CLIMATE AND THE DISTRIBUTION OF MARINE ANIMALS 



325 



fluctuations in mackerel landings indicate tliat 

 this decline was almost certainly due to natural 

 causes rather than to fishing. 



After 1890 and to about 1925, there is a weak 

 tendency for the mackerel landings to vary with 

 temperature. After 1925 the landings are quite 

 out of phase with temperature fluctuations, al- 

 though both are at a much higher level than at the 

 beginning of the period. The correlation between 

 landings and temperature deviations for the period 

 1890-1954 is 0.207, which is about at the 10- 

 percent level of significance (0.211). 



The spawning of mackerel, on botli tiie eastern 

 and western sides of the Atlantic, appears to take 

 place from south to north as temperatures reach 

 a fairly well-defined critical level duriiig the vernal 

 warming (Allen 1897-99; Orton 1920; Sette 1943). 

 The length of the incubation period is greatly 

 affected by the temperature of the waters in the 

 surface layer. Sette reported that eggs hatch in 

 2 days at 70° F. and in 8K daj's at 50° F., and that 

 the rate of development of the larvae also depends 

 to some extent on the temperature. He estimated 

 the survival rate of the 19.32 j-ear class during its 

 planktonic existence to be in the order of magni- 

 tude of only 1 to 10 fisli per million of newly 

 spawned eggs. The period of planktonic life is 

 shortened in warmer waters, and there is experi- 

 mental evidence (Worley 1933) that the rate of 

 mortality is less at higher temperatures. There- 

 fore, relatively minor differences in the tempera- 

 ture of the waters in which mackerel develop may 

 produce wide fluctuations in the strength of the 

 year classes. Furthermore, the spawning of 

 mackerel in the western North Atlantic is largely 

 confined to a coastal belt 10 to 30 miles in widtli 

 (Sette 1943), where fluctuations in water tempera- 

 tures may be expected to show greater correlation 

 with land air temperatures than would water 

 temperatures farther offshore. 



The influence of sea temperature on the move- 

 ments of mackerel has been documented by Sette 

 (1950). With regard to the appearance of mack- 

 erel in the spring, and the northward advance, 

 Sette considered that temperature has a limiting 

 rather than a causal influence, water colder than 

 7° to 8° r. (44.6°-46.4° F.) acting as a barrier 

 (op. cit., pp. 292-294). With a general warming 

 of coastal waters, one might expect earlier arrival 

 and later departure dates. Unfortunately, data 

 on times of first arrival and corresponding tem})er- 



ature data are too fragmentary to determine this 

 directly, but possible indication that this has 

 occurred is shown in the monthly landing statistics 

 for Gloucester. Between 1901 and 1935, nolandings 

 of mackerel in April are shown in the Gloucester 

 landings. Between 1935 and 1950, April landings 

 of mackerel appear in 5 of tlie 15 years. Between 

 1901 and 1930, December landings of mackerel 

 at Gloucester appear in 9 of the 25 years for which 

 statistics are reported. From 1932 to 1950, 

 December landings appear in 14 of tlie 19 years 

 reported. Prior to 1939, no landings of mackerel 

 are shown for the montli of January. Between 

 1939 and 1950, January landings are reported for 

 7 of the 12 j'ears. 



We have no satisfactory hypothesis to explain 

 the great fluctuations in mackerel landings over 

 the period of record; however, it is rather clear 

 that the behavior of the mackerel is govern etl to a 

 considerable degree by temperature. Data cover- 

 ing fairly long periods of time show that the mack- 

 erel has responded to fluctuations and trends in 

 temperature, but in a maimer sufficiently complex 

 to suggest the operation of other, perhaps indirect, 

 factors. 



FLUCTUATIONS IN LOBSTERS 



Between 1940 and 1945, annual lobster landings 

 in Maine increased from 7.6 million to 19.1 million 

 pounds — a 250-percent increase and nearly three 

 times the average landings made during the 1920's. 

 Since 1945, the Maine catch lias averaged about 

 19 million pounds, a level not exceeded since the 

 1880's. The increase in lobster landings was 

 observed not only along the coast of Maine but 

 also in Massachusetts, in the Bay of Fundy, and 

 in western Nova Scotia over about the same 

 period. 



Such a widespread increase in lobster landings 

 suggests an environmental change making possible 

 the survival of greater immbers of lobsters to 

 catchable sizes. Tagging experiments in Maine 

 and length-frequency data collected by the Fish 

 and Wildlife Service over the period 1939 to 1947 

 indicate that mortality rates remained very con- 

 stant over the period of increase so that the in- 

 creased catch was caused by an increase in abun- 

 dance rather than by an increase in the amount of 

 fishing. The only change in the environment 

 known to have occurred was a general warming of 

 coastal and offshore water. We shall, therefore. 



