What's interesting about this is that tuna are not typically fished in those waters. So the recovery is not simply because the tuna themselves are not being fished. This boost is stock is specifically a response to the protect of the entire system rather than just the species itself.

Because of dolphin safe tuna regulations, relatively few tuna are fished in the eastern tropical pacific tuna region (IATTC), where this MPA resides. In the IATTC, most tuna fishing was done via sets on dolphins. These types of sets have extremely low bycatch with a high concentration of large adult tuna, but that bycatch is primarily spinner dolphins. So, most tuna fishing is done in the WCPFC (western and central pacific) region (especially western) region. Although this region has plenty of dolphin bycatch, as well as many other species, the sets are done against floating objects rather than dolphins which automatically makes them dolphin safe.

(I used to work here a long time ago. I have not kept up to date on the industry, but most of these policy implications have not changed much in that time.)