NATURE ADRIFT 



adult fish (as any trout fisherman will know) have a tendency to continue 

 feeding on the same acceptable food while the supply lasts, ignoring other 

 foods for the time being. This means that if larval fish arc plentiful and adult 

 fish are taking them they will tend to continue doing so, therefore the 

 greater the numbers of young fish the greater the hazard. 



The third major factor is the actual transport of the larvae by water 

 movements. Some fish like a sandy bottom, others a stony or a rocky bottom, 

 either because of their own likes and dislikes or because their favourite foods 

 are associated with such a bottom. Some fish, e.g. plaice, need to be in very 

 shallow sandy bays during their first year, and so their nursery grounds are 

 very close inshore. Who has not paddled in a sandy pool left by the tide and 

 not seen the tiny plaice, the size of a big-toe nail, scudding along on being 

 disturbed? Even haddock, that as babies do not mind fairly deep water, cannot 

 stand it too deep. It follows, then, that if currents carry the young fish to 

 places where, when they go to the bottom, they cannot find a suitable habi- 

 tat they will be doomed. Again, taking plaice as an example, the main spawn- 

 ing grounds of the southern North Sea are so sited that the normal currents 

 there carry the young to the very suitable sandy shallows of the Dutch, 

 German and Danish coasts. How many other species are usually so consis- 

 tently lucky, especially in areas where currents are far from constant? 



Taking all three major hazards, is it any wonder that survival rates are 

 so low, so variable and so dependent on plankton? Recruits to the cod stocks 

 in the area of the Bear Island fishery have been shown to vary with the 

 abundance of Calainis there, in turn depending on the planktonic conditions 

 and the amount of the north-flowing warm water. This is, of course, also 

 correlated with the weather conditions — a point nicely illustrated recently in 

 Norway, where it has been found that the cod stocks at the Lofoten fishery 

 vary from year to year with an extraordinary similarity to the growth of 

 the nearby pine trees, judged by the distance between the growth rings seen 

 when the wood is cut. Recent Russian work has shown that when the 

 northern sea gooseberry is abundant the rest of the plankton is poor and cod 

 stocks are affected, but when Beroi' (p. 58) is numerous this eats the sea 

 gooseberries, plankton thrives and so do the young cod. 



Turning now to the relationship between plankton and adult tish we 

 can first consider the adult pelagic fish such as herring, mackerel, pilchard, 

 sprat etc, which are plankton feeders. Here it is obvious again that the main 

 relationship is a direct one of food supply, and it must be emphasized yet 

 again that in their own turn these food organisms are dependent on others in 

 the plankton right down to basic plant production. 



The copepods, chiefly Calaiius (Plate XVII), are responsible on the average 

 for about 21 per cent of herring food but with a much higher percentage in 



130 



