BULLETIN OF MARINE SCIENCE, 58(3): 826-837, 1996 CORAL REEF PAPER

SHELTERING AND FORAGING SUBSTRATE USES OF THE ARC-EYE HAWKFISH PARACIRRHITES ARCATUS (PISCES: )

Edward E. DeMartini

ABSTRACT A combination of free-ranging snorkel surveys and focal- observations was used to evaluate substrate use by the arc-eye hawkfish Paracirrhites arcatus, a typical cirrhitid rec- ognized for its conspicuous "perching" and ambushing behavior, The arc-eye hawkfish used different substrates for sheltering and for foraging. Pocillopora rneandrina, the only common of closely branched coral in the main Hawaiian Islands, was the perch most frequently encountered (>96% of all cases) on snorkel surveys at seven sites on the leeward side of the Island of Hawaii. P. arcatus refuged within Poe. rneandrina whenever predators ap- proached closely, but darted out from coral heads to feed. Foraging strikes were directed mainly at targets on benthic substrates surrounding Poe. rneandrina refuges; strikes were primarily directed (81-96% of all strikes) at prey on rock/dead coral substrate that dominated cover (43-90%) at the study sites. P. arcatus also fed to a major extent within the near- bottom water column. The dichotomy between preferred foraging and sheltering substrates is discussed relative to the trade-off between foraging benefit and risk of predation hazard when the two substrates differ.

Hawkfishes (Cirrhitidae) derive their common name from the habit of conspic- uously "perching" on.corals and other substrates, from which they dart to capture prey (Hobson, 1974, 1994; Myers, 1989). The Indo-Pacific arc-eye hawkfish, Paracirrhites arcatus, an abundant and widespread species in the high Hawaiian islands (Randall, 1963; J. E. Randall, pers. comm.), exhibits perching behavior typical of many cirrhitids, with the anterior of the body protruding from the outermost branches of coral heads (Myers, 1989: pI. 39A). Despite the prevalence of perching behavior, its function has never been quantified in cirrhitids. More generally, the uses of substrate and other habitat by organisms typically reflect the balancing demands of sheltering and foraging, and studies of the trade-offs involved are of continuing interest (Fraser and Cerri, 1982; Cerri and Fraser, 1983; Werner et al., 1983; Gilliam and Fraser, 1987). The objectives of this study were to quantify substrate use by P. arcatus and evaluate its two most likely, but not mutually exclusive, functions of sheltering and foraging. I used an extensive set of observations, conducted at multiple sites spanning a gradient in wave exposure on the leeward side of the Island of Hawaii, to describe the types of biological and physical substrates used as perches. I provide complementary data on the use of foraging microhabitats, based on an intensive series of focal-animal observations.

METHODS

Fish Observations.-Observational data were collected on periodic surveys conducted at seven fring- ing coral reef sites on the North and South Kohala coasts of the Island of Hawaii (DeMartini and Donaldson, in press). Individuals of P. arcatus within fixed study plots (1000-2500 m2 area) were tallied on snorkel surveys conducted every 2-4 mo during the period December 1989 to June 1994. Individuals were viewed at distances of 1-3 m, and total lengths (TL) estimated by eye to the nearest em by comparison with a measured rule. Individuals =£4cm TL and 2:5 em TL were post-classified as "small" and "large", respectively. Estimated lengths of a subsample of censused fish were verified against distances directly measured between substrate features (DeMartini and Donaldson, in press).

826 DEMARTINI: SHELTERING AND FORAGING BY ARC-EYE HAWKFISH 827

The relation between fish TL (mm), body width (BW, 0.1 mm), and body depth (BD, 0.1 mm) was determined for fish speared from non-study reef areas adjacent to the study sites.

Habitat Characterizations.-All small and large individuals of P. arcatus were classified by micro- habitat use, defined as the substratum upon which an individual reposed when first seen. The six substrata categorized were live colonies of Poe. meandrina, plus two other common corals (Porites lobata, Par. compressa); "other corals"; lava rock/dead coral; and sand. Substrata were quantified as density (number of discrete coral colonies per unit area; for Poe. meandrina) or as percentage cover (for Porites spp., other two-dimensional encrusting corals, lava rock/dead coral). In the spring of 1991, I surveyed four replicates of a 5-m x I-m (5-m2) band transect centered on a line intercepted by 25 random points (Ohlhorst et aI., 1988), at each of four core sites that ranged from exposed to semi-protected from ocean swell. Live coral cover varied from about 10- 50% among sites at surveyed (1- to 5-m) depths, and was inversely related to cover of lava rock/dead coral, with the latter proportional to the degree and frequency of physical disturbance (Grigg, 1983). Rock and dead coral substrata were sparsely covered by turfing and crustose coralline algae. Average diameter (AD in cm, as [length + width + height] 13: Kuwamura et aI., 1994) was estimated for a random sample of Poe. meandrina colonies. Interbranch (IB, mm) distance was mea- sured for a subsample of 40 colonies of a range of sizes, so that mean IB could be related to colony diameter. For each colony, mean IB was estimated based on five measurements of the shortest distance between adjacent branches, with location of the measurement points randomized within the centermost, middle, and outermost thirds of the colony surface. Also noted were the presence and number of P. arcatus occupants and their estimated body lengths. Fish TL was compared to AD of the Poe. mean- drina coral colony that the fish occupied.

Focal-animal Observations.-Social and other overt behaviors were recorded for individuals of a range of body sizes during 1991. Between seven to nine fish were chosen at random and observed at each of the four core sites for a total of about 120 min·fish-I. Each fish was observed for about 60 min during two periods of the year-spring-summer (April-July) and fall (October-November). During each period, each focal individual was observed for one, 15-min interval during 0800-1030, 1030- 1300, 1300-1530, and 1530-1800, usually on different days, to sample during all daylight hours of activity. Fish were not tagged; when observed at distances of 1-3 m, fish were individually recogniz- able by unique combinations of body size, color morph (melanistic or white-striped: DeMartini and Donaldson, in press), color intensity (hue), and topography-specific behaviors. For fish that disappeared between spring-summer and fall (10%), replacements were chosen if available from among like-size individuals within the same area of reef. Total observation times differed slightly between sites and periods because of variations in the numbers of individuals monitored. The following behaviors were recorded: the number and duration of intra- and inter-specific aggressive bouts (defined as any discrete, agonistic interaction), including the identity and body sizes of interactants; the types of substrate involved in apparent feeding bites; and the number of movements between, and duration at, different perches. Refuging behavior was also noted whenever it occurred, along with the identity of the pred- ator. All observations were made by the author.

Statistical Analyses.-Fish length and several measures of coral colony size were related to one another using standard statistics (parametric correlation: Sokal and Rohlf, 1981; Kolmogorov-Smirnov [K-S] 2-sample test: Siegel and Castellan, 1988). The electivity of P. arcatus for different perching and foraging substrates was evaluated using

Strauss' (1979) linear index of selection, L = (ri - p,), where ri = the relative use of the ith perch or foraging substrate, and Pi = the relative abundance of the ith substrate in the environment. Both Pi and ri can vary between + I (sole presence and exclusive use, respectively) to -1 (complete absence and avoidance). The variance of L was estimated as var L = [(r.)(1 - pi)]/n, + [(p,)(1 - p.)]/np' where n, = total measure of substrates used, and np = total measure of substrates available in the environment (Strauss, 1982). Log-likelihood ratio tests (G-tests of association; Sokal and Rohlf, 1981) were used to compare coral use between small and large fish. Subannual differences in the frequency of aggressive behavior were evaluated using a binomial test (Siegel and Castellan, 1988). Evaluation of subdaily periodicity in feeding activities and aggression was not possible because I was unable to replicate observations within a day. The effects of site and subannual period on use of perching and foraging substrates were evaluated by Model I (fixed effects) 2-way Analysis of Covariance (ANCOVA; Sokal and Rohlf, 1981). Ob- servation time was used as a covariate to standardize observations among sites and periods for slight inequalities in sampling effort. Because uses of different substrate types were interdependent, a pos- teriori multiple comparisons used Bonferroni's inequality (Penl = wm, where IX = 0.05) to adjust alpha values. A significance level of 0.05 was used in all other statistical tests. 828 BULLETIN OF MARINE SCIENCE, VOL. 58, NO.3, 1996

Table I. Summary statistics for Strauss' (1979, 1982) clectivity indiccs of substrate use for perching and for foraging by Paracirrhires area/us, at each of four focal sites," Kohala coast, Hawaii Island

Utilization Perching Foraging

Site Substratum p, L ± 2 SE r, L ± 2 SE

CCov Poe, meandrina 0,04 0,81 +0.77 ± 0,037 0.03 -0,0 I ± 0.036 Par. [obara 0.06 0.00 -0.06 ::':0.017 om -0,05 ::':0.023 Rock/dead coral 0.90 0.18 -0.72 ::':0.040 0.96 +0.06 ± 0.042 KBnch Poe, meandrina 0,09 0.68 +0.59 ± 0.039 0.08 -0.01 ± 0.074 Par. [obara 0.47 0.28 -0.19 ± 0.042 0.11 -0.36 ± 0.090 Rock/dead coral 0.43 0.04 -0.39 ::':0.027 0.81 +0,38 ::':0,110 Sand om n/a n/a 0,21 +0,20 ::':0.112 PBDrv Pac. meandrina 0.10 0.68 +0.67 ± 0.043 0.05 -0.05 ± 0.051 Par. [obara 0,08 0,21 +0,03 ± 0.044 0.13 +0,05 ± 0,080 Rock/dead coral 0.81 0.10 -0.71 ::':0.037 0.82 +0.01 ::':0.091 PuPt Pac. meandrina om 0.76 +0.75 ± 0,054 0.00 -om ± 0.007 Par. [obara 0,12 0,18 +0.06 ::':0,053 0.09 -0,03 ::':0.076 Rock/dead coral 0.87 0.06 -0.81 ::':0.037 0.91 +0,04 ::':0.076 Sand >0.00 n/a nfa 0.03 +0.03 ::':0.043 a CCov: Kai 'opae Point. North Kohala; KBnch: Kanekanaka Bench, reef inshore of Kanekanaka Point, South Kohala; PBDrv: 20 Puako Beach Drive. between Waimea and Puako Points, Soulh Kohala; PuPt: end-of-road on Puako Beach Drive, Kapuniau Point-Pauoa Bay, South Kohala. The three non-focal study sites were: Lapa (Koaie Cove, Subzone B of Lapakahi Marine Life Conservation District, North Kohala); KRCov (Kohala Ranch Cove = Waiabilio Bay, North Kohala); and WaiBay (Waialea Bay reef, South Kohala).

RESULTS Perch Types.-All categories of biological and physical substrates were used for perching except bare sand (Table I). The encrusting coral Porites [obata was used for perching to an extent roughly proportional to its abundance (L = -0.19 to +0.06), whereas rock/dead coral substrates were clearly underutilized (L = -0.39 to -0.81). During 55 mo of extensive observations at seven sites, the branching coral Pocillopora meandrina was the perch occupied by P. arcatus on 96.6% of 11,335 sightings. The amount of time spent perched on Poe. meandrina was less on focal surveys at the four core sites (68-81 %), but Poe, meandrina was clearly preferred beyond its abundance (L = +0.59 to +0.77). A variable minority of all Poe, meandrina present (2-35%, depending on site) was occupied by P. arcatus; at 92% of the occupied corals, a single fish was present, while :2:2 fish were present at 8% of the occupied corals. Multiple occupancy on average differed seven-fold among the sites from <2% to nearly 15%. A maximum of four P. arcatus individuals was observed (very rarely: ten occasions) together in a single coral head. Use of Poe, meandrina was clearly different for large and small P. arcatus. During the extensive surveys, other substrates were perched upon 3.4% of 10,950 times for large fish (to 14 cm TL). For small fish, Poe. meandrina was used in all of 385 instances; small fish used branching coral with greater relative fre- quency than did large fish (2 X 2 G-test: Size X Perch effect, P < 0,0001). The extent to which Pocillopora and Porites were used for perching differed among the four focal sites and two time periods (2-way ANCOV A on time spent perched: Site X Period interaction, both P :;::;0.01), The extent to which rock/ dead coral was used for perching varied more generally among all sites (Site effect, P < 0.0001), and there were no differences between subannual periods at sites (Period effect, P > 0.2; Tables 2,3). Overall, focal fish spent >99% of all daylight hours inactively perched at all sites during both observation periods (see Foraging Behavior, below), Focal fish moved more frequently among Pocillopora DEMARTINI: SHELTERING AND FORAGING BY ARC-EYE HAWKFISH 829

Table 2. Perching data and foraging bout tallies for focal Paraeirrhites areatus observed during Apri]-July and October-November] 991, at four core sites, North and South Kohala districts, Hawaii Island. "No. obs. prd." refers to number of 15-min (approximate) observation periods. Habitat acro- nyms are "Poe." = Pocillopora meandrina, "Por." = Porites lobata. "Rock" = Rock/dead coral. For site acronyms see Tab]e 1 footnote.

Time (mins) No. fonlging bites perched in habitat No. No. Subannual obs. obs. Wafer Site period prd. min Poe. Por. Rock Poco Por. Rock Sand column

CCOV Apr-Jul 32 488.4 396.4 0.0 77.0 I 0 51 0 109 Oct-Nov 42 615.] 488.4 2.7 124.0 3 I 63 0 10 K.Bnch Apr-Jul 43 614.0 474.8 118.0 18.0 3 I 25 3 15 Oct-Nov 33 503.6 278.2 196.0 29.3 I 5 18 II 5 PBDrv Apr-Jul 36 530.4 371.9 120.0 38.5 2 5 39 0 20 Oct-Nov 39 563.9 377.2 113.0 73.8 2 5 24 0 33 PuPt Apr-Jul 29 433.7 309.2 94.4 30.1 0 4 32 0 15 Oct-Nov 36 533.8 417.5 76.8 24.2 0 2 26 2 10 All Apr-Jul 140 2,066.5 1552.3 332.4 163.6 6 10 147 3 159 Oct-Nov 150 2,216.4 1561.3 388.5 251.3 6 13 131 13 58

and other perches at all sites during April-July (2-way ANCOVA on number of perches used: Period effects, both P :5 0.02; Table 4). During an average IS-min observation, focal fish shifted positions among 5.7-8.3 different perches (includ-

Table 3. Summary statistics for Model I 2-way ANCOVAs of the effects of Site and subannual Period (AJ = April-July; ON = October-November) on time (sec) spent perched at (a) Pocillopora mean- drina. (b) Porites lobata and (c) rock/dead coral by focal Paracirrhites areatus. per 15-min obser- vation. The amount of time spent observing fish (Tot Sec variable) is the covariate used to standardize effort equally among sites and periods. Underlines connect insignificantly different means.

SSQ MSQ df F-ratio Pr> F

(a) POCo meandrina Tot Sec 5,398,101 5,398,101 ] 81.56 <0.000 I Site 899,614 299,87] 3 untestable Period 158,902 158,902 1 untestable Site X Period 753,381 251,127 3 3.79 0.0] Error 18,597.559 66,183 281 CCovAl = PuPtON = CCovON = KBnehAl = PuPtAl = PBDrvAl > PBDrvON > KBnehON 748 718 = 703 687 = 631= 621 > 595 > 483 (b) Par. lobata Tot Sec 31],503 311,503 1 7.04 <0.00 I Site 2,683,378 894,460 3 untestable Period 44,788 44,788 I untestable Site X Period 613.529 204,510 3 4.63 <0.001 Error 12,424.924 44,217 281 KBnchON = PBDrvAl = PuPtAl = PBDn,ON = KBnehON = PuPtON > CCovON = CCovAl 35/ 200 193 177 172 127 > 5 -6 (c) Rk/dead coral Tot sec 12,993 12,993 ] 0.49 >0.48 Site 672,537 224,179 3 8.41 <0.000] Period 40,]04 40,104 ] 1.51 >0.22 Error 7,566,694 26,643 284 CCov > PbDrv PuPt KBnch Apr-lui Oct-Nov 16/ > 90 49 39 73 96 830 BULLETIN OF MARINE SCIENCE, VOL. 58, NO.3. 1996

Table 4. Summary statistics for Model I 2-way ANCOVAs of the effects of Site and subannual Period on the number of different (a) Pac. meandrina perches and (b) total perches occupied by focal Par- acirrhites arca/us, per 15-min observation. The amount of time spent observing fish (Tot Sec variable) is the covariate used to standardize effort equally among sites and periods. Underline designations as in Table 3.

SSQ MSQ df F-ratio Pr>F (a) Pac. meandrina perches Tot Sec 9.45 9.45 I 8.89 0.003 Site 76.89 25.63 3 24.11 <0.0001 Period 5.90 5.90 I 5.55 0.02 Error 301.91 1.06 284 KBnch > CCov > PBDrv PuPt Apr-Jui > Oct-Nov 2.52 > 2.09 > 1.46 1.21 1.96 > 1.68 Ib) Total perches Tot Sec 408.66 408.66 1 19.19 <0.0001 Site 705.82 235.27 3 11.05 <0.0001 Period 148.57 148.57 I 6.98 0.009 Error 6,048.41 21.30 284 KBnch 2: CCov 2: PBDrv > PuPt Apr-Jui > Oct-Nov 8.42 2: 6.74 2: 6.41 > 3.94 7.10 > 5.66

ing 1.3-2.7 Poe. meandrina) during April-July but repositioned only 2.5-8.7 times (including 1.1-2.3 Pocillopora) during October-November. Fish-coral Size Relations.- The relation between diameter of coral head and length of P. arcatus occupant was significant, but not particularly strong (r = +0.40, N = 273, P < 0.001; Fig. 1). Much scatter occurred among median-sized and larger fish (2::8cm TL) that perched on small coral heads, particularly at two of the seven sites at which larger individuals were relatively abundant (Fig. IF). P. arcatus usually did not perch on Poe. meandrina colonies with AD <15 cm (2-sample K-S test: P < 0.001; Fig. 2). Interrelations of Coral Dimensions.- There was a significant and moderately strong (r = +0.76, N = 40, P < 0.000l) correlation between average diameter and IE spacing in Poe. meandrina coral heads (Fig. 3). Mean IE spacing in the average-sized (20 cm diameter) Poe. meandrina head was about 17 mm (Fig. 3). Fish Size Interrelations.-There were very strong (,-2 = 0.97-0.99) correlations between TL, BW and BD in P. arcatus (N = 27 fish; range: 45-125 mm TL, 5- 16 mm BW, 13-40 mm BD). The BW and BD of median-sized (7-8 cm TL) P. arcatus were 8-9 mm and 21-25 mm, respectively, or about 14-16 mm in average cross-section. Foraging Behavior.-A total 546 feeding bites were observed in 71.4 h of focal observations (34.4 h April-July, 36.9 h October-November). Of these, 329 (60%) involved contact with benthic substrates and 217 (40%) were directed towards prey in the water column at distances 0.1- to 3-m above the substrate (Table 2). Most of the substrate bites (81-96%, depending on site) were directed at prey on lava rock/dead coral; this proportion was equivalent to or greater than (L = +0.01 to +0.38) the percentage area cover of rock/dead coral (43-90%) at the respective site (Table 1). Relatively fewer bites (0-13%; L = -0.36 to +0.05) were directed at prey on live Porites lobata coral surfaces than expected based on the area cover (maximum 47%) of Por. lobata (Table 1). Poe. meandrina, a trivial to minor proportion (1-10%) of all available substrata, was used for foraging at similarly DEMARTINI: SHELTERING AND FORAGING BY ARC-EYE HAWKASH 831

14 , 14 (A) PBOrv n=35 (B) Pupt n=57 12 12

10 10 o 0 8 o ClD 0 8 0aD) 0 o cc:oo 0 o ClDClIIlXlDlIIlDO 0 6 o iIIIIDOIDlCllll) (X) 0 6 m'C'CclIIIIoJIIIID 00 occ:oo0 4 o 4 o , 2 2 5 10 15 20 25 30 35 40 5 10 15 20 25 30 35 40 ".-.., 14 , 14 E (C) Lopo n=37 (D) KronchCv n=38 U 12 12 '-" ..c ..•...•10 10 0\ C o o .3 8 o 0 00 8 o 0

14 , 14 , (E) CCOY n=23 (F) WoiB~y n=40 KBnch n=43 o 12 12 00 • o • 0 10 10 •••• 0 •• o~ •• 8 o 8 _~.o. 0 0000 _0. •. 6 o 000 6 IIX) • 0 at CXXlllO 0 • 0 0 0 4 o iIIIID 0 4 o o , 2 2 5 10 15 20 25 30 35 40 5 10 15 20 25 30 35 4() Mean Coral Head Diameter (em)

Figure 1. Scatterplots of the relations between average diameter (cm) of Pocillopora meandrina coral head and body size (TL, cm) of Paracirrhites arcatus occupant at each of seven north and south Kohala, Hawaii, sites. Sites are ordered from most wave-exposed (A) to most wave-protected (F); data for the WaiBay (hollow circles) and KBnch (solid circles) sites are illustrated together in panel (F). N = sample size. 832 BULLETIN OF MARINE SCIENCE, VOL. 58, NO.3, 1996

150

100

50 >. (J c Cll :J a 0" •...Cll lL. 50

100

150 0 5 10 15 20 25 30 35 Mean Coral Head Diameter (5 cm class)

Figure 2. Frequency distribution of the average diameter (cm) of Pocillopora meandrina coral heads that were unoccupied by Paracirrhites arcatus (top) and occupied by ;;:,one P. arcatus (bottom). low extents (0-9%; L = -0.01 to -0.05). Foraging occurred on sand substratum at both sites at which sand occurred, and was nontrivial in extent at one site (Table 1). The rate of attempted feeding on benthic substrates (primarily lava rock/dead coral) differed among the four focal sites (2-way ANCOVA, Site effect, P < 0.001), but not between subannual periods (Period effect, P = 0.15; Tables 2,5). Fish foraged on the benthos at similar rates during April-July and October-No- vember (mean = 1.1 bites·fish-I.}5 min-I). The effects of site and subannual period on levels of water-column and total (water-column plus benthos) feeding activities were confounded by site-by-period interactions for water-column feed- ing (Tables 2, 5). Inordinately high levels of water-column feeding were observed at the CCov site on a single date during the April-July period (levels then were three- to tenfold greater than at the other sites or at the same site in October- November). If the CCov data for April-July are excluded, water-column feeding occurred at a mean rate of about 0.4 bite·fish-I·15 min-I. A reasonable estimate of overall prey capture activities (benthic plus water column) thus would be <2 bites·fish-I·I5 min-I, or <8 bites·fish-I·h-I. A typical foraging bout lasted <5 sec. Sheltering Behavior.-On nine occasions at two sites (Lapa, PuPt), focal individ- uals responded to the approach of piscivorous fishes (most commonly the jack Caranx melampygus and the goatfish Parupeneus cyclostomus, often in foraging association) by completely withdrawing their bodies among the branches of Poe. meandrina. Fish typically resumed their usual perching posture within seconds to several minutes of predator departure. Agonistic Behavior.-Aggressive behavior was rare (0.32 bout·h-I) and directed equally at conspecifics and other species (14 and 9 bouts, respectively). Intraspe- cific aggression was too rare to allow evaluation of the potential influences of body size or season. An apparent trend towards more frequent total aggression DEMARTINI: SHELTERING AND FORAGING BY ARC-EYE HAWKFISH 833

30 r--r--,-- l--r-'Ir--,..--'-,...-,---r-.,----.--,--,...--.-- I~-'"

0

r-.. 0 E 25 E '-" Q) u c 0 00 +-' 0 (fl 0 20 0 00 ..c 0 0 U 0 c o 000 0 L CD ?Jg;~ 0 I 0 L o Q) 00 00 +-' 15 f- 0 - C 0 eD C 0 «D 0 0 0 Q) 0 2

10 0

, I 5 10 15 20 25 30 35 40 Mean Coral Head Diameter (em)

Figure 3. Scatterplot of the relation between interbranch spacing ·(mm) and average diameter (cm) of Pocillopora meandrina coral heads.

(16/23 bouts) during April-July, a time that includes the principal breeding season in Hawaii (March-May; Walsh, 1987), was significant (binomial test, P < 0.05).

DISCUSSION Substrate Electivities.-P. arcatus used the closely branched coral Poe. mean- drina as a perch more frequently than the proportionate abundance of this coral among potential perching substrates. Depending on site, lava rock/dead coral sub- strate was either preferred for foraging or used in proportion to its relatively great abundance. These apparent patterns, however, are only as real as the accuracy of the measure used to describe them. 834 BULLETIN OF MARINE SCIENCE, VOL. 58, NO.3, 1996

Table 5. Summary statistics of Model I, 2-way ANCOVAs of the effects of Site and subannual Period on the numbers of (a) substrate-directed bites, (b) water-column directed bites, and (c) total bites by focal Paracirrhites areatus, per 15-min observation. The covariate used to standardize observation effort is the Tot Sec variable (see Table 3 caption). Underline designations as in Table 3.

SSQ MSQ df F-ratio Pr>F (a) Substrate bites Tot Sec 30.12 30.12 I 17.87 <0.0001 Site 32.59 10.86 3 6.44 <0.001 Period 3.58 3.58 I 2.13 0,15 Error 478.89 1.69 284 CCov > PbDrv PuPt KBneh Apr-Jui Oct-Nov 1.61 > 1.05 0,98 0.69 1.19 0,97 (b) Watr. colmn. bites Site 109.63 36.54 3 untestable Period 48.97 48.97 I untestable Site X Period 135.87 45.29 3 8.85 <0.0001 Error 1,443,78 5,12 282 CCovAJ > PBDrvON = PBDrvAJ = PuPtAJ = KBnehAJ = PuPtON = CCovON = KBnehON 3.41 > 0.85 = 0.56 0.52 0.35 = 0.28 = 0,24 0,15 (c) Total bites Tot Sec 33.94 33,94 I 5.54 <0.02 Site 250.95 83.65 3 untestable Period 79.88 79.88 I untestable Site X Period 106.46 35.49 3 5.80 <0.001 Error 1,720.53 6.12 281 CCovAJ > CCovON = PBDrvAJ = PuPtAJ = PBDrvON = KBnehAJ = PuPtON = KBnehON 4,97 > 1.85 = 1.84 = 1.74 = 1,68 = 1.08 = 1,05 = 0.82

Strauss' (1979) electivity index has been criticized for being based on ratios whose values can alter with changes in preference at constant resource abundance, with changes in resource abundance at constant preference, or with both (Chesson, 1983). Indices based on ratios such as Strauss' L provide an unambiguous measure of preference only if resource availability remains constant (Chesson, 1983). DeMartini and Donaldson (in press) and DeMartini (in prep.) observed that the relative abundances of major substrate types remained the same between spring 1991 and winter-spring 1994 at the Kohala, Hawaii, study sites. Thus, using Strauss' (1979) index to analyze substrate use is warranted. The strongly positive index values for Poe. meandrina (as perching substrate) indicate real preference rather than an artifact of fluctuating resources. An analogous preference for rock! dead coral as a foraging substrate is only slightly less suggestive, Substrate Use and Function.-Poc. meandrina, as the only closely branched coral that occurs commonly in the main Hawaiian Islands (Maragos, 1972), is clearly preferred as a perch because of the unique shelter that its ramose growth form provides. More than one P. arcatus individual sometimes cohabited Poe. mean- drina corals at the study sites, despite the generally low level at which Poe. meandrina was occupied. Anecdotal observations by Walsh (1983) off the Kona coast of Hawaii attest to its regional desirability as a perch. At his study site, as many as 11 individual P. arcatus occupied a single Poe. meandrina coral head after a 100-yr winter storm in January 1980 destroyed most of the corals at the site. P. arcatus uses numerous species of branched pocilloporid and acroporid corals elsewhere in the South Pacific where coral species diversity is much greater than in Hawaii (DeMartini and Donaldson, in press), DEMARTINI: SHELTERING AND FORAGING BY ARC-EYE HAWKFISH 835

The positive relation between shelter size and fish size is generally recognized in small-bodied reef fishes (references in Wilkins and Myers, 1993; Kuwamura et aI., 1994). Observations of P. areatus confirm that refuging behavior has an anti-predator function in which the sizes of fish and Poe. meandrina coral heads are related. Larger Poe. meandrina have greater IB spacing, and cross-sectional body dimensions of median-sized P. arcatus are only slightly less than average IB spacing. Body lengths of P. areatus tend to increase with the size of occupied coral heads, particularly among smaller fish most susceptible to predation by piscivorous fishes. Small P. arcatus might perch exclusively in Poe. meandrina because its closely spaced branches provide the only safe refuge against predators. Hobson (1974) commented that larger-bodied P. arcatus are more apt to perch on substrates other than Poe. meandrina near Kona, Hawaii. That P. arcatus >8 cm TL rely less strongly on Poe. meandrina coral heads for shelter, and not infrequently perch on two-dimensional Por. lobata and lava rock/dead coral sur- faces, is not unexpected, because larger fish surely escape predation by size or have fewer predators. The refuging function of Poe. meandrina likely varies both temporally and spatially. Shelter may be particularly important for P. arcatus when individuals are aggressive and more actively moving among perches during spring-summer. The importance of Poe. meandrina as shelter also undoubtedly varies among sites that vary in predator abundance. Foraging Substrates and Feeding Behaviors.-Most feeding in P. areatus occurs on rock/dead coral substrata surrounding Poe. meandrina perches, during rela- tively infrequent, short-duration forays away from shelter. Little foraging occurs either within or at neighboring Poe. meandrina shelters. The low electivity of Poe. meandrina as a foraging substrate is unexpected for several reasons. First, this coral provides perch and shelter resources of disproportionate importance, and foraging immediately near, if not in, such structure might be used as an adaptive compromise between sheltering and foraging (Gotceitas and Colgan, 1990). Second, xanthid crabs of the Trapezia, obligate symbionts of live Poco meandrina (Coles, 1980, 1986), contribute to the diet of P. arcatus. Hiatt and Strasburg (1960) found xanthid crab remains in three fish out of the 17120 P. arcatus specimens (46-72 mm standard length, SL) examined that contained food; one of these fish contained recognizable Trapezia. Xanthid crabs occurred with 60% frequency occurrence and contributed >43% volume to the diet of 20/ 45 P. arcatus specimens (49-101 mm SL) with food collected off Kona (Hobson, 1974). Immature and small adult Trapezia characteristically inhabit crevices at the base of Pocillopora corals (Castro, 1976) that likely provide spatial refuges from P. arcatus and other predators. Perhaps Trapezia are more susceptible when exposed near, but not when sheltering within, coral heads. If so, a temporal as well as spatial refuge from predation may exist. Preston (1973) and Castro (1978) re- corded migration and dispersal of Trapezia among coral heads for feeding as well as mate-seeking. Movements among host corals, however, occur at night (Castro, 1978), when diurnal predators like P. arcatus (Hobson, 1974) are inactive. Spe- cifics on the relative availabilities of different sizes of Trapezia, and on diel variations in availability are lacking. Data on the time of ingestion and the size and caloric value of Trapezia consumed by P. areatus would be useful. Studies of relative prey availabilities on the various substrates used for foraging by P. areatus are needed. The frequency with which P. arcatus feeds on water-borne prey also seems 836 BULLETIN OF MARINE SCIENCE. VOL. 58. NO.3. 1996 surprising. Hobson (1974) describes P. arcatus as a diurnal ambusher of benthic prey-only 6120 P. arcatus from Kona that he examined had eaten organisms known or likely to be planktonic. Perhaps meroplanktonic stages of benthos (e.g., crab megalops) are seized in the near-bottom water column instead of, or in addition to, directly from the substrate. Diversity of Habitat Functions.-That different substrates provide key sheltering and foraging microhabitats is not without precedent in reef fishes and other or- ganisms. Although often coincident, optimal microhabitats for feeding and shel- tering sometimes differ (references in Hughes, 1990). One major contemporary issue in population ecology is the frequency and extent to which less-than-optimal foraging microhabitats are utilized when predation hazard is unacceptably high where prey are most available (Gilliam and Fraser, 1987; Gotceitas and Colgan, 1990). Shelter-dependent ambushers like hawkfishes seem ideal subjects for such studies, particularly species like P. arcatus in which distinctly different substrates provide principal sheltering and foraging functions. Potentially important differ- ences exist between ambushers and searchers in the relative tradeoffs between refuging and foraging. Refuging ambushers like P. arcatus for which handling (pursuing, capturing) prey is more hazardous than searching should have much narrower diets than searchers that refuge (Gilliam, 1990). This study provides the first quantitative data necessary to design studies that can meaningfully test pre- dictions such as whether the extent of water column foraging by P. arcatus is inversely proportional to predator density.

ACKNOWLEDGMENTS

I thank T. Anderson, C. Boggs, T. Donaldson, D. Greenfield, E. Hobson, R. Rosenblatt, H. J. Walker and two anonymous reviewers for constructive criticisms of a draft manuscript. Special thanks go to J. Leung whose field assistance made this study possible.

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DATEACCEPTED:May 11, 1995.

ADDRESS: Honolulu Laboratory, Southwest Fisheries Science Center, NMFS (NOAA), 2570 Dole Street, Honolulu, Hawaii 96822-2396.