COD AND HYDROGRAPHY 

 A Review 



Scattered through the literature of 

 fishery science is much information on the 

 relation between cod and hydrographic condi- 

 tions where they live. In some instances, 

 notably the water temperatures frequented 

 by cod of marketable sizes, there are many 

 papers on the specific relationship. But 

 often only the results of single experiments 

 or isolated observations have been reported. 



This paper is an attempt to organize 

 and summarize the information available, 

 both in the longer works and in the scat- 

 tered bits of data. It is without doubt 

 incomplete, but may perhaps serve as a 

 starting point for the conducting of experi- 

 ments or the making of observations. It 

 may prove useful in helping the researcher 

 who is not completely familiar with the 

 literature to interpret observations made 

 by himself or others. 



All papers quoted have been read by 

 the author, with one or two exceptions 

 where sources that seemed too important to 

 omit were unavailable. These exceptional 

 instances are noted in the bibliography 

 along with the secondary source from which 

 the information was actually derived. 



For the sake of uniformity and to 

 accord with modern practice, temperatures 

 have been rendered in the Centigrade system. 

 Where conversion from Fahrenheit has been 

 necessary, the calculation has betr carried 

 to the first decimal place; otherwise tem- 

 peratures are quoted as given in the origi- 

 nal. If the temperatures are given in whole 

 degrees they are so stated without the addi- 

 tion of a decimal point and zero or zeroes. 

 If the paper cited gives temperatures to 

 the nearest tenth or hundredth degree, they 

 are so quoted. This makes for an appearance 

 of inconsistency, particularly in tables, 

 but so long as different systems of mensura- 

 tion prevail, inconsistency is inevitable. 



Salinities are handled similarly, 

 quoted exactly as given, except when they 

 have been expressed in units of specific 

 gravity or density. These latter have been 

 converted to salinities in parts per thou- 

 sand (°/oo) by the use of Knudsen's Tables 



(Knudsen 1901) to the nearest hundredth of 

 a part per thousand by weight of salt. In 

 one or two older papers, salinities have 

 been cited in percent; these have been 

 changed by moving the decimal point. 



As a convenient method of handling 

 the information and relating hydrography to 

 the cod, the various phases in the biology 

 of the fish are taken up in turn. Under 

 each segment of the life history the appro- 

 priate hydrographic phenomena are considered. 



SPAWNING 



Temperature 



Authorities agree that the spawning 

 of cod is influenced by temperature. 

 Tining has described on more than one occa- 

 sion (1949, 1953) a warming trend in West 

 Greenland waters during recent years which 

 has been associated with an increase of 

 spawning in that area, and Thompson (1943) 

 mentions that in eastern Canada the cod 

 spawn earlier in the year in areas where 

 the water is warmer. 



Fridriksson (1949) recounts a change 

 in spawning habits at Iceland — from the 

 earliest recorded times until the 1920's 

 cod spawned only off the south and south- 

 west coasts, moving to the colder waters of 

 the northeast and east coasts after spawn- 

 ing season. However, in the early 1930' s 

 cod were found spawning all around Iceland 

 as the waters became warmer. One effect of 

 temperature on spawning, and indirectly on 

 the fishery in this case, was that yield 

 declined even though the numbers of fish 

 probably increased, because the fish were 

 more dispersed during the spawning season. 



The consideration that temperature 

 per se at spawning time may not be the 

 critical factor for spawning success has 

 been advanced by Eggvin (1934), Sund (1936), 

 Poulsen (1944), and Heegaard (1947). Hee- 

 gaard and Sund agree that spawning in some 

 areas coincides with great variation in 

 temperature in the water column, and con- 

 tend that it actually takes place in the 



