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Biology

Hooded seals of the Greenland Sea

Øigård, T. A., Haug, T., & Nilssen, K. T. (2014). Current status of hooded seals in the Greenland Sea. Victims of climate change and predation?. Biological Conservation, 172, 29-36. Link

Introduction

Did you know it is hooded seal whelping season?  In the North Atlantic, there are two genetically different stocks of hooded seals.  The Northwest Atlantic stock whelps on an ice drift off of Newfoundland while the Greenland Sea stock whelps in the Davis Straight, between Greenland and Arctic Canada.

The Newfoundland stock significantly declined in the 1920s.  A study in 1960 (Rasmussen 1960) suggested that climate change caused a reduction in suitable breeding habitat and thus forced an eastward shift of the Newfoundland seal stock into the Greenland Sea.  However, this is heavily debated.  It is known from recent tagging studies that seals found in the Greenland Sea during breeding, remain within the Greenland, Norwegian, and Icelandic seas for most of the year (Fig. 1).

The Greenland seal stock has been commercially exploited for centuries up until 1920 when the fishery began to experience a significant increases in fishing efforts, subsequently causing a significant decline in the stock size.  Regulatory efforts were implemented in 1958 to reduce exploitative pressure on the stock.  The Greenland stock was assumed to be recovering since, however, recent survey efforts indicate that pup production in 2005 and 2007 were much lower than in 1997.

The International Council for the Exploration of the Sea (ICES) management for hooded seals requires a “data rich” time series for developing population estimates.  This means that estimate models should include at least three pup production estimates, data on fertility, and catch statistics.  This study used a new survey to estimate the hooded seal stock in March 2012.  The results of the new survey were then input into a model to predict the future of the stock under different scenarios such as different fishing pressures.

Methods

An ice-strengthened expedition vessel complete with helicopter platform and helicopter were used for reconnaissance and pup aging surveys. Additionally, two fixed wing aircrafts were used for reconnaissance and photographic surveys.  Due to ice drifts, most areas were repeatedly surveyed to minimize the chances of missing concentrated whelping events.

Counts of photographic images were tallied by two people experienced with reading images of seals. Estimations of pup abundances were derived from the counts. A correction was applied to the counts for seal pups that had not been counted because they had either left the ice already (born early in the season) or had not been born yet at the time of the survey (born late in the season).  This was performed by using counts of the pups in different age stages.  This survey separated pups into 4 stages: newborn, thin blueback, fat blueback, and solitary blueback.

The stock assessment used an age-structured population model that has been used in previous assessments of the Greenland hooded seal stocks.  The model uses current estimates of pup production (calculated in this survey), historical catch data, and reproductive data (also calculated in this survey) to estimate current total production.  The model also explores three catch scenarios: (1) no hunt, (2) current catch level, (3) equilibrium catch level.

Figure 1. Area covered by photographic survey over seal whelping patches on March 28 and areas covered by reconnaissance flights conducted by air-crafts (22 March-1 April) and helicopter (18 March -1 April).  Ice drift in the Greenland Sea during the period 21 March-11 April and 23 March-12 May, as observed from two satellite based GPS beacons deployed on the ice (Oigard et al. 2014).

Figure 1. Area covered by photographic survey over seal whelping patches on March 28 and areas covered by reconnaissance flights conducted by air-crafts (22 March-1 April) and helicopter (18 March -1 April). Ice drift in the Greenland Sea during the period 21 March-11 April and 23 March-12 May, as observed from two satellite based GPS beacons deployed on the ice (Oigard et al. 2014).

Results

Whelping areas found in the survey were typically more scattered in the northern area.  In the southern area there was an increasing number of hooded seals during the reconnaissance period.  As shown in table 1, there was also temporal variation in birthrate as well.  Because the stages for newborn and thin blueback have shorter duration than the other two stages, the observations were pooled.  Figure 2a shows the model fit of the proportion of pups observed in each of the age-stage groups; the fit was poor.  By tuning the mean length of certain age stages, the model fit was improved.  Figure 3 shows the modeled pup abundance (Fig. 3a) and modelled total population (Fig. 3b).

Table 1. Number of hooded seals in individual age dependent stages in the Greenland Sea during March 2012.

Table 1. Number of hooded seals in individual age dependent stages in the Greenland Sea during March 2012.

Figure 2. (A) Observed proportions and estimates of the probability of a hooded seal pup being classified as belonging to the various stages, and (B) predicted proportion of hooded seal pups on ice as a function of time.  The dotted vertical line shows the proportion of pups visible on ice on 28 March when the photographic survey was carried out.  Dashed lines denote the improved model fits by tuning of duration of stages.

Figure 2. (A) Observed proportions and estimates of the probability of a hooded seal pup being classified as belonging to the various stages, and (B) predicted proportion of hooded seal pups on ice as a function of time. The dotted vertical line shows the proportion of pups visible on ice on 28 March when the photographic survey was carried out. Dashed lines denote the improved model fits by tuning of duration of stages.

 

Figure 3. (A) Modeling Modelled hooded seal pup abundance with pup production estimates (dots with 95% confidence intervals) and (B) modelled total production.  Shaded area denotes 95% confidence interval.  The reference levels N70, N50, Nlim denote the 70%, 50%, and 30% of the historical maximum hooded seal population size, respectively.

Figure 3. (A) Modeling Modelled hooded seal pup abundance with pup production estimates (dots with 95% confidence intervals) and (B) modelled total production. Shaded area denotes 95% confidence interval. The reference levels N70, N50, Nlim denote the 70%, 50%, and 30% of the historical maximum hooded seal population size, respectively.

In sum, 777 hooded seal pups were counted from the 2792 photographs taken.  This was used to estimate seal pup production at 10,982 (+/- 1460) pups, without correction.  With reader correction, pup production is estimated at 13,655 (+/- 1900) pups.

The stock assessment model trajectory suggests a substantial decrease from the late 1940s through the early 1980s.  The population has apparently reached a stable low level for the last 20 years. The model output was relatively stable over a range of initial values.  The adult hooded seal population is predicted to decrease by about 7% over the next 10 years assuming there is no hunt.

Discussion

Surveys of the Greenland hooded seal population using similar methods and models were also conducted in 1997, 2005, and 2007.  The estimate of pup production from this survey, although lower than the 2005 and 2007 estimates, is not statistically different at the 0.05 significance level.  However, this is still significantly lower than the 1997 estimate.

Typically, low population levels are attributed to excessive hunting pressure.  But the hooded seals have been under limited exploitation for nearly three decades.  Additionally, the model indicates that even with no hunting pressure, the population will continue to decline by 7% over the next decade.  The population is currently under the N30 point (Fig. 3b) which denotes 30% of the largest observed population size.  The model suggests that even with the modest hunt occurring over the past three decades, the seals were too heavily exploited.

Climate change may also be affecting the Greenland hooded seal breeding habitat by reducing the size and thickness of the ice floes.  Greenland seals prefer thick ice for breeding.  The authors could not say whether this has caused the seals to relocate breeding grounds.  Additionally, thinner and smaller ice floes could make pups more susceptible to predation from other species such as polar bears and killer whales.  Another study of polar bear diets suggests that polar bears are consuming more hooded and hard seals in recent years. A similar study showed the killer whales are also consuming more hooded seals.

 

Lis Henderson
I am studying for my doctoral degree at the Stony Brook University School of Marine and Atmospheric Sciences. My research addresses fisheries and climate change in the Northwest Atlantic. In my free time, I like to cook and spend time outdoors, sometimes at the same time.

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