Question

Topic: Extinction of caspian seals
Analyse different approaches to address the environmental problem by utilizing theoretical and analytical tools of International Relationships. (Garret Hardin Theory, Paul Ehrlich Theory, etc)
Max. 500 words

Answer 1

Abstract

Understanding historical roles of species in ecosystems can be crucial for assessing long term human impacts on environments, providing context for management or restoration objectives, and making conservation evaluations of species status. In most cases limited historical abundance data impedes quantitative investigations, but harvested species may have long-term data accessible from hunting records. Here we make use of annual hunting records for Caspian seals (Pusa caspica) dating back to the mid-19th century, and current census data from aerial surveys, to reconstruct historical abundance using a hind-casting model. We estimate the minimum numbers of seals in 1867 to have been 1–1.6 million, but the population declined by at least 90% to around 100,000 individuals by 2005, primarily due to unsustainable hunting throughout the 20th century. This collapse is part of a broader picture of catastrophic ecological change in the Caspian over the 20th Century. Our results combined with fisheries data show that the current biomass of top predators in the Caspian is much reduced compared to historical conditions. The potential for the Caspian and other similar perturbed ecosystems to sustain natural resources of much greater biological and economic value than at present depends on the extent to which a number of anthropogenic impacts can be harnessed.

Introduction

High removal levels of keystone species may push ecosystems into new equilibria from which they are unlikely to return to historical states. In marine ecosystems such regime shifts often result from unsustainable harvesting of commercially important fish or marine mammal species.Determining the past role of such populations can have important implications for reconstructing the historical state of ecosystems in terms of the biomass concentrated at different trophic levels, help with understanding long term human impacts, and provide goals for restoration and management. Demographic history is also vital for conservation evaluations since the rate of decline is one of the main criteria used in placing taxa in International Union for the Conservation of Nature (IUCN) threat categories. In contrast to most species, harvested species and populations may have time series of hunting or catch data. In this paper we reconstruct the historical abundance and demography of Caspian seals (Pusa caspica) based on exceptionally complete hunting records spanning 140 years from the 1860s to the late 20th century. We chart a catastrophic decline in Caspian seals, primarily driven by over-harvesting, and discuss the implications for the Caspian ecosystem and the current conservation status of the species. Our approach should be applicable for analyses of histories for other key species where some current census, harvesting and life history data are available, and therefore a tool for assessments of species against IUCN threat criteria and examining historical changes in ecosystem structures.

Caspian seals are endemic to the Caspian Sea, and have been isolated since diverging from the ancestral Pusa genus around 1.3 million years ago. They are one of the main large piscivores in the Caspian and large-scale changes in their abundance may therefore impact the structure of the whole ecosystem. The seals range throughout the entire Caspian Sea, which covers an area of 393,000 km². The northern ice fields constitute the critical breeding habitat, where pups are born at the end of January to the beginning of February, and weaning after 4–5 weeks. Ice coverage has gradually diminished over the past three decades due to climate warming, and the north-eastern part of the ice-field also overlies one of the world's largest oil fields, which is currently being developed for exploitation. Other issues currently considered as threats to the population include unsustainable levels of hunting and mortality from fisheries by-catch, mass mortalities due to canine distemper virus (CDV), habitat loss and disturbance from industrial development, and possible low prey abundance owing to over-fishing and recent invasion of the Caspian by the comb jellyfish Mnemiopsis leidyi.

The earliest known evidence for utilisation of Caspian seals by humans dates to around 20,000 years BP in northern Iran. Significant commercial hunting started as early as 1740, and average annual harvests exceeding 100,000 were reported from at least since 1800. These numbers indicate that Caspian seals were once abundant, but aerial surveys during the 2005 and 2006 pupping seasons showed a decline to around 21,000 breeding females.

In this paper we use the unique and extensive hunting data for Caspian seals to reconstruct the minimum population sizes that could have sustained the recorded hunting pressure over the past 140 years up to the year 2005, when an estimate of pup production was determined from an aerial survey of the breeding population on the ice [17]. We then discuss how changes in abundance of seals may have affected the Caspian ecosystem and vice versa. We consider how the relationship between the seal population and the overall Caspian ecosystem may have altered over this period and consider prospects for recovery of this and other depleted seal populations.

Results

Changes in population size

Using an age-structured projection model (eqns 1–2 3) and the annually recorded harvest (Fig. 1) of Caspian seals over the period 1867–2005, we estimate the minimum initial female population size in 1867 at 572,800 females, of which 245,830 were breeding (Fig. 2). Given this starting point we estimate about 354,210 females in 1945 of which 193,140 were breeding, and 30,200 in 2005, where the 21,000 breeding females produced the same number of pups, which was approximately the number estimated from the survey in 2005. In simulations employing 20% lower and higher pup survival rates, as a test of the sensitivity to estimates of pup mortality, the estimated population sizes in 1867 were 510,400 and 676,700 females, respectively. Since the sex ratio in Caspian seals is close to parity [9], total initial population size was in the range from 1.0 to 1.6 million seals. Mean population growth was 0.983 for the entire period, using the average juvenile survival rate (Fig. 3), and between 0.982 and 0.986 for low and high pup survival rates, respectively. The population was reduced by about 66% between 1867 and 1964, and by a further 73% between 1965 and 2005 (Fig. 3).

Figure 1

Total registered harvest of Caspian seals (solid line) and the number of pups (dashed line) for the period 1867–2005.

Figure 2

Estimated minimum total female population size (solid line) and the number adult females (dashed line) in the Caspian for the period 1867–2005 as based on historical hunting records (Fig. 1).

Figure 3

The population growth rate of the Caspian seal population from 1867 to 2006 has fluctuated significantly because of the variable hunting pressure.

Changes in population structure

The intrinsic population growth for the period 1867–1964 was 1.10. However, due to intense hunting there were great fluctuations in the population structure and the realised population growth rate during that period (Fig. 4). A lower figure for the instrinsic population growth for the period 1965 to 2005 was assumed to allow for reported lower fertility due to OC contamination (Table 1). At the estimated population structure in 2005 the 21,000 pupping females in 2005 would represent 20% of the total population size, which would therefore be about 104 thousand seals. Hunting reduced the simulated mean population growth to 0.971 (Fig. 3). Since hunting after 1965 up until the early 1990s was consistently high and focussed on pups, the age structure in the population during this period was strongly skewed towards adults (Fig. 4). Hunting was reduced in the mid 1990s, resulting in an age structure by 2005 which is close to initial conditions in the 19th century (Fig. 4.).

Figure 4

Temporal changes in age structure before pupping of the Caspian seal population.

Table 1

Vital rates for Caspian seals during the periods 1930–1964 and 1965–2005.

Human impacts on seal populations

Human exploitation of pinnipeds has resulted in the extinction of the Caribbean monk seal (Monachus tropicalis), the Japanese sea lion (Zalophus californianus japonicus) and the extirpation of the Faroese harbour seal (Phoca vitulina).

Grey seals (Halichoerus grypus) were extirpated from the European mainland North Sea coast in the Middle Ages, from the Skagerrak in the 1750s and the Kattegat in the 1930s. A combination of hunting and other human impacts brought northern elephant seals (Mirounga angustirostris) to the brink of extinction , and have severely depleted populations of Mediterranean monk seals (Monachus monachus), and Hawaiian monk seals (M. schauinslandi). Detailed historical hunting records are lacking for many formerly depleated pinnipeds e.g. Antarctic fur seals (Arctocephalus gazella), but such data are available for the Northern fur seal (Callorhinus ursinus), Saimaa ringed seal (Pusa hispida saimensis), Baltic ringed seal (Pusa hispida botnica), Baltic grey seal and the harbour seal (Phoca vitulina vitulina) in the Wadden Sea, Kattegat and Skagerrak  Analyses of these hunting records documented collapses in all populations, which were depleted to about 5–10% of pristine abundances before protective measures were taken. The very detailed hunting records for Caspian seals enables a more thorough analysis where we find that numbers of breeding females have decreased from a minimum of 245,800 in 1867 to around 21,000 in 2005, which is a decrease by at least 90%.

Population estimates for the Caspian seal

An earlier reported population size estimate of about one million Caspian seals in the beginning of the 20th century, are fairly consistent with our results, which suggest a minimum of 1.2 million seals in 1900 (Fig. 2). However, an estimate of about 360–400,000 for total population size and 46,800 for the size of the reproductive female stock in 1989s, which is frequently cited in international compilations, deviates substantially from our calculations (Fig. 2). Data from our study suggest a total population size of about 128,000 and 30,000 for the number of reproductive females for 1989. The estimates for 2005 (104,000 in total and 21,000 reproductive females) therefore indicate a 19% and 30% decline in total numbers of seals and reproductive females respectively, from our estimate for 1989.

In the projections of earlier population sizes we systematically used high parameter values that resulted in under-estimations of population sizes in the past – hence we take a conservative approach and estimate minimum population sizes. We also assumed that the 20th century hunt killed equal numbers of males and females, when in reality the hunting on ice was mainly targeted at females and pups, while the hunting in spring and late autumn was focused on adult animals of both sexes. Consequently, the average rate of decline in numbers of breeding females for the period 1965–2005 of 3.0% (λ = 0.97) is probably an under-estimate.

Biases in hunting statistics

The annual catch in the Caspian sealing industry in the first half of the 20th century was registered at the seal oil processing plant on the NE coast of the Caspian. This registration was probably reasonably accurate, since the annual harvest fluctuated considerably, rarely reaching the set quota,suggesting catches were not over-reported in order to meet targets. The early Russian authors suggest that the inter-annual fluctuations in numbers of seals hunted in the 19th and first part of the 20th Century primarily reflect variation in hunting effort and access to seals according to winter conditions

Factors affecting recovery of depleted pinniped populations

Many depleted pinniped populations have shared common combinations of factors which have driven their decline. Recovery of populations depends on the extent to which threats persist and on ecological changes following declines.

Most formerly over-exploited pinniped populations have recovered when hunting ceased. Examples include the northern elephant seal, most species of fur seals as well as populations of harbour seals and grey seals. Consequently, protection from hunting has been the single most important factor allowing recovery of formerly depleted seal populations. However, in some cases (such as the northern fur seal and the Saimaa ringed seal) recovery has been inhibited by a combination of new threats, such as by-catch and food chain alterations, which were probably less important during the hunting era.

Conclusions

Our study shows that the collapse in the Caspian seal population was primarily driven by overharvesting. The distribution of the Caspian seal in a completely closed ecosystem, from which individuals cannot disperse to or from adjacent habitats, makes it extremely vulnerable to some or all of the many threats it currently faces, which include mortality caused by hunting and by-catch, reduction in stocks of prey fish and oil industry activities in the ice breeding grounds combined with potential ice-field reductions due to climate change. Until this array of threats can be resolved by the implementation of effective conservation measures, as laid out in the Caspian Seal Conservation Action Plan, further rapid declines of this species are likely in the short term.

Reference

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3446954/