Trillmich, Fritz

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EFFECTS OF THE 1982183 EL NINO 
ON GALAPAGOS FUR SEALS AND SEA LIONS 
by 
Fritz Trlll",id! 
Max-Pionck-11l.ftillltjiir VerlUlltensphysiologie, [).8i31 Suwiesen, W. Gemwny 
The catastrophic 1982183 EJ Nino (EN) bas left its mark onatmostall formsoflife on the Galapagos,be it 
terrestrial or marine. While the event was (almost) paradise for some species of terrestrial animals and 
plants (Graot and Grant 1984) it proved a serious disaster to marine (onns (Laurie 1984. Schreiber and 
Schreiber 1984). Fur seals and sea lions were hard hit (Limberger et al. 1983) by the dramatic decrease of 
numbers and the accessibility and quality oftbeirfood resources(Barber and Chavez 1983, Santanderand 
Zuzunaga 1984). 
The effec:tsare best documented for the Galapagosfurscal (Trillmicb and Limbergerin press). In late 1982 
mothers stayed at sea seeking food for ever-increasing periods and returned to their young only so 
sporadically that pups and. lacer on, yearlings and 2-year aids lost weight and eventually died. AppaccntJy 
mothers found it more and more difficult to find enough food for themselves, let alone for their young.and 
had to spend very lona tiJDe$ at sea to support tbemselves. We know from dive records (Kooyman and 
Trillmich in press) and scat analysis that fur seals bunt mosdy in the upper 40 metres of tbe ocean, where 
they take advantage of the vertical migration of organisms from the deep scattering layer( e.g. lantern fish. 
small cephalopods) which come close to the surface during the night. Under EN conditions the upper layer 
of the ocean wanned so much that presumably many of these cold water organisms did not rise so close to 
the surface. Measurements on Peruvian fishes showed tbat the nutritive value of the surviving fish 
decreased by 30-40% (Santander and Zuzunaga 1984). The dependent young fur seals (yearlings and 2-
year olds), which are mucb smaller than tbeirmothtrs and therefore less efficient divers. were unable to get 
enough food for themselves by their own foraging. With neither sufficient maternal milk nor enough 
independent foraging success they died during the Jatter half of EN. 
Even the Iaraer, and mostly weaned, 3-year aids were unable to find enough food forthemselves and died 
to almost 100%. Of the adult females about 30% died and thesame proportionofsubadult males seems to 
have suuumbed to this climatic disaster. Hardest hit of all age and sex groups were the males which were 
territorial in the 1982 breedingseasoD (Aug.-Nov.). Of these animals we could not find a single one when 
we returned in AUlust 1983. Males normally lose about 2S% of their body weight as they fast while 
defendin&: their territory and apparendy they were unable to rtCOYeI' from this weight loss under EN conditions. 
The EN ended in July 1983. Duril1B the immediately following reproductive season from AUI. to Nov. 
1983, very few of the survivill8 females save birth. Only about 10% of them had carried theirfoetus tofull 
term under the previous food stras, and even those gave birth to unusually light pups. With the Iossofall 
the very large territorial males from 1982, somewhat smaller males found themselves with huge territories 
and extremely numerous females which they were unable to defend against small, 4-1 year old, intruding 
males. Essentially the territorial system broke down and was replaced byoneof space-related dominance. 
The largest survivil1B males established themselves on areas about 4-.5 times the size of a normal territory 
and chased smaller males away from estrus females when they encountered them. instead of preventina 
them from intruding altogether. 
When I returned in 1984 the situation looked much better for the fur seals. 1984 was an unusually cold year 
and this apparently provided the fur seals with plenty offood. Females were almost 20% heavier than in 
previous years, the few pups born in 1983 had grown to IalJe yearlings, about the size of a 2-year old in 
former years, and the smaller males had grown tremendously durina the intervening period. Thus more 
males held territories in the 1984 breeding season. The most obvious feature of the last season was, 
however, that despite the reduction in female numbers the colony was full of pups. How did this happen? 
In normal years only half of the female population produces pups, because females nurse their young for 
approximately two years. Ouring EN all females had lost their young and were therefore synchronously 
beginning to reproduce again when they had recovered from the effects offood stress during EN. Theage 
structure of the fur seal population has thus become a very rugged curve: the 1980-1982 classes are en tirely 
missing, the 1983 class is very small and the 1984 class, if not killed by another EN event, will become a 
'Ieep peak. On the other end of the age distribution, old males are entirely missing. 
" 
Noticias de Galapagos, vol. 42 1985
Galapagos Sea Lions 
Photo: Fritz Pol king 
We kno w much less about the effect of EN on the Galapagos sea lions but what little has been documented 
indicates that they were hit similarly ailhough perhaps less violently: a ll pups bom in 1982 died and pup 
production in 1983 was about one third of the usual. while in 1984 pups abounded everywhere. Theeffe<:1 
of EN on the :ldul!s is unknown. but f would expectlhat older. physically 1c'ss fit animals and perhaps quile 
a few territorial males died as well. 
It is clear from these: observations that recurrc:nt strong EN events must great ly influence the population 
dynamics of these sjX'cies by (I) changing the age structure and (2) strongly reducing the average ca rrying 
capacity of the environment. It may be that ENs contribute in this way to maintain the population 
densi ties of Gllapagos fur 5('als and sea lions .:11 much IO~'er 1c'vels than those: of more temperate or sul).. 
pola r fur seal and sea lion species. 
REFERENCES 
Barber. R.T. and Chaves. F.P. (1983) Biological consequences of EI Nino. Sci~nCt' 11]; 1203· 1210 . 
Grant. B.R. and Granl. P.R. (1984) Plant a nd Finch responses on Isla Genovesa 10 tht EI Nino e\'t'nt of 
1982-83. NOlicias dt Galapagos No. ]9. 1984: 12·14. 
Kooyman . G.L. and Trillmich, F. (In press) Diving behaviour of Galapagos fur sea ls. In: Gentry. R.l. 
and Kooyman. G .L. (eds.) Fur seals: maternal strattgic:s on land and at sea. Princeton University 
Press, Princeton NJ . 
laurie. A. (1984) Interim report on the Marine Iguana si tuation in the aftermath of the 1982·3 EI Nino. 
Noticias de Galapagos No. 40, 1984: 9_11. 
Limberger, D., Trillmich, F .• Kooyman, G.L. and Majluf, P. (1983) ReproduCli\'e (airurt of fur sc:als in 
Galapagos and Peru in 1982-8]. Trop. Ocean·Almos. Newsl. no. 21. Oct. 198]: ,6· 17, 
San tander. H. and Zuzunaga. J . (1984) Impact of tht 1982-8] El Nino on tht pelagic reSoufCtS off Peru. 
Trap. Ocean-Almos. News!' no. 28, Nov. 1984: 9- 10 . 
Schreiber, R.W. and Schreiber. E.A. ( 1984) Pacific seabirds and the EI NifloSout htrn Oscilla tion: 198210 
1983 perspectives. Science 225: 713-716. 
Trillmich, F. and Limbtrger. D. (In press) Drastic t(feets ofEI Niftoon Galapagos pinnipeds. Oecoioaia. 
Noticias de Galapagos, vol. 42 1985


Effects of the 1982/83 El Niño on Galapagos Fur Seals and Sea Lions

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Effects of the 1982/83 El Niño on Galapagos Fur Seals and Sea Lions

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