The Impact of Food Resources, Reproductive Rate and Hunting Pressure on the Baltic Grey Seal Population in the Finnish Sea Area

Abstract

Grey sealRinged seal Harbour sealThe indicator includes all species of seals that occur in the Baltic Sea and is applicable in the whole HELCOM area. The status evaluation is presented separately for the three seal species. The grey seal in the Baltic proper is evaluated as a single unit, whereas the Kattegat grey seals are evaluated separately. The status of ringed seals is evaluated for two management units and harbour seals for four units. The evaluation of abundance of seals is based on data from 2014.Grey seal occurs in the entire Baltic Sea except for the Kattegat where the species has not been breeding since the 1930s except for a few observations from recent years. Grey seals achieve GES with regard to population growth rate in the entire Baltic when evaluated as one single population. The abundance of grey seals achieves GES. The confidence of evaluation is high.Ringed seals occur in the Bothnian Bay, which is one management unit, and the Gulf of Finland, the Archipelago Sea, the Gulf of Riga and the Estonian coastal waters, which is a second management unit. The ringed seal population growth rate is considerably below GES levels in both units. Subpopulations in the Gulf of Finland and the Gulf of Riga are stable or declining. The size of the population is in GES only in the Bothnian Bay. Confidence of evaluation is high for both the Northern and Southern management units.Harbour seals are confined to the Kalmarsund, Southern Baltic, the Kattegat and the Limfjord, which all are separate management units. The Kattegat and Limfjord sub-populations may be approaching carrying capacity, since the annual growth rates are levelling off. As no decline exceeding 10% has been detected over the last 10-year period, these sub-populations have achieved GES with regard to population growth rate. However, more information is needed for the Limfjord on the connectivity of this stock with the Wadden Sea population. The harbour seal population in the southern Baltic is growing at 9.1% per year,

The effect of prey quality and ice conditions on the nutritional status of Baltic gray seals of different age groups

“…The increasing herring weight in recent years thus likely caused the increase in the blubber thickness of adult females in the Bothnian Bay. Furthermore, an earlier study (Kauhala et al 2016) showed that the birth rate (% mature females giving birth each year) of gray seals increased during recent years with increasing herring weight. Blubber thickness of hunted adult females also partly explained the variation in the birth rate of gray seal females (birth rate from Kauhala et al 2016: Table 2; r 2 = 0.53, F = 8.9, df = 1, 6, p = 0.024) in recent years.…”

“…Furthermore, an earlier study (Kauhala et al 2016) showed that the birth rate (% mature females giving birth each year) of gray seals increased during recent years with increasing herring weight. Blubber thickness of hunted adult females also partly explained the variation in the birth rate of gray seal females (birth rate from Kauhala et al 2016: Table 2; r 2 = 0.53, F = 8.9, df = 1, 6, p = 0.024) in recent years. Nutritional status (or body mass) of female seals may thus have an impact on reproduction (age at sexual maturity, ovulation rate, implantation of embryos, and maintenance of pregnancy) of seal females (e.g., Boyd 1984;Boyd et al 1999;Harwood et al 2000;Bowen et al 2006Bowen et al , 2015.…”

Delayed effects of prey fish quality and winter temperature during the birth year on adult size and reproductive rate of Baltic grey seals

“…Ovaries were removed, and cross sections made by knife were examined by naked eye for the presence of corpus albicans (CA). Birth rate was determined as the proportion of females with CA from the spring sample (April-June; see also Kauhala et al 2014Kauhala et al , 2016. Corpus luteum atrophies to CA after parturition and remains until mid-summer when a new embryo implants into the uterus but may disappear after mid-summer if the female is not pregnant (Hewer 1964 .…”

“…It is a generalist predator preying on several fish species (Lundström et al 2007(Lundström et al , 2010Kauhala et al 2011;Scharff-Olsen et al 2018;Tverin et al 2019). In the Finnish sea area, Baltic herring (Clupea harengus membras) is the most common prey fish for grey seals, and herring quality is related with body condition and reproductive rate of seals (Kauhala et al 2016(Kauhala et al , 2017(Kauhala et al , 2019. Another common prey fish for grey seals is the sprat (Sprattus sprattus), especially in the southern part of the Finnish sea area (Kauhala et al 2011).…”

“…Another common prey fish for grey seals is the sprat (Sprattus sprattus), especially in the southern part of the Finnish sea area (Kauhala et al 2011). Changes in the Baltic food web which influence the populations of the prey fish species of seals (Casini et al 2006(Casini et al , 2008(Casini et al , 2010 also have demonstrable effects on seal populations (Kauhala et al 2016(Kauhala et al , 2019.…”