Baltic seal monitoring and population trends Markus Ahola Swedish - - PowerPoint PPT Presentation

Baltic seal monitoring and population trends

Markus Ahola – Swedish Museum of Natural History

Seals’ annual cycle

• Grey seal and ringed seal pups are born in February-March,

harbour seal pups in June, lactation period 3-6 weeks

• In the end of lactation period mating for the next reproduction

takes place

• Moulting - RS: April-May, GS: May-June, HS: August)
• From moult to next breeding season: foraging, resting, travelling,

gathering a fit condition for next breeding and winter

• Delayed implantation of the embryo a couple of months after

moult

History of seal populations in the Baltic

• High hunting pressure &

bounty practises lead to population crash in early 1900’s

• After the largest crash

environmental contaminants such as PCB caused low reproductive success and prevented the populations to recover

Harding & Härkönen, Ambio 1999

Current monitoring methods for abundance and distribution

• Based on aerial surveys during

peak moulting period

– Ringed seal after mid-April – Grey seal in late May - early June – Harbour seal in mid-August

• Moult is the time when seals spend

largest proportion of their time hauling out on land or ice and are available for counting

• The monitoring in Baltic is

internationally coordinated among people in HELCOM SEAL EG

Grey seal

240

• Grey seals gather to islets and skerries in outer archipelago

and coastal zone

• All haulouts are surveyed annually 2-3 times within a

commonly agreed two-week period in the end of May- beginning of June, new locations searched during the surveys

• The groups of seals are photographed and their numbers

are counted from the photos

• The largest daily result for each sea area is used as the

abundance index

2 600 +20.6 % 2 000 +3.9 % 800 +1.0 %

Abundance index in 2017 and average annual increase (%/year) during 2003-2017

30 000 +4.9 %/v

Grey seal abundance trends

11 100 +8.1 % 2 100 +9.0 % 8 100 +0.9 % 4 000 +3.8 %

The whole Baltic ~ 30 000, 60-80 % of the total population size of 37 500 – 50 000. Geographically describes the situation during the peak moult!

Harbour seal

• Two-three aerial surveys covering all haulout sites within two weeks in August
• Seals are photographed and number of individuals are counted from the

photographs

• Abundance index, ”a trimmed mean” = average of two largest daily results

6 500 +7.5 % 10 500 +6.9 % 900 +6.9 %

Abundance index in 2016 and increase (average %/v) during 2005-2016 Four management units:

• Kattegat, Skagerrak, Kalmarsund

ja Southern Baltic

900 +7.8 %

Harbour seal abundance trends

Abundance index estimated to represent 65 % of total population size. ⇒ Total population sizes: Skagerrak 10 000 Kattegat 16 000 Southern Baltic 1 400 Kalmarsund 1 400

Ringed seal

• Bothnian Bay:

– Aerial transect line methodology – Transects evenly spread over the ice- covered area – Flying altitude 90 m, survey strips 400 m on both sides – Survey strips give a sample of min 13 % of the ice-covered area – All seals on the survey strips are photographed and counted from the photos – The number is multiplied from the sample to the whole area – The result is used as abundance index

Härkönen & Lunneryd, Ambio 1992

Ringed seal

• The Archipelago Sea, Gulf of Finland and Western Estonia:

– Aerial surveys possible only on good enough ice-years – In the Archipelago Sea the whole area between the transect lines is observed – Monitoring methods for ice-free circumstances under development

Two management units:

• Bothnian Bay
• The Archipelago Sea, Gulf of Finland and W-

Estonia

> 20 000 ~1 000 ~200-300 ~100

Annual increase for Bothnian Bay population in 1988-2016: on average 4,5 %/year

Ringed seal trends

In other areas: no increase