Commercial recaptures of 26% (22.5% in years after the tagging year) of 18,822 Atlantic cod (Gadus morhua) 50 cm or more in length that were tagged mainly in the feeding season at 13 localities in the Newfoundland area in 1954–55 elucidated migrations and intermingling of cod of various stocks or stock complexes. Cod of the northern Gulf of St. Lawrence stock migrated southward from the Strait of Belle Isle to winter on the northern side of Cabot Strait and vicinity, mingling there with the Burgeo Bank stock. Cod of the Newfoundland–Labrador complex tagged at three sites mingled along the east coast of Newfoundland and off southern Labrador and with cod of the northern Gulf stock near the Strait of Belle Isle, of the Avalon–Burin stock complex off the Avalon peninsula, and of a Grand Bank stock on the northern part of the bank. Though cod tagged off the Avalon peninsula and on St. Pierre Bank were recaptured mainly near the tagging sites, some moved as far north as Labrador, to the southern Grand Bank, and to Cabot Strait. Fish tagged on the northwestern Grand Bank were recaptured mainly at the tagging site but also elsewhere on the bank and off the east coast of Newfoundland. Those tagged on the Southeast Shoal of the Grand Bank were recaptured mainly on the southeastern part of the bank though some moved as far as southeastern Newfoundland. Those tagged on Burgeo Bank mingled with the Avalon–Burin stock complex, including the St. Pierre Bank component, but mainly with the Northern Gulf stock in winter–spring near Cabot Strait.Cod 90–129 cm long at tagging were recaptured at shorter distances from the tagging sites than 50–89-cm cod. Tagging–recapture distances also increased with increasing age for cod of the smaller length ranges on tagging. In their vertical movements, most tagged cod moved to shallow water in late spring or early summer and retreated gradually to deeper water from late summer to early spring. Some remained in deep water throughout the year.Recapture rates at different size ranges varied with area but were not greatly different at length ranges from 50–59 to 80–94 cm, which included 95% of the tagged cod.

A total of 416 cod vertebral samples collected from the Newfoundland and adjacent areas during 1947-71, are analyzed in relation to their use for delineating the cod stocks of the region. Differences greater than two vertebrae were found between the highest and lowest vertebral means for different stocks. The overall vertebral mean for females exceeded that for males by 0.10. The main portion of the Labrador-East Newfoundland stock (Div. 2G to 3K) was well separated from the southern Grand Bank stock (Div. 3N and 30) by the presence in the northern area of high vertebral means and of higher percentages of high vertebral numbers and the lack of low vertebral numbers. This contrasted with vertebral means about one to two vertebrae lower in the southern Grand Bank stock, with higher percentages of low vertebral numbers and a lack of high vertebral numbers. Cod from the offshore area in Div. 2G to 3K had somewhat higher vertebral numbers than those of the inshore area. The high vertebral-count cod of the Labrador-East Newfoundland stock also extended to the northern slopes of Grand Bank and around the Avalon Peninsula (Div. 3L), but in this area they usually intermingled with cod of the Avalon stock complex and with some migrants from southern Grand Bank as indicated by vertebral means intermediate between the northern and southern extremes. In the Labrador-East Newfoundland stock, vertebral means were slightly lower in 1961-71 than in 1947-60. Vertebral means for cod off West Greenland were considerably lower than those for Labrador cod and similar to those for the southern Grand Bank stock. On St. Pierre Bank (Subdiv. 3Ps), some samples had as low vertebral means as those on Grand Bank whereas other samples had intermediate vertebral means similarto those in the adjacent coastal area. The northern Gulf and southern Gulf stocks, north and south of the Laurentian Channel in the Gulf of St. Lawrence (Div. 4RST), had low vertebral means, except for evidence of the intrusion of some cod with high vertebral numbers in and near the Strait of Belle Isle. Vertebral means declined from north to south on the Scotian Shelf (Div. 4VWX) and to Georges Bank (Div. 5Z). Spawning times of cod in various parts of the Northwest Atlantic are reviewed, and vertebral numbers for the different areas and stocks are related to the surface temperatures and current drifts following spawning. The lowest surface temperatures during egg and early larval development were associated with the longest period of egg and larval drift, the highest vertebral means, and the largest, most-widely distributed stock.

Haddock otolith lengths decline from over 5% of the total length of the fish at 14 to 17 cm. to almost 3% at 74 to 77 cm. The otolith lengths of slow-growing haddock decline less rapidly than those of fast-growing haddock. Little difference was found between otolith lengths in male and female haddock of the same length.Slow-growing haddock both from the Grand Bank and from St. Pierre Bank have heavier otoliths than fast-growing haddock at the same fish lengths. This is true not only for fish of different year-classes in the same year but also for fish of the same year-class in different years.The otoliths of male haddock of the Grand Bank exceed progressively in weight the otoliths of females of the same length after the males become sexually mature. The relative increase in otolith weight of the male fish is attributed to a probably slower growth rate after sexual maturity, which occurs earlier than in females. Similarly, the heavier otoliths of mature fish in sizes overlapping with immature fish are due to a greater age of the mature fish at these sizes. Otolith weight relative to fish weight decreased considerably with increase in fish size, and no distinct differences were apparent between otolith weights as a percentage of fish weights in slow-growing and fast-growing fish of the same length.

The lumpfish, Cyclopterus lumpus, was found to be a common intermediate host of the larvae of the copepod, Lernaeocera branchialis, in the Newfoundland and neighboring areas. Large numbers of these larvae occurred on the gills of lumpfish from inshore Newfoundland areas from the latter half of June to the first half of August, whereas only minor infection was found in any month in offshore areas. Some larvae of the year reached the final or seventh stage on the intermediate host in May on the west coast of Newfoundland and in June on the east coast. By July on the west coast and early August on the east coast, this was by far the most numerous stage present. The larvae were attached mainly near the tips of the gill filaments. Most larvae were attached to the gills of the first two branchial arches, less to those of the third, and much less to those of the fourth. There were more larvae on the right than on the left gills. Infection of the final host Atlantic cod (Gadus morhua), by stage-7 female L. branchialis was apparently mainly inshore.Adult L. branchialis on Atlantic cod, typically located near the anterior ventral apex of the branchial arches, were numerous throughout the year in the areas of high infection. Infection rates of cod with the adult parasite were usually considerably higher near the coast than farther offshore, the rates declining with distance from the coast. Rates of infection with the adult parasite usually increased from the smaller to intermediate cod lengths and declined rapidly at greater lengths. Infection rates were found useful as evidence of inshore and offshore migrations of cod. Infection with the copepod apparently delayed sexual maturity in cod. Most infected cod (86%) had one adult copepod and declining numbers (12–0.05%) possessed two to five adult copepods. The Greenland cod, G. ogac, was the only other fish of the area found to be highly infected with adult L. branchialis.

Observations made as early as 1947 and data collected in 1953 to 1954 and 1956 to 1957 indicate that there is considerable variation in the size and appearance of the drumming muscles of the swim-bladder of haddock with ffsh length, sex, sexual maturity, and season on the Grand Bank and St. Pierre Bank. For haddock of the same length the data indicate that the drumming muscles of mature male haddock are larger than those of immature males and also of immaiure and mature females of the same length. In addition the data indicate that a periodicity is exhibited in the size of the muscles of the mature male, the muscles being nearly twice as large during the pre-spawning and spawning period in May and June as they are in October and November. This phenomenon does not occur in female haddock, the drumming muscles of which are considerably smaller than those of male fish of the same size and show no variation in size with the season.It is suggested that the difierence in size of the drumming muscles of male and female haddock can serye as a mearls of determining the sex of haddock landed in the gutted condition by commercial trawlers, at least for the mature ffsh which make up the bulk of Lhe commercial trawlers' catches. There is some difference between male and female haddock in the colour of drumming muscles, but this seems to be of secondary importance for distinguishing between the sexes.Male haddock probably use the drumming muscles chiefly to produce low-frequency swimbladder sounds whicl-r serve as rallying calls for female and other male haddock to join in spawning activities.