Recoveries of 477 (11.8%) of 4,048 tagged parasitic-phase sea lampreys showed that lampreys moved extensively and that interchange between the Great Lakes was common. Of 93 recoveries from 2,265 tagged lampreys released below the navigation locks in the St. Marys River, 80 were in Lake Huron, 12 in Lake Superior, and 1 in Lake Erie; of 359 recaptures from 1,666 released in northern Lake Huron, 331 were in Lake Huron, 25 in Lake Michigan, and 3 in the St. Marys River; and of 25 recaptures from 117 released in Lake Michigan, 21 were in Lake Michigan and 4 in Lake Huron.
Of the 477 sea lampreys recovered, 209 (44%) were captured more than 15 km from the locality of release; 42 had traveled 160-628 km (average, 275 km) before they were recaptured.
Among recaptures from sea lampreys released in northern Lake Huron, 240 (67%) were recovered within 15 km of the release point in less than 50 days after release; only 12 were recaptured within this radius after 50 or more days of liberty.
Probability of recapture was not influenced by size among sea lampreys longer than 205 mm (total length), but the lampreys recaptured at the greater distances from the tagging localities tended to be large. All of 34 lampreys recaptured more than 150 km from the point of release were longer than 254 mm (average, 418 mm) when tagged and 9 recaptured more than 300 km away were longer than 354 mm (average, 441 mm).
Four sea lampreys 345-445 mm long when tagged in November or early December were recovered in mid-September of the following year, at least 2 months after the end of the normal spawning season. Suggested possible reasons for this unusual occurrence are the extension of the parasitic phase of the life cycle beyond the usual 12 to 20 months, late-season maturation of the gonads, disease, or deleterious effects of the tags.