Marine species and habitats
Midway Atoll location in subtropical waters offers unique
opportunities to study the effect of colder waters on the
growth, development, and ecology of coral reefs. The atoll
drops off steeply outside the perimeter reef, making it possible to observe within a relatively small area the different
organisms and communities associated with pelagic, reef
crest, ocean reef slope, deep reef, and lagoon habitats.
The lagoon contains circular patch reefs in the deep central
lagoon (Figure 1) and shallow back reefs near the lagoon
perimeter. Both impede water circulation and trap sand
washed over the reefs. These sediments limit coral development in much of the lagoon, except in the deeper central
lagoon where stands of the endemic finger coral (Porites
compressa) flourish. Corals are more abundant on some
leeward ocean facing reefs and shallow back-reefs and
lagoon pinnacles, but overall, corals are scarce on reefs
exposed to ocean waves. A total of 33 species of stony
corals and two non-stony corals (Palythoa and Sinularia)
have been recorded at Midway, based on surveys from
2000–2006 and are dominated by Pocillopora, Porites,
and Montipora species. These totals are slightly less than
the 34–46 species found at the four nearest NWHI atolls
and much less than the 66 species at the furthest atoll,
French Frigate Shoals (FFS), 1,300 km to the southeast
within the MNM. The close proximity of FFS to Johnston
Atoll may facilitate dispersal of coral species between the
Hawaiian and Line archipelagos and help explain the
much higher species totals at FFS. The dominant blue
encrusting coral at Midway is now tentatively identified
as Montipora dilatata, an endemic species that displays
spectacular formations in the lagoon and back reef habitats
at Midway and several neighboring atolls, but which is
rare in the larger MHI. Approximately 12 of the Midway
coral species are endemic ($40%), an above-average level
for the NWHI but comparable to the remaining NWHI.
In September 2002, marine biologists were surprised to
find extensive bleaching of corals at Midway and two
neighboring atolls (Kure and Pearl and Hermes) in these
subtropical waters. Less severe bleaching occurred again
at the same atolls during the late summer of 2004. Corals
at Midway, where the 2002 bleaching was especially
severe, must adapt to colder winter water temperatures
of 18
C, as well as adapting to summer temperatures, normally 8–10 degrees higher. During the bleaching event of
2002, temperatures reached 29
C, 11
higher than the
winter lows, and the corals responded by bleaching.
Corals have recovered well during the past several years
although there has been a net loss of lobe coral (Porites)
compared to levels at permanent transects in the lagoon
prior to the bleaching events at Midway.
Patches of the seagrass (Halophila) are reported in the
lagoon, and calcareous green algae, brown turban algae,
and crustose coralline algae are common on the outer
reef crests. Large numbers of rock boring sea urchins
(Echinometra, Heterocentrotus, and Echinostrephus) predominate, hollowing out the dead skeletal portions of live
lobe corals (Porites) and riddling the coralline algal covered reef crest with numerous holes and burrows. Scientists
of the University of California at Santa Cruz are now determining whether reef erosion primarily from the boring
urchins is outstripping reef growth, and whether selective
removal of urchins would prevent serious erosion of the
perimeter reef crests, the main natural barrier protecting
the lagoon and islands. Beyond the reef massive spurs and
grooves high in coralline algal cover face the open ocean
along the northwest to southwest perimeter reefs, providing
clear evidence of the importance of coralline algae as
a major reef builder and bulwark in the far end of the
NWHI. More than 100 species of algae are now known
from Midway, including 35 new records for Midway and
one species new to science, Dudresnaya babbittiana.
A thorough 1997 survey reported no alien algal species at
Midway. Quantitative surveys in 2006 revealed that
Lobophora, Laurencia, Microdictyon, Padina, crustose
coralline algae, and Halimeda in descending order were
the most abundant algae at Midway.
The first systematic marine invertebrate survey was
done at Midway in 1997. It documented 316 invertebrate
species, 250 of which had not been previously recorded
at Midway. Crustaceans dominated the larger invertebrate
species, composing 46% of the total species. Other conspicuous marine invertebrates included sea urchins, with
the burrowing sea urchin Echinostrephus responsible for
the highest densities in the NWHI reported at Midway
and Kure (>12 individuals/m
2
). The most common sea
cucumber was Actinopyga and the most common sea star
was Linckia multifora. The crown-of-thorns sea star,
Acanthaster plancii, has been reported at Midway at low
levels. The black-lipped pearl oyster, Pinctada
margaritifera, has also been recently reported at Midway.
A total of 258 species of fish, including 7 pelagic
species, have been recorded on Midway reefs by 1993.
Despite its low species diversity, Midway’s reef fish biomass is higher than in the MHI, largely due to reduced fishing pressures attributed to the protective mandate of the
NWR. Midway ranked fifth in terms of species richness,
but ranked first in numerical density (2.7 individuals/m
2
,
and ranked second in fish biomass, (2.5 t/ha) among all
locations in the NWHI. Many Midway species grow to
larger than average size, and all trophic levels are well
represented, including jacks and four species of sharks.
Some of these species are either not found on shallow
reef habitats in the MHI or are very rare there. Several
of these are limited to deep waters in the MHI, but are
found at shallow diving depths (15–50 m) at Midway,
including the only endemic Hawaiian grouper, the
hapu‘upu‘u, (Epinephelus quernus) a species commercially harvested at greater depths (150–200 m) in the
MHI, but fully protected within Midway NWR. This disparity in depth distribution is explained by the fish staying
in the same isotherms extending upward to shallower
depths at Midway compared to their greater depth regimes
in the MHI.
704
MIDWAY ATOLL (HAWAIIAN ARCHIPELAGO)
Midway Atoll location in subtropical waters offers unique
opportunities to study the effect of colder waters on the
growth, development, and ecology of coral reefs. The atoll
drops off steeply outside the perimeter reef, making it possible to observe within a relatively small area the different
organisms and communities associated with pelagic, reef
crest, ocean reef slope, deep reef, and lagoon habitats.
The lagoon contains circular patch reefs in the deep central
lagoon (Figure 1) and shallow back reefs near the lagoon
perimeter. Both impede water circulation and trap sand
washed over the reefs. These sediments limit coral development in much of the lagoon, except in the deeper central
lagoon where stands of the endemic finger coral (Porites
compressa) flourish. Corals are more abundant on some
leeward ocean facing reefs and shallow back-reefs and
lagoon pinnacles, but overall, corals are scarce on reefs
exposed to ocean waves. A total of 33 species of stony
corals and two non-stony corals (Palythoa and Sinularia)
have been recorded at Midway, based on surveys from
2000–2006 and are dominated by Pocillopora, Porites,
and Montipora species. These totals are slightly less than
the 34–46 species found at the four nearest NWHI atolls
and much less than the 66 species at the furthest atoll,
French Frigate Shoals (FFS), 1,300 km to the southeast
within the MNM. The close proximity of FFS to Johnston
Atoll may facilitate dispersal of coral species between the
Hawaiian and Line archipelagos and help explain the
much higher species totals at FFS. The dominant blue
encrusting coral at Midway is now tentatively identified
as Montipora dilatata, an endemic species that displays
spectacular formations in the lagoon and back reef habitats
at Midway and several neighboring atolls, but which is
rare in the larger MHI. Approximately 12 of the Midway
coral species are endemic ($40%), an above-average level
for the NWHI but comparable to the remaining NWHI.
In September 2002, marine biologists were surprised to
find extensive bleaching of corals at Midway and two
neighboring atolls (Kure and Pearl and Hermes) in these
subtropical waters. Less severe bleaching occurred again
at the same atolls during the late summer of 2004. Corals
at Midway, where the 2002 bleaching was especially
severe, must adapt to colder winter water temperatures
of 18
C, as well as adapting to summer temperatures, normally 8–10 degrees higher. During the bleaching event of
2002, temperatures reached 29
C, 11
higher than the
winter lows, and the corals responded by bleaching.
Corals have recovered well during the past several years
although there has been a net loss of lobe coral (Porites)
compared to levels at permanent transects in the lagoon
prior to the bleaching events at Midway.
Patches of the seagrass (Halophila) are reported in the
lagoon, and calcareous green algae, brown turban algae,
and crustose coralline algae are common on the outer
reef crests. Large numbers of rock boring sea urchins
(Echinometra, Heterocentrotus, and Echinostrephus) predominate, hollowing out the dead skeletal portions of live
lobe corals (Porites) and riddling the coralline algal covered reef crest with numerous holes and burrows. Scientists
of the University of California at Santa Cruz are now determining whether reef erosion primarily from the boring
urchins is outstripping reef growth, and whether selective
removal of urchins would prevent serious erosion of the
perimeter reef crests, the main natural barrier protecting
the lagoon and islands. Beyond the reef massive spurs and
grooves high in coralline algal cover face the open ocean
along the northwest to southwest perimeter reefs, providing
clear evidence of the importance of coralline algae as
a major reef builder and bulwark in the far end of the
NWHI. More than 100 species of algae are now known
from Midway, including 35 new records for Midway and
one species new to science, Dudresnaya babbittiana.
A thorough 1997 survey reported no alien algal species at
Midway. Quantitative surveys in 2006 revealed that
Lobophora, Laurencia, Microdictyon, Padina, crustose
coralline algae, and Halimeda in descending order were
the most abundant algae at Midway.
The first systematic marine invertebrate survey was
done at Midway in 1997. It documented 316 invertebrate
species, 250 of which had not been previously recorded
at Midway. Crustaceans dominated the larger invertebrate
species, composing 46% of the total species. Other conspicuous marine invertebrates included sea urchins, with
the burrowing sea urchin Echinostrephus responsible for
the highest densities in the NWHI reported at Midway
and Kure (>12 individuals/m
2
). The most common sea
cucumber was Actinopyga and the most common sea star
was Linckia multifora. The crown-of-thorns sea star,
Acanthaster plancii, has been reported at Midway at low
levels. The black-lipped pearl oyster, Pinctada
margaritifera, has also been recently reported at Midway.
A total of 258 species of fish, including 7 pelagic
species, have been recorded on Midway reefs by 1993.
Despite its low species diversity, Midway’s reef fish biomass is higher than in the MHI, largely due to reduced fishing pressures attributed to the protective mandate of the
NWR. Midway ranked fifth in terms of species richness,
but ranked first in numerical density (2.7 individuals/m
2
,
and ranked second in fish biomass, (2.5 t/ha) among all
locations in the NWHI. Many Midway species grow to
larger than average size, and all trophic levels are well
represented, including jacks and four species of sharks.
Some of these species are either not found on shallow
reef habitats in the MHI or are very rare there. Several
of these are limited to deep waters in the MHI, but are
found at shallow diving depths (15–50 m) at Midway,
including the only endemic Hawaiian grouper, the
hapu‘upu‘u, (Epinephelus quernus) a species commercially harvested at greater depths (150–200 m) in the
MHI, but fully protected within Midway NWR. This disparity in depth distribution is explained by the fish staying
in the same isotherms extending upward to shallower
depths at Midway compared to their greater depth regimes
in the MHI.
704
MIDWAY ATOLL (HAWAIIAN ARCHIPELAGO)