Ancient Aquaculture

Fishpond aquaculture is a practice that exists all over the world, from Europe to North America to East Asia.  While practices vary by location depending on the landscape and geography of each region, Hawaiian aquaculture is especially unique and has been used since the first Polynesians made their way to the islands.  Fish remains a vital source of protein in the diets of islanders, and fishing success is largely dependent upon weather and ocean conditions.  Aquaculture acts somewhat as an insurance policy, guaranteeing that protein will be readily available when needed, even if conditions are not favorable for fishing.

Fishpond aquaculture is different from the aquaculture we typically think about because it relies on the natural structures that are already present in the environment, rather than farming fish in a man-made system.  A wall is built on a shallow reef flat circling the mouth of a freshwater stream or river as it reaches the ocean.  Constructed from coral, lava, and other semi-porous materials, the water contained within the wall remains uniform with the water level of the ocean, as it is allowed to ebb and flow with the tide.  Sluice gates formed from sticks and branches are the crucial components that allow the fishpond to be successful.

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Illustration of the components of a Hawaiian fishpond system

Arial view of a fishpond

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Sluice gate

The gate is just the right size to keep large fish and predators out but allows young fish to get in.  The protected brackish water creates an ideal, safe site for the fish to grow up, and once they reach a certain size, they are unable to leave and are eventually harvested.  Herbivorous species are targeted because they are easy to feed and grow quickly compared to their carnivorous counterparts.  Mullet and milkfish are a few of the most common species found in Hawaiian fishponds.

What makes Hawaiian fishponds different from other fishponds is that limu (algae) is cultivated to guarantee an abundant supply of food for the fish to maximize their growth.  Similar to a rancher caring for the grass in the pastures, the cultivation of algae allows herbivorous fish to grow without being fed and attracts young fish to the pond so it is restocked naturally.

During my time in Hawaii, I had the chance to volunteer with the restoration of an ancient fishpond on the East side of Oahu, where we worked to extend the fishpond wall, and remove invasive mangroves.  It was a lot of work, but with a devoted and enthusiastic team, we were able to accomplish a lot.  The project was a lot of fun, and it was extremely rewarding to be able to see all the progress we made.



The section of the fishpond wall that we built. Thanks for the photo, Dawn!




The inside of the He’eia fishpond, in gorgeous Kaneohe Bay.



Help for Honeycreepers

While we already know how destructive invasive species can be on native populations, when the advancing threat of climate change is added to the picture, we encounter some extraordinarily devastating consequences.  Hawaiian honeycreepers are a unique group of birds belonging to the finch family, found exclusively in the Hawaiian Islands.  An equivalent to Darwin’s finches in the Galapagos, over 50 distinct species have evolved from the original colonists to fill a variety of ecological niches.

Each honeycreeper is a specialist, meaning that they have specific diet and habitat requirements. The diverse phenotypes are informative as to what the preferred food source is.  A slender curved beak indicates an insect and nectar-feeding bird, while a short, broad beak is a sign of a bird that feeds on nuts and seeds.

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Hawaiian Honeycreeper Diversity

Before the introduction of mosquitos in 1826, bloodsucking insects did not exist in Hawaii.  Mosquitos act as effective vectors for pathogens, transmitting them between hosts as they feed.  By the early 1900s, at least two avian diseases were present – avian pox and avian malaria.  Ironically, these were most likely introduced by imported birds that were released in an attempt to replace the declining populations of native birds.  Songbirds like honeycreepers are especially susceptible to avian malaria, which was spread very quickly via mosquito vectors.  Unlike most diseases where pathogens and hosts coevolve, allowing the host to develop some resistance, honeycreepers are naive hosts without any previous exposure to this type of disease.  With mortality rates as high as 90% and increased predation pressure by invasive predators, honeycreeper populations have been devastated over the past several decades.

In addition to the disease itself, healthy forest habitat is disappearing and being degraded due to human development and competition with non-native plants, which creates a more stressful environment.  This lowers the bird’s immune systems, as energy that would normally be spent investing in defense against disease must be allocated to finding food and appropriate habitat.  Invasive wild pigs have contributed to a steep rise in the number of mosquitos by digging up forest floor and creating ideal breeding grounds, further increasing disease transmission.

Since these birds live in such an isolated location and can’t escape the disease by moving elsewhere, their only option is to flee to higher altitudes where mosquitos cannot survive.  While this offers temporary refuge, the increasing global temperature is allowing mosquitos to expand their range and move into higher elevations.  The “mosquito line” falls around 1500 meters, but is steadily rising as it essentially squeezes the birds into a continually shrinking habitat.  Only 19 species of honeycreepers remain in upland forests, and several are listed as critically endangered by the International Union for the Conservation of Nature.

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Amakihi honeycreeper

Some creative management plans are currently in the works in an effort to help honeycreepers deal with avian malaria, and aid in population recovery.  Genetically manipulated, sterile male mosquitos are being released into forests to hinder reproductive success of the mosquitos, thus reducing their population size over time. Ex situ breeding programs are currently breeding honeycreepers as a way to preserve them in an artificial environment.  This strategy is useful as it allows researchers to learn specifics about the physiology of the birds and can provide insight as to what can be done to help wild populations.  Additionally, predatory mammals like mongoose, cats, rats and wild pigs are being trapped and removed as a way to keep non-disease related mortality as low as possible.

And finally a bit of good news.  It has recently been discovered that at least two species of amakihi honeycreepers have adapted some resistance to avian malaria, and have begun repopulating lowland forests where malaria is rampant.  This affirms that these birds do have the ability to evolve and have a chance at fighting the disease, given adequate time.  The critical unknown factor is the rate at which globally increasing temperatures will allow mosquitos to advance into higher altitude forests and reach the birds.  The hope is that the current management actions will mitigate the disease long enough that the birds will have a chance at developing resistance.

Feral Felines

Cats – people either love them or hate them, but cats have been a major problem for Hawaiian wildlife since their introduction in the 1800s.  European colonists kept cats on their ships in order to control mice and rats that plagued the ships and contaminated food supplies.  Like the rats, cats made their way to shore and established populations throughout the islands.  Birds are now facing predation from both mongoose and cats.  Native birds and other species evolved in ecosystems where these types of predators did not exist, therefore have not adapted any defenses against them.  As many of these birds do not have the natural instinct to flee, they will remain still even when predators approach – quite literally, sitting ducks.  Unlike mongoose, cats are more agile and can access animals in a greater variety of habitat types, including wetlands, forests, and lava fields.  Mortality of native forest birds, seabirds, and other ground nesting birds soared as cat populations continued growing.

One species that was hit especially hard is the ‘Ua’u, the Hawaiian petrel.  These large seabirds nest in rock crevices on the ground, with hatchlings remaining flightless until they are around 15 weeks old.  Parents spend their days at sea hunting for food to bring to the chicks, leaving them unprotected and vulnerable during the day.  Since petrels create burrows on the ground to nest in, they are easy targets for cats and mongoose, which are even known to attack and kill adults.  These introduced predators have caused severe declines in the petrels, to the point where only a few isolated nesting locations remain.  These birds are not able to breed until they reach sexual maturity at around 5 years of age, so even without considering the predation pressure by cats, it would take several decades for populations to return to healthy levels.


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Hawaiian petrel chick in its burrow


Conservationist John Muir once said, “When one tugs at a simple thing in nature, he finds it attached to the rest of the world”.  This quote is tremendously relevant when analyzing the huge cascade of ecosystem effects caused by the addition of just one species.  Petrels, like other seabirds, are vital players in the nutrient cycle, as their phosphorus and nitrogen-rich excrement provides fertilization for trees and forest plants.  The disappearance of the petrels is challenging other aspects of the environment, which will bring larger problems that have no simple solution.

Several management strategies are currently in play to attempt to protect the birds and give populations a chance to rebound back to more sustainable levels.  Predator proof fences have been built surrounding nesting sites of seabirds, and while a fence may not necessarily sound like the most secure way to guard the nests, they have been incredibly effective.  Additionally, trapping and euthanizing cats and mongoose is practiced at most nesting sites as well.

As a personal side note, I was recently offered a job at Haleakala National Park on the island of Maui, where one of the largest breeding populations still exist.  I would be participating in management and protection of the birds by monitoring population sizes and trapping mammal predators on a daily basis.  I would be thrilled to be a part of something so important and exciting!



The predator-proof fence at Kilauea Point, Kauai


A new, more intensive management approach began just over a year ago in November 2015, which has left researchers and conservationists with high hopes for the program’s success.  Petrel chicks and eggs are collected from nests located in especially vulnerable areas and are translocated to a fenced in nesting spot at Kilauea Point on the island of Kauai.  The chicks are monitored and cared for daily and are fed a nutritious diet of fish and squid.  All chicks in the first translocated group fledged within about a month of being moved, and will spend the next 3 to 5 years of their lives at sea, returning to breed once they reach sexual maturity.  Most seabirds imprint on their nesting location and will return there as adults to raise their own chicks.  In order for this project to be successful, the chicks will need to have imprinted on the new, safe nesting site at Kilauea Point rather than the site where the chicks were hatched and collected.  The next 2 to 3 years will be crucial in evaluating how successful the project will be, as chicks should begin returning to lay their eggs.  Experts are confident that their efforts will be worthwhile and will allow petrel populations to begin the extensive recovery process.

I’d like to note that the introduction of cats has had severe repercussions for more than just birds; even aquatic animals like monk seals and sea turtles are impacted in different ways.  For the purpose of this blog, I chose to focus on Hawaiian petrels.

Paradise Threatened

Hawaiian culture is composed of people who are extremely connected with the land – malama ‘aina means to care for and live in harmony with the land, and is one of the core values in Hawaiian culture, from ancient times through today.  Essentially, if you give to the land, the land will give back to you.  Hawaiians are proud of their natural resources and pay them a great deal of respect, and many ancient land use practices are still continued today.

The Hawaiian Islands are the most isolated populated land mass on earth, located 2,400 miles from the next closest piece of land, California.  Because of this, the islands are able to support an incredible amount of diversity, unlike anywhere else in the world, and are especially vulnerable to intrusion by non-native species.  The spectacular flora and fauna have coevolved with one another in a way that allowed them to persist without the need for harsh defense mechanisms that are seen in most places.  These ecosystems remained untouched, allowing nature and man to persist peacefully.  In order for new species to be introduced, they would be required to swim across the ocean – an unimaginable task, or be brought over by humans.  Large numbers of settlers began moving to Hawaii in the 1800s, which is when the delicate balance of the ecosystem began experiencing disruptions that are still causing major problems over 200 years later.

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Rats were unintentionally introduced via shipping traffic, and the numerous new sugar cane farms provided the rats with vast resources and allowed them to become established.  Much to the discontent of sugar cane farmers, crops were decimated by rat predation.  A seemingly simple solution emerged in 1872 in Jamaica, which was experiencing similar issues with rats at the time.  The decision was made to introduce several dozen Asian mongoose, small carnivores that are known to feed on rats in their native habitat.  Upon hearing of this strategy, hopeful sugar cane plantation owners decided to import mongoose to Hawaii in an attempt to remove rats and protect their crops.  Mongoose were shipped to all the islands, excluding Lana’i and Kaua’i.

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Unfortunately, the mongoose minimally impacted rat populations due to a variety of unforeseen factors.  Firstly, the rats and mongoose were active at different times of day, as rats are nocturnal and mongoose are not.  Interaction between the two species was limited therefore preventing any manageable predation on the rats.

Additionally, it turned out that the plentiful supply of ground nesting bird eggs and hatchlings offered an abundant, of easy to find food source for the mongoose.  As a result, native Hawaiian and migratory birds began experiencing severe population declines due to the new and intense nest predation.  Mongoose reproduce rapidly and have become widely distributed throughout the islands.  They remain on all but two islands, and are partially to blame, along with other human-induced factors, for the near extinction of several Hawaiian bird species.