Health assessment in the Baltic ringed seal (Phoca hispida botnica)

	Authors: Mervi Kunnasranta1, Marja Isomursu2, Britt-Marie Bäcklin3, Riikka Puntila1 and Charlotta Moraeus3, of Ad hoc HELCOM SEAL Expert Group 1) Finnish Game and Fisheries Research Institute, Itäinen Pitkäkatu 3, FIN-20520 Turku. 2) Finnish Food Safety Authority Evira, Fish and Wildlife Health Research Unit, P.O.Box 517, FIN-90101 Oulu. 3) Swedish Museum of Natural History, dept. of  Contaminant Research, Box 50007, S-104 05 Stockholm.

Key messages

Reproductive health of the Baltic ringed seal females has improved since the 1970s. Especially the prevalence of uterine occlusions, has been steadily decreasing from 48% in the late 70’s to 30% in years 1991-2004 (Helle 1980a, 1980b, Helle et al. 2005). The proportion of adult females (4 years-old or older) with occlusions was 8% (N=49) in the period 2001 to 2009.  A majority of the health problems observed in the Baltic ringed seals have been suggested to be related to the high levels of environmental toxins such as organochlorines (e.g. Helle et al. 1976, Bergman 2007). A recent study indicates that PCB and DDE levels have decreased in the Baltic ringed seals and the health status of the seals has subsequently improved during the last decades (Routti 2009).

About 75% of the Baltic ringed seal population is found in the northern Baltic Sea and in the Bothnian Bay (Härkönen et al 1998), where the population has been increasing at a rate of a 4,5% per year since 1988 (Hårding & Härkönen 1999, Karlsson et al. 2009). However, in the southern breeding areas, the Gulf of Riga, the Gulf of Finland and the Archipelago Sea, increasing trend has not been observed.

Blubber thickness, which is a measure of the nutritional status of the seal, has remained somewhat steady during the sampling period (1981 to 2009) among the examined adult ringed seals (4 to 20 year-old). However, young individuals (1 to 3 year-old) have shown a recent (1996 to 2009) decrease in the blubber thickness. This trend is similar to the trend observed among Baltic grey seals.

Results and Assessment

Relevance of the indicator for describing developments in the environment

Ringed seals are among the top predators in the Baltic Sea mainly preying on fish and crustaceans, and their health condition reflects the overall state of the Baltic Sea environment. Ringed seals are distributed throughout the Baltic Sea, although the majority of the population is located in the Gulf of Bothnia. Therefore, a vast majority of the examined ringed seals in this study were obtained from that area.

Baltic ringed seals have been suffering from various health defects, especially uterine occlusions, during the past few decades, which have been associated with environmental toxins (Helle et al. 1976, Bergman & Olsson 1985; 1986, Bergman 2007). Reproductive disorders have lowered their fecundity and resulted in limited population growth.

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 the health problems due to environmental toxins of have been decreasing during the past decades (Nyman et al. 2002, Routti 2009).

Policy relevance and policy references

The HELCOM recommendation on Conservation of seals in the Baltic area 27-28/2 2006-07-08. In the Baltic Sea Action Plan (adopted 2007-11-15, Poland) where seal health is appointed as an indicator of wildlife health status in the hazardous substances segment.

Assessment

Ringed seal health status is assessed on the population level, where the accuracy of patterns for the cause of death and health variables depend on sample sizes of examined animals For the interpretations of the results it is important to record the cause of death in examined ringed seals.

The population health status can be defined as the sum of the following available indicators:

1.      Blubber thickness as an indicator of overall nutritional state

2.      Cause of death of examined seals. This indicator is divided into three classes (hunting, by-catch and disease/other cause)

3.      Rate of pregnancy as an indicator of the reproductive health of the population

4.      Rate of fecundity, fraction of females ovulating (same as above)

5.      Occurrence of uterine pathology (same as above)

6.      Parasites (species diversity and abundance) as an indicator of the infection pressure, foraging patterns and the Baltic environmental condition

Common nutritional state

Blubber thickness is a commonly used indicator for the nutritional state of marine mammals (Ryg et al. 1990). In all of the examined ringed seals (N=335), consisting of samples mainly from the Gulf of Bothnia 1981-2009, mean sternum blubber thickness was 3,44 cm. Blubber thickness exceeded 0,8 cm in all months among both sexes and age classes. The mean blubber thickness among the studied ringed seals was the thickest in the autumn except among the young of the year (Table 1).  This pattern emerges likely due to their low success in independent food supply during the post-weaning period, first summer compared to steady food supply from nursing in the spring. Springtime mean blubber thickness has varied significantly (F_{2, 50}: 21,556, p<0,0001) over the study period in examined 1- to 3-year-old seals, being thickest in 1991-2000 (Fig. 1 and Table 1). The reason for the observed change is not known.

Table 1. Mean blubber thickness (cm, sternum) in examined young of the year (0 yr old), young (1-3 yr old) and adult (4-20 yr old) Baltic ringed seals during spring (January-June) and fall (July-December) in Finland and Sweden.

Human impact on ringed seal population size

As a consequence of bounty hunting, Baltic ringed seals declined from a minimum of 180 000 at the beginning of the 20^th century to 25 000 in the 1940s. Over the period 1940 to 1965, the hunting maintained the population at the latter level, after which it dropped to about 5000 individuals in the 1970s as a consequence of reproduction difficulties (Hårding & Härkönen 1999). Ringed seal hunting was prohibited in 1986 in Sweden and 1988 in Finland (Anonymous 2007). Recently, a limited number of hunting licenses (N=30) have been issued for the Baltic ringed seal in Finland.

Currently, the main threats to population growth are direct mortality, mainly as by-catch in fishing gear, and reproductive disorders due to environmental toxins (Härkönen et al. 1998). The magnitude of by catch mortality is largely unknown, but it may be a notable source of mortality especially for the young individuals. Furthermore, poor ice and snow conditions, due to the climate change, result in low pup survival and may be a potential threat for the ringed seal population in the future (Meier et al. 2004).

The most common causes of death of the ringed seals examined for this study were scientific sampling (78%) and by-catches (19%).

Reproductive health

The health status in wild animal populations is commonly assessed by the reproductive capacity of the females, and the proportion of pups in the population. In several pinniped species, ovarian anatomy has been used as a proxy to determine the present and past reproductive status in females. 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.  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 environmental pollutants (DDTs, PCBs) (Helle et al. 1976, Bergman & Olsson 1986).

Although the population 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). Prevalence of uterine occlusions in collected samples has significantly decreased in the study period (F_{2, 26}: 1,979, p=0,019) and the decline is the most significant since year 2000 (Tukey A vs. C, p=0,018). The prevalence has decreased from 48% in the late 70’s to 30% in years 1991-2004 (Helle 1980, Helle et al. 2005). Most recently, in years 2001-2009, proportion of adult females (4 years or older) with occlusions has decreased to 8% (N=49) (Fig. 2). The proportion of sexually mature females (4 years or older females with observed Corpus luteum in their ovaries) has remained steady throughout the years 1981 to 2009 and has been approximately 90% (Fig. 3).

Parasites

Ringed seals harbor several species of parasites. Monitoring prevalence of various endoparasites species can be a good indicator of which prey species seals consume.  Parasite infections can also be an indicator of how resistant seals are against parasite infections since parasite infections often increase as a result of other stressors such as environmental toxins (Lafferty & Kuris 1999).

Parasites found in ringed seals are listed in Table 2. Schistocephalus solidus, a tapeworm infecting the three-spined stickleback (Gasterosteus aculeatus), is commonly found in ringed seal alimentary tract. The final hosts for S. solidus are fish-eating birds and rodents and it has no adult stadium or effect on the ringed seals. Contracaecum osculatum is common in Baltic grey seals but occurs only sporadically in ringed seals.  Only one lice species Echinophthirius horridus has been detected on ringed seals (Durden & Musser 1994) and it has also been found on Baltic ringed seals.

There is no continuous monitoring of parasite prevalence in Baltic ringed seals. Previous studies indicate that Corynosoma spp. are the most prevalent endoparasites (Helle & Valtonen 1980, Helle & Valtonen 1981). Also heartworm (Dipetalonema spirocauda) is relatively common in the southern Baltic ringed seal population (Westerling et al. 2005).

Table 2. Baltic ringed seal endoparasites

Data

Figure 1. Mean blubber thickness (sternum) (mean ± SE) in examined, young of the year (0 year olds), young (1-3 year old) and adult (4-20 year old) Baltic ringed seals in Finland and Sweden. 

Figure 2. Prevalence of uterine occlusions in examined ringed seals older than 4 years (Finland and Sweden).

Figure 3. The prevalence of pregnant, and sexually mature (presence of Corpus luteum) females in examined Baltic ringed seals (4 years or older) in Finland and Sweden.  Data of prevalence of pregnancies was limited to samples acquired in the implantation period between August to February.

Metadata

Technical information

Data source: The National Swedish Monitoring Program of Seas and Coastal areas, Swedish EPA, Swedish Museum of Natural History.  Baltic ringed seal necropsy data of Finnish Game and Fisheries Research Institute and Finnish Food Safety Authority, years 1977-2002. Baltic ringed seal necropsy data of Finnish Food Safety Authority years, 2002-2009. Baltic ringed seal reproduction health data of Finnish Game and Fisheries Research Institute, years 2000-2009.

Description of data:  Necropsy of by-catch, stranded 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.

Geographical coverage: The Swedish and the Finnish coast of the Baltic Sea

Temporal coverage: see figures.

Methodology and frequency of data collection: see Bergman (1999)

Methodology of data manipulation: Prevalences of occlusions, pregnancies and occurrence of ovarian corpora lutea was calculated as percentages and mean blubber thicknesses as arithmetical means. All calculations were made using MS Excel.

Statistical analyses: Change in prevalence of uterine occlusions was analysed using one-way analysis of variances ANOVA. Original binary variable was trasnformed to yearly prevalences and then classified into decades. Since the variable was proportional it was arcsine transformed prior to the analysis (Zar 1974). Tukey’s post hoc test was used in multiple comparisons. Change in blubber thickness was analysed using one-way ANOVAas well and post hoc tests were performed using Tukey’s test. All results were considered significant with p < 0,05. Tests were performed using xlstat software (Addinsoft 2009).

Quality information

Sweden: During these three decades two persons (veterinarian and patho-biologist) have performed the necropsies, the first one between 1977 and 2002 (WTF). Finland: During these three decades several persons (veterinarians, seal biologists) have performed the necropsies.

National consultations and synchronizations are made continuously between persons. Age determinations of the ringed seals were performed by counting growth layer groups (GLGs) in the cementum of teeth according to a well established method. Age determinations were conducted independently by at least two persons.

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For reference purposes, please cite this Baltic Sea Environment Fact Sheets as follows:
[Author’s name(s)], [Year]. [Baltic Sea Environment Fact Sheets title]. HELCOM Baltic Sea Environment Fact Sheets 2010. Online. [Date Viewed], http://www.helcom.fi/environment2/ifs/en_GB/cover/.

Last updated 30 September 2010