10 U. S. BUREAU OF FISHERIES 
Although no direct measure of mackerel abundance has been found, 
their availablity to the purse-seine fishery has been taken as roughly 
indicative of abundance under actual conditions. Of course, this 
fails to measure adequately the availability except where the fleet is 
fishing. The strictly shoreward dispersal of the species, and its 
distribution on distant banks, has not been measured. 
During the course of the investigation it was found that the make- 
up of the commercial fishery in any year was predominantly mack- 
erel of one or two year classes. Sometimes a single year class has 
been important to the catch for as long as 5 years. In other seasons 
the year class of most importance has been of a transitory nature, 
appearing for but one year and then dropping out of the catch quite 
rapidly. 
Observations indicated two principal factors that control mackerel 
abundance—the recruitment of new year classes to the fishery, and 
the losses due to mortality. Having obtained a measure of these 
factors, a series of fairly successful catch forecasts was begun in 
1931. In 1937, however, the catch by the vessel fishery fell about 
40 percent below the prediction. Since this circumstance indicated 
the operation of unknown factors, forecasts have been discontinued 
pending further research to discover them. 
Progress during 1939.—TYhe mackerel investigation in 1939 was 
under the direction of John R. Webster, who was assisted by Frank 
E. Firth. Analysis of the commercial landings showed three size 
groups in the mackerel populations during 1938—the year class of 
1937, the year class of 1936, and a mixed group of year classes from 
1934 to 1930. No one group dominated the catch for an entire year. 
All groups exhibited marked increases in availability compared to 
the previous year which averaged about 500 percent for the 1937 and 
1936 year classes, and more than 100 percent for the mixed group of 
older fish. 
The causes of such an increase in availability must be found in 
environmental conditions. To be caught by purse seiners, mackerel 
must be sighted while they school at the surface. Records of surface- 
water temperatures at lightships along the New England coast during 
the summer of 1937 were above normal, sometimes by as much as 7° 
F. It was at this time that the mackerel catch fell off the most. 
Temperatures during the summer of 1938 were near or slightly 
below normal. 
The spring purse-seine fishery shifts its activity from the Middle 
Atlantic area to the Gulf of Maine as rapidly as mackerel become 
commercially available there. The question arises as to whether 
abundance of the shore run of mackerel keeps pace with that of off- 
shore populations. Progress in solving this problem has been made 
through the assembling of tr ap-catch records for 1937 and 1938 cov- 
ering ‘the coast from Virginia to Massachusetts. The records indi- 
cate that considerable numbers of mackerel persisted in the Middle 
Atlantic region for at least 2 months after seers in the spring fish- 
ery quit the region. 
Field work in 1939, conducted principally by Mr. Firth, was con- 
fined largely to securing measurements, scale samples, and interviews 
with boat captains who landed catches. Data were secured in New 
Jersey and New York in April and May, and at the Boston Fish Pier 
