Processed data on blubber thickness are only available for grey seals, and hence the present indicator evaluation only covers this species. Currently, the results are based on combined Swedish and Finnish data but future evaluations will also include German, Danish, Estonian and Polish data.
The current evaluation of the nutritional status of grey seals indicates that good status has not been achieved (Results figure 1). The status evaluation is based on 257 individuals from Swedish and Finnish monitoring programmes (Results table 1).
Results table 1. Number of grey seals – hunted, by-caught and total used in the current assessment.
Results figure 1. Baltic grey seal population does not attain good status with regard to nutritional status, since observed data fails the blubber thickness threshold values of 40mm for hunted seals and 35mm for by-caught seals (in standardised samples).
A strict Bayesian analysis has not been carried out yet, but it is evident that a steep decline in blubber thickness has occurred. Such an analysis would likely support that good status has been achieved for the time periods 1993-2001 and 2002-2005, whereas the status would not be good for data from 2006 onwards for both hunted and by-caught grey seals (Results figure 2), when tested against the threshold values (40 and 35 mm for hunted and by-caught seals respectively).
However, it should also be considered that if the grey seal was identified to be at carrying capacity, which may be statistically confirmed in future assessments of population trend, and status was evaluated against a provisionally proposed threshold value of 25mm (applicable in populations close to carrying capacity) then good status would be achieved.
Results figure 2. Grey seal blubber thickness. Three-year moving average of autumn/winter blubber thickness ± SE in examined sub-adult by-caught and hunted grey seals in Finland and Sweden. All were by-caught or shot between August and December. Dashed horizontal lines indicate the thresholds used in the current evaluation for hunted (black) and by-caught (red) grey seals. Adapted from Kauhala et al 2017.
There was significant (p = 0.014) annual variation in the blubber thickness of hunted sub-adults from 2002 to 2016 without a significant trend (no significant covariates) (Kauhala et al 2017). No significant annual variation was found in by-caught sub-adults during the same period (original values, sea region and sex were significant covariates). There was, however, a decreasing trend from 37.0 mm in 2002 to 25.3 mm in 2016 when calculated from the smoothed values (p = 0.001). Smoothed values were used due to small sample sizes in some years. In addition, in can be mentioned that in a population segment, pups of the year, not included at present in the indicator the same trend can be observed: The blubber thickness of hunted pups (< 1 year) decreased from 37.2 mm in 2002 to 24.6 mm in 2010 (p < 0.001, n = 86), and that of by-caught pups declined from 29.4 mm in 2003 to 20.2 mm in 2010 (p <0.001, n = 159). However, blubber thickness of pups has declined in recent years in the Gulf of Finland but increased in Baltic Proper and the values were calculated after adjusting the covariate effect of sea area. The declining trend in the blubber thickness of pups in the Gulf of Finland and by-caught sub-adults is alarming, and its causes should be examined. It may be a natural trend, if the population numbers are approaching the carrying capacity but it could potentially indicate a major change in the Baltic Sea ecosystem.
Although data are currently too scarce to establish a threshold value for ringed seals, the available data indicate that the nutritive condition of ringed seals is also deteriorating (Results figure 3). Decreasing blubber thickness is seen both in juveniles (1-3 year old) and adults (Kauhala et al 2018).
Results figure 3. Blubber thickness of ringed seals: (A) total data (n = 489) with month and cause of death as covariates and (B) hunted adults (n = 208) with month and sex as covariates (3-year moving averages of means and S.E.) (Kauhala et al 2018).
In the overall ringed seals data there was significant annual variation in the blubber thickness (with month and cause of death as covariates), and a significant declining trend from 41.0 mm in 1986 to 31.0 mm in 2007 (p < 0.001). After 2007 the blubber thickness fluctuated. There was also significant annual variation (with month and sex as covariates) in the blubber thickness of hunted adults, and a stronger declining trend in blubber thickness, from 45.6 mm in 1986 to 32.0 mm in 2002 (p < 0.001). Data including only autumn samples are too small to perform equivalent analysis for this indicator evaluation or to definitively identify specific causes and these data should be considered as a preliminary suggestion of the nutritional status of ringed seals.
Results figure 4. Blubber thickness of ringed seal pups and sub-adults with month and cause of death as covariates (mean of 3-year moving averages ± S.E.).
There was significant annual variation (p = 0.036, n = 228) in the blubber thickness of pups and sub-adults, though no clear trend was present (Results figure 4). No significant annual variation existed among hunted pups and sub-adults (p = 0.214, n = 138). There was, however, a declining trend among by-caught pups and sub-adults from 36.5 mm in 2007 to 27.5 mm in 2015 (p < 0.001, n = 90) (Results figure 5).
Results figure 5. Blubber thickness of by-caught pups and sub-adults (with month as a covariate) from 2007 to 2015 (mean of 3-year moving averages ± S.E.).
A number of possible developments are planned for this indicator to expand the current approach and hone the accuracy of the assessment. For example, indicator evaluations could be based on animals of several age classes in addition to the current subadults (ages 1-4) to increase sample sizes, i.e. age class 0 (pups of the year) and sexually mature females and males could be included if care is taken to also account for their reproductive status). Should new scientific evidence determine that grey seals spend sufficient time in restricted areas (i.e. utilize only limited areas and thus are only affected by the conditions in those areas) to also represent regional differences, though to do this the thresholds would need to be re-evaluated, and alternative model assumptions should be tested to account for the cyclic nature of blubber thickness. Furthermore, the indicator will also be incorporated into discussions on future developments of a more wide-ranging nutritional status assessment that should be sensitive and able to inform more strongly on specific drivers (health status indicators). Such approaches could also encompass a wider sampling scope, for example also including stranded seals and assessing the impact of parasites or other factors on blubber thickness. Importantly, data collection and reporting through agreed national monitoring programs, to a designated database, need to be developed and expanded for all seal species in all relevant areas of their distribution (i.e. to increase spatial coverage of data underlying the assessment). Secondly, major methodological developments are required to develop and agree suitable thresholds for species of seals other than the grey seal and for populations that are at carrying capacity. Aspects of this work will require new methodological approaches on existing data and also primary research initiatives.
Sufficient material is collected annually for grey seals in Finland and Sweden to enable a status assessment to be made, and the methodological approach is sound, thus the confidence of the indicator status evaluation for the grey seal in the central and northern parts of the Baltic Sea is high. Samples used in this evaluation also include Swedish material from the southern Baltic Sea, and considering the know dispersive nature of the grey seal and the single population of grey seals in the Baltic Sea region the scaling of the evaluation to the whole Baltic Sea is considered to have moderate confidence. In the future it would be highly beneficial to also include existing and future collected data from other countries (e.g. Denmark, Germany and Poland).
The high confidence in the indicator evaluation for grey seals is supported by earlier studies, which have shown that the autumn/winter blubber thickness has decreased significantly in Baltic grey seals since the beginning of the 2000s, especially in 1-3 year-old by-caught and hunted seals (Bäcklin et al. 2011). There could be several reasons for a thin blubber layer in the autumn/winter season, e.g. disease, contaminants, decreased fish stocks, dietary changes, a change in the quality of the diet, or increasing population density. Recent studies of grey seal suggest that the herring quality (weight) may play in an important role. The reason for the decreasing trend in blubber thickness in seals is unknown but so far no correlations to disease have been found.
One important consideration in the future when/if grey seals are shown to reach carrying capacity would be the need for the implementation of alternative threshold values that reflect the population density, environmental conditions and dynamics that would be different from the current situation. While signs of the grey seal approaching carrying capacity may be occurring currently there is not sufficient data to confirm this situation and this is also reflected in the moderate confidence value assigned to the current assessment.
A decreasing trend in autumn/winter blubber thickness has also been observed in young Baltic ringed seals (Kunnasranta 2010), although the latest material from Finland might indicate improved conditions. Data are still scarce for ringed seals in both management units, resulting in a low confidence in the preliminary evaluation results for this species.
For harbour seals, although material is collected annually, further studies and analysis are required before status can confidently be assessed.