What is the fine-grained topic of the following text: Dr Gordon began his research in the shallow waters of the Firth of Lorn but, with the commissioning of the RRS Challenger in 1974, he moved into deeper waters (500 to 1,500m) to the west of Scotland’s continental slope, working on the biology of the bottom-living fish which were later to be commercially fished. Only return the topic.
Dr. Gordon's research on the biology of deep-water bottom-living fish in Scotland.