This is the 12400th most frequent Russian word.
каспийских
"Каспийских" means "Caspian" (relating to the Caspian Sea).
In this sentence, 'Каспийских' is used as an adjectival modifier to describe 'морей' (seas), referring specifically to the seas connected to or that are part of the Caspian region.
Глубина Каспийских морей поражает исследователей.
The depth of the Caspian seas astonishes the researchers.
Here, 'Каспийских' modifies 'рыб' (fish), attributing the fish species that are found in the Caspian region.
Каспийских рыб ловят местные рыбаки.
Caspian fish are caught by local fishermen.
'Каспийских' describes 'горах' (mountains), specifically indicating that these mountains are part of the Caspian geographical area.