Kunnasranta, Mervi
Behavioural biology of two ringed seal (Phoca hispida) subspecies in the large European lakes Saimaa and Ladoga
(Saimaan- ja laatokannorpan käyttäytymisbiologiaa)
Abstract
Key words: behaviour, breeding habitat, diet, diving, hauling-out, Ladoga ringed seal, lair, Phoca hispida ladogensis, Phoca hispida saimensis, radiotelemetry, Saimaa ringed seal, vocalisation
The last ice age created two freshwater populations of the ringed seal (Phoca hispida) in northern Europe; one in Lake Saimaa and another in Lake Ladoga. During the last 8000-9000 years these populations have lived in isolation and are now considered separate subspecies. The aim of this thesis was to describe some aspects of the behavioural biology of these poorly known sub-species. The diving behaviour, activity patterns and diet of the Saimaa ringed seal (P. h. saimensis) were studied. In addition, underwater vocalisation and the northern breeding habitat of the Ladoga ringed seal (P. h. ladogensis) were determined.
The Saimaa ringed seal typically spent 65-80% of the total time submerged during the open-water season. The dive duration of the seals ranged from 2.8 min to 7.1 min, with a maximum duration of 23 min. Long duration resting dive series of several hours occurred mostly at night or around midday. The mean dive depth ranged from 9.8 m to 15.7 m, with a maximum of 39.6 m. Five different dive types were observed. The functions of these dives were defined as travelling, feeding and resting. Saimaa ringed seals showed nocturnal hauling-out during the open-water season, after moulting. Adult seals exhibited a high degree of place fidelity.
Small schooling fish were the main prey species for the Saimaa ringed seal. Among the nine prey species identified, perch (Perca fluviatilis), roach (Rutilus rutilus), vendace (Coregonusalbula), smelt (Osmerus eperlanus) and ruff (Acerina cernua) were the most common. The importance of crustaceans in the diet was insignificant. In cafeteria tests with a captive seal the preferred species were vendace and smelt. The captive Saimaa ringed seal displayed clear seasonal variation in its feeding activity. The length of the intestinal tract in relation to body length was shorter in the Saimaa ringed seal than in marine ringed seals.
Underwater vocalisation of the Ladoga ringed seal was rich and six different sound types were identified, among them two sound types that have not been described previously for the ringed seal. The northern part of Lake Ladoga is an active breeding habitat. All birth lairs and 88% of haul-out lairs were located in snowdrifts on the shorelines of islands. Some large haul-out lairs were used by several seals. The predators, such as red fox (Vulpes vulpes) and wolf (Canis lupus), attacked or marked some lairs. The colour of the Ladoga ringed seal pup natal hair is dark grey.
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