The status of biodiversity is assessed using several core indicators. Each indicator focuses on one important aspect of the complex issue. In addition to providing an indicator-based evaluation of the nutritional status of seals, this indicator will also contribute to the next overall biodiversity assessment to be completed in 2018 along with the other biodiversity core indicators.
The core indicator on nutritional status of seals addresses the Baltic Sea Action Plan's (BSAP) Biodiversity and nature conservation segment's ecological objective 'Viable populations of species'.
The core indicator is relevant to the following specific BSAP target:
The HELCOM Recommendation 27/28-2 'Conservation of seals in the Baltic Sea area' outlines the conservation goals which the indicator's threshold value is based on. The explicit long-term objectives of management plans to be elaborated are: Natural Abundance, Natural Distribution, and a health status that ensures the persistence of marine mammals in the Baltic.
The core indicator also addresses the following qualitative descriptors of the MSFD for determining good environmental status (European Commission 2008):
Descriptor 1: 'Biological diversity is maintained. The quality and occurrence of habitats and the distribution and abundance of species are in line with prevailing physiographic, geographic and climatic conditions' and
Descriptor 4: 'All elements of the marine food webs, to the extent that they are known, occur at normal abundance and diversity and levels capable of ensuring the long-term abundance of the species and the retention of their full reproductive capacity'.
Descriptor 8: 'Concentrations of contaminants are at levels not giving rise to pollution effects'
and the following criteria of the draft Commission Decision on GES criteria (European Commission 2016):
Marine mammals were recognized by the MSFD Task Group 1 as a group to be assessed.
In some Contracting Parties the indicator also has potential relevance for implementation of the EU Water Framework Directive (WFD, Chemical quality) and Habitats Directive. The WFD includes status categories for coastal waters as well as environmental and ecological objectives, whereas the EU Habitats Directive (European Commission 1992) specifically states that long-term management objectives should not be influenced by socio-economic considerations, although they may be considered during the implementation of management programmes provided the long-term objectives are not compromised. All seals in Europe are also listed under the EU Habitats Directive Annex II (European Commission 1992), and member countries are obliged to monitor the status of seal populations.
Being top predators in the Baltic ecosystem, seals are exposed to ecosystem changes in lower trophic levels, but also to variations in climate (length of seasons and ice and snow conditions) and impacts of human activities. Specific pressures such as changes in fish stocks, levels of harmful substances, boat traffic, noise pollution, harmful algal blooms, as well as direct mortality caused by hunting or by-catch are considered significant. The vulnerability of seals to these pressures makes them good indicators for measuring the environmental status of ecosystems.
The nutritional status of seals can be regarded as a direct link between the environment, individual fitness and population growth rate (Relevance figure 1). Seals fight a constant struggle to reach a critical limit of fat storage each autumn (Relevance figure 2). Failure to reach this level will result in failed reproduction in adults and high mortality rates in juveniles. Ecosystem effects (e.g. reduced food supply or poor quality of food) are readily visible in the blubber layer in a few weeks or months. If poor nutritive conditions persist for a prolonged time period also total body growth rate in sub-adults is stunted and eventually the asymptotic adult body lengths of the entire population decline. This results in delayed sexual maturity, smaller females that can transfer less energy to their pups, which will in turn have reduced chances of survival. All this will have dramatic effects on the population growth rate and health of seals. The latter because leaner seals are more exposed both to parasites and diseases.
Relevance figure 1. Seal population dynamics is linked to the rest of the ecosystem through individual fitness which in turn is determined by the blubber thickness as an indicator of the nutritional status of individuals.
The nutritional status of seals reflects many processes in the Baltic Sea ecosystem, especially quality and quantity of different fish stocks. It can also reflect levels of pollutants and other stressors, since diseased animals are in poor body condition. Baltic seal nutritional status can potentially act as an early warning when new hazardous substances begin to accumulate in the food chain since seals are at the top of the food chain and likely show early symptoms, as was the case with PCBs in the 1970s (Bergman & Olsson 1986).
The three seal species included in this indicator are all phocid seals that have a life history where they rely on stored fat reserves for over-winter survival and reproduction. Their pups are lactated during a few weeks in the spring (grey and ringed seals) or summer (harbour seals) and female weight loss during this short period is massive, up to 30-50% of total body weight (Kovacs & Lavigne 1986; McCann et al. 1989; Haller et al. 1996). During summer and autumn, seals intensively search for prey to build up their fat reserves (Relevance figure 2; Nilssen et al. 1997; Hauksson 2007). Failure to reach a critical fat reserve in late autumn may result in decreased survival and failed reproduction, including foetal mortality. Thus, food abundance/quality and other factors that influence feeding success during the autumn are important. Blubber thickness is one vital component indicative of nutritional status and is most informative during late autumn and winter as it is at it annual maximum (however, samples collected year round can be used by treating month of collection as a covariate in the statistical testing). Also, body length and weight at age are important parameters to monitor year round for evaluation of nutritional status (examples below).
Relevance figure 2. Schematic figure of the blubber thickness during the annual cycle of an adult female harbour seal in the Baltic Sea and Kattegat. The grey circle illustrates the strong flux in blubber thickness that is connected to the indicated major 'annual cycle events'. The figure would be similar if drawn for any true seal (Phocidae), if the months are rotated to fit the cycle for each population. The true size of the weight loss is exemplified in Relevance table 1. Modified from Harding 2000. The best sampling period in winter is difficult to apply to sample collection in Finland because the hunting season ends in December, thus most samples are collected earlier in autumn (or in spring).
Grey seal females spend on average 85% of their energy reserves during lactation (Fedak & Anderson 1987). In harbour seal females the associated mean weight loss is about 40% (Härkönen & Heide-Jørgensen 1990), but females need to forage during the last weeks of lactation to successfully wean their pups. Loss in blubber thickness and weight in ringed seals during the breeding season is also dramatic (Relevance figure 3).
Relevance figure 3. Weight of ringed seal females caught in the open water season. Filled circles refer to data from a ringed seal satellite tag study, and open circles are data compiled from the seal database at the Swedish Museum of Natural History (NRM). A linear regression of pooled data (y = 9.16x27.97, R2 = 0.83) suggests that ringed seal females gain 9.2 kg per month from May up to December (Härkönen et al 2006).
A study on individually marked harbour seals also showed that winter survival in the young of the year was highly dependent on the autumn weight (Härkönen & Harding 2001, Harding et al. 2005). The range in survival was large, from 96% in well fed pups to only 65% in lean pups (Relevance figure 4). Similar fluctuations in life history parameters have also been observed in e.g. harp seals, Canadian harbour seals and ringed seals (Harwood & Prime 1978; Fowler 1981; Kjellqwist et al. 1995; Bowen et al. 2003; Kraft et al. 2006).
Relevance figure 4. Body weight in pups reflects nutritive conditions. First year winter survival of Harbour Seal pups in the northern Skagerrak is significantly related to their body mass in the autumn. Error bars denote 95% confidence limits for each given weight. From Harding et al (2005).
Nutritional status also affects body length, and just as in other mammals the nutritive condition during childhood affects the final adult body size after sexual maturation (Harding et al 2018, Fig 6). Large declines in the average body length of adult seals have been documented in harbour seals and harp seals during periods of limited food supply (Kjellqwist et al. 1995). There is, for example, a statistically significant difference of almost 10 cm in average adult mean lengths of harbour seals collected during the last decades in the Kattegat-Skagerrak (Relevance figure 5).
Relevance figure 5. Overall body growth curves and final adult body length respond to food availability and stress and reflect the nutritive condition of seals over longer time periods (decades). In the graph above is an example from harbour seals at different population densities. In 2002 adults were 10 cm shorter on average, presumably due to higher population density (Harding et al 2018).
For long-term trends, total body length can provide very important information. The benefit of length as a parameter is that available sample size is increased since all animals collected can be incorporated (all seasons, all sampling methods). An additional feasible parameter is pup weaning weight in grey seals, where reference data is available from repeated studies. For harbour seals pup autumn weight is a sensitive parameter (Harding et al. 2005) that could be elaborated in the future.
Disturbance causing stress.
Ecosystem changes (food web, introduction of pathogens and non-indigenous species).
Fishery and food availability.
Effects of climate change are a threat to the ringed seal that breeds on sea ice, and also to grey seal pups.
Boat traffic, hunting, under water noise, ice breaking.
Theme: Substances, litter and energy
Input of other substances (e.g. synthetic substances, non-synthetic substances, radionuclides) Must be confirmed by further analysis.
Historically, hunting of seals has been a major human pressure on all the seal species in the Baltic Sea. A coordinated international campaign was initiated in the beginning of the 20th century with the aim of exterminating the seals (Anon. 1895). Bounty systems were introduced in Denmark, Finland and Sweden over the period 1889-1912, and very detailed bounty statistics provide detailed information on the hunting pressure. The original population sizes were about 180,000 for ringed seals, 80,000 for Baltic grey seals and 5,000 for the Kalmarsund population of harbour seals (Harding & Härkönen 1999; Härkönen & Isakson 2011). Similar data from the Kattegat and Skagerrak suggest that populations of harbour seals amounted to more than 17,000 seals in this area (Heide-Jørgensen & Härkönen 1988).
By-catches are known to have substantial effects on the population growth rate in species like the Saimaa and Ladoga ringed seals (Sipilä 2003). The current knowledge on the level of by-catches of Baltic seal species is limited to a few dedicated studies which suggest that this factor can be substantial. An analysis of reported by-caught grey seals estimated that more than 2,000 grey seals are caught annually in the Baltic fisheries (Vanhatalo et al. 2014), but numbers of by-caught ringed seals and harbour seals are not known. Both hunting and by-catches will affect population density and thereby the nutritional status of seals via mechanisms described in the above section 'Ecological background of the indicator concept'.
In the beginning of the 1970s grey seals were observed aborting near full term foetuses, and only 17% of ringed seal females were fertile (Helle 1980). Later investigations showed a linkage to a disease syndrome including reproductive disorder, most probably caused by organochlorine pollution, in both grey seals and ringed seals (Bergman & Olsson 1986). This disease syndrome also included adrenocortical hyperplasia, reduced bone mineral density, loss of teeth, claw deformation (Bergman & Olsson 1986). These manifestations should have had severe effects on the general nutritive condition of seals.
Climate change poses a pressure on species breeding on ice because shorter and warmer winters lead to more restricted areas of suitable ice fields (Meier et al. 2004). This feature alone will severely affect the Baltic ringed seals and the predicted rate of climate warming is likely to cause extirpation of the southern subpopulations (Sundqvist et al. 2012). Grey seals are facultative ice breeders and their breeding success is considerably greater when they breed on ice as compared with land (Jüssi et al. 2008). Furthermore, the weaning weight of grey seal pups was substantially greater when born on ice as compared with land. When a larger proportion of the grey seal pups are born on land in the future, they will be leaner and experience greater juvenile mortality. Consequently, both ringed seals and grey seals are predicted to be negatively affected by a warmer climate.
By-caught grey seals are significantly leaner as compared with hunted seals (Bäcklin et al. 2011; Kauhala et al. 2015), which may suggest that food is a limiting factor for by-caught grey seals. It is possible that food limitation is becoming an important factor also for the entire population since data of blubber thickness in Baltic grey seals (also hunted) shows a significant decline during the last decade (Bäcklin et al. 2011). These factors could indicate that the population is at the early stages of reaching carrying capacity and that seals unable to compete for food in 'risk-free' areas will approach fishing gear in search of food, however, both subjects currently require greater scientific exploration.