The migrations and concentrations of nektonic animals are governed largely by two biologic factors, reproduction and search of food. Mention need only be made of such truly phenomenal alternating wanderings as are witnessed in the salmon and the eel. Many other fishes also migrate, although the reasons for the migration of some of them have not been ascertained even in a general way. As a matter of fact, even in the
Among the great wanderers of the seas are the marine mammals. The habits of the Alaskan fur seal in returning to the Bering Sea to breed are widely known, and some whales migrate regularly to warm waters for breeding and between breeding times to the feeding grounds of the polar regions for food.
It will be realized from the above that any study of migratory animals and their habits as related to these biologic factors is complicated to the extent to which breeding and feeding grounds do not coincide.
It is well known among whalers and scientists that blue, fin, sei, and humpback whales feed wholly or partly upon planktonic life. Scientific investigations carried on in cooperation with whaling companies have shown, as might be expected, that the occurrence of plankton-feeding whales is correlated with the distribution of the planktonic life upon which they are known to subsist. In Davis Strait it was shown by Hjort and Ruud (1929) that young euphausiid crustaceans (the young are considered to reflect numbers of old specimens not so readily caught with vertically hauled nets) had their maximum concentration over certain coastal banks, and it is precisely in these waters that the maximum numbers of whales also occurred (fig. 244). The euphausiids tend to collect in swarms during breeding and the appearance of blue whales coincides with the periods of spawning of the dominant species in the region. Sperm whales, which feed largely on squids and not directly on plankton, are found to be present farther offshore, as indicated in the figure. The same authors compare the numbers of sei whales (Balaneoptera borealis) with the concentration of Calanus finmarchicus during the same period from the coastal banks. Figure 245 shows a striking correlation in the abundance of these animals, with the increase in Calanus preceding somewhat the appearance of the whales. Data also
Correlation of the catch of sei whales (Balaneoptera borealis) with the number of copepods. (After Ruud.)
Investigations by the Discovery likewise showed a positive correlation between distribution of blue and fin whales and abundance of zooplankton, especially with their favorite food Euphausia superba (Mackintosh. 1934, Hardy and Gunther, 1935).
Histograms showing (in black) the individual catches of herring in half-monthly periods arranged in each period from left to right in the order of ascending values of Calanus in the associated plankton samples. The average catches of herring are shown as shaded histograms. The left half of each series represents catches in the poorer, and the right half in the richer Calanus water as indicated in the graph for associated Calanus values. (From Lucas.)
Numerous other examples might be given showing the concentration of whalebone whales in coastal areas rich in plankton food, and their instinctive migrations into these waters at the swarming season of such forms as euphausiids is truly phenomenal. One is impressed with the efficiency of the metabolism of some of the large animals which feed upon the animal plankton. For instance, at birth the blue whales are about 7 m long and weigh about 2000 kg (4400 lb), and at weaning, seven
Among the pelagic fishes the best material illustrative of correlation with plankton comes from studies of the herring. It has long been believed that the movements and concentrations of these fish are associated with the relative concentrations of the zooplankton upon which they feed. Hardy, Lucas, Henderson, and Fraser (1936) have recently investigated the plankton by means of the plankton indicator (a quick-sampling device) and, in correlating the numbers of Calanus copepods of the plankton with the amount of herring caught by fishermen in the same areas, they have shown in most instances that the greatest number of adult herring are caught in Calanus-rich waters. Theoretically the gains derived through fishing exclusively in Calanus-rich waters may be as high as 21 per cent. In Fig. 246, from Lucas, is shown the general trend of the relation that exists when the Calanus series are arranged in ascending order of numbers and each half of the series compared with the average catch of herring for the period.
A negative correlation is found between heavy phytoplankton and herring. North Sea fishermen have observed this in practice, and designate the heavily diatom-populated waters as “weedy water” or “stinking water” and consider it a bad omen to fishing.