I've fished this Lake along with the river that flows down through Medford in the summer you can catch plenty of bass ... Good veg.
Note that there are parts of this description referencing mystic on the cape, not the one in medford
Mystic Lake, 149 acres, has an average depth of 20 feet and a maximum depth of 42 feet. Mystic Lake and Middle Pond (total area 257 acres) are two natural kettlehole ponds connected by a shallow channel and have similar characteristics. The bottom of these ponds are primarily sand with scattered areas of gravel and rubble. Transparency is very good, extending to 18 feet, and aquatic vegetation is sparse. The ponds 4.7 miles of shoreline (Mystic Lake =2.5 miles, Mystic Pond=2.2 miles) are moderately developed with seasonal and permanent residences. These ponds together with Hamblin Pond (just to the south) are collectively known as the Indian Ponds or Cotuit Ponds. The ponds are fed by groundwater and cranberry bog drainage and serve as the headwaters of the Marstons Mills River. Access: Mystic Lake and Middle Pond are located just west of Route 149 and just north of Hamblin Pond. Public access to Mystic Lake is limited to a small roadside area off Race Lane and through the connection to Middle Pond. There is an unimproved town of Barnstable ramp located at the south end of Middle Pond suitable for canoes, cartop boats and possibly lightweight trailered boats. To get to the access take Mistic Drive from Route 149 around the south end of Hamblin Pond. The access is on the south end of Middle Pond, at the end of Flume Avenue. There is a 10 horsepower limit on boat motors enforced by the town. The channel which connects the two ponds is only about a foot deep. As a result, those planning to fish Mystic Lake should plan on using a canoe or very shallow draft craft. This shallow access keeps recreational pressure on Mystic Lake quite light, even during the summer. Fish Populations: largemouth bass, smallmouth bass, chain pickerel, pumpkinseed, white perch, yellow perch, white sucker, banded killifish, brown bullhead, golden shiner and alewife (sea-run). The alewives run up the Marstons Mills River from Nantucket Sound via Cotuit Bay.
Due to the poor access and relatively light fishing pressure (particularly on Mystic Lake) these two ponds should offer some excellent fishing opportunities. At the time of the last survey both smallmouth and largemouth bass populations were healthy and there were considerable numbers of fish in the 12 to 18 inch range. The best resource in the ponds at the time of the last sample was a bonanza of yellow and white perch. Large numbers over 10 inches long were recorded.
The Mystic Lakes, consisting of Upper Mystic Lake and Lower Mystic Lake, are closely-linked bodies of water in the northwestern suburbs of Boston, Massachusetts.