RELATIONSHIP WITH FISHERIES 



Two arrow-worms are of special interest here as one, Sagitta cicgaiis, is 

 associated with mixed waters and the other, Sagitta setosa (Plate XXX VI), with 

 unmixed water in the North Sea and English Channel. These associations 

 will be more fully dealt with in the next chapter but here it is sufficient to 

 note the relationship with the survival of fish larvae. Sometimes there is an 

 incursion of western water, a mixed water, into the English Channel which 

 pushes the local water farther east, and we can distinguish the two masses by 

 the arrow-worms present. When 5. clegaiis is present, i.e. with a greater 

 inflow and mixture of western water, young fish are fairly abundant in the 

 Plymouth area: when S. setosa is present and Channel water is unmixed the 

 young fish are scarce. The same feature is also found in the northern North Sea 

 where, again, S. clegans indicates the presence of the North Sea water mixed 

 with (not replaced by) oceanic water entering from the north. Over a period 

 of years the number of larvae of bottom-living fish found in the plankton 

 associated with 5. clcgaiis always averaged more than if 5. elcgaiis were absent. 

 These differences are, of course, nothing to do with the presence of the 

 arrow-worms, but with the mixed conditions that S. chgaiis also needs. Just 

 as too httle inflow of new water to mix with the local water leads to poorer 

 food supply so does too great an inflow which actually displaces the local 

 water, at least until it has had time to mix. 



The previous biological history of the water is in part responsible for the 

 types of planktonic organisms that thrive and these may or may not be suit- 

 able as food. This is particularly important for those young fish which are 

 very selective in their food, e.g. plaice and lemon sole which feed so much on 

 Oikopleiira (Plate XIX). 



Another condition that reduces the available food supply is an abundant 

 population of salps (Fig. 23) which filter off the plants extremely effectively. 

 An incursion of Salpa fiisiforiiiis, such as that which came into the northern 

 North Sea in 1958 and also flooded the area south-west and west of Iceland, 

 can virtually deprive hundreds and hundreds of square miles of water of its 

 phytoplankton, thus leaving too little food for the animals which in turn 

 would have been eaten by the young fish. 



The second major hazard for the young fish larvae is being eaten by 

 predators. The jeflyfish, sea gooseberries and arrow-worms are the worst 

 offenders in the plankton. The method of feeding of the jellies and sea goose- 

 berries has already been described (pp. 48, 57). When they swarm they can have 

 a very serious predatory effect and this is increased by the severe depletion of 

 other animals that could be used by fish larvae as food. Swarms o£ Pleiiro- 

 hrachia in particular can be so dense that after a short time there is almost 

 nothing else to be found. Other important predators of young fish are adult 

 pelagic fish such as herring and mackerel. A point to remember here is that 



129 



