The claim that 'seals need culling' in order to reduce their numbers is not justified.
In modern times, most seal numbers have been drastically reduced along with apex predators such as cestaceans and elasmobranchs. Furthermore, fish stocks which are seals main diet have also followed the trend along with plankton that complete the food web.
Damming, overfishing, whaling, shark fin souping, clubbing and humans indiscriminate taking of any species has knock on effects. If the sea was left to it's own devices - mainly the weak and old in each species would be eaten in the food chain therefore leaving the strongest and most fecundant individuals to survive.
By taking the strong and virile individuals it leaves weaker animals to reproduce which do not produce as many or as adaptive young, so the gene pool becomes weaker and thus numbers reduce.
This is Darwinism in a nutshell and shows why over population of humans could eventually lead all humans to cannabalism as there will be few animal species left