Detecting Density Dependence in Recovering Seal Populations

Svensson, Carl-Johan; Eriksson, Anders; Härkönen, Tero; Hårding, Karin C.

doi:10.1007/s13280-010-0091-7

Cited by 15 publications

(15 citation statements)

References 47 publications

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“…Survival rates of the adults in our study (0.87-0.92 in 4-15-year-old individuals) fall into the range found earlier for the Baltic grey seal (Harding et al 2007, Svensson et al 2011, and for other seal populations (e.g. Harwood & Prime 1978, Smith 1987, Wickens & York 1997, Schwartz & Stobo 2000.…”

Section: Other Sources Of Mortalitysupporting

confidence: 82%

“…The survival rate of pups during their first year of life (0.53 for males and 0.55 for females) in 2005-2009 was a little lower as compared with the value (0.62 for females) given for British grey seals by Hall et al (2001), and 0.70 given for Baltic grey seals without density dependence (Svensson et al 2011), whereas the survival rate of female pups in 2000-2004 (0.76) was higher than that of British seals. Survival rates of the adults in our study (0.87-0.92 in 4-15-year-old individuals) fall into the range found earlier for the Baltic grey seal (Harding et al 2007, Svensson et al 2011, and for other seal populations (e.g.…”

Section: Other Sources Of Mortalitycontrasting

confidence: 51%

“…According to Harding et al (2007), the risk for extinction increases as the occurrence of unpredictable events increases. We must, however, keep in mind that juvenile survival rates are also dependent on population density: they may decrease rapidly with increasing density (Svensson et al 2011). Because the growth rate of the Baltic grey seal population seems to be ceasing and there are changes in the age structure, pup production and survival rates of the population (e.g.…”

Section: Other Sources Of Mortalitymentioning

confidence: 99%

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Demographic Structure and Mortality Rate of a Baltic Grey Seal Population at Different Stages of Population Change, Judged on the Basis of the Hunting Bag in Finland

Kauhala¹,

Ahola²,

Kunnasranta

2012

Annales Zoologici Fennici

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We examined the demographic structure and mortality rate of the Baltic grey seal (Halichoerus grypus) population from the early 2000s when the population increased rapidly to the late 2000s when the growth rate slowed down. We calculated life tables based on the age structure of hunted grey seals in the Finnish sea area. The catch was treated as a sample of the dying part of the population. The catch was male biased and the proportions of female pups and mature males in the catch increased from the early to late 2000s. Annual mortality rate of the youngest age classes was high and higher among males than females, which resulted in low sex ratio (males to females) of the population. Sex ratio was, however, higher in the latter years due to the increased mortality rate of females, especially those < 10 years of age. Accordingly, the proportion of mature females in the population decreased and relatively fewer pups were produced during the late 2000s than in the early 2000s.

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Section: Other Sources Of Mortalitysupporting

confidence: 82%

Section: Other Sources Of Mortalitycontrasting

confidence: 51%

Section: Other Sources Of Mortalitymentioning

confidence: 99%

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Demographic Structure and Mortality Rate of a Baltic Grey Seal Population at Different Stages of Population Change, Judged on the Basis of the Hunting Bag in Finland

Kauhala¹,

Ahola²,

Kunnasranta

2012

Annales Zoologici Fennici

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“…The initial periods were the intuitively obvious ones: late on in the growth of the population while it was decelerating towards the carrying capacity. The others seem to be associated with an overcompensatory response in the model (Svensson et al 2011, Fig. 2b), a feature those authors noted.…”

supporting

confidence: 60%

“…

A recent simulation study (Svensson et al 2011) proposed the use of changes in age-structure as an indicator of density dependent effects on the growth of the Baltic grey seal (Halichoerus grypus) population. That approach provides a clever alternative to the direct examination of abundance estimates, and sidesteps the well-known difficulty of detecting trends in marine mammal populations from survey data (Taylor et al 2007).

…”

mentioning

confidence: 99%

Detecting Density Dependence in Recovering Seal Population is Difficult: A Response to Svensson et al. (2011)

Lonergan

2011

AMBIO

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A recent simulation study (Svensson et al. 2011) proposed the use of changes in age-structure as an indicator of density dependent effects on the growth of the Baltic grey seal (Halichoerus grypus) population. That approach provides a clever alternative to the direct examination of abundance estimates, and sidesteps the well-known difficulty of detecting trends in marine mammal populations from survey data (Taylor et al. 2007). However, there are three features of Svensson et al.'s analysis that limit its utility as a demonstration of the practicality of their methodology as a tool for investigating real-world problems. ARTEFACTSThe first issue is that the results, as presented, seem to contain artefacts, patterns produced by the analysis rather than arising from the data. Simulations where the environmental carrying capacity was quite stable and density dependence only acted on populations close to their carrying capacity contained multiple periods when density dependence was detectable (Svensson et al. 2011, Figs. 3c, 4c). The initial periods were the intuitively obvious ones: late on in the growth of the population while it was decelerating towards the carrying capacity. The others seem to be associated with an overcompensatory response in the model (Svensson et al. 2011, Fig. 2b), a feature those authors noted. Such effects are sensitive to many factors including: the pattern of density dependent mortality; whether it exclusively affects pups; and the distribution of ages of first reproduction of females. Changes in any of these could alter the timing, duration and number of periods in which density dependent effects are detectable. None of them are easy to investigate, and the prevalence of overcompensatory effects in actual pinniped population trajectories seems an unexplored area. The examples in Svensson's paper suggest that density dependence is detectable for longer periods when abundance, rather than age-structure, is used as the indicator (Svensson et al. 2011, Figs. 3, 4), but it is difficult to anticipate how the relative durations would be affected by modifications to the model's structure. METHOD FOR DETECTING DENSITY DEPENDENCEA second difficulty arises out of a mismatch between the population model and the regression used to identify density dependence. The population model appears almost deterministic, with stochasticity only occurring in the environmental carrying capacity. No uncertainty was described in the assessment of abundance. A significant quadratic term in a regression of log(abundance) against time was used to indicate the occurrence of density dependent effects. Regression assumes that each datapoint is independently drawn from a distribution centred on the appropriate point on an overall trendline (Gerrodette 1987), but these data are not independent: each year the abundance lies below the The online version of the article commented upon can be found at

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Risk for overexploiting a seemingly stable seal population: influence of multiple stressors and hunting

Silva

Bottagisio

Härkönen³

et al. 2021

Ecosphere

Self Cite

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Conservation efforts have mainly been focused on depleted species or populations, but many formerly reduced marine mammal populations have recovered to historical abundances. This calls for new management strategies and new models for ecological risk assessment that incorporate local density dependence and multiple environmental stressors. The harbor seal metapopulation in Swedish and Danish waters has increased from about 2500 to 25,000 over the past 40 yr. Trend analysis based on aerial survey data and somatic growth curves indicates that the population is close to carrying capacity. We performed a population viability analysis based on realistic life history parameters and investigated a range of potential scenarios caused by future stressors. If the population is able to resume its high intrinsic rate of increase at about 11% annually, when pushed down below carrying capacity, it can also sustain additional mortality such as modest hunting and infrequent epizootics. However, if xenobiotics will cause even a slight reduction in average fecundity, the population becomes significantly more vulnerable. In the absence of epizootics, and given full reproductive capacity, hunting of a few hundred animals annually is not harmful to the long-term persistence of the population. Nevertheless, a slight decrease in growth potential, for example, caused by exposure to endocrine disruptors, makes even limited hunting risky. Our study shows how an apparently stable and abundant marine mammal population can be close to a point of rapid population decline. Thus, careful monitoring of population size, growth rate, health, and exposure to xenobiotics as well as recording of the age and sex structure of the hunt is required to avoid repeating the history of overexploitation and another population collapse.

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Detecting Density Dependence in Recovering Seal Populations

Cited by 15 publications

References 47 publications

Demographic Structure and Mortality Rate of a Baltic Grey Seal Population at Different Stages of Population Change, Judged on the Basis of the Hunting Bag in Finland

Demographic Structure and Mortality Rate of a Baltic Grey Seal Population at Different Stages of Population Change, Judged on the Basis of the Hunting Bag in Finland

Detecting Density Dependence in Recovering Seal Population is Difficult: A Response to Svensson et al. (2011)

Risk for overexploiting a seemingly stable seal population: influence of multiple stressors and hunting

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