50km Asteroid or Siberian Traps v2.0
An enormous event that completely clouds over the surface of the Earth for decades, perhaps centuries. Primary productivity of land plants and surface plankton stops completely. All land-based multi-celluar life (excepting fungus) ends.
The oceans, on the other hand, have some better options. First, they have pockets of life where primary production is not provided by the sun. Second, there are extensive eco-systems that rely mostly on dying biological waste from above. Lastly, that dying biological waste will last a long time on anoxic deep sea floors.
Several vertebrate fish are candidates to survive in this scenario. Depending on the extend which which deep sea ecosystems stay intact, there are a great number of medium-small predatory deep sea fish. There are long-living scavengers like hagfish and sleeper sharks who may be able to survive for decades on rotting fish remains. Finally, there various eels and ophidiiformes that live on the ecosystems of black smoker vents and will almost certainly survive.
Humans are too widespread and too adaptable to not survive longer than any other surface dwelling vertebrates. It will have to be a whole-land extinction that removes humans, and leaves specialist fish around.