No. Decomposers in an ecosystem are generally organisms like bacteria and fungi: they feed on or absorb dead tissue and waste products, and they also break it down so that other creatures can benefit from it. Most science textbooks consider mackerel "consumers"-- they are scavengers that eat other sea animals (like jellyfish and shrimp).
mackerels are carnivores
A shoal of mackerels
There are not many differences between female jack mackerels and male mackerels. However, the female jack mackerels are able to lay eggs and reproduce.
a shark
mackerels are carnivores
Mackerel or mackerels
Mackerels are saltwater fish. Tadpoles live in freshwater. Meaning a mackerel wouldn't have much chance of eating tadpoles.
A shoal of mackerels
Spotted Dolphins Spotted Dolphins
Yes, some species of sharks do eat mackerels. Mackerels are a common prey for many shark species due to their abundance and high nutritional value. Sharks use their sharp teeth and powerful jaws to capture and consume mackerels as part of their diet.
Seals are a lot bigger than mackerels. It is far more likely for a seal to eat a mackerel than the other way around.
Mackerels have several predators, including larger fish such as tuna, sharks, and billfish. Marine mammals like dolphins and sea lions also prey on mackerels. Seabirds, such as gulls and pelicans, are known to feed on mackerel as well.