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Adult fish are dwindling across many marine protected areas

A new study led by the Smithsonian Environmental Research Center has found that marine protected areas (MPAs) may not be fully serving their purpose in rebuilding dwindling fish populations, specifically among adult fish. 

This study provides the first analysis of the age composition of reef fish within marine protected areas, revealing a concerning trend of stagnating or declining adult fish populations.

Adult fish are critical to rebuilding populations 

“Adult fish are really important,” said co-lead author Steve Canty, a marine ecologist at the Smithsonian. “They are the primary catch within commercial fisheries that support livelihoods and food security of coastal communities.” 

“Additionally, it is the adults that breed and rebuild the fisheries, and here size matters. Bigger and older female fish produce more and better eggs, and therefore are critical to rebuilding fish populations.”  

Protecting 30% of Earth’s oceans

This discovery comes at a time when global efforts are intensifying to meet the ambitious “30 by 30” conservation target, set by delegates at the United Nations Biodiversity Conference in December 2022, which aims to protect 30% of Earth’s oceans by 2030. 

However, the effectiveness of this target hinges on the selection of sites conducive to marine life prosperity and the consideration of coastal community needs.

Fish on the Mesoamerican Reef

The study focused on the Mesoamerican Reef, the longest reef in the Western Hemisphere, which spans over 600 miles across the coasts of southern Mexico, Belize, Guatemala, and Honduras. 

This region, a biodiversity hotspot, is not only home to over 500 fish species, 60 coral species, and one of the largest whale shark congregations but also supports the livelihoods and food security of over one million people.

Effective protection and strong enforcement are needed

By analyzing fish biomass across 139 reef sites, including both protected and unprotected areas, the researchers categorized the MPAs based on their level of protection. They found that only 24% of the examined MPAs offered full or high protection, underscoring the challenge of restricting fishing activities in the sea.

“But fully protecting and enforcing these areas is the best way to grow bigger fish that can rebuild the populations and actually increase overall fish catches outside the fully protected areas,” said co-author Melanie McField, the director of the Healthy Reefs Initiative.

Widespread decline of adult fish

Data for the study was sourced from the Healthy Reefs Initiative, which employs scuba divers to conduct fish surveys across the region’s reefs. Over a 12-year period from 2006 to 2018, the findings indicated that a mere 11 sites exhibited significant increases in adult fish biomass, with most sites showing no change and 28 sites experiencing declines. 

Notably, declining sites shared characteristics such as proximity to heavy coastal development, higher water temperatures, and weak protection levels with inadequate enforcement.

Minimal protection is more detrimental 

“Minimal protection in the weakest MPAs proved to be more detrimental than no protection at all, because it allowed for continued and concentrated exploitation without effective management,” said co-author Abel Valdivia, a marine conservation scientist at the World Wildlife Fund

“Unlike open-access zones, where fishing pressure may be spread out, weakly protected areas faced unchecked coastal development, warming waters and inadequate enforcement, leading to the decline in adult biomass.”

However, sites where adult fish populations rebounded shared positive attributes: full protection, adequate enforcement, fewer temperature spikes, and a lower human impact in adjacent areas.

Broader implications of the study

These findings clarify the path forward for global “30 by 30” conservation efforts. Effective management, strong enforcement, community support, and climate-conscious site selection are paramount to MPA success, emphasizing the need to consider the distinct needs of adult and juvenile fish.

“Our study shows that adults and juvenile fish respond very differently to management and environmental change. Adults were more sensitive to stressors like climate change, which can undermine the local benefits of MPAs,” said study co-lead author Justin Nowakowski, an ecologist at the Smithsonian. 

“The big question now is where to locate new MPAs to maximize positive impacts for the regional fishery. To do this, we need to consider the unique requirements of adult and juvenile life stages – otherwise we are missing the full picture.” 

The study is published in the journal Global Change Biology.


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