Mystery Islands have higher fish spawns than your island. Though the layout is random - some could have a river cliff, while others will only spawn trash for you to catch - it can help you better catch fish in certain locations, especially if you use the 'scaring' tactic mentioned earlier.

In Animal Crossing: New Horizons, Mystery Islands (visited via Nook Miles Tickets) do not inherently have "better" fish in terms of catch rates for every species, but they often provide a significantly higher concentration of rare fish due to their specialized layouts. Some islands are specifically designed as "Big Fish Islands," where only large shadows appear, or "Fin Islands," where every shadow has a dorsal fin, making it much easier to catch sharks and sunfish. Additionally, because Mystery Islands have a smaller surface area than your home island, it is much faster to "cycle" the spawns by running around and scaring away common fish to force new shadows to appear. Certain islands also feature unique geography, like "Trash Island," which actually makes it harder to fish, or "Gold Tool Island," which has a high spawn rate for rare river fish like the Stringfish. While your home island has the same seasonal fish available, Mystery Islands act as a concentrated hunting ground that removes many of the environmental distractions found back home, significantly increasing your efficiency for filling out your Critterpedia.