Indigenous aquaculture system responses to climate change, nutrient enrichment, and hatchery-based restocking

Historically in Hawaiʻi, Indigenous aquaculture systems termed loko iʻa (fishponds) produced abundant seafood for hoaʻāina (residents) of the surrounding ahupuaʻa (watershed). Now, there are efforts to restore these systems, but climate change, development, and regulatory barriers inhibit restoration. The objective of this study is to 1) assess how loko iʻa restoration affects fish populations and fisheries in the surrounding estuary and 2) test how climate change may affect loko iʻa productivity, as well as how strategies like nutrient enrichment and restocking (adding juvenile fish from hatcheries to the loko iʻa) may mitigate some of these effects. To meet these objectives, we created a food web model representing nutrients, phytoplankton, fish (ʻamaʻama or striped mullet), and fisheries inside a loko iʻa and in the surrounding estuary, parameterized for Kāneʻohe Bay, Oʻahu, Hawaiʻi. First, we simulated different loko iʻa restoration scenarios (different amounts of area designated as loko iʻa), and found that larger loko iʻa areas not only increased fish production inside the pond but were capable of supplementing estuary fisheries harvest. Then, we used this model framework to test 1) how different climate change emission scenarios affect loko iʻa and estuary fish production, through incorporating temperature-dependent fish and phytoplankton physiology, 2) how different levels of external nutrient inputs affects fish populations and fisheries harvest, and 3) the viability of various restocking scenarios in promoting fish population growth inside the loko iʻa and potentially spillover effects to the estuary fish populations. We found that although rising water temperature may lead to declines in fish density, the loko iʻa fish population was more resilient to rising water temperatures than those in the surrounding estuary, likely due to temperature regulation via freshwater inputs from streams. We also found that increased nutrient flow into the loko iʻa and estuary, such as through watershed restoration, can substantially increase fisheries harvest levels in the bay. Finally, our results showed the viability of certain restocking scenarios in terms of speeding up fish population growth and increasing fish production in the short-term. In the face of rising water temperatures and other climate change impacts, biocultural restoration may be especially important in terms of enhancing fish populations and increasing social-ecological resilience.