[2][4] She had a cruising speed of 7.75 knots (14.4 km/h; 8.9 mph)[3] or 9 knots (17 km/h; 10 mph)[4] (sources disagree), and her large fuel capacity gave her a cruising range of 8,000 nautical miles (15,000 km), a longer range than commercial fishing boats of her size made necessary by the vast size — 13,000,000 square miles (34,000,000 km2) — of the ocean area in which she was to operate and the limited refueling options in the area.

[4] After her commissioning, John R. Manning departed Seattle, Washington, on 21 February 1950, called at San Pedro, California, then proceeded to her home port, Pearl Harbor, Hawaii, from which she operated in support of the FWS′s Pacific Oceanic Fishery Investigations (POFI).

[1] POFI tasked her to search for commercially viable populations of fish around the Hawaiian Islands and in the waters of the Pacific Ocean and the Philippine Sea between Hawaii and Palau in cooperation with the United States West Coast fishing industry, as well as to test alternative capture methods because of the lack of live bait in the open ocean.

[4] Her crew reported disappointing results with purse-seining and gillnetting, but also that the use of modified longline gear yielded a 40 percent increase in the take of yellowfin tuna.

[4] John R. Manning underwent an overhaul at Seattle in early 1963, during which shipyard workers discovered a substantial dry rot problem which required the replacement of entire planks and timbers.

[4] After completion of these repairs, she had an eventful year in 1963, engaging in exploratory scallop fishing in the Gulf of Alaska and later carrying scientists on an expedition to tag king crabs around Kodiak Island.

[4] During most of the 1960s, John R. Manning conducted halibut and other bottomfish surveys and fisheries patrols, including observation of foreign fishing activities in the Bering Sea.

[4] Her patrols took place mostly in the Gulf of Alaska,[17] and a 1964 BCF publication on foreign fishing activities in the Bering Sea and Gulf of Alaska reported very negatively on her patrol work,[17] describing her crew as "inept"[17] and the vessel herself as "inadequate,"[17] concluding that she was "severely lacking as a law enforcement vessel"[17] and that her "very presence among the most modern fishing fleets in the world is damaging to US prestige,"[17] and recommended her replacement.

In 1967, she supported the BCF's Exploratory Fishing and Gear Research (EF&GR) program by assessing the mid-water populations in the Bering Sea of Alaskan pink shrimp, using echo-sounding transects and test drags with a Cobb pelagic trawl to locate schools of Alaskan pink shrimp at night well above the sea floor in inshore waters.