Assessment Protocol

This core indicator assesses the reproductive status of seals in the Baltic Sea.

Seals in each assessment unit are evaluated against the set threshold values. Samples from opportunistically collected, hunted, by-caught and diseased seals can be used in the analysis. Observed data for 3-year intervals for each species are regarded as three independent datasets and tested if they deviate from the set threshold values using non-parametric tests. Also 3-year-moving averages can be used to be able to test the trends in the reproductive rate (at least for birth rate of grey seals).

The indicator evaluation is assessed in the following stages. Firstly the annual pregnancy rate (Swedish data) and inferred birth rate (Finnish data) are calculated as a percentage of the sampled individuals. Secondly the value is assessed against the threshold value (of 90%). The ratio for each parameter for each year in the assessment period is calculated as the percentage of pregnant animals or animals inferred to have given birth divided by the threshold value or 90%. The ratios for each parameter are then aggregated by calculating a mean value and assessed against the threshold value of 1. If the value of the aggregated ratios is above 1 the indicator achieves the threshold value (i.e. good status) and if below 1 the threshold is failed (not good status). The mean values for the overall assessment period are then calculated and this value gives the status evaluation for the assessment period.

 

Parameter calculation

Pregnancy rate is measured as the proportion of 5/6–24-year-old females, depending on the seal species, with an embryo or foetus during the pregnancy period (post-implantation period). Postpartum pregnancy rate is calculated from the pre-implantation sample as the proportion of 6/7–25-year-old females with post-partum signs, i.e. a corpus albicans/placental scar.

Grey seals

Pregnancy rate is measured as the presence of an embryo/foetus in the pregnancy period in 6–24 year-old (or ≥ 6 yr) seals. Birth rate is calculated from the pre-implantation sample as the proportion of 7–25-year-old (or ≥ 7 yr) females with a corpus albicans/placental scar. From Finland about 30 females (7-25-year-old) are analyzed in spring each year for inferring birth rate.

The reason for using the age interval 6-24 years is that estimated age-specific birth rates increase steeply from the age of four to six (Hamill & Gosselin 1995). The birth rates for six-year old females in the Northwest Atlantic, British, Norwegian and Baltic populations ranged between 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; Harding et al. 2007).

In the Baltic grey seal population, the pregnancy rate was 88% in 4–20-year old females in 2008–2009 (Assessment protocol figure 1). Thus, a pregnancy rate of 88% pregnancy seems to be normal in 4–20-year old Baltic grey seals in an increasing population (Assessment protocol figure 1 and Assessment protocol figure 2). This rate is also close to the pregnancy rate of Northwest Atlantic grey seals older than five years.

 

Assessment figure 1.png

Assessment protocol figure 1. Pregnancy rate in 4–20-year old female Baltic grey seals (August to March). Finnish data for inferred birth rates is included in the period 1997–2007, in addition to Swedish pregnancy rate data.

 

The pregnancy rate for the 4–5-year old individuals was 65% and for the 6–20-year old individuals it was 95.5% among hunted and by-caught grey seals in 2002–2009 in Sweden (Assessment protocol figure 2).

 

Assessment figure 2.png

Assessment protocol figure 2. Pregnancy rate in 4-6 year-old females (first column), 6–20 year-olds (second column), and all age classes 4-20. Based on by-caught and hunted seals during 2002–2009.

 

About 4–19 dead grey seal females of ages between 4–20 years are collected annually in Sweden during the pregnancy period. From Finland about 5-51 females (7–25-year-old) wereanalysed in spring for calculating inferred birth rates between 2011 and 2016. If the females are divided into younger and older, the annual Swedish contribution will be further reduced. However, the status assessment should be based on females six years or older (for pregnancy rate) to avoid effects from young females with late sexual maturity. Consequently, threshold values should be based on material sampled from age classes 6–24 for pregnancy rate and 7–25-year-old females for birth rate.

 

Ringed seals

Life history data of ringed seals is similar to grey seals (Harding et al. 2007), which would imply that the threshold value for ringed seals should be similar to that of grey seals. The threshold value of 90% is tentatively suggested also for this species until proven false. Age classes to be included in the analysis should encompass six years and older.

The annual number of investigated 6-20-year old Baltic female ringed seals and Baltic harbour seals during the pregnancy period is very small. Assessment protocol figure 3 shows the pregnancy rate of a total number of 19 ringed seals examined during 1981-2009. The pregnancy rate in ringed seals was 68% in 2001-2009, but the sample size is confined to 9 animals. Although later material is limited, some individual ringed seals sampled are still suffering from uterine occlusions.


Assessment figure 3.png

Assessment protocol figure 3. The prevalence of pregnant females (blue columns) sampled in the implantation period August to February (Kunnasranta 2010). Proportion of sexually mature (red columns) encompass females with presence of Corpus luteum (4 years or older) sampled year round in Finland and Sweden. Sample sizes must be increased before assessments of status can be performed.

 

Harbour seals

The harbour seal historical pregnancy rates are based on samples from Danish and Swedish sampling programs in the Kattegat in 1988. When evaluating the threshold value at 90%, the age classes to be included are females of five years and older.

Large data sets were collected during the 1988 and 2002 phocine distemper virus (PDV) epidemics and 2014 influenza die-off that killed thousands of harbour seals. Pregnancy rates were determined either by signs of late abortions or the presence of pregnancy indicators (Heide-Jorgensen & Härkönen 1992). The pregnancy rate was found to be 94% in the 59 females older than 5 years that were sampled, and three of four females that were older than 25 years and senescent. This dataset can be used to establish a threshold value, and there are many samples available from the 2002 PDV epidemic as well as from later years in Sweden which is stored at the Swedish Museum of Natural History. However, most of is the samples are from the Kattegat, and only few are available from the Southern Baltic Sea and the Kalmarsund.

 

Assessment units and management units

This core indicator evaluates the reproductive status of seals using HELCOM assessment unit scale 2 (division of the Baltic Sea into 17 sub-basins). The assessment units are defined in the HELCOM Monitoring and Assessment Strategy Annex 4.

Existing management plans for seals operate according to management units that are based on the distribution of seal populations. The management units typically encompass a handful of HELCOM scale 2 assessment units. Evaluations are therefore done by grouping HELCOM assessment units to align with the management units defined for each seal population.

  • The Baltic grey seal (excluding Kattegat) is a single management unit, although genetic data show spatial structuring (Fietz et al. 2013). Also behavioural data suggest some large scale structuring. However, grey seals show extensive migration patterns.
  • The Baltic Ringed seal is distributed in the Gulf of Bothnia on the one hand and Southwestern Archipelago Sea, Gulf of Finland and Gulf of Riga on the other, and is represented by two different management units. This sub-division is justified by ecological data that indicate separate dynamics of these stocks. Since ringed seals from both areas show a high degree of site fidelity, as seen in satellite telemetry data (Härkönen et al. 2008), it is unlikely that extensive migrations occur at current low population numbers, although some individuals can show more extensive movements (Kunnasranta 2010). See Oksanen et al. 2015 for ringed seal movements.
  • Harbour seals in the Kalmarsund, Sweden constitute a separate management unit and is the genetically most divergent of all harbour seal populations in Europe (Goodman 1998). It was founded about 8,000 years ago, and was close to extinction in the 1970s as a consequence of intensive hunting, and possibly also impaired reproduction (Härkönen et al. 2005). The genetic diversity is substantially reduced compared with other harbour seal populations.
  • Harbour seals in the southwestern Baltic (Danish Straits, Danish, German, Polish Baltic and the Öresund region including Skåne county in Sweden) should be managed separately as this stock is genetically distinct from adjacent populations of harbour seals (Olsen et al. 2014).
  • Harbour seals in the Kattegat are also genetically distinct from adjacent populations (Olsen et al. 2014). This population has experienced dramatic declines in 1988 and 2002 caused by phocine distemper epidemics. A third epidemic caused by an unknown virus caused substantial mortality in 2007 (Härkönen et al. 2008).

Harbour seals in the Limfjord form the fourth management unit and is genetically distinct from the Kattegat harbour seals (Olsen et al. 2014).