BACKGROUND
multidecadal fluctuations, or biological regime shifts, have basin-wide effects on sea surface temperature  and upwelling that are associated with large-scale changes in biological productivity  <cit> .
they are similar to the 3– <dig> year cycles of the el niño southern oscillation  but on longer time scales, and as background states, may amplify enso effects  <cit> .
the pacific decadal oscillation , which addresses the north pacific/north american sector, comprises events that persist for  <dig> to  <dig> years  <cit> .
in the 20th century, "cool" pdos prevailed from  <dig> to  <dig> while "warm" pdos were recorded from  <dig> to  <dig> and from  <dig> through at least the mid-1990s.
chavez and colleagues  <cit> , focusing on the last two regimes, report that in the mid-1970s the pacific changed from a cool "anchovy regime"  to a warm "sardine regime" , which lasted until the middle to late 1990s before shifting back to an anchovy regime.
the changing ocean temperatures associated with these regimes exert strong effects on marine food webs  <cit> .
in the northeastern pacific many marine birds and mammals flourish during anchovy regimes and are nutritionally stressed during sardine regimes.
it is well documented that short-cycle ensos cause drastic reductions in survival and reproduction of several marine mammal species in the temperate waters of the pacific  <cit> .
non-migrating sea lions and fur seals are especially vulnerable to ocean warming because nursing females alternate feeding and nursing, which ties them closely to their coastal rookeries and results in pup starvation when prey move out of the area  <cit> .
it is not known, however, to what degree these processes impact deep-diving marine mammals that obtain their prey far below the surface as they range widely in the open ocean of the northeastern pacific far from the coast.
the condition of newly weaned northern elephant seals, mirounga angustirostris, is expected to be sensitive to these processes because of the location where adults feed and the close link in energy exchange between mother and pup.
in this species weaning weight reflects total energy transfer from the mother because pups feed solely on mother's milk prior to weaning.
because the mother gives birth annually to a single pup and fasts during lactation, her pup's weight at weaning correlates positively with her resource accruing ability at sea during the eight-month period of pregnancy  <cit> .
the mean weaning weight of the colony reflects the general foraging success of parturient females in that year.
it follows that mean weaning weight may oscillate annually with ocean processes that exert strong effects on marine food webs and impact prey availability.
a similar logic has been used in studies of the southern congener, m. leonina, that breeds and feeds in antarctic waters  <cit>
northern elephant seals are among the deepest diving marine mammals.
females disperse widely in the northeastern pacific twice a year, from  <dig> to 60°n and from the coast to 188°e , foraging at modal depths of 300– <dig> m and maximum depths of  <dig> m  <cit>  on prey such as mesopelagic squid and pacific hake, merluccius productus  <cit> .
after continuous foraging over the 8-month gestation period , females return to rookeries along the coasts of mexico and california to give birth and nurse their pups.
in four weeks a pup triples its birth weight and is weaned when the mother returns to sea in february for a  <dig> 1/ <dig> month post-partum bout of foraging to recover the one third of the body mass lost during nursing and begin the next pregnancy  <cit> .
milk energy transferred to the pup varies with its sex and the age, mass, blubber reserves and overall condition of the mother  <cit> .
by maternal condition, we mean the total body energy available to the female at the time of parturition.
on average,  <dig>  ±  <dig> %  of a female's body reserves is depleted during lactation.
the size and blubber reserves obtained by the female during her biannual foraging migrations determine the level of reproductive expenditure in the subsequent breeding cycle.
pup weight at birth and weaning is correlated positively with the mother's age and mass  <cit> .
male pups are about 8% heavier than females at birth and at weaning  <cit> , suggesting that males require more maternal energy than female pups.
pregnant females are capital breeders that store energy in blubber laid down over months of continuous foraging at sea and expend much of this energy later in reproduction  <cit> .
acquisition of insufficient body reserves owing to low foraging success directly impacts reproductive expenditure resulting in low weaning weight of pups  <cit> .
additional data supporting these points are reported in studies of southern elephant seals  <cit> .
here we show that annual weaning weights of northern elephant seal pups and resource accrual of adult females associated with a rookery in central california fluctuate with ocean temperature cycles.
RESULTS
weaning mass
the pattern of fluctuation in weaning weight over the years was similar in both sexes even though males were, on average, 5% heavier than females .
the regression of weaning mass on year showed a statistically significant decline in both males  and females .
the regression slopes of males and females were not significantly different from each other .
the weaning mass data of both sexes, combined, which we address in the rest of this paper, declined significantly from  <dig> to  <dig> as illustrated by the scatter plot and regression of weaning mass against year in fig.
2a .
a loess smoothing plot, however, reveals two distinct slopes .
mean weaning weight declined significantly from  <dig> to the mid to late 1990s, decreasing from highs of  <dig> kg in  <dig> and  <dig> kg in  <dig> and  <dig>  to lows of  <dig>   <dig> and  <dig> kg in  <dig>   <dig> and  <dig>  respectively.
a linear regression line through the data over the  <dig> year period from  <dig> to  <dig>  shows a significant mean decline of  <dig>  kg/yr .
a significant decline is also observed if the end data point is anywhere in the interval 1995– <dig>  similarly, the decline is statistically significant when years with a sample size of less than  <dig> are omitted, i.e.
<dig>   <dig>   <dig> and  <dig> .
in the late mid 1990s, the declining trend ends abruptly and the mean weaning weight increases up to  <dig> to  <dig> kg in the new century .
although the slope of weaning weight from  <dig> to  <dig>  shown in fig.
2c, is positive, it is not statistically significant  owing in part to the small sample size.
the slope of weaning mass versus year, however, varied significantly between the two regimes .
the period of declining weaning weight was associated with higher than normal ocean temperatures associated with the long-term sardine decadal cycle and the prevalence of el niños in the short-term enso cycles .
weaning weight declined increasingly with the tenure of the sardine regime, a trend that ended abruptly with the shift to the anchovy regime in the late 1990s.
with the regime shift, a reverse trend to increasing weaning weights is evident.
mean weaning weight between the two regimes was not significantly different   but this analysis is confounded by the declining slope of weaning mass over the course of the sardine regime and the small and incomplete sample size for the anchovy regime.
a similar lack of association was found between weaning mass and pdo.
declining weaning weight over the course of the decadal regime from  <dig> to the late 1990s is consistent with the overlapping pattern of short-term enso cycles during this period, represented in fig.
<dig> by annual mean monthly values of the standardized southern oscillation index   <cit> .
this was clearly a period of extended warmth in the pacific.
warm el niños were more frequent than cold la niñas, the el niños of 1982– <dig> and 1997– <dig> were the most intense of the century, and the el niño in the early 1990s  was unusually long.
the global temperatures in  <dig> and  <dig> were the warmest years on record  <cit> .
the lowest weaning weights were observed from  <dig> to  <dig>  an unusually warm period dominated by two el niños coming at the end of the sardine regime.
the decline in weaning mass was not linearly related to increasing ocean temperatures during the sardine regime.
although the period from  <dig> to  <dig> was warmer than usual, ocean temperatures did not increase significantly over the course of this period, as revealed by soi and pdo annual averages regressed on year .
consequently, a linear regression plot reveals that annual mean weaning mass was not closely associated with annual soi  or annual pdo averages .
it is notable, however, that the mean weaning weight over the period 1990– <dig>  which included the longest el niño and the most intense one of the century, was  <dig>  ±  <dig>  kg, significantly lower than the mean weaning weight of  <dig>  ±  <dig>  kg recorded in the period 1975– <dig>  which included only the strong el niño of 1982– <dig> .
female foraging success: foraging duration and mass gain
to examine the relationship between weaning weight, ocean temperature and female foraging success, we used the spring post-breeding foraging trip duration, and mass gain over this period, as the metric of foraging success.
during this foraging trip females can recover approximately 85% of the mass lost during parturition and nursing.
we assume that foraging success in the spring is associated with and reflects foraging success in the fall.
the duration of the fall foraging trip, however, is an unsuitable measure of foraging effort because its length is set by gestation, and is relatively invariant at  <dig>  ±  <dig>  days .
the duration of spring foraging trips of females was inversely related to mass gained .
the regression of mass gain on foraging duration was statistically significant .
over the course of the sardine regime, post-partum females spent increasingly more time attempting to accrue resources  <cit> , increasing the mean foraging trip duration by approximately 36% from the late 1970s to the mid to late 1990s .
with the shift to the anchovy regime in the late 1990s, foraging durations decreased from  <dig> days or more at sea to approximately  <dig> days at sea from  <dig> to  <dig>  the same level observed at the beginning of the sardine regime.
association of foraging trip duration with shorter term el niño cycles is also evident, such as the longest foraging trip and lowest mass gain occurring in  <dig>  an exceptionally strong el niño year.
mass gain during foraging trips was inversely related to trip duration , decreasing from approximately  <dig> kg/day at sea during the late 1980s to approximately  <dig>  kg/day in 1993– <dig>  to a low of  <dig>  ± .
<dig> kg/day in  <dig>  and returning to  <dig> kg/day in  <dig>  in spite of small annual sample sizes, variation around the overall mean was small .
weaning weight of pups was inversely related to the foraging trip duration of females .
linear regression of pup weaning mass on female foraging duration shows that this relationship was statistically significant .
weaning weight and mass gain of females covaried in time  but the sample size was small and the association was not statistically significant .
colony composition and sex ratio
the año nuevo colony increased over the course of the study period from  <dig> pups produced in  <dig> to  <dig> pups produced in  <dig>  the decline in weaning mass from  <dig> to  <dig> was not associated with changes in the age composition of the colony such that more young females gave birth to smaller pups.
rather, the opposite was observed; mean age at primiparity of females increased over the study period from a mean of  <dig>  years in the early 1970s  <cit>  to over  <dig> years in the 1990s.
the decline in weaning mass was not due to a sex ratio increasingly biased to smaller female pups.
there were marginally more females in the sample from  <dig> to  <dig> , when mean weaning weight was highest, and more males in the sample from  <dig> to  <dig>  when mean weaning weight was lowest.
the sex ratio of weaned pups in the entire colony has not deviated significantly from unity during the last four decades  <cit> .
discussion
our data show that the weaning mass of northern elephant seal pups varies with ocean temperature cycles.
weaning mass declined over the course of an unusually warm 24-year period that coincided with the warm decadal or sardine regime and included several long and strong short-term warm enso events.
the decline in weaning mass ended abruptly with the regime shift to the cold anchovy period.
weaning mass increased with the return to the cold regime but monitoring throughout an entire anchovy period is required to reveal the entire pattern and determine whether the positive trend of increasing mass continues.
the link between ocean temperature and weaning weight is strengthened by the finding that weaning mass was inversely related to the foraging effort of females and positively related to the mass they gained.
over the course of the warm period from  <dig> to about  <dig>  females spent increasingly longer periods searching for food; the more time spent searching, the less mass they gained.
evidently, females attempted to compensate for reduced foraging success by staying at sea longer searching for food.
this would buffer the effect of reduced foraging success and would obviously complicate any expectations of a linear relationship between soi and weaning mass and rates of female mass gain and pup weaning mass.
the ability of a given female to extend her pre-implantation foraging trip, however, is limited by her need to implant in time to give birth during the breeding season and is impacted by her weaning date the previous year.
females do not have the ability to compensate in this way during the gestational foraging trip.
the relationship between soi and weaning mass is further confounded by changes in colony age distribution.
as the colony has aged, weaning mass should increase based on our understanding of the relationship between maternal mass and investment  <cit> .
soi helps define the strength and duration of enso cycles but it does not predict weaning mass of pups well except during unusually strong or long el niños.
we emphasize that it is important that we detected such a strong trend in pup weaning mass despite the trip duration compensation by females and the changing colony demographics.
this implies that reduction in resources were greater than could be compensated for by increased foraging duration in any given year or by increasing maternal body size with age.
by this logic, it is reasonable that we did not observe a linear relationship from soi and pdo to rate of foraging success to pre-implantation trip duration to weaning mass.
it is clear that foraging effort was greatest and foraging success was lowest at the end of the sardine regime during the late 1990s.
with the return to the cold regime in the late 1990s, female foraging effort returned to the level observed in the mid-1970s, the time of the previous regime change.
we reason that warmer than usual ocean temperatures influenced elephant seal prey, either through recruitment, competitive advantages or predation, effecting a redistribution or diminution of populations.
as a result, female seals were less successful forging, and consequently, at parturition and during lactation, they had less energy to transfer to their young.
this logic is supported by observations during el niño years.
during the strong el niño of 1997– <dig>  foraging females spent more time travelling from one prey patch to another and they spent less time on each prey patch than in non-el niño years  <cit> ; mass gain in  <dig> was by far the lowest value we have recorded.
moreover, enduring effects of these "hard times" are suggested by the observation that female pups born in  <dig>  a strong el niño year, postponed breeding for the first time relative to adjacent cohorts in the years  <dig> to  <dig>  <cit> .
the link between seal condition and physical measurement of ocean temperature is complicated.
the progressive decline in weaning mass over the course of the sardine regime is not simply explained by increasing ocean temperatures.
the soi and pdo indices did not increase systematically over the course of this period.
evidently, other factors such as the duration of exposure to warming or warm temperature spikes during enso cycles overlaid on a warm decadal cycle may have been operative.
anthropogenic perturbations, or global warming, may amplify the effect of these natural cycles  <cit> .
in any case, the temporal link between temperature changes, effects on prey, and the weaning mass of pups is unclear.
substantial lag effects may operate, as has been observed in the link between the pdo index and salmon catches  <cit> .
moreover, female seals spend  <dig> of the  <dig> months of the year foraging and it is not clear whether foraging is affected during all or part of this time.
an additional complexity is the difficulty of separating the effects of short-term ocean temperature cycles from the long-term cycles.
for example, in addition to the long period of warm water associated with the sardine regime, an unusually long el niño occurred during the early 1990s that had a more severe impact on marine mammals and birds than was predicted from its strength.
overall, the data suggest that the duration and persistence of warm water may have a cumulative negative effect on seal prey and indirectly on seal foraging behaviour and, ultimately, on pup weaning mass.
it is also likely that the long-term cycle, as background state, may amplify or dampen the effect of short-term cycles, e.g.
the impact of an el niño on seal prey and foraging may be amplified during a sardine regime and dampened during an anchovy regime.
monitoring weaning mass and foraging behaviour over the entire course of the current cold anchovy regime may shed light on these issues.
this study shows that understanding the relationship between ocean climate and biological processes depends on having a long and complete time-series.
in the present study, single year records yielded estimates of weaning mass ranging from  <dig> to  <dig> kg, a  <dig> % change from lowest to highest estimate.
even samples of five consecutive years would have provided quite disparate estimates, e.g.
means of  <dig>   <dig> and  <dig> kg for the years 1975– <dig>  1985– <dig> and 1995– <dig>  respectively.
this study, ranging over  <dig> years, not only provides a more valid and reliable estimate of weaning weight of northern elephant seals from central california, it also presents a wide range of observed values and the circumstances associated with this variability.
even so, the present study suggests that representative sampling of pup weaning weight should cover at least two successive multidecadal regimes, about  <dig> continuous years!
CONCLUSIONS
elephant seals, one of the deepest divers in the ocean and a migratory mammal that spends 83% of the year feeding far from the coast in pelagic waters beyond the thermocline in the modal temperature range  <dig> – <dig> °c  <cit> , are impacted by ocean warming.
females spend more time searching for prey and are less successful in acquiring it when ocean temperatures are warmer than usual.
body stores laid down during long foraging periods at sea set limits on energy transfer to pups.
mean weaning mass of pups, consequently, fluctuates from year to year and reflects the impact of cyclic warming on the availability of elephant seal prey in the northeastern pacific in that year.
weaning mass declined significantly during the unusually warm period from  <dig> to  <dig>  it is well documented that offspring mass at the time of nutritional independence affects the probability of survival in many animals and is a crucial variable in population growth  <cit> .
it is notable that the decline in elephant seal weaning mass we report here paralleled in time the  <dig> to 80% decline of steller sea lions, eumetopias jubatus, northern fur seals, callorhinus ursinus, and non-migratory harbor seals, phoca vitulina, in the northern gulf of alaska that began in the late 1970s and only abated to approximately 5% in the late 1990s  <cit> .
our results provide additional support for the hypothesis that nutritional stress  <cit>  is a partial explanation for population declines such as these.
lastly, our findings support the assertion that monitoring aspects of ecosystems, in addition to monitoring climate data, facilitates early identification of regime shifts  <cit> .

 multidecadal fluctuations, or biological regime shifts, have basin-wide effects on sea surface temperature and upwelling that are associated with large-scale changes in biological productivity. they are similar to the 3– year cycles of the el niño southern oscillation but on longer time scales, and as background states, may amplify enso effects. the pacific decadal oscillation, which addresses the north pacific/north american sector, comprises events that persist for to years. in the 20th century, "cool" pdos prevailed from to while "warm" pdos were recorded from to and from through at least the mid-1990s. chavez and colleagues, focusing on the last two regimes, report that in the mid-1970s the pacific changed from a cool "anchovy regime" to a warm "sardine regime", which lasted until the middle to late 1990s before shifting back to an anchovy regime. the changing ocean temperatures associated with these regimes exert strong effects on marine food webs. in the northeastern pacific many marine birds and mammals flourish during anchovy regimes and are nutritionally stressed during sardine regimes. it is well documented that short-cycle ensos cause drastic reductions in survival and reproduction of several marine mammal species in the temperate waters of the pacific. non-migrating sea lions and fur seals are especially vulnerable to ocean warming because nursing females alternate feeding and nursing, which ties them closely to their coastal rookeries and results in pup starvation when prey move out of the area. it is not known, however, to what degree these processes impact deep-diving marine mammals that obtain their prey far below the surface as they range widely in the open ocean of the northeastern pacific far from the coast. the condition of newly weaned northern elephant seals, mirounga angustirostris, is expected to be sensitive to these processes because of the location where adults feed and the close link in energy exchange between mother and pup. in this species weaning weight reflects total energy transfer from the mother because pups feed solely on mother's milk prior to weaning. because the mother gives birth annually to a single pup and fasts during lactation, her pup's weight at weaning correlates positively with her resource accruing ability at sea during the eight-month period of pregnancy. the mean weaning weight of the colony reflects the general foraging success of parturient females in that year. it follows that mean weaning weight may oscillate annually with ocean processes that exert strong effects on marine food webs and impact prey availability. a similar logic has been used in studies of the southern congener, m. leonina, that breeds and feeds in antarctic waters northern elephant seals are among the deepest diving marine mammals. females disperse widely in the northeastern pacific twice a year, from to 60°n and from the coast to 188°e, foraging at modal depths of 300– m and maximum depths of m on prey such as mesopelagic squid and pacific hake, merluccius productus. after continuous foraging over the 8-month gestation period, females return to rookeries along the coasts of mexico and california to give birth and nurse their pups. in four weeks a pup triples its birth weight and is weaned when the mother returns to sea in february for a 1/ month post-partum bout of foraging to recover the one third of the body mass lost during nursing and begin the next pregnancy. milk energy transferred to the pup varies with its sex and the age, mass, blubber reserves and overall condition of the mother. by maternal condition, we mean the total body energy available to the female at the time of parturition. on average, ± % of a female's body reserves is depleted during lactation. the size and blubber reserves obtained by the female during her biannual foraging migrations determine the level of reproductive expenditure in the subsequent breeding cycle. pup weight at birth and weaning is correlated positively with the mother's age and mass. male pups are about 8% heavier than females at birth and at weaning, suggesting that males require more maternal energy than female pups. pregnant females are capital breeders that store energy in blubber laid down over months of continuous foraging at sea and expend much of this energy later in reproduction. acquisition of insufficient body reserves owing to low foraging success directly impacts reproductive expenditure resulting in low weaning weight of pups. additional data supporting these points are reported in studies of southern elephant seals. here we show that annual weaning weights of northern elephant seal pups and resource accrual of adult females associated with a rookery in central california fluctuate with ocean temperature cycles.  weaning mass the pattern of fluctuation in weaning weight over the years was similar in both sexes even though males were, on average, 5% heavier than females. the regression of weaning mass on year showed a statistically significant decline in both males and females. the regression slopes of males and females were not significantly different from each other. the weaning mass data of both sexes, combined, which we address in the rest of this paper, declined significantly from to as illustrated by the scatter plot and regression of weaning mass against year in fig. 2a. a loess smoothing plot, however, reveals two distinct slopes. mean weaning weight declined significantly from to the mid to late 1990s, decreasing from highs of kg in and kg in and to lows of and kg in and respectively. a linear regression line through the data over the year period from to shows a significant mean decline of kg/yr. a significant decline is also observed if the end data point is anywhere in the interval 1995– similarly, the decline is statistically significant when years with a sample size of less than are omitted, i.e. and. in the late mid 1990s, the declining trend ends abruptly and the mean weaning weight increases up to to kg in the new century. although the slope of weaning weight from to shown in fig. 2c, is positive, it is not statistically significant owing in part to the small sample size. the slope of weaning mass versus year, however, varied significantly between the two regimes. the period of declining weaning weight was associated with higher than normal ocean temperatures associated with the long-term sardine decadal cycle and the prevalence of el niños in the short-term enso cycles. weaning weight declined increasingly with the tenure of the sardine regime, a trend that ended abruptly with the shift to the anchovy regime in the late 1990s. with the regime shift, a reverse trend to increasing weaning weights is evident. mean weaning weight between the two regimes was not significantly different but this analysis is confounded by the declining slope of weaning mass over the course of the sardine regime and the small and incomplete sample size for the anchovy regime. a similar lack of association was found between weaning mass and pdo. declining weaning weight over the course of the decadal regime from to the late 1990s is consistent with the overlapping pattern of short-term enso cycles during this period, represented in fig. by annual mean monthly values of the standardized southern oscillation index. this was clearly a period of extended warmth in the pacific. warm el niños were more frequent than cold la niñas, the el niños of 1982– and 1997– were the most intense of the century, and the el niño in the early 1990s was unusually long. the global temperatures in and were the warmest years on record. the lowest weaning weights were observed from to an unusually warm period dominated by two el niños coming at the end of the sardine regime. the decline in weaning mass was not linearly related to increasing ocean temperatures during the sardine regime. although the period from to was warmer than usual, ocean temperatures did not increase significantly over the course of this period, as revealed by soi and pdo annual averages regressed on year. consequently, a linear regression plot reveals that annual mean weaning mass was not closely associated with annual soi or annual pdo averages. it is notable, however, that the mean weaning weight over the period 1990– which included the longest el niño and the most intense one of the century, was ± kg, significantly lower than the mean weaning weight of ± kg recorded in the period 1975– which included only the strong el niño of 1982–. female foraging success: foraging duration and mass gain to examine the relationship between weaning weight, ocean temperature and female foraging success, we used the spring post-breeding foraging trip duration, and mass gain over this period, as the metric of foraging success. during this foraging trip females can recover approximately 85% of the mass lost during parturition and nursing. we assume that foraging success in the spring is associated with and reflects foraging success in the fall. the duration of the fall foraging trip, however, is an unsuitable measure of foraging effort because its length is set by gestation, and is relatively invariant at ± days. the duration of spring foraging trips of females was inversely related to mass gained. the regression of mass gain on foraging duration was statistically significant. over the course of the sardine regime, post-partum females spent increasingly more time attempting to accrue resources, increasing the mean foraging trip duration by approximately 36% from the late 1970s to the mid to late 1990s. with the shift to the anchovy regime in the late 1990s, foraging durations decreased from days or more at sea to approximately days at sea from to the same level observed at the beginning of the sardine regime. association of foraging trip duration with shorter term el niño cycles is also evident, such as the longest foraging trip and lowest mass gain occurring in an exceptionally strong el niño year. mass gain during foraging trips was inversely related to trip duration, decreasing from approximately kg/day at sea during the late 1980s to approximately kg/day in 1993– to a low of ±. kg/day in and returning to kg/day in in spite of small annual sample sizes, variation around the overall mean was small. weaning weight of pups was inversely related to the foraging trip duration of females. linear regression of pup weaning mass on female foraging duration shows that this relationship was statistically significant. weaning weight and mass gain of females covaried in time but the sample size was small and the association was not statistically significant. colony composition and sex ratio the año nuevo colony increased over the course of the study period from pups produced in to pups produced in the decline in weaning mass from to was not associated with changes in the age composition of the colony such that more young females gave birth to smaller pups. rather, the opposite was observed; mean age at primiparity of females increased over the study period from a mean of years in the early 1970s to over years in the 1990s. the decline in weaning mass was not due to a sex ratio increasingly biased to smaller female pups. there were marginally more females in the sample from to, when mean weaning weight was highest, and more males in the sample from to when mean weaning weight was lowest. the sex ratio of weaned pups in the entire colony has not deviated significantly from unity during the last four decades. discussion our data show that the weaning mass of northern elephant seal pups varies with ocean temperature cycles. weaning mass declined over the course of an unusually warm 24-year period that coincided with the warm decadal or sardine regime and included several long and strong short-term warm enso events. the decline in weaning mass ended abruptly with the regime shift to the cold anchovy period. weaning mass increased with the return to the cold regime but monitoring throughout an entire anchovy period is required to reveal the entire pattern and determine whether the positive trend of increasing mass continues. the link between ocean temperature and weaning weight is strengthened by the finding that weaning mass was inversely related to the foraging effort of females and positively related to the mass they gained. over the course of the warm period from to about females spent increasingly longer periods searching for food; the more time spent searching, the less mass they gained. evidently, females attempted to compensate for reduced foraging success by staying at sea longer searching for food. this would buffer the effect of reduced foraging success and would obviously complicate any expectations of a linear relationship between soi and weaning mass and rates of female mass gain and pup weaning mass. the ability of a given female to extend her pre-implantation foraging trip, however, is limited by her need to implant in time to give birth during the breeding season and is impacted by her weaning date the previous year. females do not have the ability to compensate in this way during the gestational foraging trip. the relationship between soi and weaning mass is further confounded by changes in colony age distribution. as the colony has aged, weaning mass should increase based on our understanding of the relationship between maternal mass and investment. soi helps define the strength and duration of enso cycles but it does not predict weaning mass of pups well except during unusually strong or long el niños. we emphasize that it is important that we detected such a strong trend in pup weaning mass despite the trip duration compensation by females and the changing colony demographics. this implies that reduction in resources were greater than could be compensated for by increased foraging duration in any given year or by increasing maternal body size with age. by this logic, it is reasonable that we did not observe a linear relationship from soi and pdo to rate of foraging success to pre-implantation trip duration to weaning mass. it is clear that foraging effort was greatest and foraging success was lowest at the end of the sardine regime during the late 1990s. with the return to the cold regime in the late 1990s, female foraging effort returned to the level observed in the mid-1970s, the time of the previous regime change. we reason that warmer than usual ocean temperatures influenced elephant seal prey, either through recruitment, competitive advantages or predation, effecting a redistribution or diminution of populations. as a result, female seals were less successful forging, and consequently, at parturition and during lactation, they had less energy to transfer to their young. this logic is supported by observations during el niño years. during the strong el niño of 1997– foraging females spent more time travelling from one prey patch to another and they spent less time on each prey patch than in non-el niño years; mass gain in was by far the lowest value we have recorded. moreover, enduring effects of these "hard times" are suggested by the observation that female pups born in a strong el niño year, postponed breeding for the first time relative to adjacent cohorts in the years to. the link between seal condition and physical measurement of ocean temperature is complicated. the progressive decline in weaning mass over the course of the sardine regime is not simply explained by increasing ocean temperatures. the soi and pdo indices did not increase systematically over the course of this period. evidently, other factors such as the duration of exposure to warming or warm temperature spikes during enso cycles overlaid on a warm decadal cycle may have been operative. anthropogenic perturbations, or global warming, may amplify the effect of these natural cycles. in any case, the temporal link between temperature changes, effects on prey, and the weaning mass of pups is unclear. substantial lag effects may operate, as has been observed in the link between the pdo index and salmon catches. moreover, female seals spend of the months of the year foraging and it is not clear whether foraging is affected during all or part of this time. an additional complexity is the difficulty of separating the effects of short-term ocean temperature cycles from the long-term cycles. for example, in addition to the long period of warm water associated with the sardine regime, an unusually long el niño occurred during the early 1990s that had a more severe impact on marine mammals and birds than was predicted from its strength. overall, the data suggest that the duration and persistence of warm water may have a cumulative negative effect on seal prey and indirectly on seal foraging behaviour and, ultimately, on pup weaning mass. it is also likely that the long-term cycle, as background state, may amplify or dampen the effect of short-term cycles, e.g. the impact of an el niño on seal prey and foraging may be amplified during a sardine regime and dampened during an anchovy regime. monitoring weaning mass and foraging behaviour over the entire course of the current cold anchovy regime may shed light on these issues. this study shows that understanding the relationship between ocean climate and biological processes depends on having a long and complete time-series. in the present study, single year records yielded estimates of weaning mass ranging from to kg, a % change from lowest to highest estimate. even samples of five consecutive years would have provided quite disparate estimates, e.g. means of and kg for the years 1975– 1985– and 1995– respectively. this study, ranging over years, not only provides a more valid and reliable estimate of weaning weight of northern elephant seals from central california, it also presents a wide range of observed values and the circumstances associated with this variability. even so, the present study suggests that representative sampling of pup weaning weight should cover at least two successive multidecadal regimes, about continuous years!  elephant seals, one of the deepest divers in the ocean and a migratory mammal that spends 83% of the year feeding far from the coast in pelagic waters beyond the thermocline in the modal temperature range – °c, are impacted by ocean warming. females spend more time searching for prey and are less successful in acquiring it when ocean temperatures are warmer than usual. body stores laid down during long foraging periods at sea set limits on energy transfer to pups. mean weaning mass of pups, consequently, fluctuates from year to year and reflects the impact of cyclic warming on the availability of elephant seal prey in the northeastern pacific in that year. weaning mass declined significantly during the unusually warm period from to it is well documented that offspring mass at the time of nutritional independence affects the probability of survival in many animals and is a crucial variable in population growth. it is notable that the decline in elephant seal weaning mass we report here paralleled in time the to 80% decline of steller sea lions, eumetopias jubatus, northern fur seals, callorhinus ursinus, and non-migratory harbor seals, phoca vitulina, in the northern gulf of alaska that began in the late 1970s and only abated to approximately 5% in the late 1990s. our results provide additional support for the hypothesis that nutritional stress is a partial explanation for population declines such as these. lastly, our findings support the assertion that monitoring aspects of ecosystems, in addition to monitoring climate data, facilitates early identification of regime shifts. 
the weaning weight of northern elephant seal pups, mirounga angustirostris, being closely linked to maternal condition, indirectly reflects prey availability and foraging success of pregnant females in deep waters of the northeastern pacific. the aim of this study was to examine the effect of ocean climate on foraging success in this deep-diving marine mammal over the course of three decades, using cohort weaning weight as the principal metric of successful resource accrual. the mean annual weaning weight of pups declined from to the late 1990s, a period characterized by a large-scale, basin-wide warm decadal regime that included multiple strong or long-duration el niños; and increased with a return to a cool decadal regime from about to increased foraging effort and decreased mass gain of adult females, indicative of reduced foraging success and nutritional stress, were associated with high ocean temperatures. despite ranging widely and foraging deeply in cold waters beyond coastal thermoclines in the northeastern pacific, elephant seals are impacted significantly by ocean thermal dynamics. annual fluctuations in weaning mass, in turn, reflect the foraging success of females during the year prior to giving birth and signals changes in ocean temperature cycles.
 the condition of many marine mammals varies with fluctuations in productivity and food supply in the ocean basin where they forage. prey is impacted by physical environmental variables such as cyclic warming trends. the weaning weight of northern elephant seal pups, mirounga angustirostris, being closely linked to maternal condition, indirectly reflects prey availability and foraging success of pregnant females in deep waters of the northeastern pacific. the aim of this study was to examine the effect of ocean climate on foraging success in this deep-diving marine mammal over the course of three decades, using cohort weaning weight as the principal metric of successful resource accrual.  the mean annual weaning weight of pups declined from to the late 1990s, a period characterized by a large-scale, basin-wide warm decadal regime that included multiple strong or long-duration el niños; and increased with a return to a cool decadal regime from about to increased foraging effort and decreased mass gain of adult females, indicative of reduced foraging success and nutritional stress, were associated with high ocean temperatures.  despite ranging widely and foraging deeply in cold waters beyond coastal thermoclines in the northeastern pacific, elephant seals are impacted significantly by ocean thermal dynamics. ocean warming redistributes prey decreasing foraging success of females, which in turn leads to lower weaning mass of pups. annual fluctuations in weaning mass, in turn, reflect the foraging success of females during the year prior to giving birth and signals changes in ocean temperature cycles. 