midwater fauna may vertically migrate (Young, 1983) but in others 

 the proportion is much less ( Pearcy et al., 1977). Although the 

 exact causes of diel vertical migrations are unknown there is 

 good evidence that they are mediated by light and are related to 

 feeding behavior (Zaret and Suffern, 1976). 



Possibly if we could determine what factors affect midwater 

 vertical distributions we might be able to better understand 

 vertical migrations. The major environmental parameters which 

 affect vertical distributions are light, temperature, and 

 abundance of prey and predators (Roe, 1974; Merrett and Roe, 

 1974; Zaret and Suffern, 1976; Enright, 1977; Young, 1983; 

 Larson, 1986; Pugh, 1986). It appears that physical factors, 

 most importantly temperature, may determine the overall vertical 

 limits of a given midwater species but biological parameters, 

 mainly food availability and predation pressure, determine the 

 normal range. Vertical migrators must be able to cope with a 

 broad range of temperatures, up to 25°C in the tropics. They 

 probably do this so that they can feed on the more abundant 

 zooplankton in the upper 100 m (Roe, 1972). 



Although light is important in determining vertical 

 distributions it mostly acts indirectly through feeding success 

 and predation (however, we have observed the lethal effects of 

 light on siphonophores ) . As a result the mesopelagic may act as 

 a partial refuge from visual predators. However, even in the 

 lower mesopelagic where vision may be limited, tentaculate, 

 gelatinous predators are abundant and they may be significant 

 causes of mortality. 



Direct observations on the behavior and associations between 

 midwater organisms are needed to provide essential data for 

 understanding the vertical distributions and ecology of midwater 

 organisms. Our results will be the subject of future papers. 



This contribution is number 16 of Direct Studies of 

 Mesopelagic Communities. 



LITERATURE CITED 



Backus, R. H. and J. E. Craddock. 1982. Mesopelagic faunal 



provinces and sound-scattering levels in the Atlantic Ocean. 

 In: N. R. Anderson and B. J. Zahuranec (eds.), Ocean sound 

 scattering production . Mar. Sci . , Vol. 5, , pp. 529-547. 



Badcock, J. 1970. The vertical distribution of mesopelagic 



fishes collected on the SOND cruise. J^ Mar. Biol . Assoc. 

 U.K., Vol. 50, pp. 1004-1044. 



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