Body size of female calves and natality rates of known-aged females in two adjacent Alaskan caribou herds , and implications for management

We studied body mass of female calves and natality rate of adult females in two adjacent Interior Alaskan caribou (Rangifer tarandus granti) herds during 1991-2001. Mass of newborn calves was similar in both herds, but Delta calves gained significantly more mass over summer than Nelchina calves. In contrast, Nelchina calves consistently maintained their mass during winter while Delta calves lost mass. Metatarsus length was similar in both herds in 4-month-old and 10-month-old calves, and it increased over winter in both herds. Natality rates of females >3 years old were consistently higher in the Delta Herd than in the Nelchina Herd, primarily because natality in 3to 5-year-old Nelchina females was low. Although body mass of Delta Herd calves consistently declined over winter, we concluded that nutrition was not significantly limiting herd growth. Managers are more likely to maximize harvest by maintaining the Delta Herd near its present size (i.e., 3500), or allowing it to increase only slightly. The only real option for increasing harvestable sur¬ pluses of caribou in the Delta Herd is reducing predation during calving and summer. In contrast, we conclude that sum¬ mer nutrition significantly limits potential population growth and body mass in the Nelchina Herd, and managers are more likely to maximize harvest by maintaining herd size at or below 30 000 than by allowing the herd to grow to near historical highs (i.e., 60 000-70 000).


Introduction
During the late 1970s and 1980s most caribou herds in Alaska grew significantly, and many herds reached relatively high densities (Valkenburg et al., 1996).During this period, the emphasis in caribou research in Alaska broadened from primarily studying preda¬ tion as a limiting factor to determining the influence of weather and population density on nutrition and productivity (Russell et al., 1993;Valkenburg et al., 1996;Adams & Dale, 1998;Lenart et al., 2002;Valkenburg et al., 2002, in press).This work has been of particular importance in the few caribou herds where the primary management goal is to maximize harvest and where managers have the abil-ity to control herd size through harvest.In these few herds it is important to be able to estimate optimum population sizes that might provide the highest har¬ vests over the long term.Therefore, in the early 1990s, Alaska Department of Fish and Game biolo¬ gists began monitoring the mass and size of female caribou calves and natality rates of known-age females in several economically important herds (Valkenburg et al., 2002).We chose this approach because changes in body size and natality rate have been shown to be useful indices of nutrition in ungu¬ lates and sensitive to changes in climate and popula¬ tion density (McEwan & Wood, 1966;Klein & Strandgaard, 1972;White et al., 1981;Clutton-Fig 1. Location of Delta, Nelchina, and Fortymile caribou herds.Brock et al., 1982;Peters, 1983;Reimers, 1983;Reimers et al., 1983;Skogland, 1983Skogland, , 1984Skogland, , 1985;;Beninde, 1988;Crete & Huot, 1993;Gaillard et al., 1996;Reimers, 1997).We concentrated our efforts on female calves because they are inexpensive to han¬ dle, they can be collared with an adult-sized radio collar, and they are subsequently recruited into the population as known-aged females.Furthermore, the mass and size of 4-and 10-months old calves is largely a function of quality and quantity of available food during late gestation, and during the calf's first summer of life, so calves primarily reflect annual changes in nutrition (Skogland, 1983(Skogland, , 1984;;Reimers, 1997;Valkenburg et al., 2000).
Research on calf size and natality has been partic¬ ularly important to managers of the Delta and Nelchina caribou herds where access for hunters is good, there is a strong hunting tradition, demand for wild meat production is high, and where the caribou have approached or exceeded previous population highs.In 1995, we increased research emphasis on the Nelchina and Delta herds in the hope of deter¬ mining the relative importance of summer and win¬ ter nutrition as limiting factors and providing man¬ agers with estimates of optimum population sizes for these herds.In this paper we compare changes in body size of female calves during summer and win¬ ter, and natality rates of females, and make inferences about the relative importance of winter and summer nutrition as limiting factors in these two herds.We also discuss management implications and provide initial estimates of optimum population sizes for these herds.

Nelchina Herd
The Nelchina Herd has been relatively well studied 204 since 1948, and it has fluctuated considerably in size since then (Van Ballenberghe, 1985;Tobey, 1999).During the late 1940s and early 1950s the herd numbered less than 10 000 but it increased rapidly to about 70 000 by the early 1960s following inten¬ sive wolf (Canis lupus) control.By the early 1970s the Nelchina Herd had once again declined below 10 000 and density dependent factors, predation, and overhunting were implicated in the decline (Doerr, 1979;Van Ballenberghe, 1985;Eberhardt & Pitcher, 1992).During 1975-1995 the Alaska Department of Fish and Game allowed the herd to grow while range conditions, and later, body condition, were being monitored.In the late 1980s, as the herd approached 30 000, Nelchina caribou began actively searching for new winter range.In 1987 many cari¬ bou moved northeast of traditional winter ranges in the Nelchina Basin to new winter ranges north of the Nutzotin Mountains (Tobey, 1999) (Fig. 1).This movement expanded, and within a few years a major¬ ity of the herd began using winter range on both sides of the Yukon-Alaska border (Tobey, 1993).Subsequently, most Nelchina caribou settled on win¬ ter range in eastcentral Alaska.These ranges are also used in some years by Fortymile Herd caribou.Until the mid-1990s, about 25-33% of the Nelchina Herd remained on traditional winter ranges in the Nelchina Basin, but since then, only about 10% of the herd continues to use this traditional winter range (Tobey & Scotton, 2001).
By the mid-1990s, the Nelchina Herd numbered about 50 000 and evidence of density-dependent effects on body size of calves and natality rate of adults began to appear (Tobey & Scotton, 2001).High caribou numbers obviously began to affect the distribution and biomass of lichens and other plants on primary summer range in the Talkeetna Mountains.After 1995 the Nelchina Herd declined from reduced calf production and survival and delib¬ erately heavy hunting (Tobey & Scotton, 2001).From 1997 to 2001 the herd varied between 29 000 and 39 000 and hunting was greatly reduced.The newer winter ranges used by the Nelchina Herd after 1987 obviously have a much higher lichen biomass than traditionally used ranges in the Nelchina Basin.Proportion of lichens in the winter diet of caribou on these new ranges is also comparatively high (Valkenburg et al., 2002).

Delta Herd
The Delta caribou herd has been intensively studied since 1979 (Valkenburg et al., 2002).Like most other herds in Interior Alaska, numbers were low (<2500) in the early 1970s.Following wolf control in the mid-1970s, the herd increased rapidly and  , 1996;Valkenburg et al., 1996).Wildlife managers had deliberately allowed the herd to grow to determine if density-dependent factors would eventually regulate herd size.As the herd increased, caribou changed winter ranges frequently and used nontraditional winter range in the Tanana Flats.Following severe summer and winter weather in the early 1990s, the herd declined because of heavy pre¬ dation and reduced calf survival (Valkenburg et al., 1996).Between 1995 and 2001 the herd remained relatively stable at about 3500-4500 caribou (Valkenburg et al., 2002).During the decline in the early 1990s, it was clear that nutrition was relative¬ ly poor compared with the late 1970s and early 1980s -body size and survival of calves was low, and natality rate in adults declined.After the population was reduced in the early 1990s and weather patterns moderated, nutritional condition of the herd largely recovered (Valkenburg et al., 2002).However, the proportion of lichens in the winter diet has remained relatively low compared with other Interior herds, and caribou have continued to pioneer new winter ranges (Valkenburg et al., 2002).

Methods
During  (Valkenburg et al., 1999).Four-month-old calves were captured during 27 September-14 October, and 10-month-old calves were captured during 1-25 April.Calves were weighed with calibrated electron¬ ic or spring scales, and metatarsus length of 4month-old and 10-month-old calves was measured with calipers.We monitored natality rates of radiocollared female caribou during mid to late May by documenting the presence of hard antlers and/or distended udders (Bergerud, 1964;Whitten, 1995).
We used a linear model of mixed effects to exam¬ ine potential differences in newborn, 4-month-old, and 10-month-old female calf mass.We used the same model to examine differences in metatarsus length in 4-month-old and 10-month-old female calves.The following model was used: where Zijk is the mass (or metatarsus length) for the ith herd, i = Delta or Nelchina, for the jth year, and Rangifer, Special Issue No. 14, 2003 k indicates the replicate for the ith herd in the jth year; u is an overall mean effect, ^i is a fixed effect for herd, Yj is a random effect for year, and (^y)ij is an interaction term that allows separate random effects among years for each herd.We used this model for each age class: newborns, 4-month-olds, and 10month-olds.We compared age-spe¬ cific natality rates of radiocollared females between herds by calculating confidence limits for the binomial distribution.

Discussion
Even though female Delta caribou calves consistently lost mass over winter, at 10 months of age they remained heavier than Nelchina calves because Nelchina calves gained significantly less mass over summer, and they were not able to gain mass over winter.Because of the apparently superior winter nutrition of the Nelchina caribou we would  \O--HC\lC0VOW^ i/-Ncd C\l oooZ;Z;og have expected to see consistently higher newborn calf mass (cf.Skogland, 1984), but mass of newborn calves was similar in both herds.Because of the apparently superior summer nutrition of Delta cari¬ bou we expected to see consistently higher natality in Delta females (cf.Reimers, 1997).Natality rates of 3-to 5-year-old Delta females were higher than natality rates of 3-to 5-year-old Nelchina females.Despite higher natality and better summer nutri¬ tion in the Delta Herd, relatively few calves remained in the herd in autumn because of heavy predation by wolves, grizzly bears (Ursus arctos), and golden eagles (Aquila chrysaetos) (Valkenburg et al., 2002).Despite the higher natality of the Delta Herd, autumn calf:cow ratios in the Nelchina Herd were consistently higher than in the Delta Herd.During winter, mortality of the radiocollared calves was similar in both herds (i.e., about 40%) (Tobey & Scotton, 2001;Valkenburg et al., 2002).
Historically, the Nelchina Herd reached a popula¬ tion high of about 70 000 during the early 1960s, followed by a major decline to less than 10 000 by 1972 (Van Ballenberghe, 1985;Eberhardt & Pitcher, 1992).There has been much debate about causes of the decline, but there was clear evidence that nutri¬ tion was limiting (Eberhardt & Pitcher, 1992).In view of the strong evidence of nutritional limitation on summer range while the herd has recently fluctu¬ ated between 50 000 and 30 000, it seems even more unlikely now that the high caribou population pres¬ ent on the Nelchina range in the 1960s was sustain¬ able.Similar strong evidence of limiting summer nutrition was not documented in the Delta Herd during its population high in 1989, although the herd peaked and declined so rapidly that there may not have been sufficient time for evidence of poor summer nutrition to become obvious (Valkenburg et al., 1996).

Management implications
At present, harvestable surpluses of caribou are rela¬ tively low in the Nelchina and Delta herds and har¬ vest must be restricted largely to males to keep herd sizes from declining.To increase harvestable surplus¬ es of caribou in the Delta Herd it may be desirable to increase herd size slightly (perhaps to about 4000¬ 5000) even though there are indications that winter food is not abundant.At the present time there is no evidence that winter range is significantly limiting population growth either through production or sur¬ vival.However, if herd size is increased we expect that body condition of females would decline during winter (particularly during severe winters), and neonatal calf survival would eventually decline (Adams et al., 1995).It appears therefore, that reduc¬ ing predation is the only real option for increasing harvest --the herd is currently stable or declining slowly because of high mortality of calves in summer and this mortality is not related to nutrition (Valkenburg et al., 1999;Valkenburg et al., 2002).
In the Nelchina Herd, reducing herd size further or maintaining it at about 30 000 may alleviate over¬ use of summer range and thus improve natality in 3to 5-year-olds.The dilemma for managers of the Nelchina Herd is that predation is probably already a significant limiting factor, and reducing herd size further might exacerbate the problem.However, it seems inadvisable at present to allow herd size to increase because of the already strong effect of the heavily used summer range on natality.

Table 1 .
Mean mass with standard deviations (s) in kg of female newborn, 4-month-old, and 10-month-old caribou calves in the Delta caribou herd.

Table 2 .
Mean weights and standard deviations (s) in kg of female newborn, 4-month-old, and 10-month-old caribou calves in the Nelchina caribou herd.

Table 3 .
Model predictions for mean mass and standard deviation (s) in kg of newborn, 4-month-old, and 10 month-old female caribou calves in the Delta and Nelchina caribou herds.