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Estimating the Juvenile Survival Rate of Male Northern Fur Seals ( Callorhinus ursinus )

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Abstract

Three methods for estimating the survival rate of juvenile northern fur seals (Callorhinus ursinus) are developed from the earlier works of Chapman, Smith and Polacheck, and Lander. Each of the methods I propose divides the estimated number of males alive at 2 yr of age by the estimated number of pups born in their year class. The number of surviving juveniles are reconstructed by back calculation using the number of males killed during the commercial harvest and the subsequent counts of bulls. The three methods differ in their assumptions concerning subadult survival and escapement from the harvest, although all produce similar estimates when applied to the St. Paul Island fur seals. These new estimates of juvenile survival (1950-80) are strongly correlated with the ratio of cohort kill to pup production and with estimates from the currently-used Lander procedure. This is because the harvest mortality of males is large compared with natural mortality. The new methods perform acceptably over a wider class of data than Lander's. Their greatest advantage over current procedures is that they provide a better insight into the reliability of the survival estimates they produce. Trois mCthodes pour estimer le taux de survie des jeunes adultes d'otarie a fourrure (Callorhinus ursinus), ont ete mises au point d'aprPs les travaux anterieurs de Chapman, Smith et Polacheck, et Lander. Chacune des methodes proposkes divise le nombre estirne de rnbles vivants i I'ige de deux ans, par le nombre estirne de jeunes nes
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... Attempts to determine these coefficients have been made repeatedly, and there is a large volume of literature on this issue (Chapman, 1964;Ichihara, 1972;Lander, 1975;Andreev et al., 1978;Trites, 1989;Corey et al., 2003;etc.). As the initial information, data are used on the size of the offspring, commercial extraction of seals, and the number of bulls. ...
... To calculate the survival rate of male seals up to the age of two years, we used the methods of Lander (1975) and Trites (1989) and their modifications. For the remaining age groups, the survival rates were calculated in a manner similar to the procedure proposed by Frisman et al. (1985), and completed by the quality assessment of the parameters obtained at the stage of calculating the survival of bulls. ...
... The reliability of all three methods was studied in detail by Trites (Trites, 1989); the Lander method was criticized the most, because, as the study showed, its estimates are biased and correlate more with the volume of the hunting than with the real survival of the young; i.e., a large amount of hunting provides a high survival rate, and a decrease in captures inevitably leads to a decrease in the estimated survival rates of the young. This effect can also be observed in our study for the late period , when the hunting strat- egy changed, as extraction became more sparing in order to help the herd out from depression. ...
Article
We discuss and improve the methods proposed previously for estimating the survival rate of male northern fur seals representing different age groups. Estimates of the number of pups and bulls per rookery are given, as well as information on the age structure of animals hunted during the coastal harvest. The data were obtained by researchers from the Pacific Research Fisheries Center over 56 years of observation of the fur seal (Callorhinus ursinus) herd on Tyulenii Island, located in the southwestern part of the Sea of Okhotsk, south of Cape Terpenia, 15 km from Sakhalin Island. Lander’s method and its modifications were used for estimating the juvenile survival rate of male fur seals. This methodology was found to have not been working properly for the past several decades (since the end of the 1980s) due to changes in population harvesting. Satisfactory estimates for all characteristics of the male life cycle were obtained. Structural changes in survival ability were revealed as having occurred at the end of the 1980s, i.e., the survival of subadult and adult males increased slightly. New estimates of survival rates allow us to create a model of the dynamics of bull numbers that is in good agreement with that observed, its mean error of approximation equaling 3.2%.
... It makes the methods of estimating the survival rates for different age groups essential for such prediction. There have been many attempts to determine these rates, and a large body of literature has ensued (Andreev et al., 1978;Chapman, 1964;Corey et al., 2003;Ichihara, 1972;Lander, 1975;Trites, 1989;etc.). Numbers of newborn pups, bulls, and harvest information were used as the input in most of these works. ...
... The survival rates of fur seal males from birth to two years of age were estimated using a modified Lander's method (Lander, 1975), and using a new method based on Trites' approach (Trites, 1989). For all other age groups, the survival rates were calculated using the procedure suggested by Frisman et al. (1985). ...
... In particular, besides the most commonly used Lander's method (Lander, 1975), there are also Chapman's (Chapman, 1964) and Smith and Polacheck (Smith and Polacheck, 1984) methods, which only determine the lower bound of the male survival rate from birth to three years of age, and are based on certain assumptions on harvest structure. The reliability of all three methods is studied in details by Trites (1989), with the most critique directed to Lander's method due to correlation of his estimates with the harvest size. ...
Article
Following many years of managed harvest, the population size of Northern fur seal on Tyuleniy Island appeared to be depressed. In order to preserve the population, the harvest was first significantly limited, and then banned altogether. However, the birth rates have never recovered. We perform the estimation and analysis of survival rates of different groups of males in the population with the goal of elucidating the underlying changes in intra-population dynamics. We use the estimated numbers of adult males and pups in the rookery along with the exact numbers and ages of animals, harvested over the years. Our investigation shows that the unexpectedly slow growth of the population is most likely not caused by an abrupt decline of survival rates of any age groups. There is no substantial change in juvenile survival, and the survival rates of the older animals are actually increasing. However, the analysis of density-dependent factors of juvenile survival demonstrates significant increase in intra-species competition. This may be caused by change in environmental conditions such as quality and availability of food. Also, the dynamics of pups’ viability is telling: it was the highest at the beginning of the period of observations, and then it sharply decreased following intensification of harvest, and finally started growing again in response to strong conservation measures, but never achieved its original level. These facts may be consequence of changes in the population gene pool, which shows the importance of research of the evolutionary consequences of harvest.
... The rate of mortality from weaning to 2 years old is a parameter that exerts a strong influence on NFS population stability, and variability in this parameter has been hypothesized to influence population trends 25,26 . NFS survival from weaning to 2 years old is variable and often low, with estimates of cohort survival for males from the EPS ranging from roughly 15% to 50% 25,27,28 . Knowledge of what causes mortality during this time is limited, because pup (4-to 12-month-old) and juvenile (1-to 2-year-old) NFS are highly pelagic and spend very little time on the breeding islands before two years of age 3,9,17 . ...
... The fitted values for mean trip durations were 8.4 d (95% CI: 6.5-10.8 d), 25.0 d (19.7-31.8 d), and 28.4 d (17.7-45.5 d) for trips originating from BG, PRB, and SM, respectively, on 21 October (the median observed pre-migratory trip departure date). ...
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In species exhibiting differential migration by sex and age, understanding what differences exist, and the adaptive reasons for these differences is critical for determining how demographic groups will respond to environmental variability and anthropogenic perturbations. We used satellite-telemetered movement and diving data to investigate differential migration and its ontogeny in a highly migratory North Pacific Ocean predator, the northern fur seal (Callorhinus ursinus; NFS), with a focus on understudied juvenile (1-to 2-year-old) animals. We instrumented 71 juvenile NFS in two years (2006-07 and 2007-08) at three major North American breeding sites and compared their migratory strategies with pups and adults. Although sexual dimorphism is strong in adult NFS, only weak differences in body mass between sexes were found in juveniles, which had similar body mass to pups (~3-4 months). However, unlike widely-dispersed pups, juvenile male and female NFS dispersed in different directions, and used different habitats characterized by distinct hydrography and prey assemblages during migration, similar to breeding adults. Juvenile diving behavior differed only modestly among habitats and between sexes, consistent with weak differences in body mass. Evidence of habitat sexual segregation by juvenile NFS contradicts previous hypotheses that physiological differences predominantly drive the ontogeny of differential migration.
... Juvenile male survival from birth to 2 years of age w 02 [21,23,24] is estimated by determining its upper and lower limits (w 02U and w 02L ) based on assumption that the annual average survival of subadult males in the generation of interest (aged 2-7 years, w 27 ) is constant: w 27 (n) = w 2,3 (n) = w 3,4 (n + 1) = w 4,5 (n + 2) = w 5,6 (n + 3) = w 6,7 (n + 4). It should be emphasized that coefficient w 02 characterizes their survival during the first two years of life, whereas w 27 , during one year (each year between 2 and 7 years). ...
... Changes in the pattern of harvesting made it necessary to modify previous methods for estimating w 02 [23,24]. After excluding the assumption concerning the size of harvest and including information that males of this generation replenish the group of bulls upon reaching the age of 7 years, we managed to obtain adequate estimates of the survival of juvenile and subadult males [25,26]. ...
Article
Tendencies in the dynamics of harvested northern fur seal (Callorhinus ursinus) population on the Tyulenii Island have been analyzed in detail. The results show that retardation of reproduction (decrease in the numbers of pups) and decline in the survival of young females (up to 3 years of age) by the late 1980s resulted in a reduction of the total number of females and a significant increase in the proportion of older females. This tendency changed during the later observation period (after 1988–1989) due to increase in the survival of young females: the female population has gradually recovered, with the proportion of young females increasing at the expense of old females (aged over 10 years). The age composition of males has also changed: the proportion of young animals has decreased, while that of large mature males (bulls) has increased. Moreover, the number of bulls continues to increase and has already exceeded the level that formerly provided for the well-being of the population. This, a paradoxical situation has arisen: the numbers of females and bulls are increasing, whereas pup production remains at a low level.
... Надежность работы этих трех методов детально исследована А.Тритесом (Trites, 1989); наибольшей критике подвергся метод Ландера, поскольку, как показало исследование, его оценки смещены и коррелируют больше с объемом промысла, чем с реальной ювенильной выживаемостью; т.е. большой объем промысла дает высокую выживаемость, а уменьшение изъятия неизбежно приводит к снижению расчетных значений ювенильной выживаемости. ...
... А. Тритес [23] предложил свою методику оценки ювенильной выживаемости, которая в меньшей степени подвержена влиянию предположений о возрастной структуре совокупности особей, изымаемых в процессе промысла, чем метод Ландера. В основе методики Тритеса лежат оценки верхней (w 02U ) и нижней (w 02L ) границы ювенильной выживаемости (самцов от рождения до двух лет), а также предположение, что среднегодовая выживаемость холостяков рассматриваемого поколения (самцов от двух до семи лет, w 27 ) постоянна: w 27 = w 23 = w 34 = w 45 = w 56 = w 67 . ...
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Для рационального использования запасов северного морского котика немаловажное значение имеет оценка и прогнозирование численности основных возрастных групп самцов. Данные непосредственных наблюдений позволяют оценить лишь величину приплода (количество новорожденных щенков) и суммарную численность секачей, также известен возрастной состав животных, добытых в ходе промысла. Для прогнозирования по этим данным требуется вычислять коэффициенты выживаемости отдельных возрастных групп самцов котиков. В данной работе обсуждаются и модернизируются предложенные ранее методики оценки коэффициентов выживаемости самцов. Для сужения расчетных интервалов ювенильной выживаемости используется дополнительная информация (в частности, расчетные выживаемости самцов на последующих стадиях жизненного цикла), а также корректируется процедура оценки. Предложены два метода расчета коэффициентов выживаемости самцов на различных этапах жизненного цикла: метод итераций и метод определения границ выживаемости сразу всех возрастных групп из уравнения динамики. Оба метода базируются на имеющихся данных наблюдений и особенностях жизненного цикла рассматриваемого вида, они не требуют выполнения каких-либо предположений о характере промысла. С их помощью получены оценки коэффициентов, характеризующих основные этапы жизненного цикла самцов котиков. Оба метода дают похожие регрессионные оценки средней по периоду выживаемости секачей. При этом первый метод предсказывает более высокую ювенильную выживаемость и низкую выживаемость холостяков; а второй, наоборот, завышает выживаемость холостяков и занижает ювенильную выживаемость. По результатам регрессионного анализа оба набора оценок хорошо совместимы с данными наблюдений. В итоге мы получили наборы оценок, которые являются адекватными границами параметров модели для проведения полномасштабных многовариантных вычислительных имитационных экспериментов.
... year at sea (Trites and Bigg, 1996). Mortality rates are up to 60% in the first 2 years of life for northern fur seals (Trites, 1989;Gentry, 2002), but are hypothesized to have been lower prior to the population decline (Trites, 1989). In contrast, early mortality rates of Antarctic fur seals are around 30% (although it has never been precisely calculated from birth to first return on land, Boyd et al., 1995a;Wickens and York, 1997). ...
... year at sea (Trites and Bigg, 1996). Mortality rates are up to 60% in the first 2 years of life for northern fur seals (Trites, 1989;Gentry, 2002), but are hypothesized to have been lower prior to the population decline (Trites, 1989). In contrast, early mortality rates of Antarctic fur seals are around 30% (although it has never been precisely calculated from birth to first return on land, Boyd et al., 1995a;Wickens and York, 1997). ...
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Efficient extraction of energy from the environment is key to the survival and reproductive success of wild animals. Understanding the ratio of energy gained to energy spent of different foraging strategies (i.e., foraging efficiency) can shed light on how animals cope with environmental changes and how it affects population trajectories. I investigated how female foraging strategies during the breeding season impact the foraging efficiencies and reproductive successes of two fur seal species—one declining (NFS–northern fur seals, St. Paul Island, Alaska) and one increasing (AFS–Antarctic fur seals, Kerguelen Island, Southern Ocean). I also sought to develop new accelerometry-based methods to easily determine fine-scale energy expenditure at sea (VeDBA and flipper stroke metrics). Twenty lactating females of each species were captured and equipped with biologging tags to record GPS locations, depth and tri-axial acceleration. Energy expenditure for each foraging trip was measured using the doubly-labeled water method, and energy gained while foraging was determined from 1) diet composition (scat hard-parts and DNA) and blood stable isotope ratios; and 2) numbers of prey capture attempts (from head acceleration). Maternal investment in pups was determined from pup growth rates or from energy content of milk samples. Results showed acceleration metrics were only accurate at predicting energy expended by fur seals when time-activity budgets were taken into account (i.e., time spent performing different types of activity). Foraging strategies of AFS females resulted in efficiencies of ~3.4, with more efficient females producing bigger pups at weaning that had greater chances of survival. NFS females employed two foraging strategies with very different efficiencies (~1.4 vs ~3.0) that were associated with different foraging habitats and diet qualities. However, NFS with the more efficient strategy (3.0) undertook longer foraging trips than the other NFS (1.4) or AFS (3.4), and thus fed their pups ~20% less frequently. As a consequence, the declining NFS (unlike the increasing AFS) had to compromise between the rate of energy acquisition and the pup feeding frequency. Such reductions in energy intake and time allocated to nursing pups can ultimately lower juvenile survival, and may explain the population decline of NFS in Alaska.
... A new method for estimating both upper and lower bounds of the juvenile survival rates: w02L and w02U (Equations A1 and A2) were developed in [23], which was based on Trites' approach [61] without any assumptions about harvest. At that, the larger the number of harvested animals the narrower estimated intervals (w02L, w02U) for the value of w02. ...
Article
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We examine population trends in light of male harvest data considering the long-time series of population data on northern fur seals at Tyuleniy Island. To answer the question has the way males were harvested influenced the population trajectory, we analyzed the visual harem size and birth rate dynamics of the population, as well as the strategy and intensity of the harvest. We analyzed the dynamics of the sex ratio in the early (1958–1988) period to estimate parameters in the late period (1989–2013) based on the observed number of bulls and pups, while utilizing the distribution of reproductive rates obtained from pelagic sealing. Using a matrix population model for the observed part of the population (i.e., the male population), we analyzed the population growth rate associated with changes in both birth and survival rates considering the stochastic effects. Observations allow us to reject the hypothesis of nonselective harvest. Among the variety of natural and anthropogenic factors that could contribute to the decrease in the birth rate in the population, the effect of selective harvesting seems to be the most realistic.
... Estimated pup survival on land from birth to weaning averaged 0.901 (range: 0.786-0.974) from 1950 through 1987 [35]. Annual survival rates from ages 2 through 14 years are 0.80 or higher, and males and females rarely live longer than 17 years and 24 years, respectively [5]. ...
Article
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The Pribilof northern fur seal (Callorhinus ursinus) herd in the eastern Bering Sea has declined by ~70% since the 1970s, for elusive reasons. Competition for pollock (Gadus chalcogramma) with the commercial fishery has been suspected as a contributing factor, but no correlative relationship between fishing activity and fur seal population declines has heretofore been demonstrated. Here, we present evidence for a moderately strong inverse relationship between fishery catches of pollock and first-year survival of fur seals, based on three different approaches to evaluation. We suspect this relationship results from the dependence of lactating female fur seals on locating dense and extensive schools of pollock near the Pribilof Islands to efficiently provide nutrition for their pups, because the pollock fishery also targets these same schools, and when fished, the remnants of these schools are fragmented and dispersed, making them more difficult for fur seals to locate and exploit. Inadequately fed pups are less likely to survive their initial independent residence at sea as they migrate south from the Pribilof Islands in the fall. Our results imply that pollock catches above ~1,000,000 t within ~300 km of the Pribilof Islands may continue to suppress first-year survival of Pribilof fur seals below the estimated equilibrium survival value of 0.50, leading to continued decline of the population.
Article
The dynamics of the survival and number of female northern fur seals, Callorhinus ursinus, on Tyuleniy Island have been studied. The data on the age structure and physiological condition of females (obtained during observations in 1958–1988) were used to estimate the model parameters. The adequacy of the model estimates of female number was evaluated using the data on the number of newborn pups at the rookery in 1958–2013. It has been found that a reduction in the survival rate of individuals of younger age groups, a decrease in the proportion of females, as well as aging of the population occurred in the 1958–1988 period. The use of numerical simulation methods shows that the rate of survival of females among individuals under 3 years of age increased substantially after 1988, indicating a positive trend in the dynamics of their number.
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A mathematical model incorporating the basic life history features of the North Pacific fur seal (Callorhinus ursinus) approximated the decline of the Pribilof Islands population by reconstructing pup estimates and counts of adult males over the period 1950 to 1987. Simulation results suggest that commercial female harvesting and a series of poor juvenile survival rates were responsible for causing and maintaining the observed decline in pup pro- duction on St. Paul Island from 1956 to 1970. A more recent drop in pup production since 1976 is also attributed to poor juvenile survival, but with the addition of higher natural mortalities of adult females. It appears that the natural mortality of adult females may have increased by 2 to 5% beginning in the mid 1970s. We suspect reductions in the fur seal food base and entanglement-related mortality associated with commercial fishing in the North Pacific are contributing to the current decline, although neither possibility has yet been clearly demonstrated.
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Three methods of estimating average age at first reproduction of the northern fur seal, Callorhinus ursinus, are presented. For the first, the percentage of females pregnant for the first time is estimated from the percentage of primiparous pregnant females in the population. For the second, the percentage of females pregnant for the first time is estimated from the difference in the percentages of females pregnant at least once at successive ages. The jackknife variances of both estimates are presented. The estimates are calculated for the 1954–64 year-classes of northern fur seals using data collected at sea by the United States and Canada during 1958–74. By fitting parametric curves to the fraction pregnant at least once as a function of age and applying the second method to that resulting curve, the average age at first reproduction is estimated in a third way; this method is applicable to year-classes for which data are incomplete and estimates are given for the 1952, 1953, 1965, and 1966 year-classes. Age at first reproduction was lowest for the 1952–55 year-classes, increased sharply beginning with the 1956 year-class, and then gradually decreased almost to the level of the 1952–55 year-classes. Average age at first reproduction is negatively correlated with juvenile survival (birth to age 2 yr).Key words: fur seal, northern fur seal, Callorhinus ursinus; average age at first reproduction, cohort analysis, population dynamics
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Mathematical simulation huas been used to analyse the population dynamicsm of sixteed sex and age groups of a northern fur-seal herd. Principal attention is given to determining the model parameters on the basis of the results of catches and observations of the state of the herd. Detailed methods are suggested for estimating survival coefficients for males and females from various age groups. The dependence of the number of pups born upon the size of and the ratio of the sexes in the reproductive part of the population has also been studied.