In Nuxt 2, you have 2 fetch hooks.

The old one, before Nuxt 2.12, fetch(context) which acts a lot like asyncData. It's executed before the component creation, so you don't have access to it (data, props, options... nothing).
This one is deprecated, use asyncData instead.

The new one, from Nuxt 2.12, fetch() (with no parameters). It's executed at the same time as the created() hook. It has access to the component's context (props, data, etc.).

fetch(context) {
  // "this" doesn't exists
  // context is the Vue global context
}

fetch() {
  this.myProp // "this" exists and have access to props
}

As shown in the documentation, you can access the context with this when using the fetch() hook like

async fetch() {
  await this.$axios // or whatever this.$i18n etc.....
}

Don't use fetch(context) as explained here.