72 H. B. Hachey 



The water replacements in the Halifax area associated with the formation and 

 subsequent movement of a tropical cyclone have been described (Hachey, 1934), 

 and it has been shown that bottom water temperatures in some of the inshore areas 

 increased from 2-0" C to greater than 15-0° C in less than one week. Under these 

 circumstances, in these areas, all fishing for cod and haddock ceases, and does not 

 resume until temperatures return to more normal values (Hachey, 1934; McKenzie, 

 1934; Vladykov, 1933). 



Incursions of warmer slope water over the Scotian Shelf have been observed 

 (Hachey, 1953), when bottom temperatures reached values as high as 12-0° C, 

 about five degrees above normal seasonal temperatures. While no records of fishing 

 effort are available for periods in which such incursions have been observed, it is well 

 known that such bottom water temperatures are unfavourable to cod and haddock, 

 the groundfish probably moving out of the areas subjected to the incursions of waters 

 of such temperatures. 



BARREN SEA FLOOR 

 In the Gulf of St. Lawrence comparatively extreme variations in temperature and 

 salinity have been observed at depth during the summer months (Lauzier, 1952) 

 brought about by oscillations of the various water layers of contrasting characteristics 

 on a gradually shoaling sea floor. It has been pointed out by Lauzier that organisms 

 which cannot tolerate these sudden changes in temperature and salinity will not form 

 important populations along the margins of the Magdalen Shallows, and Hunttsman 

 (1918) has shown that there are bands of the sea floor between Cape Breton and the 

 Magdalen Shallows which are comparatively barren. The oscillation of these water 

 layers of contrasting temperatures is probably responsible for the periodic complete 

 destruction of scallop beds which become temporarily established and fishable under 

 marginally satisfactory conditions in the Magdalen Shallows (Annual Report for 

 1953, Fisheries Research Board of Canada, 34). 



MARGINAL FISHERIES 



In Canadian Atlantic waters, which are contained in the area of confluence of three 

 current systems with waters of contrasting characteristics, extreme contrasts in water 

 characteristics, chiefly temperature, are to be observed. Very sharp boundaries, both 

 vertically and horizontally, are thus encountered, and various marine organisms 

 suitable to one environment (frigid) or another (temperate) exist on a marginal basis. 

 A small vertical or horizontal change in the margin sometimes exerts a very pronounced 

 effect on a fishery. 



With progressive warming of the waters during recent years we have witnessed 

 the northerly expansion of successful oyster production in the Gulf of St. Lawrence, 

 and the northerly extension of the green crab to the Bay of Fundy (Annual Report 

 for 1953, Fisheries Research Board of Canada, 25). As mentioned earlier in this 

 paper, the expansion of the west Greenland cod fishery is probably the most outstand- 

 ing result of the increased Atlantic influence in northern waters. It is to be expected 

 that a downward trend of temperatures would bring about a recession of these 

 extensions. 



In some cases, man is responsible for changes in environmental conditions, and this 

 is particularly true in estuaries where pollution problems arise with present-day 



