Thresholds and Status evaluation

Good status for the population trends and abundance of seals in the Baltic Sea is determined by comparing population data with threshold values that have been defined based on concepts developed for the conservation of seals, in particular the HELCOM Recommendation 27/28-2 'Conservation of seals in the Baltic Sea area'.

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Thresholds figure 1. Good status is achieved when the population growth rate trend and abundance of seals are above the threshold value.

 

Good status is achieved for abundance of seals in a management unit if the population is above the Limit Reference Level (LRL) with a steady increasing trend towards the Target Reference Level (TRL), where TRL is the level where the growth rate starts to level off and the population asymptotically approaches the current carrying capacity level.

HELCOM set a LRL of 10,000 individuals for all the Baltic seal species for each ecologically and genetically isolated population. For ringed and grey seals, the LRL of 10,000 individuals is the minimum abundance for a species to achieve the threshold value. Harbour seal subpopulations that are connected by migrants should be treated as a metapopulation, where numbers of seals in the metapopulation should exceed 10,000 seals to achieve good status. Harbour seals in the Kalmarsund are isolated and with the current abundance estimate of approximately 1,000 individuals the subpopulation does not reach good status. Harbour seals in the Southern Baltic are considered as a part of a metapopulation including the Kattegat and achieve good status with regard to abundance.

The growth rate aspect of the threshold value is assessed separately for populations at and below the TRL:

  • For populations at target reference level (TRL), good status is defined as 'No decline in population size or pup production exceeding 10% occurred over a period up to 10 years'
  • For populations below TRL, good status is defined as 3% below the maximum rate of increase for seal species, i.e. 7% annual rate of increase for grey seals and ringed seals and 9% for harbour seals. For good status, 80 % statistical support for a value at or above the threshold is needed. Subpopulations of harbour seal metapopulations are evaluated separately for trends, making different statuses possible for subpopulations of the same metapopulation.

The concept for defining a threshold value for the population size of seals is derived from the general management principle in the HELCOM Recommendation 27/28-2, which states that the population size is to be managed with the long-term objective of allowing seal populations to recover towards carrying capacity.

The limit reference level (LRL) corresponds to the safe biological level and minimum viable population size. HELCOM has set a LRL of 10,000 individuals for grey seals, ringed seals and harbour seals in each of their management units, respectively, understanding that the haul-out fraction during moult surveys is 70%. The LRL of 10,000 implies a population with approximately 5,000 adult seals (and thus 2,500 adult female seals). LRL has been calculated based on estimates of minimum viable population sizes of each seal species based on different extinction risk levels (1, 3, 5 and 10%) for genetically and ecologically isolated populations. The LRL is applicable to Baltic ringed seals, grey seals and harbour seals, corresponding to management units defined in HELCOM Rec. 27/28-2. Some management units of harbour seals (Southern Baltic Sea, Kattegat and possibly the Limfjord) show distinct genetic differences, these populations are affected by immigration/emigration, which is why LRL is not applicable in these units and population sizes of adjacent stocks are included in the evaluation of the LRL. For some isolated harbour seal units, the LRL may not be achievable. This may be the case for the Kalmarsund and the Limfjord units (degree of reproductive isolation of the harbour seals in the inner Limfjord is being resolved by an ongoing project). For these management units, the abundance criterion will be considered achieved when they have attained carrying capacity, even if it is at a level below the LRL.

The approach used for defining the threshold value for population trends is based on the principles of the HELCOM Recommendation 27/28-2 as the population is to increase until it reaches carrying capacity. The 'good status' concept also follows principles applied in Ecological Quality Objectives (EcoQOs) that were developed for marine mammals in the North Sea by the Convention for the Protection of the Marine Environment of the North-East Atlantic (OSPAR Convention). This core indicator is similar to the EcoQO element of the same name used in the ICES and OSPAR frameworks, with the distinction that the two latter EcoQOs include 'No decline in population size or pup production exceeding 10% over a period up to 10 years' for populations 'minimally affected by anthropogenic impacts'. This condition is, however, also deemed appropriate for this core indicator when seal populations are close to natural abundances, i.e. close to carrying capacity.

The OSPAR and ICES frameworks provide some guidance also for populations far below 'natural' or 'pristine' abundances. Applying the term 'anthropogenic influence is minimal' would imply that a population should grow close to its intrinsic rate of increase when not affected by human activities. The theoretical base for this measure is outlined below and compared with empirical data from seal populations.

 

Approach for defining the threshold value for growth rate for populations close to carrying capacity (target reference level)

All growing populations will eventually be affected by density dependent factors (such as decreased availability of food and lack of haul-out sites) and the population size will stabilize at the carrying capacity of the ecosystem. Population sizes of marine mammals can be expected to fluctuate around the carrying capacity due to annual changes in food abundance and other external factors (Svensson et al. 2011). In this situation, the ICES and OSPAR frameworks proposed that good status is achieved when there is 'No decline in population size or pup production exceeding 10% occurred over a period up to 10 years'. The same level is used in the Baltic Sea for the purposes of this core indicator.


Approach for defining the threshold value for growth rate for populations below carrying capacity

Long-term maximum growth rates in seals

The maximum rate of population growth is limited by several factors in grey seals and ringed seals. Females have at most one pup a year, of which 48% are female pups, and first parturition occurs at about 5.5 years of age. It is also evident that not all adult females bear a pup each year, especially not young females (Bäcklin 2011). Of females older than six years, 95.5% ovulate each year (NRM database, Bäcklin et al. 2011), and not all of them will complete a successful pregnancy. An additional limitation for the population growth rate is given by the survival of adults. In most seal species, the highest measures of adult survival are about 0.95-0.96, and for grey seals the best estimate available is 0.935 (Harwood & Prime 1978). An additional constraint is the observation that pup and sub-adult survival is always found to be lower and more variable compared to adult survival in all studied species of seals (Boulva & McLaren 1979; Härkönen et al. 2002). 


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Thresholds figure 2. Biological constraints delimit the maximum possible rate of increase in populations of grey and ringed seals. The shaded area denotes unlikely combinations of adult and juvenile survival rates. Any given point along the 6 lines shows a combination of adult survival and juvenile survival that produces a given growth rate (λ). The two uppermost lines are for λ=1.10, the two lines in the middle for λ=1.075, and the lowest two lines show combinations that result in λ=1.05. The stippled lines show combinations of adult and juvenile survival rates given that the mean annual pupping rate is 0.95. The bold full lines show the possible combinations given that the pupping rate is 0.75.

 

These biological constraints impose an upper ceiling of possible rates of long-term population growth for any seal species and can be found by manipulations of the life history matrix (Caswell 2001; Härkönen et al. 2002). Thresholds figure 2 illustrates how fertility and mortality rates known for grey and ringed seals can combine to produce different long-term population growth rates. It is found that growth rates exceeding 10% (λ= 1.10) per year are unlikely in healthy grey seal populations (top stippled line in Thresholds figure 2). Reported values exceeding 10% should be treated sceptically since they imply unrealistic fecundity and longevity rates. Such high growth rates can only occur temporally, and can be caused by e.g. transient age structure effects (Härkönen et al. 1999; Caswell 2001), but are also to be expected in populations influenced by considerable immigration.

The upper limit of individual reproductive rate is reflected at the population level, and gives an upper theoretical limit for the rate of population increase (Thresholds figure 2). The mean values of fecundity and mortality will always be lower than the theoretical maximum, also for populations which live under favourable conditions. Chance events such as failed fertilization or early abortions reduce annual pregnancy rates, and in samples of reasonable sizes, mean pregnancy rates (or rather annual ovulation rates) rarely reach 0.96 (Boulva & McLaren 1979; Bigg 1969; Härkönen & Heide-Jørgensen 1990).

Another factor that will decrease mean pregnancy rates is senescence and pathological changes in the reproductive organs (Härkönen & Heide-Jørgensen 1990). Further, environmental factors reduce fecundity and survival rates. The impact from extrinsic factors may occur with different frequency and amplitude. Environmental pollution and high burdens of parasites can decrease population-specific long-term averages of fecundity and survival (Bergman 1999), while epizootic outbreaks and excessive hunting have the capacity to drastically reduce population numbers on a more short-term basis (Dietz et al. 1989; Harding & Härkönen 1999; Härkönen et al. 2006). Fluctuations in food supply and availability of breeding grounds can cause energetic stress affects survival and fecundity. The type of variation in fecundity and survival rates will determine the demographic structure of a population. In a population with a constant rate of increase (thus no temporal variability), the age- and sex-structures quickly reach stable distributions, where the frequencies of individuals at each age class are constant. Populations with low juvenile survival typically have steeper age distributions compared to populations with higher juvenile survival rates (Caswell 2001). Skewed age structure can cause a temporal flux in the population growth rate.

Harbour seals mature about one year earlier than grey seals and ringed seals, which is why maximum rate of increase in this species is 12-13% per year (Härkönen et al. 2002).

 

Empirical evidence

With few exceptions, most populations of seals have been severely depleted by hunting during the 20th century. Detailed historical hunting records for pinnipeds are available for the Saimaa ringed seal, Baltic ringed seal, Baltic grey seal and the harbour seal in the Wadden Sea, Kattegat and the Skagerrak. Analyses of these hunting records have documented collapses in all populations, which were depleted to about 5-10% of pristine abundances before protective measures were taken (Heide-Jørgensen & Härkönen 1988; Kokko et al. 1999; Harding & Härkönen 1999). After hunting was banned and protected areas were designated, most populations started to increase exponentially.

Harbour seal populations in the Kattegat and outside the Baltic increased by about 12% per year between epizootics in 1988 and 2002 (Olsen et al. 2010, Teilmann et al. 2010), whereas harbour seals and grey seals in the Baltic showed lower increase compared with exponentially increasing oceanic populations (Wadden Sea Portal). A Bayesian approach (below) is used to evaluate if observed rates of increase close to intrinsic rates are supported. The threshold value for population growth rate is set to a value 3% lower than the maximum rate of increase.

 

Thresholds table 1. Rates of increase in seal populations recovering after over-hunting. Grey seals from the UK, Norway, and Iceland are not included here since they have been consistently hunted over the years. Canadian grey seals have a life history similar to harbour seals.

Species Area Annual growth rate Period Reference
Harbour seal Skagerrak+12%1978-1987Heide-Jørgensen & Härkönen (1988)
Harbour seal Skagerrak+12%1989-2001Härkönen et al. 2002
Harbour seal Kattegat+12%1978-1987Heide-Jørgensen & Härkönen (1988)
Harbour seal Kattegat+12%1989-2001Härkönen et al. 2002
Harbour seal Baltic+ 9%1972-2010Härkönen & Isakson 2011
Harbour seal Wadden Sea+12%1980-1988Reijnders et al. 1994
Harbour seal Wadden Sea+12%1989-2001Wadden Sea Portal