fish, a minimum environment to where the fish is raised.
1. For wild fish, not endangered species.
I hope this is clear to understand. Most of Bluefin Tuna is endangered. So is Unagi, Fresh Water Eel due to overfishing. And the key part is where the fish is caught, because according to the Seafood Watch Guide, at the time of this writing, some of Atlantic Bluefin tuna is sustainable.
2. For wild fish, minimum bycatch, or minimize damage to the ocean.
If you ever heard of "Dolphin safe" tuna, that is what "bycatch" means (technically, not really, but again, I am making this simple.) What it all means is so that we only catch what we intend to catch, or if we end up catching, Dolphin and Tuna release dolphins from the net. Minimum damage to the ocean is something like won't destroy rocks, seaweeds at the bottom of the sea with the fishing net.
3. For farm-raised fish, a minimum environment to where the fish is raised.
This means when you farm-raised Salmon, keep the ocean clean is what it means. Also, it has to do with the "healthy" feed to the fish- something similar to USDA organic standard, where there are no antibiotics and so on.
Well, I hope it made sense to you? Even if it didn't, that is OK. That is what the Seafood Watch Guide is for. All the scientists, researchers, are working hard to update it constantly so that all we need to do is to lookup using the guide.