THE BIOLOGY OF OITHONA SIMILIS IN THE GULF OF MAINE AND BAY OF FUNDY

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Abstract
1. Oithona similis, although almost world-wide in distribution, is centered largely north of Cape Cod on the western Atlantic coast, and south from this point is gradually replaced by O. brevicornis. 2. The adult stock in the Gulf of Maine appears to approach the coast from offshore with rising temperatures in the spring. 3. Propagation begins in the Gulf in March and continues in a succession of generations until September. 4. Two breeding stocks are distinguishable, due to an average difference of about one month in the time of vernal propagation in the eastern and western parts of the region. This difference is reflected in subsequent generations after dispersal. 5. There appear to be four broods of the western stock, in March, May, July, and September, and three in the eastern, in April, June, and August. 6. A developmental period of two months during the early season and about six weeks in summer is indicated. 7. Oithona is at times far more prolific than either Calanus or Pseudocalanus, but mortality is relatively much higher, depletion being greatest in nauplius stages. 8. The western stock remained for the most part concentrated offshore, but the eastern, particularly in advanced stages, was more generally distributed, sometimes being more abundant inside than outside of the 100-meter contour. 9. There is some evidence that in June the local stock in the inner Gulf is supplemented by important contributions from another production area, probably in the vicinity of the outer banks. 10. There is no indication of successful propagation in the turbulent coastal region of the Gulf east of Mt. Desert or in the Bay of Fundy. 11. The larvæ of Oithona afford the most abundant source of food in the region in mid-summer for those animals feeding on larval planktonic organisms. Of added importance in the natural economy of the region is the fact that the seasonal maxima of the three numerically dominant species occur at successively later periods during the summer, Calanus in May, Pseudocalanus in June, and Oithona in August.

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