HELCOM Core Indicator of Biodiversity
Pregnancy rates of marine mammals
Pregnancy rates of the marine mammals
- Particular emphasis on Baltic grey and ringed seals
Authors
1)
1)
2)
Britt-Marie Bäcklin , Charlotta Moraeus , Kaarina Kauhala and Marja Isomursu
Group
1)
3)
of Ad hoc HELCOM SEAL Expert
2)
Swedish Museum of Natural History, dept. of Contaminant Research, Box 50007, S-104 05 Stockholm. Finnish Game
3)
and Fisheries Research Institute, Turku Game and Fisheries Research, Itäinen Pitkäkatu 3 A, FIN-20520 Turku. Finnish Food
Safety Authority Evira, Fish and Wildlife Health Research Unit, P.O.Box 517, 90101 Oulu, Finland.
Acknowledged persons:
Stefan Bräger, Anders Galatius, Britta Knefelkamp, Anna Roos, Ursula Siebert and Stefanie Werner as well as Members
of the HELCOM SEAL EG.
Reference to this core indicator report: [Author’s name(s)], [Year]. [Title]. HELCOM Core Indicator Report. Online.
[Date Viewed], [Web link].
© HELCOM 2013
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HELCOM Core Indicator of Biodiversity
Pregnancy rates of marine mammals
Contents
Key message ........................................................................................................................................................................ 3
Description of the indicator ................................................................................................................................................ 4
Policy relevance .................................................................................................................................................................. 4
What is the reproductive status of marine mammals?....................................................................................................... 4
Reproductive health of grey seals ................................................................................................................................... 4
Reproductive health of ringed seals ............................................................................................................................... 6
How the indicator describes the Baltic marine environment? ........................................................................................... 7
The Baltic seal species and harbour porpoise ................................................................................................................. 7
The assessment approach ............................................................................................................................................... 7
Pregnancy rates in Baltic grey seals and ringed seals ..................................................................................................... 8
Methods to evaluate the pregnancy rate ....................................................................................................................... 8
Approach for defining GES .............................................................................................................................................. 9
Metadata............................................................................................................................................................................. 9
Data source ..................................................................................................................................................................... 9
Description of data .......................................................................................................................................................... 9
Geographic coverage .................................................................................................................................................... 10
Recommendations for monitoring and assessment ..................................................................................................... 10
Temporal coverage ....................................................................................................................................................... 11
Methodology and frequency of data collection ............................................................................................................ 11
Methodology of data analyses ...................................................................................................................................... 11
Determination of GES boundary ................................................................................................................................... 12
Strengths and weaknesses of data ................................................................................................................................ 12
Quality information ................................................................................................................................................... 12
Weaknesses/gaps...................................................................................................................................................... 12
References ........................................................................................................................................................................ 13
View Data .......................................................................................................................................................................... 14
© HELCOM 2013
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HELCOM Core Indicator of Biodiversity
Pregnancy rates of marine mammals
Key message
The reproductive health of the Baltic ringed seals and grey seals has improved significantly since the 1970’s and mid1980s, respectively. Pregnancy rate of grey seal can be considered as good.
For both the grey seals and ringed seals the prevalence of uterine obstructions has decreased and the number of
pregnant animals increased significantly in recent decades. The improved status is related to the decreased levels of
Figure 1. Pregnancy rates, mean values and one -sided 95% confidence intervals for a proportion, in 4 -20year-old female Baltic grey seals (August to repro ductive season) from 1977 to 2011. Finnish data is
included in the period 1997–2007. GES boundary has been set to 80%
organochlorines, such as PCB and DDE (Routti 2009).
Figure 2. Pregnancy rate of ringed seals (blue bars) and the maturity rate (red line) during 1981–2011. Good
environmental status is assessed against the pregnancy rate and is proposed to be set at 80% (green line). Sample
sizes are given above the bars and the line. The data is predominantly from the Bothnian Sea
© HELCOM 2013
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HELCOM Core Indicator of Biodiversity
Pregnancy rates of marine mammals
Description of the indicator
This indicator considers fertility and is assessed by increasing pregnancy rate and supported by assessments of
maturity of females and prevalence of uterine obstructions. The pregnancy rate of mature females can be affected by
hazardous substances, particularly organochlorines. The boundary for good environmental status considering
pregnancy rate is based on data from within and outside the Baltic Sea.
Policy relevance
Marine mammals may be negatively affected by changes in the food web, contaminants, and anthropogenic activities.
The pregnancy rate can be reduced by hazardous substances.
Health status of grey seal, ringed seal, harbour seal and harbour porpoise is referred to in several environmental
policies in the Baltic Sea:
-
-
-
-
HELCOM has a recommendation on Conservation of seals in the Baltic area (27-28/2 2006-07-08) and in the
Baltic Sea Action Plan (adopted 2007-11-15, Poland) seal health was defined as an indicator of a healthy
wildlife in the Hazardous substances segment.
The grey seal, ringed seal and harbour seal and harbour porpoise are listed in the EU Habitats Directive
Annexes II and V as species of community interest whose conservation requires the designation of Special
Areas of Conservation.
The EU Marine Strategy Framework Directive requires an assessment of environmental status of populations
of marine species (EC Decision 477/2010/EU), and marine mammals were recognized by the MSFD Task
Group 1 as a group to be assessed.
The conservation of the harbour porpoise has been specifically set by the Agreement on the Conservation of
Small Cetaceans of the Baltic, North East Atlantic, Irish and North Seas (ASCOBANS).
What is the reproductive status of marine mammals?
Reproductive health of grey seals
Results from previous decades show that the Baltic grey seal fertility has improved markedly. In the Baltic grey seal
population, the pregnancy rate was 87% in 4–20-year-old females in 2008–2011, representing Good Environmental
Status, GES (Figure 1). This pregnancy rate seems to be normal in the 4–20-year-old Baltic grey seals and indicates that
the population is healthy. The rate is also close to the pregnancy rate of Northwest Atlantic grey seals older than five
years.Separation of the pregnancy rate to 45-year-old individuals and 6–20-year-old individuals gives pregnancy rates
of 65% and 95.5%, respectively, in 2002–2009 in Sweden (Figure 3).
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HELCOM Core Indicator of Biodiversity
Pregnancy rates of marine mammals
Figure 3. Pregnancy rates, mean values and one-sided 95% confidence intervals for a proportion, in
examined 4–20-year-old grey seal females from by-catch or hunt in Sweden 2002–2009, divided into 45year-old, 6–20-year-old and all seals. GES boundary has been set to 80%.
The prevalence of uterine obstructions in grey seals has decreased which supports the assessment of GES (Figure 4).
Ovulation was recorded in almost all (91 %) examined females which are 4 years or older in 2008–2010. Among the 3year-old females 19 % were sexually mature in the same time period.
Obstructions by narrowing (stenosis) and closure (occlusion) of the uterine lumen were earlier common in Baltic grey
seals older than six years (Bergman & Olsson 1985). Remnants of foetal membranes have been found associated with
these lesions as signs of abortion (Bergman & Olsson 1985). These uterine disorders can prevent or impede
Figure 4. Prevalence of uterine obstructions and uterine leiomyomas in bycaught/ stranded and hunted
examined grey seals older than 4 years in Sweden and Finland. The decreased prevalence of uterine
obstructions is significant (p<0.05). N is the number of investigated animals. The prevalence of leiomyomas in
the last decade is uncertain since the number of examined seals, above 22 years of age, has decreased since
1996.There was one case of uterine occlusion in Finland in 2009 (of a 13-year-old female).
pregnancies. Uterine leiomyomas have been common in female grey seals above 22 years of age. These benign
© HELCOM 2013
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HELCOM Core Indicator of Biodiversity
Pregnancy rates of marine mammals
smooth muscle cell tumours are mostly localised in the wall of the uterine corpus. They are often multiple and
sometimes up to 10 cm in diameter (Bergman & Olsson 1985, Bergman 1999; Bäcklin et al. 2003). These tumours,
depending on localisation, may make pregnancy more difficult to take place. There are also indications that PCB is
involved in uterine leiomyoma development and/or growth (Bredhult et al. 2008). Trends in percent of sexual
maturity (ovulation) in 3-year-old females and ovulation in 4-year-old or older females are interpreted as signs of
normal seasonal reproduction. It is defined by the presence of a corpus luteum in the ovaries in spring, i.e. between
the time of ovulation and implantation.
Reproductive health of ringed seals
The pregnancy rate determined from Corpus albicantia (CA) from the spring sample (between ovulation and
implantation) in 2001–2011 was 68% for females ≥ 4 years old (n = 38) (Figure 2). The rate has more than doubled
since the 1980s. The difference between the three analysed periods is statistically significant (p = 0.039 for ≥ 4 years
old females). It is known that the ringed seals are still suffering from uterine occlusions (see below) and therefore the
species may well be below Good Environmental Status (sub-GES). GES boundary is suggested to be the same as for
grey seals, i.e. 80%.
Although ringed seal is still suggested to be affected by exposure to environmental toxins, the health status of the
Baltic ringed seal has clearly improved during the last decades (Nyman et al. 2002, Routti 2009). The proportion of
sexually mature females (4 years or older females with observed Corpus luteum (CL) in their ovaries) has increased
slightly since the period 1981–1990 (p = 0.037, Figure 2). According to the spring samples, the proportion of females ≥
4 years old with CL was 93% (n = 44) in 2001–2011
Prevalence of uterine occlusions in collected samples among females ≥ 4 years old has significantly decreased in the
study period (chi2 = 11.8, df = 2, p = 0.003) and the decline is the most significant since year 2000 (Figure 5, Helle
1980, Helle et al. 2005). Most recently, in years 2001–2011, proportion of adult females (4 years or older) with
occlusions has decreased to 8% (N=52) (Figure 5). The last case was a 17-year-old female in 2011. Altogether there
have been four cases since 2000. The ages of these females were 16–26 years.
Figure 5. Prevalence of uterine occlusions in examined ringed seals older than 4 years (Finland and Sweden).
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HELCOM Core Indicator of Biodiversity
Pregnancy rates of marine mammals
How the indicator describes the Baltic marine environment?
The Baltic seal species and harbour porpoise
The species considered by this indicator are grey seals (Halichoerus grypus) and ringed seals (Pusa hispida botnica),
whereas we do not have yet data for harbour seals (Phoca vitulina) and harbour porpoise (Phocoena phocoena). Seals
and harbour porpoise are top predators and fish eating mammals and their health condition reflects the state of the
Baltic environment.
Grey seals are distributed more or less in the entire Baltic Sea, with the largest populations in the southern parts of
the Gulf of Bothnia and the northern parts of the Baltic proper. The number of grey seals has been increasing with
about 8% per year between the early 1990s and the mid-2000s. Ringed seals are among the top predators in the Baltic
Sea mainly preying on fish and crustaceans. It has been assumed that the species has four populations in the Baltic
sea, the largest one being in the Bothnian Bay (75% of individuals) and the other three in the Gulf of Finland, Gulf of
Riga and the Archipelago Sea, but we need more information about the gene flow between populations to draw firm
conclusions of the number of separate populations. The ringed seal population in the Bothnian Bay has been
increasing at a rate of 4.5% per year since 1988 (Hårding & Härkönen 1999, Karlsson et al. 2009), whereas in the
southern breeding areas in the Gulf of Riga and the Archipelago Sea increasing trend has not been observed and in the
Gulf of Finland the population decreases.
Harbour seal consists of two distinct populations, one around the island of Öland and the other on the Swedish West
Coast. Harbour porpoise has two populations in the Baltic Sea: the western population in the Belt Sea and the
population of the Baltic Proper.
The Baltic seals have been suffering from various health defects during the last decades, which have been associated
with the deterioration of the general status of the Baltic Sea and environmental toxins (Helle et al. 1976a, b, Bergman
& Olsson 1985, Bergman 2007). Reproductive disorders have lowered their fecundity and resulted in limited
population growth.
Several health parameters in seals are investigated by the HELCOM Contracting Parties (CPs). Baltic countries have
different possibilities and access to conduct marine mammal necropsies. Therefore pregnancy rate has been
prioritised to roughly reflect the health in marine mammals since it is routinely measured in several CPs (see section
on monitoring). Agents that are lethal to foetuses or endocrine disrupting causing a decreased pregnancy rate, and
also agents or starvation causing a thin blubber layer (see separate core indicator report), could seriously affect the
survival of the population.
The assessment approach
In this report, grey seals and ringed seals are assessed on a population level by assessing the population health status,
in particular the pregnancy rate of the females. This can only be recorded in examined animals and conclusions for the
whole population are statistically depending on the number of seals examined. For the interpretations of the results it
is important to record the cause of death in examined seals. Assessments for harbour seals and harbour porpoises are
pending on better data.
Pregnancy rate in seals is measured as the proportion of females with presence of a foetus after the delayed
implantation period in sexually mature seals (4–20 years old). The same results should be obtained using the
proportion of females with CA from the spring sample, and this is the only possibility for ringed seals (not enough
samples from autumn). For ringed seals the data is pooled for all females at least 4 years old, because of small sample
sizes. The pregnancy rate in grey seals in 2008–2009 is used as a reference state for a healthy population. The same
GES boundary has been suggested for ringed seal. The levels that can be considered to represent GES for harbour
seals and harbour porpoises remain to be compiled or investigated.
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HELCOM Core Indicator of Biodiversity
Pregnancy rates of marine mammals
In addition to the pregnancy rate, the following parameters can support the assessment:
1.
2.
3.
Rate of fecundity, i.e. the proportion of females with CL from the spring sample;
Occurrence of uterine pathological changes (this parameter is divided into three classes: hunt, drowning
and disease/other cause); and
Cause of death in examined seals (this parameter is divided into three classes: hunt, drowning and
disease/other cause).
The blubber thickness – describing the common nutritional state of the population – is being proposed as a separate
core indicator and the supporting parameters for that indicator are parasite abundance and abundance of intestinal
ulcers.
Pregnancy rates in Baltic grey seals and ringed seals
In several pinniped species, ovarian anatomy has been used as a proxy to determine the present and past
reproductive status in females.
The reproductive health of the Baltic grey seal population has been assessed as the prevalence of uterine obstructions
and uterine tumours (leiomyomas), pregnancies after the implantation period and ovulation in seals at age 4-20, and
by the prevalence of sexual maturity in 3-year-old individuals.
During the 1970s and the first half of the 1980s, uterine obstructions causing sterility were commonly found in
necropsied Baltic grey seals and Baltic ringed seals (Helle et al. 1976a,b; Bergman & Olsson 1985). Since then, the
pregnancy rate in the examined Baltic grey seals has increased from 9% (1977–1986) to 60% (1987–1996) (Bergman
1999) and 87% (2008–2011, this report). The most recent cases of uterine obstruction in grey seals waere found in
1993 in Sweden and in 2009 in Finland. In the 2000s, about 8% of the examined Baltic ringed seals still suffer from
uterine obstructions and the pregnancy rate of 68% in ringed seals in 2001–2011 is therefore probably lower than
‘normal’ (Helle et al. 2005, Kunnasranta et al. 2010, this report). The low gynecological health among the Baltic seals
was most probably explained by high concentrations of polychlorinated biphenyls (PCBs) (Olsson et al. 1975, Helle et
al. 1976a, b; Bredhult et al. 2008).
No observations of uterine obstructions in Baltic harbour seals or harbour porpoises have been reported, but there is
data on pregnancy rates for these species that needs to be compiled and evaluated.
Ringed seal population’s reproductive health has been mainly assessed as the prevalence of uterine obstructions.
These uterine disorders are non-lethal, but can prevent or impede pregnancies and cause lifetime sterility. Occlusion
of the uterine horn was the most frequent reproductive disorder in the 1970’s. The underlying mechanism behind the
development of these pathological changes is not thoroughly known, although it has been proposed that occlusions
can develop as a result of fetal death due to hazardous substances, e.g. DDTs and PCBs (Helle et al. 1976a, b, Bergman
& Olsson 1985).
Methods to evaluate the pregnancy rate
Pregnancy rate is measured as the presence or absence of an embryo or foetus during the pregnancy period in mature
females (presence of an ovarian Corpus luteum, CL). It is expressed as the percentage of pregnant females in all
mature females (age 4–20 years in seals) and it is investigated during the relevant season(s). Pregnancy rate can also
be determined on the basis of Corpus albicantia (CA) in the spring sample (between ovulation and implantation).
Especially the data relative to ringed seals is not big enough to calculate the pregnancy rate from fetuses in autumn.
When calculating the pregnancy rate from CA, autumn sample should be excluded, because CA may disappear, if the
female fails to become pregnant again (Boyd 1984). For grey seals, pregnancy rate should be calculated both from the
spring sample using CA and from the autumn sample using fetuses, and the results should be compared. The
© HELCOM 2013
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HELCOM Core Indicator of Biodiversity
Pregnancy rates of marine mammals
proportion of mature females, i.e. females with CL, should also be calculated from the spring sample, because CL soon
disappears if the females failed to become pregnant (Boyd 1984).
Grey seals. It is estimated that age-specific birth rates increase steeply from the age of four to six (Hamill & Gosselin
1995). The birth rates for the six-year old females in the Northwest Atlantic, British, Norwegian and Baltic populations
ranged from 60–91%. In a sample of 526 female grey seals from the Northwest Atlantic, pregnancy rates were
estimated from the presence/absence of a foetus. The pregnancy rate for the Northwest Atlantic population was
relatively stable at about 90% after the age of six (Hamill & Gosselin 1995; Hårding et al. 2007). Annually, Sweden does
not receive more than 4–13 grey seal females between 4 and 20 years of age in the pregnancy period. Therefore it
would be useful to calculate pregnancy rate also from the proportion of females with CA in spring.
Ringed seals. The number of 4-20-yearold Baltic female ringed seals that are investigated annually during the
pregnancy period is very small (too small to calculate the proportion of pregnant females). Pregnancy rate of ringed
seals were therefore calculated from the proportion of females with CA in spring, using all females which are at least 4
years old to get larger data.
Data from harbour seals both from the Baltic (Kalmarsund population) and the Swedish West coast and harbour
porpoise investigations remains to be compiled and evaluated.
Approach for defining GES
Pregnancy rate is measured as presence or absence of a foetus in the pregnancy period in 4–20 years-old grey seals
and/or from the proportion of females with CA in spring. GES is proposed to be assessed every third year (pooling the
data for each 3-year period) for 4–20 years old, and every sixth year pooling the data for each 6-year period,
separately for young (4–5 years-old) and adult (≥ 6-year-old) females. For ringed seals, a period of 10 years to get
enough data may be needed. Today’s figures suggest that in 4–20 years old grey seal GES could be set at the lower
limit of the 95% confidence interval i.e. at about 80%, referring to the period 2008–2009 which is proposed to be
defined as representative of a healthy population in Figure 1. The same GES boundary is proposed for the ringed seal.
Data should also be presented as trends.
Whether or not similar GES limits for pregnancy rates can be suggested for harbour seals, and harbour porpoises
remains to be investigated.
Metadata
Data source
The National Swedish Monitoring Program of Seas and Coastal areas, top predators, pathology in seals, Swedish EPA,
Swedish Museum of Natural History 1977–2011.
Baltic grey seal necropsy data of Finnish Game and Fisheries Research Institute and Finnish Food Safety Authority,
years 1977–2011.
Baltic ringed seal necropsy data of Finnish Game and Fisheries Research Institute, years 2000–2011.
Description of data
Necropsy of by-caught and hunted grey seals and ringed seals, sample preparation and evaluation of results has been
carried out by the dept. of Contaminant Research at the Swedish Museum of Natural History and by Finnish Game and
Fisheries Research Institute and/or Finnish Food Safety Authority.
Samples have been collected from various sources, e.g. samples taken by hunters, fishermen and researchers.
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HELCOM Core Indicator of Biodiversity
Pregnancy rates of marine mammals
Stranded animals were not included.
Geographic coverage
The current assessment is done with data from the Swedish and the Finnish coast of the Baltic Sea. For grey seal,
which is very mobile across the Baltic Sea basin, this limited monitoring is considered representative for the whole
population. Nevertheless, samples from other countries would support the indicator.
The ringed seal data is predominantly from the Bothnian Bay, whereas there are not enough specimens from the
southern subpopulations, and therefore the assessment result is considered to be geographically limited. Every new
sample from the southern sub-populations increases the understanding relative to the state of those sub-populations.
Health of the Baltic marine mammals is investigated in Finland, Lithuania, Poland, Germany, Denmark and Sweden.
Recommendations for monitoring and assessment
The monitoring of pregnancy rate can be done in areas where hunting of seals is permitted. For grey seals this gives an
adequate assessment result for the entire Baltic Sea. For ringed seals this gives an adequate assessment result for the
Bothnian Bay subpopulation. For harbor seals, if permits are given, the results are applicable to the area where of the
given subpopulation. Animals drowned in fishing gears give additional (supplementary) data to the indicator.
Assessment should be carried out in accordance to management units defined in HELCOM RECOMMENDATION 27–
28/2 i.e. 1) harbour seals in the Kalmarsund region (Sweden) ;2) Southwestern Baltic harbour seals (Denmark,
Germany, Poland, Sweden); 3) Gulf of Bothnia ringed seals (Finland, Sweden); 4) Southwestern Archipelago Sea, Gulf
of Finland and Gulf of Riga ringed seals (Finland, Estonia, Latvia, Russia); 5) Baltic Sea grey seals (all Contracting Parties
to the Helsinki Convention).
Table 1. Monitoring of the proposed indicators in the Baltic Sea. Information from several countries is missing.
Country
Area
Coastline
Species
Germany
Western
Sea
Hiddensee
Westküste
Harbour
porpoise
MecklenburgWestern
Pomerania
Bay
of
Mecklen-burg
&
Pomeranian
Bay,
internal
lagoons
Harbour
porpoise
All
always
stranded
and
bycaught
1990
Lithuania
Southeastern
Baltic sea
Lithuania
coastline
Sweden
whole Baltic Sea
Swedish
Grey seal
All
always
by-caught,
stranded,
hunt
1977
© HELCOM 2013
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Baltic
Page 10
Month
Interval
Type
carcass
of
Start of
data
series
stranded
HELCOM Core Indicator of Biodiversity
Pregnancy rates of marine mammals
Baltic proper
Swedish
Harbour
seal
All
always
by-caught,
stranded
1977
Western
Sea
Swedish
Harbour
seal
All
always
by-caught,
stranded,
hunt
1977
Swedish
Ringed seal
All
always
by-caught,
stranded
1977
Baltic
whole Baltic Sea
hunt*
scientific
hunt
Western
Sea and
proper
Finland
Baltic
Baltic
Swedish
Harbour
porpoise
All
always
by-caught,
stranded
1977
Baltic Sea
Finnish
Grey seal
16.AprilDecember
always
hunted
1998
Baltic Sea
Finnish
Ringed seal
16.AprilDecember
always
hunted
2010
Baltic Sea
Finnish
Grey seal
All
always
by-caught
1999
Baltic Sea
Finnish
Ringed seal
All
always
by-caugh
1999
Baltic Sea
Finnish
Grey seal
All
sporadic
stranded
2010
Baltic Sea
Finnish
Ringed seal
All
sporadic
stranded
2010
Temporal coverage
The assessment includes data since 1977 (grey seals) and 1981 (ringed seals).
Methodology and frequency of data collection
The pregnancy rates of marine mammals can be obtained from institutional necropsies. By sampling the female
reproductive organs (reproductive status), the lower jaw (age determination) and measuring the sternum blubber
thickness and reporting the date of death, position, and sending it to an institute, it should be possible to collect more
data (e.g. from hunters and fishermen) than at present. See Bergman (1999).
The core indicator report is always updated with data from seal hunt and by-caught seals, resulting in yearly updates
of the last two years.
Methodology of data analyses
For statistical evaluation two tailed Fishers exact test, linear regression or t-test were used.
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HELCOM Core Indicator of Biodiversity
Pregnancy rates of marine mammals
Determination of GES boundary
Pregnancy rate is measured as presence or absence of a foetus in the pregnancy period in 4–20 years-old grey seals
and/or from the proportion of females with CA in spring. GES is proposed to be assessed every third year (pooling the
data for each 3-year period) for 4–20 years old, and every sixth year pooling the data for each 6-year period,
separately for young (4–5 years-old) and adult (≥ 6-year-old) females. For ringed seals, a period of 10 years to get
enough data may be needed. Today’s figures suggest that in 4–20 years old grey seal GES could be set at the lower
limit of the 95 % confidence interval i.e. at about 80 %, referring to the period 2008–2009 which is proposed to be
defined as representative of a healthy population in Figure 1. The same GES boundary is proposed for ringed seal.
Data should also be presented as trends.
Whether or not similar GES limits for pregnancy rates can be suggested for harbour seals, and harbour porpoises
remain to be investigated.
Strengths and weaknesses of data
Quality information
Sweden: During these three decades two persons (veterinarian and patho-biologist) have performed the necropsies.
Finland: During these three decades several persons (veterinarians, seal biologists) have performed the necropsies.
National consultations and synchronisations are made continuously between persons. Age determinations of the grey
seals are performed by counting growth layer groups (GLGs) in the cementum of teeth according to a well-established
method. Readings of tooth sections are made independently by two persons.
Weaknesses/gaps
Monitoring of the Baltic marine mammals started in the 1970s when the health of the seal populations was seriously
threatened by contaminants, especially organochlorines. The populations have slowly recovered but new threats have
arisen (e.g. other contaminants). Therefore, it could be said that the knowledge of normal pregnancy rate and blubber
thickness is deficient in Baltic marine mammals.
Data from outside the Baltic could be used to determine normal limits, but the possible issue here is that the
ecosystem outside the Baltic Sea is different with dissimilar opportunities to forage. In the Baltic, grey seals also have
a smaller body size than in the northeast Atlantic (UK and Norway) which in turn are smaller than in the northwest
Atlantic (McLaren 1993).
There is a lack of data, especially for harbor porpoises and harbor seals but also for ringed seals. Trends of the overall
health status of the Baltic ringed seal are uncertain due to low numbers of necropsied whole animals. Health
investigations have been focused on female reproductive tracts, which have been collected systematically since the
late 1970’s. However, recent findings indicate that the overall health problems due to environmental toxins have been
decreasing during the past decades (Nyman et al. 2002, Routti 2009).
Data from investigations on the western population of harbour seals could probably serve as normal data also to
determine GES in the Kalmarsund harbour seal population.
Other gaps:
-
Finnish data for grey seal is not included since 2008,
Ringed seal GES boundary requires agreement within the group,
the issue of using CL only in the spring or also in the autumn needs to be agreed on,
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Pregnancy rates of marine mammals
Coordinated database for the results should be made under the HELCOM SEAL health team, after which the indicator
could be considered operational
References
Bergman A. 1999. Health condition of the Baltic grey seal (Halichoerus grypus) during two decades. APMIS 107:270–
82.
Bergman A. 2007. Pathological changes in seals in Swedish waters: The relation to environmental pollution.
Tendencies during a 25-year period. Thesis No.2007:131, Swedish University of Agricultural Sciences, ISBN 978-9185913-30-5.
Bergman A. & Olsson M. 1985. Pathology of Baltic grey seal and ringed seal females with special reference to
adrenocortical hyperplasia: Is environmental pollution the cause of a widely distributed disease syndrome? Finn
Game Res 44:47–62.
Boyd, I. L. 1984. Development and regression of the corpus luteum in grey seal (Halichoerus grypus) ovaries and its
use in determining fertility rates. Canadian Journal of Zoology 62: 1095–1100.
Bredhult, C., Bäcklin, B-M., Bignert, A., & Olovsson, M. 2008. Study of the relation between the incidence of uterine
leiomyomas and the concentrations of PCB and DDT in Baltic grey seals. Reproductive Toxicology, 25: 247–255.
Bäcklin, B., Eriksson L., and Olovsson M. 2003. Histology of uterine leiomyoma and occurrence in relation to
reproductive activity in the Baltic gray seal Halichoerus grypus. Vet. Pathol. 40:175–180.
Bäcklin B-M, Moraeus C, Kunnasranta M, Isomursu M. 2010. Health assessment in the Baltic grey seal (Halichoerus
grypus). HELCOM Indicator Fact Sheets 2010. Online. [Date Viewed],
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© HELCOM 2013
www.helcom.fi
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HELCOM Core Indicator of Biodiversity
Pregnancy rates of marine mammals
View Data
Pregnancy rate 1977–2011 of pregnant 4–20 years old grey seals
Period
Mean-%
95% CI
GES boundary
N
1977–1986 (N=11)
9
12
80
11
1987–1996 (N=20)
60
7
80
20
1997–2007 (N=55)
76
2
80
55
2008–2011 (N=47)
87
?
80
47
Pregnancy rate 1981–2009 of pregnant 420 years old ringed seals
Period
Pregnant 420 years old Sample size
GES
Mature females,
>4 years old
Sample size
1981–1990
30
10
80
80
10
1991–2000
46
37
80
98
39
2001–2011
68
38
80
93
44
© HELCOM 2013
www.helcom.fi
Page 14