Streamflow and Stream Biodiversity Prioritization
The main Hawaiian Islands (Niihau to Hawai‘i) make up the lower one fourth of the Hawaiian
archipelago, but represent nearly 96% of the land mass. Only five of these eight islands are high
enough to capture rain clouds carried by the prevailing trade winds. Consequently, Hawai‘i’s
freshwater ecosystem is mostly limited to 376 perennial streams on the windward side of the
islands and these streams are home to our native freshwater animals.
Hawaiian streams outwardly resemble US mainland trout streams but are generally shorter and
steeper in slope. Only about 7% of our streams are 10 miles or longer and are punctuated by
numerous waterfalls. An extreme case is Akaka Falls on the Big Island, which is estimated to be
about 420 ft. high. Many tourists are awed by these spectacular waterfalls, but are unaware that
some of our native stream animals ascend and descend these falls during their migrations.
The native aquatic fauna is represented by three groups of animals, two mollusks or wi, two
crustaceans or opae, and five species of fishes, known collectively as ‘o‘opu. These nine native
animals all share a unique life cycle, called amphidromy. Adults live and reproduce in streams
where their eggs hatch and the larvae drift out to sea. After several months, the juvenile animals
migrate inland to their adult habitats. This life cycle requires these animals to cross the estuary,
or muliwai, twice. These migrations are usually associated with episodic floods resulting from
heavy rainfall.
Hawai‘i’s streams once held healthy populations of wi, opae, and ‘o‘opu. As Hawai‘i’s
population expanded in the early 1900’s, rapid agricultural development and urbanization
followed and contributed to the decline of native stream animals. Water diversions, stream
channelization, dumping of waste, sediment runoff and alien fish introductions are still major
threats, especially near heavily populated areas such as those on Oahu and Maui.
There are still many streams, especially in rural and remote areas of the state, which contain
reasonably healthy populations of native stream animals. Efforts are being made to protect these
healthy streams and use them as benchmarks for water resource planning and management and
for developing methods for rehabilitating degraded streams.
Native Hawaiian people’s dependency on the land, stream and nearshore resources led to the
development of the ahupua‘a (watershed) ecosystem management concept. The Department of
Land and Natural Resources (DLNR) is embracing this mauka (mountain) to makai (ocean)
connection approach to treat the watershed’s land, stream, estuary and nearshore ocean as one
management unit.
Maintaining natural flow patterns in streams is an important requirement for the protection of
native stream biota. These natural flows will keep the river mouth open and provide a gateway
for these animals to complete their life cycle thus sustaining the mauka-makai connection.
DLNR is faced with mounting challenges regarding natural resource management. For example,
DLNR’s Division of Aquatic Resources (DAR) must balance the mandate to conserve and
protect biodiversity and habitat, and mitigate the threat of aquatic nuisance species, while
simultaneously attending to the demands of the consumptive users of these resources.
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Similarly, the Commission on Water Resource Management (Commission) is faced with the
particularly difficult duty of establishing instream flow standards (IFS) on a stream-by-stream
basis to protect the public interest in state waters. An IFS is the amount of water that is required
to protect fish, wildlife, recreational, aesthetic, scenic, and other beneficial instream uses.
However, establishment of an IFS also requires that the Commission “weigh the importance of
the present or potential uses of water from the stream for non-instream purposes, including the
economic impact of restriction of such uses.” The fact remains that data on instream uses and
stream conditions are presently insufficient to make well-informed management decisions for
many streams in the State.
The Division of Aquatic Resources and the Commission staff continue to work closely together
in addressing the biological aspects associated with IFS, and have been able to leverage private
and public funds to cooperatively fund studies to address the hydrological conditions for selected
streams statewide. Such projects have included partnerships with various groups including, the
U.S. Geological Survey, the County of Maui, Office of Hawaiian Affairs, Bishop Museum, the
Nature Conservancy, the Audubon Society, Kamehameha Schools, Hawai‘i Nature Center,
university researchers and community watershed groups.
From these cooperative studies, we have recently discovered that recruiting goby larvae use
chemical signatures of other adults as guideposts to locate available stream habitat. Another
study pointed out that the quality of algae in the estuary and lower stream sections may be an
essential “fuel source” to complete the inland migration phase of their life cycle. A recent
breakthrough is the development of habitat models that can be used as predictive tools for
managing aquatic resources. The models will be beneficial for assessing stream and biotic
conditions, including cases where field data are minimal or not available.
DLNR plans to continue and expand upon these coordinated efforts in support of its management
and data collection responsibilities, continue stream monitoring and survey functions, conduct
additional selected scientific studies, and enhance public participation efforts. An excellent way
to further enhance collaboration and optimize current resource and staffing limitations may
include the creation of the proposed Center for Island Stream and Estuarine Studies at the Wailoa
Fishery Research Station in Hilo.
The Center would support DAR’s Fisheries and Alien Nuisance Species programs and DLNR
programs under the Commission on Water Resource Management, Natural Area Reserve System
and State Parks by providing integrated, scientifically-defensible solutions to difficult
management decisions. As an amalgamation of partnerships with other agencies and institutions,
the Center will provide both increased research opportunities and access to additional funding
mechanisms.
Additionally, networking with the Instream Flow Council, an organization of state (US) and
provincial/territorial (Canada) agencies with public trust responsibilities for fish and wildlife
management, will provide a wider perspective of the difficult issues of stream biodiversity
conservation and instream flow management.
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