118 



D. G. AINLEY AND G. A. SANGER 



mon merganser (Mergus merganser). Most 

 likely then, they select different-sized fish. 

 Another example of this phenomenon is pro- 

 vided by the eight neritic gulls, which are 

 largely scavengers and show a remarkably 

 even progression in bill and body size. Finally, 

 as shown clearly by Bedard (1969o> 19696) 

 and Harris (1970), alcids of different sizes 

 select different-sized prey, often of the same 

 species. 



A final important way in which seabirds 

 partition available resources is by inhabiting 

 different zones. Zonation is especially evident 

 during the breeding season when species com- 

 mon to the same breeding site sort themselves 

 out according to the distances they range for 

 food. This phenomenon was discussed by 

 Murphy (1936), Shuntov (1974), Sealy (1972), 

 Cody (1973), and Scott (1973). 



Trophic Relations and 

 Seabird Conservation 



The species that appear to have specialized 

 food habits (if further research confirms that 

 indeed they do) are probably very sensitive to 

 vagaries in food availability or are, at least, 

 much more sensitive than other species. Some 

 specialists which also have very restricted dis- 

 tributions would, therefore, be susceptible to 

 localized catastrophes occurring where 

 specialists are concentrated around the food 

 resource. This is proved in the case of the 

 scoters, which are both specialized and rather 

 restricted to nearshore beds of molluscs and 

 have fallen victim to local oil slicks (Smail 

 et al. 1972). An example of another poten- 

 tially critical situation is that of the black 

 brant, which at certain times of the year con- 

 centrate their entire population around eel- 

 grass beds in Bristol Bay, Alaska, where 

 much offshore oil drilling may soon occur. 



Birds adapted to feed by diving, with the 

 exception of cormorants, spend most of their 

 time in the water. These species are therefore 

 most susceptible to oiling (Smail et al. 1972), 

 but pursuit plungers (the shearwaters) are 

 also highly susceptible (Point Reyes Bird Ob- 

 servatory, unpublished data). A characteristic 

 of polar and subpolar seabird communities is 

 the high percentage of birds that feed by div- 



ing and pursuit plunging. These birds are 

 mostly absent from tropical and subtropical 

 communities because feeding by these meth- 

 ods is not adaptive there (Ainley 1977). 

 Hence, oil pollution has all the potential of 

 rendering maladaptive the principal feeding 

 methods of many polar seabirds. 



Another way in which seabird feeding re- 

 lates to conservation problems concerns com- 

 petition between birds and man for commer- 

 cially valuable fishes. A related problem is the 

 mass mortality of seabirds due to man's fish- 

 ing gear. An acute situation is the drowning 

 of seabirds caught in salmon gill nets (Barto- 

 nek et al. 1974; Pacific Seabird Group 1975; 

 Ripley 1975; King et al., this volume). Imme- 

 diate action is definitely required. 



Further, competition between birds and 

 man for the same resource has the potential 

 for disastrous effects on bird populations if 

 humans out-compete the birds and overfish 

 the resource. A classic example, reviewed by 

 Idyll (1973), is the possible collapse of the 

 Peruvian anchovy (Engraulis ringens) fishery; 

 if overfishing and an El Nino should coincide, 

 the Peruvian seabird populations could col- 

 lapse as well. The California fisheries and ap- 

 parently the double-crested cormorants that 

 nest on the Farallon Islands have both suf- 

 fered from the demise of the Pacific sardine 

 (Sardinops caerula) in the California current 

 (Ainley and Lewis 1974). In regulating fish 

 harvests, fishery organizations should include 

 in their calculations the harvest by creatures 

 other than man (Schaefer 1970), rather than 

 evading the issue by referring to a vague 

 "natural mortality." 



Finally, fishing by humans can benefit sea- 

 birds by removing fish (or whales) that com- 

 pete with birds for food (Laws 1977). A poten- 

 tial example is that of northern California, 

 where salmon and seabirds both feed heavily 

 on juvenile rockfishes (Fitch and Lavenberg 

 1971; Point Reyes Bird Observatory, unpub- 

 lished data). Harvest of salmon should theo- 

 retically leave more rockfish available for 

 birds to eat. This sort of situation has not yet 

 been fully documented and definitely war- 

 rants further study, especially in such areas 

 as the Bering Sea, where some fish stocks 

 have become depressed due to overfishing 

 (Gulland 1970). 



