Diet and food selection in the endemic Hawaiian amphidromous goby, Sicyopterus stimpsoni (Pisces: Gobiidae)

Environmental Biology of Fishes 45 :199-209,1996 .© 1996 Kluwer Academic Publishers . Printed in the Netherlands.

Diet and food selection in the endemic Hawaiian amphidromous goby,Sicyopterus stimpsoni (Pisces: Gobiidae)

Michael H. KidoUniversity of Hawaii, Kauai Research Facility, 7370A Kuamoo Road, Kapaa, HI 96746, U .S.A .

Received 24 .10 .1994

Accepted 16 .3 .1994

Key words: Interspecific competition, Nonequilibrium theory, Disturbance regimes, Patch dynamics,Morphological separation

Synopsis

The ecological role of Sicyopterus stimpsoni as a nearly exclusive algal grazer in Hawaiian streams is establish-ed through gut content analysis of 192 fish from Wainiha River on the northern island of Kaua'i . Algae in threephyla (Chlorophyta, Cyanophyta, and Chrysophyta) were found to be the primary components of the fish'sdiet (94.62% of dry biomass). Aquatic insect immatures (primarily Chironomidae) accounted for most of theremaining food biomass (5 .37%). Poorly developed gill rakers and high gut-to-length ratios provide evidencefor adaptation to herbivory and morphological separation of S stimpsoni from sympatric gobiods . Interspecif-ic competition for the green alga, Cladophora sp., is suggested as an important feature of biotic interactionsamong native stream fishes but is mitigated by interspecific differences in food preference and utilization . ACategory V hurricane which devastated the island in September 1992 provided a fortuitous opportunity tostudy disturbance influences on benthic food resources in the stream . Disturbance regimes influenced foodselection of S. stimpsoni by altering the abundance and composition of stream algae . Rapid adjustment of S.stimpsoni to changes occurring in algal resource abundance illustrates its superb adaptation to life in a swift-water environment subjected to periodic flood-induced disturbance .

Introduction

Sicyopterus stimpsoni Gill is one of three native Ha-waiian gobiid fish species inhabiting mountainousstreams in this extremely isolated Pacific archipela-go. Superbly adapted to swiftly-flowing basalt-bot-tomed stream habitat, this amphidromous fish ispart of a biota shaped by insular evolution. Thefood resource base available to stream fishes in thistaxonomically simplified system is largely limited toalgae and a disharmonic aquatic insect fauna dom-inated by dipteran immatures . The availability anddiversity of food resources are factors important inthe structuring of natural communities (Tilman

1982); therefore diet and food selection data for S.stimpsoni is not only of valuable scientific interest,

but is also essential for resource management activ-ities.

Patterning of native fish populations within - andamong-Hawaiian island streams is inconsistent andnot well documented . Instances of syntopic combi-nations of two of the three mountainous gobiods,allotopic populations restricted to certain eleva-tions or stream ecotype, as well as coexisting assem-blages of all three species have been observed (M .Kido unpublished data) . In a study of habitat use,Kinzie (1988), found S. stimpsoni to be separatedfrom coexisting species by virtue of its micro-hab-

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Fig. 1. Hawaiian island of Kauai and study sites on Wainiha Riv- er.

itat utilization pattern. Habitat data collected in this study were restricted to physical variables such as water velocity, position in stream, and substratum type. In addition, similarities in certain morphological feeding traits (e.g. in dentition and gut length to body length ratios) amongst the three species (M. Kido unpublished) hint at pressure for resource partitioning due to competition (Wiens 1984). Is food selection a source of competition for S. stimpsoni with other stream gobies? Is competitive exclu- sion therefore a significant factor in the structuring of fish populations in Hawaiian streams? Diet data for S. stimpsoni (and other native gobiods) are needed to answer these questions.

Hawaiian streams are also subjected to frequent- ly occurring episodes of flood and drought which periodically reach catastrophic levels. Flow-related disturbance can be a significant source of variability causing patchiness in the food resource base of streams (Resh et al. 1988). The ensuing successional change in the benthos (Fisher 1990) has been described as a key determinant of lotic species diversity (Reice et al. 1990) and an agent of natural selection (Sousa 1984). Do disturbance events regulate food resource availability in Hawaiian streams and ultimately influence biotic interactions in non-equilibrium fashion (Reice 1994)? A better understanding of the temporal and spatial dynamics of the resource base supporting S. stimpsoni is an essential prerequisite to addressing this question.

I attempted to elucidate the diet niche and func- tional feeding morphology of S. stimpsoni in terms of food selection with an eye towards temporal or age-related shifts in diet, competitive interactions, and variation in resource abundance. Which food items does S. stimpsoni prefer and how available are these foods in its habitat? Is this fish’s feeding morphology closely linked to resource use? How closely does this species track varying resource levels within its habitat? This information will provide a basis for future work directed at understanding be- havior, resource partitioning and species interactions among native Hawaiian stream fishes.

Materials and methods

Site description

The investigation was conducted in the Wainiha River which discharges into the ocean on the northern shore of the Hawaiian island of Kaua’i (Fig. 1). Fed by the Alaka’i Swamp atop the ancient caldera of Mt. Wai’ale’ale, this fourth order stream de- scends about 21 km to the ocean from an elevation of 610 m. Rainfall on this mountain can exceed 15 m annually (Stearns 1985). Mean daily discharge for calendar year 1992 measured by a gauge at 300 m elevation was 3.5 m3 see-’ with a ‘highest daily mean’ of 39.1 m3 se? (11 September 1992) and a ‘lowest daily mean’ of 0.93 m3 se& (31 May 1992) (R. Taogoshi personal communication). A hydro- electric diversion weir at ca. 213 m elevation diverts water through a penstock to a power plant at ca. 30 m elevation where water is returned to the stream. Two fish collection sites below the diversion weir were used (Fig. l), each was ca. 100 m long and consisted of riffle-run habitat with stretches of large exposed boulders. The sites were separated by a distance of about 4 km; the ‘lower site’ was found at an elevation of ca. 37 m and the ‘upper site’ at ca. 116 m.

Sampling methodology and analyses

Sicyopterus stimpsoni were collected monthly from

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mid-February 1992 to late January 1993 at the two study sites with electrofishing gear. Fish were an- esthetized in the field with MS-222 (tricaine me- thanesulphonate), measured for standard length, weighed to the nearest 0.1 g and preserved in 10% buffered formalin. Fish collection locations within each study site were chosen randomly and fish were retained only if three or more individuals were cap- tured simultaneously at a particular locality. Each capture location was then identified with a weight- ed ribbon for subsequent bottom sampling. Fish were identified by capture location with individual tags.

To estimate the availability (biomass) of food resources within-site, bottom sampling was initiated after fish capture was completed. Over each flagged capture location, a one square meter grid was su- perimposed delineating nine 0.111 m2 subunits. From pre-generated random numbers (one to nine), two squares were selected and sampled using a standard Surber-type sampler (250 urn mesh) (Surber 1937). One Surber sample (0.093 m2) was collected in each of the two designated squares. The biotic material scraped from the substrate with a stiff brush was washed into a nitex fabric square (21 cm x 21 cm x 0.2 mm mesh) and subsequently stored in 10% buffered formalin.

In the laboratory, fish were examined for morphological features related to feeding (Lowe-McCon- nell 1978). Digestive systems from esophagus to anus were removed along with the gonads and stored separately in 10% buffered formalin. Go- nads were weighed to the nearest 0.001 g. The digestive system was examined for arrangement of folds and measured for total length. From these measurements, gut length to body length ratios and gonadosomatic indices (GSI) for the ratio between wet go- nad weight to body weight were calculated. GSI values were used to estimate length at which fish at- tained sexual maturity. Fish digestive systems were subsequently dissected and the contents separated into animal and plant food portions. Animal foods were sorted, identified to lowest possible taxonomic category, dried at 60” C for 48 h and weighed to the nearest 0.0001 g.

Composition and abundance of plant food items in gut samples are difficult variables to quantify be-

cause of the presence of both digested and undigested fractions. Plant material from the gut was dilut- ed in water in a petri dish (3.5 cm diameter) and examined under 104 x magnification of a dissecting microscope (Leica M37). Taxonomic composition (to lowest possible taxonomic category) and percent coverage of algal species in the undigested fraction was estimated through ten random scans (about 10 mm2 each) of the petri dish. From the digested fraction, six random samples were taken with a pipette (about 0.1 ml each) and mounted on slides for scanning at 400 x magnification under a compound microscope (Leica Laborlux S). Taxo- nomic composition and percent algal species coverage in the digested fraction was estimated through at least three random transects varying in length in each of the six samples (Peterson & Ste- venson 1992). The total plant food sample was subsequently dried at 60” C for 48 h and weighed to the nearest 0.0001 g. Estimates of dry biomass of algal food categories was obtained by multiplying percent coverages by total weights for digested and undigested fractions.

Statistical analysis

Diet data (dry biomass in grams) were standardized by dividing food weight by fish length (g biomass/ mm standard length) and normalized with a log(x + I) transformation. Analysis of variance (ANOVA) (GLM procedure, SAS Institute Inc., 1992) was used to evaluate for significant differences in overall quantities of animal and plant foods eaten. Sex was included in the ANOVA model to determine if males and females differed in overall diet. Diet differences within specific food groups were evaluated through ANOVA (including Site in the statistical model) to determine if significant site specific differences existed in foods consumed. Seasonal differences in diet were evaluated by sorting the data- set into seasonal categories (October to January, February to May, June to September) and applying ANOVA to test for differences in biomasses of food groups consumed between season. Temporal trends in diet were examined by plotting means of per cent abundances by month for each site. Diet data were

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re-sorted into three size classes (< 45 mm, 46 mm to 95 mm, and > 96 mm} and ANOVA performed to determine if differences in diet existed between immature and adult fish or between individuals of different size classes. A Pearson correlation coefficient (Proc Corr procedure, SAS Institute Inc., 1992) was used to evaluate the relationship between quantities of aquatic insect species consumed and algal species abundance in the gut. A similar procedure was used to evaluate the relationship between quantities of algal species consumed over the study period. The size-class groupings were used to evaluate the within-species relationship of gut length to body length and sex through ANOVA. Means were separated at p c 0.05 by using the Duncan option (GLM procedure, SAS Institute Inc., 1992) and in all cases only untransformed means are reported.

For the evaluation of resource availability, biotic benthic material collected with Surber samplers was processed as described above. The random scanning procedure used for the undigested gut fraction was applied to portions of benthic samples which were unidentifiable under 104 x magnification. Total dried weights were multiplied by percent

coverage values to yield biomass estimates (in grams) by food category. Percent abundance, used for estimation of availability of food items in the fish’s habitat, was calculated by dividing food item weight by total sample weight. The nonparametric Spearman coefficient was used for all relationship comparisons of per cent abundances (Proc Corr procedure, SAS Institute Inc., 1992).

Diet and selectivity indices

To compare the numbers of S. stimpsoni feeding on a particular food, a frequency of occurrence index (% F) (Hynes 1950) was calcualted which gives the sample proportion of individuals in the population having a particular food item in the gut. The relative importance of a food item in the diet of S. stimpsoni is expressed as a percentage of its dry weight to total dry weight of all foods found in the gut (% DW) (Zander 1982). Percent abundances of food items in the gut were used in comparisons of availability of these items in the fish’s habitat.

For comparisons of selection (Manly et al. 1993)

Table I. Food items from gut content analysis of 192 S. stimpsoni collected in Wainiha River, Kauai (% F = frequency of occurrence of food item in all fish collected; % DW = percentage of dry weight of food item to total dry biomass of all foods in gut).

Food group % F % DW Food group % F % DW

Animal foods Insecta

Diptera Chironomidae

Cricotopus bicinctus Microspectra spp.

Canaceidae Proconace spp.

Ephydridae Scatella spp.

Empididae Hemerdromia stellaris

Tipulidae (Limonia spp.) Tricoptera

Cheumatopsyche pettiti (Hydropsychidae)

Hydroptila arctia (Hydroptihdae)

Hydracarina (Hydrachnidae) Hydrozetes sp.

93.8 5.38 93.8 5.37 93.8 5.25 93.8 5.12

48.4 0.08

8.3 0.01

29.2 0.03

5.2 0.01 38.0 0.12 15.6 0.01

38.0 0.11

29.2 0.01

Plant foods 100.0 94.62 Chlorophyta 91.7 17.83

Cladophora sp. 71.9 13.28 Rhizoclonium sp. 5.2 0.44 Ulothrix sp. 7.3 0.14 Oedogonium sp. 2.1 0.17 Spirogyra sp. 67.7 3.80

Cyanophyta 87.5 22.56 OscillatoriaiSymploca/ 18.2 3.00

Phormidium sp. Nostoc sp. 21.9 1.11 Rivularia sp. 67.2 18.45

Chrysophyta 99.0 54.17 Hydrosera sp. (Centrales) 17.2 4.89 Pennales (Fragilaria) 95.8 49.28

SynedralNivaculalCymbella /Gomphosepatum/Nitzschia/Rhopalodia/ Eunotia/Gomphoneis sp.)

Miscellaneous (eggs, etc.) 6.8 0.06

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and preference (Johnson 1980) three indices were utilized. The ‘population proportion’ of resource units available in the habitat (n,) is equivalent to the per cent abundance of food items obtained through Surber sampling of the stream bottom. The ‘used sample proportion’ of resource units utilized by fish (oi) equals the per cent abundance of food items found in the gut. The ‘selection index’ (wi) represents the ratio of used to available resource units (o/u;). The ‘standardized index’ (Bi) present the selection indices in standardized form so that they add to 1. This index (Bj) gives ‘the estimated probability that a randomly selected resource unit will be in category i if all categories are equally frequent in the original population of available resource units’ (Manly et al. 1993). A chi-squared statistic with one degree of freedom was used to test for selection of available food items against a hypothesis of no selection (Manly et al. 1993). A non-significant chi- square test would indicate that animals were randomly selecting food items in proportion to their availability in the habitat.

Results

In total 193 S stimpsoni were collected from the two study sites. For the 93 fish obtained from the lower site (24% male, 66% female, 10% juveniles), mean standard length (in mm units) was 75.44 + 2.930 for males (range 51-198) 79.27 + 2.733 for females (range 29-130) and 29.78 It 1.977 for juveniles (range U-39). In the upper site, 100 S. stimpsoni were collected (40% male, 53% female, 7% juveniles). Mean standard length (in mm units) was 73.4 + 2.197 for males (range 42-105) 84.30 + 2.661 for females (range 38-ill), and 33.14 + 2.405 for juveniles (range 26-41). Only one individual from the upper site had an empty gut and was removed from the diet analysis. Elevated gonadosomatic indices (GSI) for females (GSI > 14.9) and males (GSI > 0.30) declined in early March 1992, rose again in late August and peaked through the end of the study in January 1993. These periods of elevated GSI values delineated periods of spawning for the species in Wainiha River. Since no individuals < 45 mm standard length had elevated GSI values, S stimpsani

Cbimnomidm

Fig. 2. Comparison of standardized mean weights (dry biomass g per mm fish length) of major foods found in the gut of Sicyopre- rus stimpsoni collected from Wainiha River, Kauai. (’ Chloro- phyta, ’ Chrysophyta, ’ Cyanophyta) (Oscillatoriaceae = Sym- ploca sp., Phormidium sp. & Oscillatoria sp.; Chironomidae = Cricotopus bicinctus & Microspectra spp.)

> 46 mm were assumed to be reproductively mature.

Diet

Based on the results of gut content analysis, significant differences in the types of foods consumed (by biomass) were found (df = 2,F = 512.43, p = 0.0001). Algae was determined to be the primary food of S. stimpsoni (Table 1). It comprised 94.62% of the fish’s diet by biomass, was consumed in significantly greater quantities than animal foods (p < 0.05) and was found in the gut of all individuals sampled. Ani- mal foods, in comparison, only contributed 5.38% to the total diet although they occurred in 93.8% of the fish sampled. No significant difference in foods consumed were found between male and female fish (df = 3, F = 1.18, p = 0.3177) or between different size classes of fish (df = 6, F = 1.15, p = 0.3331).

For algal foods, significant differences were found in the abundances of major taxa (df = 3, F =

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43.95, p = 0.0001) (Fig. 2). Significantly greater Immature aquatic insects were the primary ani- quantities of chrysophytes (diatoms) as compared mal foods of S. stimpsoni and accounted for 99.8% to cyanophytes (blue-gree algae) or chlorophytes of animal biomass. Biomass of chironomid larvae (green algae) were found in S. stimpsoni’s gut (p < was significantly greater (p < 0.05) than any other 0.05). Algae from five genera of Chlorophyta in animal food yet contributed only 5.12% to total dry four families (Ulotrichaceae, Oedogoniaceae, Cla- biomass (Table 1) (Fig. 2). Two chironomid species, dophoraceae, and Zygnemataceae), five genera of one alien (Cricotopus bicinctus (Meigen)) and the Cyanophyta in three families (Oscillatoriaceae, other probably native (Micropsectra spp.), were Nostocaceae, and Rivulariaceae), and nine genera found in gut samples of S. stimpsoni. Two other na- of Chrysophyta in nine families (Biddulphiaceae, tive aquatic insect species (Scatella spp. and Proca- Fragilariaceae, Eunotiaceae, Naviculaceae, Gom- nace spp.) were found in the fish’s diet but contrib- phonemaceae, Cymbellaceae, Epithemiaceae, uted little to total food biomass (0.09% DW). Chi- Nitzschiaceae, and Surirellaceae) were found in the ronomids were selected for food more often by S. diet of S. stimpsoni (Table 1). The three most signif- stimpsoni than any other stream invertebrate; how- icant algal foods (p < 0.05) found in the diet overall ever this is more likely indicative of their high abun- (81.01 % DW combined) were pennate diatoms dance in Hawaiian stream habitat rather than their (Pennales), the cyanophyte Rivularia sp. and the ‘preferred food’ status (Table 2). No native aquatic chlorophyte Cladophora sp., respectively (Fig. 2). moths (Hyposmocoma sp.) or introduced turbellar- Pennate diatom abundance in the gut of S. stimpso- ian worms (Dugesia sp.) were found in S. stimpsoni’s ni was positively correlated with that of Cfadophora diet despite their presence in benthic samples. No sp. (r = 0.30, p = 0.0001). significant correlation was found between the

Table 2. Relative probabilities of selection (Manly et al. 1993) for plant and animal foods of Sicyopterus stimpsoni. Animal and plant food categories are treated separately in calculating indices.

Food category Population proportion

Used sample Selection index Standardized Chi square proportion (0,) (w,) index (B,) value (df = 1)

(+ p c 0.05)

Plant foods Cladophora sp. 0.4553 0.2138 0.4695 0.0242 + 4.5600 Rhizoclonium sp. 0.0082 0.0073 0.8877 0.0458 0.0020 Ulothrix sp. 0.0009 0.0002 0.2427 0.0125 0.0100 Oedogonium sp. 0.0061 0.0000 0.0000 0.0000 0.0000 Spirogyra sp. 0.0253 0.0412 1.6325 0.0842 0.2009 Oscillatoria sp.lSymploca sp.lPhormidium sp. 0.1188 0.0436 0.3671 0.0189 1.0467 Nostoc sp. 0.0566 0.0085 0.1505 0.0078 0.8389 Rivularia sp. 0.1613 0.1289 0.7989 0.0412 0.1508 Anabaena sp. 0.0008 o.oooo 0.0000 0.0000 0.0000 Diatoms 0.1581 0.5235 3.3119 0.1709 + 19.4477 Hydrosera spp. 0.0029 0.0331 11.5206 0.5944 + 6.1733

Animal foods Chironomidae 0.2026 0.9731 4.8024 0.5452 2.1912 Scatella (Ephydridae) spp. 0.0052 0.0003 0.0521 0.0059 0.0028 H. stellaris (Empididae) 0.0098 0.0034 0.3464 0.0393 0.0025 Procanace (Canacidae) spp. 0.0070 0.0088 1.2498 0.1419 0.0003 C. pettiti (Hydropsychidae) 0.6119 0.0017 0.0029 0.0003 0.9351 H. arctia (Hydroptilidae) 0.0988 0.0122 0.1232 0.0140 0.0503 Limonia (Tipuhdae) spp. 0.0002 o.ooo5 2.2315 0.2533 0.0002 Hyposmocoma spp. (Cosmopterigidae) 0.0089 0.0000 o.oooo 0.0000 0.0000 Dugesia sp. (Planariidae) 0.0555 0.0000 0.0000 0.0000 0.0000

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Jung2 Jut hg OCI NOV 0.x Jon93 J”n92 Jul Wg =P Oe, NO” ._

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. . . . . . . Available Diet

Fig. 3. Monthly comparisons of mean per cent abundances for algal food availability vs. presence in the diet of Sicyopterus sfimpsoni. Dashed lines = per cent abundance in habitat; Solid lines = per cent abundance in diet.

206

abundance of aquatic insect larvae in the fish’s diet and the abundance of any algal species.

and r = - 0.576, p = 0.0001, respectively) and lower site (r = - 0.670, p = 0.0001 and r = - 0.671, p = 0.0001, respectively).

Food availability Food selection

Significant seasonal (df = 18, F = 7.75, p = 0.0001) and site (df = 9, F = 4.45, p = 0.0001) differences in quantities of algal groups consumed were detected over the twelve month study period in Wainiha Riv- er (Fig. 3). These diet differences reflect (to varying degrees) concomitant changes in availability of these foods in the fish’s habitat. A decline in the abundance of the cyanophyte Rivularia sp. in the diet of S. stimpsoni from early spring through sum- mer to low levels in the fall and winter was correlated with a similar decline in abundance of Rivularia sp. in the benthos of both lower (r = 0.683, p = 0.062) and upper sites (r = 0.634, p = 0.091). During this same period, a sharp rise in the abundance of Cladophora sp. occurred in both sites; however a correlated rise in this algal species in the diet of S. stimpsoni was only significant for fish from the lower site (r = 0.691, p = 0.058). Except for Spirogyra sp. in the lower site (r = 0.805, p = 0.016) no other significant correlations were determined for algal abundance in diet and habitat; however diet composition followed algal availability during the study period. A weakly negative correlation was found between Cladophora sp. and Rivularia sp. abundance in the diet of S. stimpsoni; however the relationship was not determined to be significant (r = - 0.08, p = 0.2944). The greater proportionate abundance of diatoms in the fish’s gut as compared to its habitat was due to the presence of diatom species epiphytic on chlorophytes and cyanophytes (e.g. Cymbella sp. and Gomphosepatum sp.). Abun- dance of these epiphytic chrysophytes was difficult to assess in the analysis of benthic samples.

Percent abundance of Cladophora sp. in the benthos of both study sites was negatively correlated over the study period with that of other algal groups and suggests that the marked rise in abundance described earlier was associated with abundance declines in other species. This negative correlation was most significant for Rivularia sp. and Nostoc sp. abundance in both upper (r = - 0.625, p = 0.0001

The resource selection indices for plant and animal foods of S. stimpsoni (Table 2) indicate that for most foods, selection is random and made according to availability. Rivufaria sp., for example, which was the single most abundant algal species in the gut overall (Table l), was selected with equal probability as Rhizoclonium sp. which only contributed 0.44% to total food biomass (Table 2). Other important algal foods such as Spyrogyra sp., Nostoc sp., and the Oscillatoriaceae were also selected randomly. Availability of food resources therefore ex- erted considerable influence in S. stimpsoni’s search for food in the two sites sampled. Radical declines in resource abundance, as noted earlier for Rivula- ria sp., limits access to preferred foods and forces animals to utilize more abundant resources. In the case of S. stimpsoni, Cladophora sp. was preferred; however Rivularia sp. was twice as likely to be chosen as Cladophora sp. A determination of preference is therefore equivocal because of this discre- pancy and their opposing differences in abundance occurring simultaneously in both sites (Fig. 3). The two resources may be equally preferred but selected based on relative abundance in the habitat. Strong evidence of preference is only demonstrated for pennate diatoms and the filamentous diatom Hydrosera sp. which was the only centric species found in S. stimpsoni’s gut (Table 2).

Morphometrics

The morphological feeding traits of S. stimpsoni corroborate the herbivorous habits indicated by the composition of its diet. Its poorly developed gill raker system, ventro-terminal mouth, and rows of tricuspid teeth for scraping algae are typical of herbivorous fishes. High gut-to-length ratios found for S. stimpsoni provide still more evidence of adaptation to herbivory. Mean gut length to standard

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length ratios for 180 individuals was 2.73 + 0.041 (range 1.12 to 4.01). Significant differences in mean ratios were found between size classes (df = 2, F = 6.278, p = 0.0023) but not between male and female fish. Large fish (> 96 mm standard length) had significantly larger ratios (longer gut length) (mean = 3.02 + 0.078) and juvenile fish (< 0.45 mm standard length) significantly smaller ratios (shorter gut length) (mean = 2.025 + 0.115) than intermediate- length adult fish (46 to 95 mm standard length) (mean = 2.78 + 0.042) (p < 0.05). As indicated earlier, however, no significant diet differences were associated with differences in fish size.

Discussion

The ecological role of S. stimpsoni as a nearly exclusive algal grazer in Hawaiian streams is clearly es- tablished. Despite its greater specialization for herbivory, which separates it morphologically from other native swift-water gobiods (M. Kido unpublished), Connell’s (1980) ghosts may not have been able to scare away interspecific competition for food in Hawaiian streams. Extensive use of algal foods (particularly Cludophora sp.), for example, has been shown for A. guamensis (Kido et al. 1993) as well as other native freshwater fishes (M. Kido unpublished); therefore competition for algae may be an important feature of biotic interactions in Ha- waiian streams. In the sites sampled, Cludophoru sp. was the predominant alga comprising 45.5% of total available food biomass. For S. stimpsoni, this alga was an important food which was selected non- randomly from its habitat and was positively correlated in per cent abundance in the benthos and in the diet of fish from the lower site. Several pennate diatom species (e.g. Gomphosepatum sp., Cymbella sp., and Gomphoneis sp.), often densely epiphytic on Cludophora sp., were also important components in the diet of S. stimpsoni. A significant correlation identified Cladophora sp. as an important source of these epiphytic diatoms. This alga likely represents a high-energy nutrient source available to Hawaiian stream fishes.

Differential resource selection is a mechanism which allows species to coexist (Rosenzweig 1981)

and may serve to reduce interspecific competition for food in Hawaiian streams. Other stream gobioids, for example, exhibit stronger preference for drifting and benthos-dwelling invertebrates (M. Ki- do unpublished) than does S. stimpsoni which spe- cializes in a diverse array of swift-water algal species. Cyanophytes and chrysophytes, algae not known to be important foods of other Hawaiian gobioids, were extensively exploited by S. stimpsoni. Species of pennate diatoms, for example, free-living in mats of cyanophytes or tufts of mucilage attached to rocks were highly preferred and important contributors to total food biomass, Second in overall abundance in the diet was Rivulariu sp., a cyanophyte which forms a dark-green crust on the surface of basalt substrate. Significant correlations of declining Rivularia sp. abundance in the benthos of both study sites with that in the diet, illustrates S. stimpsoni’s propensity to follow this resource closely.

Patchiness in the food resource base induced by stochastic disturbance events (Reice 1994) may provide the landscape in which differential resource selection is employed. These flow-related disturbances vary in intensity, occur periodically and charac- terize the hydrology of Hawaiian streams; yet little is known about their influence on Hawaiian stream communities. Two significant physical events occurred in Wainiha River during this diet study which provided the fortuitous opportunity to study disturbance effects on food availability. On 11 Sep- tember 1992, a Category V hurricane devastated the island of Kauai. High winds denuded nearly all ri- parian vegetation from the river’s banks and de- stroyed the power plant in Wainiha, forcing closure of the penstock. As a result, natural flows over the diversion weir were restored to previously dewa- tered reaches until mid-September of 1993. With the return of water, it was estimated that stream width, wetted cross sectional area and mean depth of stream habitat immediately below the weir increased by about 42.4%, 88.6% and 79.8%, respectively (M. Kido unpublished). The first significant spate (22.7 m3 set-‘) in Wainiha River after the hurricane occurred in mid-October 1992. Eight subsequent floods occurred in December 1992 (ranging in discharge from 15.9 m’ set-’ to 28.6 m” se&) (R. Taogoshi personal communication).

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The components of S. stimpsoni’s algal food base responded differently to these disturbance events. The abundance of Cladophora sp. increased visibly in Wainiha River and was recorded in the sampling of the benthos in the two study sites (Fig. 3). As its abundance in the benthos was on the rise prior to the storm, the increase may be partially seasonal; however, increases in flow, available habitat, and light levels on the water’s surface likely resulted in the attainment of unusually high resource levels. A similar but opposing scenario can be described for Rivularia sp. which was declining in abundance be- fore the storm and reached extremely low abundance levels prior to the large spate occurring in Oc- tober (Fig. 3). Declines in per cent abundances of most algal species were correlated with the rise in the abundance of Cladophora sp. in both sites and was highly significant for the cyanophytes Rivularia sp. and Nostoc sp. In its response to improved environmental conditions, Cladophora sp. may be li- kened to Connell’s (1978) ‘resident species’ becom- ing the superior competitor perhaps not so much because of its competitive abilities but by changes induced through environmental disturbance (Hus- ton 1979). Declines in Cladophora sp. abundances after the October spate (Fig. 3) illustrates the role of catastrophic disturbance in initiating population re- ductions (Reice et al. 1990) and the re-setting of the stream’s benthic landscape. Temporal change observed in the algal communities of Hawaiian streams may indeed be induced by periodic flood disturbance with subsequent re-structuring through ‘nonequilibrium dynamics’ (Reice 1994). This hypothesis begs to be falsified.

Sicyopterus stimpsoni reacted to these changing algal resource levels and patchiness in its resource base by increased selection (p < 0.05) of the most abundant resource (Cladophoru sp.) with concomitant random selection of other algal foods (e.g. Rhizoclonium sp., Spirogyra sp., Nostoc sp., and the Oscillatoriaceae) based on availability. The shift in diet from Rivularia sp. to Cladophora sp. demon- strates the ability of S. stimpsoni to respond to rapid change in food availability and it is clear that the species is well adapted to life in a disturbance-influenced environment. During the course of this study, disturbance regimes were important factors influ-

encing the trophic dynamics of S. stimpsoni and it is by having a diverse food resource base that rapid adjustments to abundance level fluctuations are ac- commodated. Preference for a narrow range of foods would clearly not be adaptive in such an environment. If selection of algal species is based primarily on availability, then food selection by S. stimpsoni may ultimately be governed by disturbance influences. Species which are better able to re- sist or recover from disturbance will be more available and therefore more likely to be selected as food by fishes. The high resilience of diatom assem- blages to floods and their quick recovery from spates (Peterson & Stevenson 1992) may then part- ly explain S. stimpsoni’s preference for it as staple food source.

The gobioid species native to moutainous Hawai- ian streams inhabit a torrential paradise where the diversity of naturally available foods has been limited by insular evolution. Variation in stream flow ranging from drought to catastrophic flood imposes dynamic patterns of environmental change which influences the abundance, composition, and distri- bution of these benthic resources. Understanding disturbance regimes (Pickett & White 1985) and patch dynamics (Pickett & Thompson 1978) within Hawaiian streams is an important area for future study. Comprehending these processes in relation to the forces affecting individuals and their interactions at different scales (Levin 1992) is an important goal of ecology and vital to effective management of these ecosystems. The specter of ever increasing human-induced disturbances to Hawaiian streams adds urgency to this need.

Acknowledgments

This study was made possible through program sup- port for environmental research in the Hawaiian Is- lands from the Agricultural Research Service, U.S. Department of Agriculture and through a grant from the Division of Aquatic Resources, State of Hawaii, Department of Land and Natural Re- sources, award number LOlO-PO5910. Much appre- ciation is extended to Mike Fitzsimons and Adam Asquith for their critical reviews and suggestions

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for improving the manuscript. I am also immensely appreciative of expert assistance from Isabella Ab- bott and Tom Iwai in identification of algal species. My gratitude also to Melissa Pacheco-Agan for her dedicated assistance in the field and laboratory.

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Diet and food selection in the endemic Hawaiian amphidromous goby, Sicyopterus stimpsoni (Pisces: Gobiidae)