Population Wide Decline in Somatic Growth in Harbor Seals—Early Signs of Density Dependence

Maëlle Salmon,Karin C Harding,Rune Dietz,Jonas Teilmann,Tero Harkonen

doi:10.3389/fevo.2018.00059

Abstract

The harbour seal populations of Danish and Swedish waters have had turbulent population dynamics during the last century. They were severely depleted by hunting in the beginning of the 20th century, followed by rapid recovery due to protective measures. They were victims to two mass mortalities caused by Phocine Distemper Virus (PDV) epidemics. Long term monitoring and intensive sampling during the last decades now allow analysis of population level phenomena in response to shifting population size. We compare somatic growth curves from several seal populations including 2 041 specimens with known age, length and population size at birth. Asymptotic body lengths of female harbour seals were 148 cm in all four regions in 1988, when seal abundances had been kept low by hunting for decades. Males were 158 cm, being 10 cm longer. However, in 2002 the asymptotic lengths of seals differed among regions. While seals in the Kattegat showed similar asymptotic lengths as in 1988, seals in the Skagerrak were significantly shorter, where both male and female asymptotic lengths declined by 7 cm. We estimate the area of available feeding grounds in the two sea regions and find the density of seals per square kilometre feeding ground to be three times greater in the Skagerrak compared to the Kattegat. Thus, the shorter body length of seals in the Skagerrak can be an early signal of density dependence. Hampered body growth is known to trigger a suite of changes in life history traits, including delayed age at sexual maturity, higher juvenile mortality and lowered fecundity. These mechanisms all point at a possible ‘’smooth route towards carrying capacity’’ with gradually reduced population growth rate as the main response to high population density. Recent aerial surveys confirm declining rates of population increase in the Skagerrak.

Highlights

Understanding the processes behind population trends is the essence of population ecology
In the first analysis we investigate the relation between age and body length and give somatic growth curves for samples collected in both epidemic years and in all regions
The abundance of seals in the Limfjord has fluctuated around a total abundance of 1,500 seals over the past decades, suggesting that this population has reached the carrying capacity of the system (Olsen et al, 2010; Teilmann et al, 2010). These findings strongly suggest that somatic growth in harbor seals is density dependent and that a change in asymptotic length of seals can indicate that a population is approaching the carrying capacity of Length Std.Error DF t-value p-value

Summary

Introduction

Understanding the processes behind population trends is the essence of population ecology. Populations of pinnipeds generally adopt exponential population growth at low numbers, which has been repeatedly confirmed in most populations after overexploitation during the twentieth century (Roman et al, 2015). What happens after this phase of exponential growth is debated, and an increasingly pressing question, since many earlier depleted marine mammal stocks are at historically high abundances. In most cases no single conclusive mechanism can be identified, but analyses suggest that combinations of factors such as predation, interspecific competition, or disease may contribute to the declines. In an interesting study competition for food, with other marine mammals, and with large fish species is suggested as a contributory factor to declining body conditions in harp seals (Pagophilus groenlandicus) (Bogstad et al, 2015)

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Conclusion