Spillover at the Papahānaumokuākea Marine National Monument

Written by: Chloe Molou, UHH Seawords Liaison

The catch rates of tuna outside the Papahānaumokuākea Marine National Monument have provided some promising evidence for the largely debated ‘spillover’ effect, as shown in a recent study published in Science. This effect highlights the benefits that marine protected areas (MPAs) can provide to the larger marine environment, helping to nurture various marine populations that inhabit the surrounding waters, as well as those within the MPA.

Papahānaumokuākea Marine National Monument (PMNM) was first established in 2000, covering an area of 360,000 square kilometers. In 2016, the reserve was expanded by U.S. President Barack Obama to cover more than four times the original area, becoming the world’s largest protected ocean reserve and covering a total of 1.5 million square kilometers, or 580,000 square miles.

PMNM was recently the focus of a study conducted by researchers from the University of Hawaiʻi and the University of Wisconsin-Madison. The team found an increase in catch rates of two tuna species within 600 nautical miles of the reserve boundary.

Data was collected from the U.S. Marine Fisheries Service Pacific Island Observer program that monitors catch numbers of species like bigeye and yellowfin tuna in Hawaiʻi. The researchers focused on deep-set longline fishing data collected from boats between 2010 and 2019, comparing catches from 2010-2013, before the expansion, to catches from 2016-2019.

The data revealed that each of the studied species showed an increase in catch rate, the largest being yellowfin tuna, or ahi in Hawaiian, with a 54% increase, followed by bigeye tuna with a 12% increase, and finally 8% for all other fish species caught outside the boundary. The team also noted that there was no significant increase or spillover effect recorded before 2016, highlighting how the expansion of PMNM has shown to have benefited the larger marine ecosystem in Hawaiʻi.

This amazing spillover effect is expected to continue and strengthen, which could mean more support for the construction of well thought out MPAs that serve larger purposes and ecosystems. The research team assumes that in this case, PMNM is either acting as a nursery for young fish populations or simply providing a safe place for the fish to aggregate and feed – or both. Whatever the reason, these results highlight the importance of these areas and their impact on fish populations found both near and far.