Until recently, scientists and fishermen had few ways to learn much about the migration of cod. Hundreds of fishing communities depended on the arrival of cod migrating from wintering areas offshore, yet the routes and the nature of this migration have been largely hidden from view. Tag returns and differences in the timing of cod's arrival inshore were almost the only way we could gain any insights into this aspect of cod behaviour.
However, the development of hydroacoustics, or high resolution echo sounders, has made it possible to locate and then follow schools of migrating cod, and to study their behaviour in relation to water conditions and the presence of prey. Sounders work by sending out a stream of high frequency pulses of sound, then interpreting the echo that bounces back from objects in the water. Sound will reflect from any surface at which there is an abrupt change in density. The sea bottom, obviously, is one such place, but so are the bodies of fish. Actually it is the swim bladder, an air sack which helps a fish maintain buoyancy, rather than the body itself, which reflects most of the echo the instruments pick up.
Acoustics made it possible to test a computer simulation of cod behaviour which predicted that cod migrating inshore from the Continental Slope would follow the "saddles" which cross the Northeast Newfoundland Shelf. These deeper pathways, normally filled with warmer, saltier "slope water", would enable cod to avoid the colder water which covers most of the Shelf. There are three such saddles between southern Labrador and the Avalon Peninsula.
In June, 1990, an acoustic survey of the southernmost of these found the largest school of cod ever documented in our waters. It covered an area of roughly 20 by 30 km, and contained an estimated 500,000 tons of cod. This was more than 80% of the estimated northern cod biomass at that time. The chartered research vessel Gadus Atlantica stayed with these fish, tracking their movements and continually plotting the position and dimensions of the school for the next two weeks. Unfortunately, the charter expired before the fish reached the coast. However, technicians had tagged several thousand of these fish. Tag returns suggest they moved right to shore and then dispersed, mostly toward to north, and returned offshore in the fall.
The following year, two research vessels tracked a smaller school of cod in the same area during the same period. However, 1991 was an exceptionally cold year, and the migration was delayed by about a month. Once again the charter expired before the migration was complete. However, cod were following almost exactly the same path as they had the year before, a route known by this time as the "Bonavista Corridor". About 10,000 cod were tagged, and again the tag returns showed a pattern of dispersal along the coast, followed by a return to the offshore in the fall.
In 1992, encouraged by having found cod in the predicted pathway for two years running, researchers set up current meter moorings and temperature - recording instruments along the corridor prior to the migration. A research vessel surveyed the eastern approaches to the saddle early in the season, and found a school of spawning cod just outside. About half the females had not yet spawned. Detailed acoustic transects across these fish over the next few weeks produced some striking echograms of spawning, migration and feeding behaviour.
For the first 10 days, the cod remained fairly stationary, packed into a very small area at a density greater than one fish per cubic metre. During the day, multiple pairs of fish would rise in "columns" above the main school, apparently as part of their courtship or spawning behaviour. Within a few weeks, 95% had completed spawning, and the structure of the school changed. The fish spread out, and were joined by smaller, immature cod which moved in from deeper water.
As the migration got underway, the school lifted off the bottom and spread out to cover an area about 20 km long by 50 - 150 m wide. Acoustic calculations give a density of about one fish per 75 cubic metres of water at this stage, compared to about one per cubic metre when the fish were spawning. Given the dim light available at the depth at which they were travelling (about 350 metres), the cod apparently spaced themselves at intervals just within the range of visibility. This would maintain the coherence of the group through visual contact, while maximizing the area swept for prey as they moved toward the shore.
The migration appeared to be led by scouts. Systematic sampling with a bottom trawl confirmed that fish at the head of the school were larger than those farther back. On several occasions they veered sharply off course in pursuit of shrimp or capelin. In each case the whole school followed.
The research cruise of 1992 produced an exceptionally rich harvest of data on cod migration and feeding. Because of the oceanographic instruments deployed earlier, sounder data on cod behaviour can be studied in the context of detailed knowledge of the temperature, currents and salinity of the water in which they were moving. Results show that migrating cod seem to follow pathways determined chiefly by water temperature and bathymetry, and that they achieve a high degree of spatial precision in their migrations.
The observations raise the question of why cod form such large migrating schools. One possibility is that larger schools maximize the chance of finding food. Another is that younger cod, which join the older fish after the latter have spawned, may need to learn the migration route from their elders. If older fish maintain migration routes and behaviour, we may anticipate some changes as a result of the loss of older fish when the stock collapsed.
The results of acoustic surveys in 1993 and 1994 were severely constrained by this disappearance of fish. The large school found in the Bonavista Corridor in 1992 was virtually absent by the following year. Almost the only adult cod found in the acoustic survey in 1993 were far to the east and south, beyond the 200 mile limit. This suggests that the fish studied acoustically in 1992 may have moved to the Nose of the Bank in the autumn and fallen prey to foreign trawlers that winter. However, it may also be that a much higher than normal proportion of the stock has remained inshore since 1992, escaping the notice of offshore trawl and acoustic surveys.
The acoustic survey in 1994 found no cod in the Bonavista Basin, at the entrance to the migration corridor. It found none on the Nose of the Bank or on the western Flemish Cap, but found a small school of mostly juveniles in St. Anthony Basin and Notre Dame Channel. It found a large but low density school of juveniles and adults in the Hawke Channel, off southern Labrador. For the first time since this work began in 1990, no high density concentration of adults was found a all.
However, it is worth noting that the largest numbers found in 1994 were off southern Labrador, and that almost half of them were over four years of age, compared to less than 15% in the schools found farther south. This was the first time the spring acoustic work extended to southern Labrador, but annual groundfish trawl surveys in the autumn have detected almost no cod in that region in the last few years. Their presence during the acoustic survey in 1994 was hard to interpret. Does it mean that cod are returning to that area, or that some were there all along, despite the inability of groundfish surveys to find them? In either case the presence of mature cod in that area, upstream from the rest of the northern cod habitat in terms of currents and the drift of eggs and larvae, is clearly welcome news.
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