FISHERY BULLETIN: VOL. 72, NO. 2 



chain of demersal piscivores (Ben-Tuvia, 1966), 

 and has declined (Ben-Tuvia, in press b) since its 

 peak bloom in the 50's. Ben-Tuvia attributes this 

 decline to the spread of the lizardfish, one of its 

 main predators. The U. asymmetricus, usually 

 small, does not occur in commercial quantities, 

 and in the catches it is classified with the other red 

 mullets. 



Another Red Sea migrant, the yellow-striped 

 goatfish, Upeneus moluccensis has not yet been 

 identified from the stomachs of the hake. There 

 are good indications that, ecologically, both the 

 Upeneus and the lizardfish are closely related in a 

 prey-predator relationship. They occupy the same 

 habitat, the goatfish being an equally rare visitor 

 at the deepwater trawling grounds (Zismann, in 

 preparation). Both species seem to increase in 

 catches during the same years (Ben-Yami, 1955; 

 Oren, 1957), which may be associated with en- 

 vironmental conditions. 



It is, thus, very likely that in areas where they 

 are both found, the hake and the Red Sea 

 lizardfish compete for food. 



THE MECHANISM OF AN INVASION 



Ecological "Barriers" to 

 Migrating Species 



A demersal fish expanding from one sea to 

 another through a man-made canal encounters 

 several barriers which it must overcome before a 

 significant population can be established in the 

 other sea (Figure 7). The "height" of an ecological 

 barrier differs for each separate species. Hypersa- 

 linity, e.g., which may be prohibitive to some 

 purely marine species, may not be a barrier or may 

 even possess attractive environmental qualities to 

 euryhaline organisms. The height of an ecological 

 barrier may also change with seasonal, annual, 

 and multiannual fluctuations in the environmen- 

 tal conditions. 



The first barrier is the canal itself which may 

 represent a less or more hostile environment for 

 the migrating species. Migration through the 

 Suez Canal must have been very difficult for some 

 and impossible for other species, because of the 

 complex hydrological conditions in the canal (the 

 high salinity of the Bitter Lakes, freshening of the 

 water due to influx of fresh water at some places, 

 and the seasonal Nile floods) (Oren, 1970; H. 

 Steinitz, pers. comm.). The nature of the Suez 

 Canal, as a barrier, has changed, however, with 



NATIVE SEA 



THE CANAL 



SEA BOTTOM & 

 FOOD DIFFERENCES 



B ARRIER I II ^ f) S;°C« '"' 



r 



1 



BARRIER IV 1 ^ COMPETITORS & 



PREDATORS 



I 



NEW SEA 



Figure 7. — "Barriers" on the path of a migrating species of 

 demersal fish the last three "barriers" may occur in any order 

 and/or overlap. 



time. Animal migration through the canal may 

 now become easier (Thorson, 1971). 



The second barrier, especially for demersal 

 species, is the difference in bottom conditions. The 

 importance in the character of the substrate for 

 the expansion of migrating benthic invertebrates 

 was emphasized by Gilat (see footnote 6) and Por 

 ( 1971). The type of bottom influences the type and 

 quality of food available. Bodenheimer ( 1966) em- 

 phasized the negative effect which lack of food 

 may have on fecundity. De Vlaming (1971) has 

 shown that starvation affected the gametogenesis 

 and gonadal regression in a goby, Gillichthys 

 mirabilis. Undoubtedly, it is not enough for a bot- 

 tom fish just to cross a canal. To survive and repro- 

 duce, it must find in its new habitat either the food 

 to which it is accustomed or a food which can 

 replace the former both quantitatively and qual- 

 itatively at all stages of its life cycle. This condi- 

 tion is, generally, controlled by the character of 

 the sea bottom. 



A third barrier is the hydrological gradient (if 

 any) between both seas. A species may cross a 

 canal, may even find an apparently suitable 

 habitat, but all its spawn may be killed by extreme 

 winter or summer temperatures. Also, adverse 



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