LONG-TERM CLIMATIC AND OCEANOGRAPHIC CYCLES 



California and southern California State, from 

 the 25-35 parallel where the California Cur- 

 rent begins to swing away from the coast to 

 the west as the North Equatorial Current, is 

 another zone of instability. 



I think that it is no accident that the south- 

 ern limit of several northern species of North 

 Pacific seabirds ends in southeastern Alaska 

 or northern British Columbia, and that the 

 northern limit of the ranges of several other 

 species occurs in Washington State or south- 

 ern British Columbia. Indeed, the west coast 

 of Vancouver Island is not rich in species, and 

 several of those that exist are not present in 

 great numbers. This is a region of rather more 

 variable conditions than elsewhere, and 

 species evidently find that it is difficult to 

 colonize and it quickly becomes unsuitable 

 again. Since 1940, indeed, there has been a 

 parallel decline in the annual mean sea-surface 

 temperature at a number of coastal recording 

 stations in British Columbia, and this seems 

 to have been a rebound from a less well-docu- 

 mented rise in sea-surface temperatures dur- 

 ing the 20 years before that, which culminated 

 in a peak around 1940. Salinity has likewise 

 trended downwards during the last 30 years. 

 The seabird colony size data before 1960 are 

 so nonquantitative that it is impossible to be 

 sure what changes in seabird populations and 

 breeding sites may have taken place in re- 

 sponse to these physical changes. 



The lesson is that we must now examine all 

 future census and distribution data with 

 trends in sea-surface temperature and salinity 

 in mind as two of several likely factors influ- 

 encing them. We must no more ignore data 

 outside our own field than a salmon ecologist 

 might. 



Conclusions 



We know little of the accuracy of censuses 

 of seabird numbers made between 1850 and 

 1950. There has been a tendency to assume 

 that numbers of seabirds at long-established 

 colonies have been relatively unchanging, 

 even though the expansion of some species 

 into previously unrecorded breeding sites in 

 low numbers is well documented. Contraction 

 of breeding ranges, likewise, has most com- 

 monly been attributed to the influence of man. 

 Recent literature from the physical sciences, 

 on the contrary, suggests that seabird num- 

 bers at particular colonies are most unlikely 

 to have been stable for any great length of 

 time, at least at high or middle latitudes and 

 particularly at points where boundaries be- 

 tween currents impinge on continental coasts. 

 Indeed, some early estimates of colony sizes 

 may not have been as much in error as we may 

 have assumed, neither when apparently too 

 large nor when apparently unlocated by pre- 

 vious visitors. 



The halving of a large colony over a period 

 of 20 to 50 years in the middle of the range of 

 a species and the establishment and disap- 

 pearance of smaller breeding groups at oppo- 

 site extremes of the range (both latitudinally 

 and longitudinally), may equally reflect natu- 

 ral long-term climatic or oceanographic 

 changes and may naturally be reversed at 

 some time in the future, perhaps within half a 

 century. The implication for conservation of 

 seabird colonies that are at the contracting 

 end of a species' range is that cultural rather 

 than biological criteria may be the best 

 determinants. 



