"Dolphins are a common by-catch in tuna fisheries, especially in the Eastern Tropical Pacific Ocean, as they commonly swim with schools of yellowfin tuna. The dolphins, who swim closer to the surface than tuna, may be used as an indicator of tuna presence. Labeling was originally intended to discourage fishing boats from netting dolphins with tuna.
The tuna fishery in the Eastern Tropical Pacific is the only fishery that deliberately targets, chases, and nets dolphins, resulting in estimates of 6-7million dolphins dying in tuna nets since the practice was introduced in the late 1950s, the largest directed kill of dolphins on Earth.[9] With the onset of the Dolphin Safe label program, started in the US in 1990 but soon spreading to foreign tuna operations, the deaths of dolphins has decreased considerably, with official counts, based on observer coverage, of around 1,000 dolphins per year.[9] However, research by the US National Marine Fisheries Service has shown that chasing the dolphins causes baby dolphins to fall behind the pod, resulting in a large "cryptic" kill, likely damaging populations of dolphins, as the young starve or are eaten by sharks while the main pod is held by the nets.[10][11] Thus, claims that tuna fishing can continue to chase and net dolphins and not cause harm are not backed by scientific research."
This is not a problem with random by-catch. The issue was the systematic netting of dolphin pods due to their close association with tuna.