For the grey seal, good status is achieved when blubber thickness of sub-adults is at least 40 mm for hunted seals and 35 mm for by-caught seals (HELCOM HOD 48-2015, outcome para 3.63, Annex 4). A provisional threshold value of 25 mm is currently proposed if the population is assessed to be at carrying capacity (Thresholds table 1) as this reflects the level below which depleted fat reserves result in interference with thermoregulatory processes.
The concept for defining a threshold value for nutritional status 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 levels. The Recommendation further states that the long-term goal is to reach a health status that ensures the future persistence of marine mammals in the Baltic Sea.
Nutritional status (i.e. blubber thickness) is an important aspect of health, affecting somatic growth, age at sexual maturity, fecundity, implantation of embryos, maintenance of pregnancy, age specific mortality as well as vulnerability to parasites and diseases. Although approaches such as body mass index (BMI) have been developed for humans, no threshold values are available for nutritional status of animal populations, although several studies have shown that seals with lower body weight and lower fat reserves show increased mortality (Kjellqwist et al. 1995, Harding et al. 2005, Bowen et al 2015) and decreased reproductive rate (e.g. Boyd et al. 1999). Currently data from 1-3 year old grey seals of both sexes are used in this indicator (for more information, see the section 'Selection of appropriate data' in the extended core indicator report). Threshold values are established for two scenarios: for populations undergoing exponential growth and for populations at carrying capacity. Future work may facilitate a fine tuning of the threshold values for populations at carrying capacity, and careful integration of this information with the ''Trends and Abundance indicators'' for seals and estimates of historical population sizes should provide a robust approach (Harding et al 1999, 2005, 2007).
The threshold value for nutritional status is defined based on what is considered to be a good condition in the current environment (Thresholds table 1). A modern baseline approach is used to set the threshold value. This is aligned with the approach used in OSPAR (Commission for the Protection of the Marine Environment of the North-East Atlantic), where baseline levels are set at pristine conditions 'where influence of human impact is minimal', or alternatively, a 'modern baseline when the former isn't applicable'.
Thresholds table 1. Threshold values set for grey seals applicable in the entire Baltic Sea as agreed by HELCOM HOD 48-2015 (outcome para 3.63, Annex 4).
To set the threshold value for grey seals, data on blubber thickness during the period 2001-2004 represents the most recent data period that indicated good status and is used to form a modern baseline for the threshold value concept for populations undergoing exponential growth. The threshold value is set at 40 mm blubber for samples from hunted seals and 35 mm blubber for by-caught seals (Thresholds table 1). This threshold is currently applicable in the entire Baltic Sea since the population is panmictic and highly migratory.
Since all growing populations eventually approach the carrying capacity of the ecosystem unless controlled by hunting, predation or by stochastic events, vital population parameters will change (see Relevance of the indicator). Nutritional status of seals will deteriorate due to limited food supply (occurring naturally or via human pressures such as fishing) or quality of food resources, and pups of the year and sub-adults (1-3) are the first to be affected. This is a natural process when populations are close to carrying capacity, and threshold values set for populations under exponential growth would no longer be applicable. To set threshold values for populations at carrying capacity thermoregulatory constraints are helpful, since lean seals will have severe problems compensating for heat loss during the winter (Harding et al. 2005). The threshold value for seal populations at carrying capacity in the whole Baltic Sea is suggested to be around 25 mm blubber for both hunted and by-caught seals, since leaner seals in both categories will have increased risk for not surviving the winter. Thus this value is a provisional threshold value, though the exact level of the lower threshold value for each seal species should be further researched.