Monday, November 19, 2007

Seal stomachs VII: Aftermath

This set of posts starts here.

Once back in Tromsø, the rest of the work started. Jaws boiled in a large pot, stench-stew in a small room with its own air supply. The teeth softened from boiling, I wrenched them from jaws, cut them with a tiny bandsaw, fixed them in epoxy to microscope slides. Then I counted the rings in teeth – rather like tree-rings – telling me how old the seals were when they died.

Lotta saw to the stomachs and intestines - thawing, washing, sieving to sort contents. Dried earbones from fish, checked and measured under a microscope for species identification. Squid beaks.

Seals can live long - harps and hoods into their thirties. The youngest harp seals reach sexual maturity at four, some female hoods mature at three. Given what's known about other, better studied, seal populations with similar life histories, about a quarter of the harps and hoods in the West Ice population should have been immature.

Of 127 seals killed on the expedition, the ages of 119 were determined from their teeth. Over two-thirds were immature (and of those, 63 were yearlings). The eight animals that weren't aged from their teeth were, judged by their weight, immature as well. We had killed too many young seals, and not enough older seals, while out on the ice. And – obviously - there was nothing that could be done to remedy the bias in age classes once we were back.

Tore wanted to use the data to make inferences on the diet of harp and hooded seals in winter, and whether the diets of the two species overlapped. What he ended up with was data that could tell him a little about the diet of very young individuals of both species, and not a lot more.

No comments: