A dense school of fish arrived at an Acapulco beach around the time the tsunami generated by the Japan earthquake was expected to reach the coast of Mexico, but it is not known whether the events are linked.
Locals said the phenomenon had not been seen before in the resort city.
The sea was teeming with sardines, anchovies, stripped bass and mackerel. Fisherman did not need their usual gear, instead scooping up fish with buckets.
The schooling was attributed to unusual currents.
At Mexico's Cabo San Lucas resort on Baja California, heavy swells that rolled in about two feet above normal rocked boats but inflicted little damage to the harbour's marinas, local authorities reported.
Strong aftershocks and a second sizable earthquake rattled Japan through the night after the island nation's worst quake in recorded history, but the tsunami waves that drowned hundreds in north-east Japan mostly spared other countries around the Pacific Rim.