Sharks having a whale of a time?

In August 2020 some fishermen off Forster went to have a look at a dead whale.

More than a dozen sharks were feeding on it, including great whites and tiger sharks.

Then one of the great whites left the whale and had a go at the fishermen's outboard propeller.

It made me wonder about something I have wondered about for years.

Have we inadvertently bred more great white sharks by saving the humpback whale?

Before commercial whaling there were perhaps 30,000 humpbacks in the east Australian population. By the early 1960s they had been reduced almost to extinction.

Now there are more than 50,000, possibly 60,000, migrating up and down Australia's east coast each year.

That is a spectacular conservation success.

But ecosystems don't contain isolated species. Change one thing dramatically and other things change with it.

Consider the amount of food those whales represent.

Take 50,000 humpbacks and assume annual mortality of 1.5%. That's about 750 dead whales a year somewhere across their migratory range.

An adult humpback weighs roughly 25-30 tonnes. Mortality will include calves and juveniles, so use a conservative average carcass weight of 20-25 tonnes.