Movements of monk seals relative to ecological depth zones in the lower Northwestern Hawaiian Islands

2006·6.4 MB·English

by  Frank A Parrish

MOVEMENTS OFMONK SEALS RELATIVE TO ECOLOGICAL DEPTH
ZONES IN THE LOWER NORTHWESTERN HAWAHAN ISLANDS
BY
FRANKA. PARRISH' and KYLERABERNATHY-
ABSTRACT
In the 1990s, adult male and female monk seals (n = 24) at French Frigate
Shoals werefittedwith satellite tags and theiractivity monitored (median 87 days). The
distribution oftheir movements was compared with the area and distribution offour
ecological zones that were used to classify the summits ofthe Hawaiian ridge. The zones
were definedby depth as reef(<30 m), bank (30-50 m), slope (51-300 m), and subphotic
(301-500 m). Geographic Information Systems (GIS) comparisons indicated that the
seals movedthroughout the region and did not focus their activities in a particularzone
or limitthemselves to shallow depths orproximity to theirhaul-out areas. Surveys offish
assemblages in each ofthe fourzones showed an overall decline in biomass with depth.
The samefish families were found in all zones except forthe subphotic zone, where other
familieswere dominant. The fish survey datawere classified into prey-evasion guilds
formonk seals, and the percent composition ofthe fourzones then was compared with
the monk seal diet data from the literature. The composition ofthe seals' diet differed
significantly from the composition offish found in each zone. However, on the basis
ofa dissimilarity index, the composition ofthefish guilds in the bank and slope zones
deviatedthe least from the monk seals' diet.
INTRODUCTION
Where andwhatmonk seals eat is a questionthat scientists andresource managers
oftheNorthwestern Hawaiian Islands (NWHI) have attemptedto address using a wide
variety ofmethods. Monk seals {Monachiisschaiiinslandi) (Gilmartin and Eberhardt,
1995) routinelymove between the reefsystems ofthe HawaiianArchipelago and dive
to awide range ofdepths (Abemathy, 1999). The scale ofthese movements challenges
some long-standing assumptions about monk seal foraging habitat and highlight the need
for information aboutprey distribution in the seals' forage grounds. Studies offoraging
behavior ofFrench Frigate Shoals (FFS) seals have included tracking ofmovements
using satellite tags (Abemathy, 1999) and analysis ofprey fragments in seal scat
(Goodman-Lowe, 1998). Inthis study, these foraging dataare compared with regional
surveys ofpotential prey assemblages.
'NOAAPacificIslandsFisheriesScienceCenter,2570Dole St.,Honolulu,Hawaii96822USA,E-mail:
Frank.Parrish(anoaa.gov
-NationalGeographicTelevision,Washington,DC USA
116
All available foraging data (Abemathy, 1999; Goodman-Lowe, 1998; Parrishet.
al., 2000, 2002, 2005) indicate that FFS seals feed on benthic and demersal fish species,
and thus their foraging grounds are limitedto the benthic habitat afforded by the shallow
portions (<600 m) ofthe HawaiianArchipelago. Modifiedby alonghistory ofsea-level
change (Grigg and Epp, 1989), the habitat ofthe lowerArchipelago is composed offour
obvious depth zones. The firstzone is the shallow "reef ofFFS (<30 m) thathosts the
sand islets where the monk seal subpopulations rest andreartheiryoung. The nextmost
prominent zone consists ofthe submerged "banks" at 30-50 m that occur SE andNW
ofFFS. These banks support minimal coral coverage and are covered primarilywith
sand and algae. At the edge ofthe reeforbank, the "slope" zone (51-300 m) begins.
m
At the base ofthe steepest slope segments, often around 60 deep, talus accumulates,
with smaller sizes ofrubble sortingbelow. At 80-100 m, there is often a terrace where
sand accumulates, and then the slope continues steeply down to 300 m. Deep-water
blackcorals (Cirripathes sp.) often are seen-200 m deep, growing on the carbonate
remnants ofprehistoric coral reefcomplexes or lithified carbonate sandfields. The slope
decreases significantly at-300 m. At this depth, light is well below the level needed for
photosynthesis; this fourthzone (301-500 m) willbe called "subphotic." Bottomtypes
include carbonate, basalt, manganese crust, and sand with occasional patches ofdeep-
watercorals in areas ofhigh currentflow.
In this paperwe consider seal movements in relationto these fourdepthzones.
We compare the prey base among the habitat zones visitedby the seals. Finally, the prey-
base data will be evaluated in relation to available monk seal diet data. The following
hypotheses will be tested: 1) seals feedmore in the nearesthabitats and less in distant
ones; 2) seal feeding is governedbythe stmcture (body size, numerical density, or
biomass density) ofthe fish community available; and 3) differentpatterns in seal feeding
found among habitats are not related to morphological orbehavioral differences in the
prey types.
METHODS
Seal MovementData
Satellite tags were fitted to 24 adult FFS seals (males and females) betweenApril
and July during 1992-94 and 1996-1997 (median 87 days)(Abemathy, 1999). Although
the distance anddive characteristics ofthe seals' movementshave been described
(Abemathy and Siniff, 1998; Abemathy, 1999), at thattime there were no data on seal-
prey assemblages with which to compare. Activitypatterns for each seal were plotted on
abase map in araster-based geographic information system (GIS)(IDRISI) representing
the 600- - km area (0.13 km-/rastercell) section oftheArchipelago from NeckerBank
to Gardner Bank- the extent oftravel documented forthe FFS seals. Isobaths from
National Ocean Survey charts were usedto delineate the fourdepth/habitat zones, reef(0-
30 m), bank (31-50 m), slope (51-300 m), and subphotic (301-500 m) as the primary test
categories (Fig. 1).
117
N26^
W168
Depth
n
< 30 m Reef Kilometers
O 30-50 m Bank 100
51-300 m Slope
300-500 m Subphotic
Figure 1.BaseGIScoverageoftheFrenchFrigate Shoalsregionwitheachofthefourhabitatzones
represented.Arrowsindicatethelocationofthefishsurveys.
Satellite tags can provide positions ofseals only ifthey are on the surface during
the dailypass ofthe orbitalARGOS satellites. Furthermore, some sampling bias maybe
introducedbythe varying degrees ofsatellite coverage throughout the course ofthe day.
Positional accuracy checkedwith independentVHF tracking ofthe satellite tags averaged
16 km± 13 km (sd). To refine confidence inthe seal positions, these datawere evaluated
using software called "Satel" provided by Loyd Lowry (Alaska Dept ofFish and Game)
that calculates the swimming speed required for a seal to travel between consecutive
118
estimatedpositions and indicates unrealistic positions given the seal's actual swimming
velocity (7.2 km/hr). These poorpositions were excluded from furtheranalysis. Finally,
even with "good"positions, it should be remembered that these are surface positions and
represent seals surfacing from dives, which can be as long as 17 min (Abemathy and
Siniff, 1998; Pamsh et a!., 2002). Itwas assumed that positions chistered tightly in one
ormore areas indicated the mostreliable focus ofthe seals' effort over a given habitat.
Clusters were definedby eye, withthe delineation ofthe boundingpolygons often
excluding wide dispersions ofpoints that were likely transits to and from feeding sites or
opportunistic searching. Limiting the polygons to exclusively represent the clusters of
positions should improve the chances ofidentifying key foraging habitats. The depth-
of-bottom contours at the positional clusters were corroborated by depth-of-dive-activity
modes transmitted from the satellite tags. The activity patterns ofthe 24 seals were
overlaid to representthe cumulative area, or"footprint," oftheir foraging.
Two comparisons were made using the GIS data. First, the amount ofoverlap
between the planararea ofeach zone and the footprint ofthe seals' foraging areawas
compared. Second, a GIS surface was generated with distance values radiating from the
seal haul outs at FFS (the six sand islets in the atoll). Distance values thenwere extracted
from each rastercell ofthe polygons ofthe fourhabitat zones and comparedto distance
values extracted from an overlay ofthe seals' footprint foreach ofthe fourhabitat zones.
Fish (Prey) Community Surveys
Fish communities ofthe fourhabitat zones were sui^veyedusing avariety of
techniques. In each survey the numerical density oftaxa and body length (to nearest
5 cm) ofa fish assemblage were recorded fora given area for standardized area-based
comparisons. Thirty-five visual sui'veys were made in each ofthe fourhabitatzones (Fig.
1), andTable 1 lists the survey methodologies for each ofthese zones. Survey stations
in the FFS reefwere established by habitat type usingpublished (NOAA, 2003) benthic
maps derived from 4-m resolution IKONOS satellite imagery. Forthe deeperhabitat
zones, no such data are available. Bank stations were placed arbitrarily across three
banks (Necker, Brooks, and Gardner). The habitat ofthe slope is deteiTnined largely by
sorting oftalus, rubble, and sand, so the 35 stations were dividedto representthe rubble
belt, the sandreservoirs, and exposed carbonate bottom. The 35 subphotic stations were
conducted fromPisces submersibles and included habitats ofcarbonate, basalt, anddeep-
watercorals.
Length estimates were used with species-specific length-weight coefficients
(FriedlanderandPamsh, 1998) to obtain an estimate ofbiomass density. Large apex
predators (e.g.,jacks, sharks, snappers) were excludedfromall the counts because they
were too large to be considered seal prey. Trawl specimens from sand bottom were
weighed to the nearestgram. No length-weight coefficients are available for subphotic
species, so size-specific weights were obtained from historical trawl catch data (unpub.
data, Pacific Islands Fisheries Science Center), orthe weight ofafish with a similarbody
shape was usedas aproxy. The estimates ofprey size, numerical density, andbiomass
density ofthe community were then comparedacross the fourzones.
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Table 1. Method, area, number ofstations, and other details forfish community surxeys
made in each habitat zone ofthe French Frigate Shoals region.
Zone Method Area No. of Years Reference for survey
(m2) stations surveyed methodology used.
Reef Divers 500 35 2002 DeMartinietal. (1996)
<30m
Banks Divers 177 35 2001-2002 Bohnsack and Bannerot
m
30-50 (1986)
Slopes Divers 85-250 16 1998-1999 DeMartinietal. (2003)
m
51-300 Trawls 4000 9 2002 Struhsaker(1973)
Sub 3600 10 2000 Moffitt and Parrish (1992)
Subphotic Sub 3600 35 1998-2002 Moffitt and PaiTish (1992)
301-500 m
Monk Seal Diet
The value ofthe fish communities as monk seal prey was derivedusing data from
analysis ofscat (Goodman-Lowe, 1998). The reported frequency oftaxon occurrence in
the scat data was used as aproxy forprey abundance, and each was classified into one
offourguilds reflecting the prey's general evasion tactic, including bottom camouflage,
hiding in shelter, fleeing along the bottom, andfleeing through midwater (Table 2).
The evasion guilds were used to compare the relative importance ofthe shallow-reef
community, whichwas best represented in the scat data, to bank, slope, and subphotic
fish communities. Afterclassifying the fish from each ofthe fourhabitat zones by
evasionguild, theirnumerical density andbiomass density thenwere comparedwith
the frequency ofoccurrence ofthe evasion guild in the seals' diet (Goodman-Lowe,
1998). We assumed that a high fraction ofa particular evasion guild found in the
seals' dietmeantthe seals wouldtargetthat evasion guildofprey across all fourzones.
Furthermore, the zone with the fractional makeup thatbest mirrors the relative fraction in
the seals' diet is the zone most usedby the seals.
Analysis
The seals'movements were tested in relation to the availability ofthe four
zones using chi-squared comparisons. The 35 stations perhabitat zone provided this
study apowerof0.80 to detect large effects at the 0.01 level (Cohen, 1988). The fish
communities ofthe fourzones were evaluatedusing a Kruskal-Wallis (K-W) analysis
ofvariance (ANOVA) andaposterioriTukey comparisons. Differences in the evasion
guilds were addressed with chi-square using the seals' diet data as the expected values.
Finally, theproportions ofthe evasionguilds in sealprey andthe fish communities were
converted into distance scores to compare theirrelative Euclidean distance from the seals
dietusing aparametric dissimilarity index.
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Table 2. Monk seal diet by functional groups derived from analysis ofscats (Goodman-
Lowe, 1998).
Evasion Guild Taxa found in seal scat Example taxa morphology
Bottom Synodontidae
Camouflage Cirrhitidae
Bothidae
BC Scorpaenidae
Octopodidae
Bottom Fleer Labridae
Scaridae
BF Acanthuridae
Muraenidae
Congridae
Kuhlidae
Ophichthidae
Mullidae
Liitjanidae
Bottom Hider Pomacentridae
Tetraodontidae
BH Pomacanthidae
Chaetodontidae
Holocentridae
Pricanthidae
Apogonidae
MidwaterFleer Kyphosidae
Monacanthidae
MF
Balistidae ^C7
12!
Figure2. MovementofmonksealswithintheFrenchFrigateShoalsregion.
RESULTS
Seals' Use ofForaging Grounds
The cumulative area orfootprintcoveredbythe 24 seals was 24% ofthe total
area available. The area coveredbythe movements ofa few individual seals made up the
bulk ofthe total footprint (Fig. 2). Overlap ofseal movements was highest closerto the
seals'haul outs in the shallows ofthe island. However, 25% ofthe atoll lagoon was left
unvisitedbythe tagged seals. The median area seals covered in their foraging compared
to the area available in each ofthe zones differed significantly (x-=58.9. df=3, P<0.01).
The seals used roughlyhalfofwhat was available in each zone except for subphotic
depths, where seals used less than 10% ofthe available area. The median distance ofthe
fourzones comparedwiththe average distance traveledby the seals did not significantly
differ{%- =3.19, df=3, P= 0.4), indicating seals generally moved overthe full extent of
grounds (Fig. 3).
122
14000
Totalarea available in FFS region
12000
Areaofsealforaging
10000
^E 8000
-^
03 6000
<
4000
2000
y / /[-
REEF BANK SLOPE SUBPHOTIC
Distancetogrounds
200
Distancesealstravelled
E 150
100
b
50
_I_
REEF BANK SLOPE SUBPHOTIC
Habitat zone
Figure3. GISderivedmeanareaanddistance(fromFFS) foreachofthehabitatzones intheFFSregion.
Thediagonal bars indicatetheavailablehabitatandthegreybarsaretheseals"movements.
Fish Community Structure
Fish size, numerical density, and biomass among stations all were found to differ
significantly from a nonnal distribution (Kolmogorov-Smimov, Z=2.4 - 4.3, df=139,
P<0.01). Significant differences in fish size, numerical density, andbiomass densitywere
detected when comparisons were made among the fourdepth/habitat zones (K-W, x' ^
26.6 - 77.5, df=3, P<0.01). Results from the aposterioricomparisons using the Tukey
tests are detailed inTable 3. As expected, the highest numerical density was in the reef
zone, and the lowest occurred at subphotic depths (Fig. 4). However, median fish size
exhibited a contrasting pattern, with the largestfish at subphotic depths and the smallest
in the reef Finally, reefbiomass density was significantly greaterthan bank and slope
biomass density, which were significantly greaterthan biomass density in the subphotic
zones.
123
Table3. Results fromK-W analysis ofvariance ofnumerical density, body size, and
biomassdensitybyhabitatzone ofthe French Frigate Shoals region with results ofa
posterioricomparisons (rf=reef. bk=bank, sl=slope, sp=subphotic).
Fish Medianvalues Tukeyaposteriori
Surveys HabitatZone comparisons
Reef Bank Slope Subphotic P 0.05 threshold
(rf) (bk) (si) (sp)
Density 0.26 0.05 0.07 0.003 <0.01 sp<bk, si < rf
(no./m"')
Size (cm) 8.80 10.7 8.5 13.9 <0.01 rf. si < bk< sp
Biomass 16.0 5.46 0.69 0.35 <0.01 sp<sl, bk< rf
(g/m')
'to'
:= 3000 -
6
c
"-
2000 -
C 1000-
D
A
.
c 10
0)
D 5 -
c
w
300 -
(«A 200
re
E
o 100
m
REEF BANK SLOPE SUBPHOTIC
Habitat zone
Figure4.Numericaldensity, standardbodylength,andbiomassdensityoffishforthefourhabitatzones inthe
FrenchFrigate Shoalsregion.
124
Prey-Evasion Guilds
Using the frequency ofprey items in scat dataprovided a fractional seal diet of
23% bottom camouflaged (BC), 49% bottom fleers (BF), 26%) bottom hiders (BH), and
2%) midwaterfleers (MF). This diet composition was used as the expected value for all
comparisons with the composition ofthe fourhabitat zones. Ofthe four evasion guilds,
only the midwaterfleers category had a notably low number offamilies in each ofthe
habitat zones (Table 4). Two dozenprey families were found in each ofthe fourhabitat
zones. Reefandbank communities were made up ofthe same families, whereas the
slope zone lacked four shallower families and included fourdeeper ones. The largest
difference in family composition was evident in the subphotic zone, where only four
families, mostlybottom camouflage, persisted from the shallow atoll depths. Chi-square
tests indicated thatthe observed composition ofthe evasion guilds for each zone
significantly differed from the composition observed in the seals' diet (density%- =37.5-
77.6 P<0.001; biomass yj =20.1-73.8 P<0.001). Failing to identify azone that was not
significantly different from the seal diet, we generated scores fornumerical density and
biomass density using the functional group compositions in a dissimilarity index (Fig. 5).
Ofthese scores, fishbiomass density in the bank and slope zones deviated least from the
seals' diet. There was no clearpattern in the density data.