BEN-YAMI and GLASER: INVASION OF SAURIDA UNDOSQUAMIS 



temperature conditions may affect prespawning, 

 reproductive processes in fish (De Vlaming, 1971 ), 

 while difference or seasonal changes in salinity 

 may affect survival of a stenohaline species. 



Oren (1970) has noticed, e.g., that after the crit- 

 ical 1954/56 years, the minimum seawater winter 

 temperatures over the Israel continental shelf 

 have never returned to their values of 15°C mea- 

 sured prior to this period. He also found that the 

 salinities in the same area increased since the 

 closure of the Aswan Dam in 1964. It seems, thus, 

 that in the long run, the hydrological gradient 

 between the Gulf of Suez, where the temperatures 

 and salinities are higher than in the Levant Basin 

 (Kosswig, 1951; Oren, 1957; Ben-Tuvia, 1966) and 

 the Mediterranean, is on the decrease. 



The fourth barrier is the predators and com- 

 petitors. Darwin (1859) emphasized the role of 

 prey, predators, and competitors on the distribu- 

 tion range of species. Obviously, the abundance 

 and distribution of the native predators and com- 

 petitors are affected by fluctuations in the hy- 

 drological conditions. Therefore, changes in the 

 hydrological conditions may affect establishment 

 of the migrant species both directly and, through 

 their competitors, indirectly. 



Human Interference May 

 Facilitate Invasion 



An invasion may succeed because of human in- 

 terference in the environment. Elton (1958) has 

 shown that such interference, especially where 

 associated with depletion of native populations, 

 considerably increases the vulnerability of an 

 area to invasions. We consider commercial 

 fisheries to be an example of an extreme interfer- 

 ence. 



The Explosion of the Red Sea 

 Lizardhsh Population 



The lizardfish, Saurida undosquamis , is an im- 

 portant component of the Egyptian trawl catches 

 in the Gulf of Suez (Latif, 1971). The early records 

 of lizardfish in the Suez Canal by Gruvel and 

 Chabanaud (1937), asS. sinaitica, S. tumbil, and 

 S. gracilis, may have been S. undosquamis 

 (Ben-Tuvia, pers. comm.). 



Fifteen years later, and 83 years after the open- 

 ing of the Suez Canal, the Red Sea lizardfish ap- 

 peared in the southeastern Mediterranean in suf- 



ficient numbers to be described as a "rare" fish 

 (Ben-Tuvia, 1953). But within 2 to 3 yr, it became 

 one of the most important commercial trawl fishes 

 forming 20% of the catch. This recalls the invasion 

 of the sea lamprey, Petromyzon marinus, in the 

 Great Lakes in North America (Elton, 1958). Al- 

 though the Welland Ship Canal was opened as 

 early as 1829, the sea lampreys were observed in 

 Lake Erie 100 yr later. Then, within 10 yr, the 

 lamprey population expanded rapidly and 

 dramatically both in space and in number, caus- 

 ing a collapse of the lake trout fishery in Lake 

 Michigan and Lake Huron. 



The population explosion of the Red Sea 

 lizardfish was much faster, for its 1959 all-time 

 record landings occurred only 4 yr after its first 

 appearance in the trawl catch as a fraction of a 

 percent. 



Although the subsequent decline in the 

 lizardfish catch may be associated with decrease of 

 the fishing effort (Tables 1 and 2), particularly in 

 the northeast Mediterranean, its relatively stable 

 proportion in the total catch indicates that an 

 ecological balance was reached within the first 2 

 yr of its appearance in the commercial catch (Fig- 

 ures 5, 8). Subsequent annual fluctuations seem to 

 be normal to natural populations. 



Table 2.— Israel trawl fishery, 1948-70. 



'Activity index (100';): 

 below 150 hp - 210 days at sea per trawler, 

 over 150 hip - 230 days at sea per trawler. 



369 



