THE BIOLOGY OF PSEUDOCALANUS MINUTUS IN THE GULF OF MAINE AND BAY OF FUNDY

¹ From the Woods Hole Oceanographic Institution

1. Pseudocalanus minutus is generally distributed both in neritic and offshore waters. Having a somewhat greater thermal range it regularly outnumbers Calanus in neritic areas south of Cape Cod and on occasions in boreal waters also.

2. Propagation begins in March and continues in a succession of generations until September. It starts earlier in the southern part of the range where there are also a greater number of annual broods.

3. Eggs appearing at progressively later periods (approximately monthly intervals) in the outer Gulf, western area, and Bay of Fundy make possible a distinction of three major breeding stocks.

4. In the western stock there are at least three and possibly four breeding periods, March-April, May-June, July-August and September.

5. In the eastern stock there is evidence of three breeding periods, April-May, June-July, and August.

6. The outer Gulf stock is not clearly traceable due to probable overlapping of subsequent breeding periods with those of the eastern stock, but four periods are indicated.

7. There appears to be a developmental period of approximately two months throughout the region.

8. Pseudocalanus is very prolific and its early larvæ greatly outnumber those of Calanus. Mortality is relatively much higher in Pseudocalanus, the depletion being greatest in nauplius stages, although a second critical period during maturation is indicated.

9. There is no evidence of successful propagation in the Bay of Fundy, and the western area declines in importance after the first brood. Subsequent propagation of all breeding stocks appears to be centered in the outer Gulf.

10. The adult stock of Pseudocalanus is second only to that of Calanus in numerical importance, averaging 9.9 per cent of the total zoöplankton population in the Bay of Fundy during the year 1931-32 and 4.9 per cent of the summer population in the Gulf.