354 EIGHTH PACIFIC SCIENCE CONGRESS 



Causes of Endemism among Hawaiian Fishes 



In the preceding sections the nature of the Hawaiian fish fauna 

 has been discussed. It remains, however, to attempt some explanation 

 to the question of why it is as it is. Wliy, on the one hand, has not 

 the fish fauna undergone the adaptive radiation found in the Hawaiian 

 terrestrial fauna (see Zimmerman, 1948) ? Why, on the other hand, 

 is there any endemism among Hawaiian fishes at all? 



The answer to these questions would seem to be found in the 

 degree of isolation of the Hawaiian Islands. Though the Hawaiian 

 chain is equally isolated in space for both fishes and terrestrial organ- 

 isms, the fishes (and other marine organisms) seem far better adapted 

 to getting there than most terrestrial forms. This is immediately in- 

 dicated by the balanced nature of the fish fauna as compared with the 

 terrestrial biota. Indeed there are a large number of central Pacific 

 fishes unrepresented in Hawaii, but on the whole it looks more as though 

 such absences were due to unsuitable conditions for survival rather 

 than to inability to arrive there. 



If the foregoing is correct, most or all suitable inshore environ- 

 mental niches have been filled by immigration, and the possibility of 

 an adaptive radiation among Hawaiian fishes is precluded. But also, 

 if the foregoing is correct, the question of why there is any endemism 

 at all among Hawaiian fishes becomes difficult to answer. 



In my opinion, the endemism among Hawaiian fishes has been 

 brought about by two factors acting together and, in general, additively. 

 The primary cause is a moderate degree of isolation. The secondary 

 cause is the slightly colder water of the Hawaiian Islands as compared 

 with the tropical central Pacific. 



With regard to isolation, it has been stated above that the vast 

 majority of central Pacific fishes have probably arrived in Hawaii at one 

 time or another. By this it was not meant that the Hawaiian fish po- 

 pulation is deluged by an influx of immigrants every year, but rather 

 that one or a small number of specimens of most central Pacific species 

 have managed to get to the Hawaiian chain from time to time. Pre- 

 sumably the original entrants, if they survived and reproduced, would 

 have had time to saturate the islands with their descendants before the 

 next immigrants arrived. There is some evidence that such a process 

 does take place. As already noted, at Johnston the pure central Pacific 

 stock of Kuhlia sandvicensis is represented, but among three other 

 fishes the pure Hawaiian form is present. Apparently what has occuiTed 

 is that in the case of Kuhlia the central Pacific stock has been able to 

 saturate the island with its representatives. Subsequent immigrants 

 from Hawaii were either unable to survive at Johnston, or if they 



