Surface water becomes warm in the summer but lower layer water just below the surface remains cold whereby warm water fish such as sardine, mackerel and sea bream is caught in the area close to the surface while cold water fish such as cutlass fish and cod is caught in deep waters and on ocean floor.
表層水は夏に高温になるが、わずか下層では寒冷となるため、表層でイワシ、サバ、鯛など温暖水魚がとれ、深海や海底ではタチウオ、タラなどの冷水魚がとれる。