This summer’s meeting in Hilo of the Society of Conservation Biology focused on island ecosystems. Not surprisingly, many of the sessions addressed issues with a strong local angle. This is our third and final report on highlights of the conference:
More Water, Happier Fish
If the native stream goby `o`opu nakea had a choice, it would live in deep water. `O`opu nopili would choose the opposite, preferring the shallower parts of the stream. `O`opu alamo`o (Lentipes concolor) doesn’t care either way.
Those are some of the findings of U.S. Geological Survey scientist Anne Brasher’s study of two Moloka`i streams, which she presented at the conservation biology conference.
Brasher also found that nakea (Awaous guamensis) used habitats with slow moving water, while nopili (Sicyopterus stimpsoni) seemed to prefer areas with rushing water.
In general, the community structure of stream life is better with more water, she found. While that may seem like an obvious conclusion, in a time where the potential for stream restoration is high, scientific data illustrating the effects of more water in streams are essential for decisions regarding water allocation.
By 1979, 53 percent of streams in Hawai`i had been diverted for plantation agriculture. With the demise of plantations over the last two decades, the opportunity for restoring flows in some streams has arisen, but the lack of data on how stream flow affects stream life has hampered restoration efforts.
Over the decades of diversion, stream fauna – finfish, snails, limpets, and shrimp – have experienced reduced habitat and increased competition and predation between species. They’ve also had to contend with barriers to their life cycle since most Hawaiian stream fishes and snails migrate between stream and ocean.
To illustrate the effects of diversion, Brasher studied one diverted and one relatively pristine stream on Moloka`i and compared how the different fauna used the different environments in the stream.
Water from Waikolu Stream on Moloka`i’s north shore has been diverted through the Waikolu Tunnel since 1962. The stream has three diversions upstream and a dam downstream. Pelekunu Stream, on the other hand, is one of the last free-flowing streams in the state.
Waikolu’s diversions have completely disrupted the stream’s flow. Brasher found that 93 percent of sampling stations below the diversions in Waikolu consisted of pools where water barely moved. At stations in Pelekunu Stream, however, “50 percent of the macrohabitat was riffle indicating high velocity and 50 percent was run habitat,” Brasher found.
Because Pelekunu had a greater range of depths and velocities, gobies had the opportunity to live where they thrived best. In Waikolu, where there was less range of depths and velocities, the different gobies all milled around in the same place.
“Higher overlap may lead to increased competition and subsequently decreased viability of the native organisms,” Brasher wrote in a report on her findings. She added that reduced water flow “not only reduces habitat availability, but periodic dewatering of the stream inhibits the ability of the native organisms to disperse downstream and to make return migrations necessary to complete their life cycle.”
— T.D.
New Grass Threatens Underwater Turf War
Covered in sand and marine organisms, the new alien sea grass discovered recently is difficult to distinguish from Hawai`i’s native sea grass. But while they make look similar to the untrained eye, the two grasses a quite different.
And while research on the alien sea grass Halophila decipiens found in Honolulu waters is “very preliminary,” Catherine Unabia of the Kewalo Basin Marine Laboratory says the grass’s physical and reproductive traits suggest that the alien species could threaten the island’s native sea grass Halophila hawaiiana.
Last October, a large population of H.decipiens was found in a dredged area off the Kahala Mandarin Hotel. More was found in an adjacent dredged bay, in a dredged area off Ala Moana, in Kane`ohe Bay, 35 meters off the airport’s reef runway, and at Kawaihae on the Big Island. Unabia says no one knows when or how it was introduced.
Sea grasses trap and hold sediment and are nurseries for fish. So why isn’t more sea grass a good thing, especially when it is so difficult to differentiate between the newcomer and the native?
One potential problem revolves around a native snail, Smargdia bryanae, which grazes on native sea grass. In lab experiments, the snail doesn’t seem to care for the new sea grass. Between the two species, there are a few differences in blade shape; the new one grows more densely, with larger clusters of leaves, and is also covered in little hairs, which may be why the snail dislikes it.
The grass is found throughout the world, in the Pacific, Indian, Atlantic oceans, and the Canary Islands. It reproduces prodigiously, bearing male and female flowers and flowering year-round. The Hawaiian sea grass only flowers occasionally and rarely produces seeds. According to Unabia, the new grass has already muscled its way into two-thirds of the Kahala Mandarin area once occupied by the native grass.
Editors Note:
Disputed Invasion
Not everyone agrees that the invasive sea grass reported by Catherine Unabia is either new or invasive, as was reported in the December issue. Karla McDermid, professor of marine biology at the University of Hawai`i at Hilo, says the grass has probably been here since the 1970s. It’s widely distributed all over the world, and it would be surprising if the grass weren’t here, McDermid says.
Albatross Numbers Take a Nosedive
Something’s fishy at Midway Atoll, and it’s not just the squid-ink-stained boluses chucked up by several hundred thousand Laysan albatross.
The birds at Midway, home base to 90 percent of the world’s Laysan albatross population, is declining, rapidly and significantly. U.S. Fish and Wildlife Service biologist Nancy Hoffman, formerly stationed at Midway, described the crash in a presentation given at the SCB meeting.
In the 1991-92 breeding season, the number of nesting pairs of Laysan albatross at Midway stood at 429,308. In the 2000-01 season, just 284,604 pairs were counted. That decline of 144,704 pairs represents a loss of a third of the breeding population.
Beth Flint, an expert on albatross with the FWS in Honolulu, elaborated on the meaning of the numbers. It is important to remember, she told Environment Hawai`i, that “we’re talking only about the birds that have decided to breed and are sitting on their eggs for at least a week or so after they lay them. So the true number of adult Laysan albatross may be quite different from those that choose to breed. So the decline may be a result of the birds deciding not to breed because the conditions were such they thought they didn’t have a good chance, or their physical condition was too poor, or food was scarce. We don’t know.”
“In that sense,” she continued, “the way we monitor these populations isn’t totally satisfactory. We can’t detect whether [the adult birds] have gone off the face of the earth. We only know that they’re not breeding.
“Clearly, though, there has been a decline in breeding birds at all three colonies we count: Midway, Laysan Island, and French Frigate Shoals. The slope of the downward curve for Laysan albatross breeding populations is very, very similar, and all of them downward. So something is going on, and whatever it is, it is archipelago-wide.”
The decline doesn’t seem to be correlated with human activity at Midway, where the Fish and Wildlife Service has partnered with a private company to run an ecotourism operation. “If you look at Sand and Eastern,” the two major islets at Midway, “the only place that stayed stable was the island with all the activity, Sand Island,” Flint said. “Possibly habitat improvement activity there has made things easier for the birds.”
— Patricia Tummons
Volume 12, Number 6 December 2001