Sex differences in gannet migration and demography – new paper out!

During spring and summer the Bass Rock is home to around 75,000 pairs of  gannets making it the largest colony of northern gannets in the world. Deploying GPS loggers on adult and immature gannets in the spring and summer is giving us a great insight about where they forage during the breeding season but we know far less about where they go once they leave the colony in the autumn. Increasing our knowledge of where gannets spend the non-breeding season is crucial for conservation and determining protected areas at a time when pressures on the marine environment from climate change, renewable energy infrastructure and fisheries, are increasing.

A recent study, authored by Zoe Deakin from the University of Exeter, has tracked the migration of male and female gannets from the Bass Rock and Grassholm (SW Wales) using geolocators (small loggers that record light levels) to see whether the two sexes differ in their wintering destinations. It has also used data from the re-sighting of adults with colour rings to test for sex-specific differences in adult annual survival and determine future impacts on colony growth.

The gannets from Bass Rock and Grassholm have diverse migratory behaviours, wintering from the North Sea down to West Africa. There were differences between the sexes with 90 % of the females travelling the furthest distances to winter in the Canary Current Large Marine Ecosystem (CCLME) compared with 57 % of males. The CCLME (shaded area off West Africa in the figure below) is currently one of the most intensively fished areas of the planet and has the world’s highest levels of illegal, unreported and unregulated (IUU) fisheries (Agnew et al. 2009 – see full paper below for reference).

Centroids of locations in December of male (blue) and female (orange) northern gannets tracked with geolocators from Bass Rock (n = 30; 17 males and 13 females) and Grassholm (n=19; 13 males and 6 females). Colony location is marked with a solid black circle. Solid black lines mark borders of Large Marine Ecosystems. Shaded area represents the Canary Current Large Marine Ecosystem (CCLME). (Deakin et al. 2019)

Survival of females from Grassholm was slightly lower than for males but there was no difference in survival between the sexes from Bass Rock despite similarly high proportions of females from both colonies wintering in the CCLME. It is possible that high levels of bycatch in the CCLME could explain the lower survival of female gannets but it is unclear why there should be a difference between females from the two colonies.

Colour rings with unique codes allow easy identification of individual birds at the colony – photo Jude Lane

The paper goes on to demonstrate the importance of accounting for sex-specific differences in survival when predicting population trajectories and highlights the importance of monitoring the behaviour of seabirds year-round.

The paper “Sex differences in migration and demography of a wide-ranging seabird, the northern gannet” has been published in Marine Ecology Progress Series and is free for anyone to download so if you’d like to read the paper for yourself then you can find it here.


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