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Little is known about microhabitat use by Caribbean parrotfishes soon after settlement from the plankton. We monitored the abundance and distribution of recently settled Sparisoma spp. (likely mostly S. viride with some S. aurofrenatum) on 6 × 6 m quadrats (n = 9) on the fringing reef front in Barbados (West Indies) every 15 d for 6 mo. Each quadrat was divided into 1 m2 cells to examine associations between substrate composition and recruit (Stegastes diencaeus and threespot damselfish S. planifrons. Whereas Sparisoma recruits overlapped with longfins and to a lesser extent with threespots, small juveniles overlapped only with threespots. Some of the microhabitat associations with recruits and small juveniles at the cell scale were also found across quadrats. We suggest that the reef front is an important habitat for recently settled Sparisoma in Barbados, where they rely on being cryptic and solitary within turf algae gardens defended by damselfishes to minimize predation. After some growth, they perform a microhabitat shift, perhaps due to increased damselfish aggression. Thus, the early post-settlement dynamics of Caribbean Sparisoma might be strongly influenced by substrate modification by algal-gardening damselfishes.

Studies of coral reef fish recruitment have focused more on factors influencing the return of larvae to reefs and the process of settlement than on factors affecting survival in the plank- ton, despite the well-documented importance of the larval environment for temperate marine spe- cies. We examined the ability of environmental factors to explain temporal patterns of recruitment in 8 taxa of coral reef fishes, distinguishing variables associated with settlement from those associated with the pelagic larval stage. At 10 d intervals over a 17 mo period, we collected newly settled fishes from settlement units in Barbados (West Indies). We collected concurrent data on 3 settlement- associated variables, lunar darkness, wind speed, and tidal amplitude, and 5 larval environment vari- ables, sea-surface temperature, cloud cover, salinity, chlorophyll a concentration, and normalized water-leaving radiance at 555 nm. The last 3 were associated with low-salinity intrusions originating in the Amazon. We used hierarchical partitioning and Mallow's C(p) model selection to identify important variables for each taxon. Temporal patterns of recruitment differed among taxa of which 6 showed significant environmental correlations. Significant settlement variables included lunar darkness and tidal amplitude. Significant larval environment variables were diverse and varied in magnitude and sign across taxa. The effects of larval variables were generally strong relative to settlement variables. The temporal scale of the strongest environmental predictor differed across taxa. Our study supports the views that planktonic factors affecting larval survival substantially influ- ence recruitment in coral reef fishes and that different species are affected differently by a common, temporally variable, planktonic environment.

We monitored the recruitment of 8 fish taxa at 10 d intervals for 1 yr on 5 fringing coral reefs along a 15 km stretch of the west coast of Barbados, using benthic standard monitoring units of recruitment of fishes (SMURFs). SMURFs are designed to sample taxa that settle onto coral rubble, including cryptic species, and to minimize post-settlement predation. We deployed SMURFs in reef habitat to separate meso-scale processes from the small-scale influences of benthic habitat. Recruit- ment of most taxa occurred year-round, but was highly variable, with on average more than 50% of recruits collected on only 14% of the sampling dates, indicating the importance of frequent sampling. Temporal patterns within taxa were concordant across reefs, indicating coast-wide synchronous set- tlement, but differed among taxa. Within reefs, 2 taxa settled more frequently on SMURFs sur- rounded by non-preferred substrate, 1 exhibited the opposite pattern, and the remaining 5 showed no substrate associations. Recruitment to individual SMURFs was seldom correlated among taxa. Among reefs, recruitment of 4 taxa was consistently lower on the central coast and higher at the north and south ends, consistent with a previously identified tidally induced flow regime that reduces onshore larval transport near the center of the west coast; the remaining taxa exhibited other spatial patterns, challenging the generality of the role of hydrodynamics. Overall, this study expands the tax- onomic range of recruitment studies, shows spatially coherent (over kilometers) but temporally vari- able (over tens of days) recruitment and reveals substantial diversity in patterns among concurrently settling taxa.

We examined the diurnal refuges occu- pied by the nocturnal squirrelfish, Holocentrus rufus, to describe refuges and the behavior associated with their use and to determine which, if any, refuge characteristics were selected. We tagged 21 H. rufus on two sites on a fringing reef in Barbados, West Indies, identified the refuges they used (n = 57), measured ten characteristics of each refuge and the surrounding microhabitat, and monitored their refuge use for 4 weeks. To evaluate refuge selection, we measured the same characteristics on a comparable number of unused potential refuges (n = 67) on the same reefs and used classification tree models to determine which characteristics separated used from unused refuges. Each fish used 1-9 refuges, which did not overlap among individuals and were defended against intrusion by conspecifics and some hetero- specifics. Fish with more than one refuge frequently moved among them. There was strong site fidelity with no immigration of untagged fish or emigration of tagged fish on either reef during the study period and no additional refuges being occupied over the 4-week period. Refuges were primarily holes, open at one or two ends, which varied in size, distance from the reef edge, entrance orientation, and vertical relief at the entrance. Holes used as refuges differed significantly from unused holes mainly in characteristics related to the vertical position of their entrance, but the classification tree models differed for the two sites. This study provides the first detailed information on characteristics of daytime refuges used by a noctur- nally active reef fish and the first evidence of selectivity of refuges. It suggests that the abundance and characteristics of holes on reefs could influence the density of H. rufus on natural reefs.

We developed a benthic standard monitoring unit for recruitment of fishes (SMURF) to sample fishes that settle in coral rubble and tested it on three fringing reefs on the west coast of Barbados, West Indies. These rubble SMURFs are inexpensive to construct and permit newly settled fishes, including cryptic, secretive and small species, to be quickly and fairly completely (> 83%) removed by divers without returning the unit to the surface. Over a 3-month period involving 9 collections, 32 units on 3 reefs yielded 948 newly settled fishes belonging to 28 taxa (3.3 ± 0.2 S.E. recruits per sample, n = 287) in addition to numerous crustaceans. Most specimens were Sparisoma spp. (Scaridae) (41.8%) and Stegastes partitus (Pomacentridae) (13.6%), with moderate numbers of Scarus spp. (Scaridae) (8.6%), Lythrypnus spp. (Gobiidae) (7.4%), and Pseudogramma gregoryi (Serranidae) (7.2%). Sampling SMURFs at 1-day rather than 11-day intervals yielded approximately twice as many fish, although some taxa were not affected by sampling frequency, indicating taxon-specific differences in post-settlement loss rates. Netting, intended to reduce predation on settlers, did not affect settlement estimates unless algae grew on it, suggesting that the SMURF design minimizes fish predation even in the absence of the netting. Over time, however, the netting resulted in substantial algal growth that was associated with an increase in abundance of Sparisoma spp. and a decrease in the abundance of S. partitus . This suggests that settler estimates are more affected by microhabitat changes from algal growth than by predation. During low settlement periods, SMURFs performed better than light-traps in assessing the daily input of new settlers of two abundant taxa. Eight SMURFs per reef were sufficient to demonstrate differences in settlement rates and temporal correlations in settlement within and among reefs. This study underscores the potential of benthic SMURFs to measure settlement of coral reef fishes at a range of spatio-temporal scales with moderate research cost and effort, and to facilitate the study of the settlement stage of several taxa.

We developed a modified visual census technique suitable for fairly sedentary, hole-dwelling species and used it to determine the diversity, density, biomass, and habitat use of morays at four sites including fringing, patch, and bank reefs in Barbados, West Indies. The method involved a careful search during two passages over transects by day and by night, noting size and position of morays seen, and using the number of new observations on the second passage to estimate the proportion of morays not seen. First passage counts were generally higher than numbers of morays recorded in previous visual censuses. These estimates were raised when we used the time period (day or night) with the highest average density visible for each species, added different individuals from the two passages, and corrected for individuals not seen. We saw more goldentail morays (Gymnothorax miliaris) during the day but more spotted (G. moringa), viper (Enchelycore nigricans), and chestnut (E. carychroa) morays at night. Counts of chain morays (Echidna catenata) did not differ significantly between night and day. Relative abundance and size of moray species varied among sites and to a lesser extent among habitat types within sites, but spotted morays were generally the most important in numbers and biomass. Mean estimated moray density and biomass were much higher than in most previous visual censuses, but similar to rotenone samples on other reefs and comparable to those of other families of predatory fishes. Our findings confirm that morays are abundant and potentially important predators that should be the focus of more attention in ecological studies of coral reefs.

We examined the abundance of blue tang surgeonfish, Acanthurus coeruleus, in each of three social modes (schooling, territorial, and wandering) in relation to size class, ecological variables, population density and time of day to discern potential determinants of social organization. We found individuals from all three social modes in all four fringing reef habitats (back reef, flat, crest and spur and groove zones) at our main site. Territorial tang density was highest in the flat zone, lowest in the spur and groove zone and negatively related to adult damselfish density. A higher proportion of tangs formed schools in reef zones with the highest densities of territorial damselfishes (spur and groove, crest). In the back reef and flat zones, where damselfish densities were very low, tangs rarely formed schools. Tangs in the wandering mode were most abundant in the back reef. The density of territorial tangs did not change with time of day, but non-territorial tangs tended to wander more in the morning and to form schools more at midday. Small yellow-phase juveniles were always territorial, and the incidence of territoriality decreased in the larger size classes, while schooling and wandering increased. Among similar fringing reefs, the incidence of territoriality increased with increasing population density. These data suggest that life history stage, damselfish density, and conspecific population density are important determinants of blue tang social organization.

Marine reef fishes may spend a significant pro- portion of their daily activity budgets visiting cleaning sta- tions. Nevertheless, the roles of ectoparasites and of inter- actions with cleaners in shaping patterns of diel activity in these fishes have been largely ignored, possibly because the importance of cleaning for the hosts remains controversial. In diurnal species, a peak in ectoparasite infestation exists at dawn, suggesting that higher rates of cleaner-seeking dur- ing this time should be beneficial to hosts. In this study, we quantified the diel pattern of visitations to cleaners, as well as other major activities, in female yellowtail damselfish Microspathodon chrysurus on a Barbadian fringing reef. Females of this species leave their territories near dawn at 3 d intervals to spawn on male territories. On non-spawning days, cleaning peaked at dawn, when fish spent >20% of their time with cleaners, and declined thereafter, averaging over 11% of the time budget during the entire day. Supple- mental observations in Puerto Rico and the Virgin Islands also revealed dawn peaks in cleaner-seeking activity. Feed- ing and territory defense together occupied less than 3% of the time budget at dawn, and never seemed frequent enough to interfere with cleaning. On spawning days, 30% of females visited cleaners at male nests or while traveling back to their territories, and 24% visited cleaners on their own territories during revisitation trips before spawning was completed; nevertheless, they spent < 1 ⁄ as much time with cleaners as did non-spawning females. Reduced cleaner- seeking indicates that spawning competes with cleaner visitation in damselfish time budgets, producing a tradeoff between cleaning and reproduction. In the 30 min after spawning, females significantly increased their rate of inter- action with cleaners, compared with the same time on non- spawning days, and regained about 68% of the missed time with cleaners; feeding and aggression did not differ between spawning and non-spawning females. Compensatory post- spawning visiting of cleaners supports the hypothesis that cleaning at dawn is important, probably because of negative consequences of the dawn peak in parasite loads.

We studied territoriality and habitat use by yellow phase juvenile blue tangs, Acanthurus coeruleus, on a small fringing reef in Barbados, West Indies. Juvenile blue tangs occurred on the reef crest, spurs, and a transition zone between the reef crest and reef flat at a density of about 8 individuals per 100 m2, but were much rarer on the reef flat. They were solitary and occupied stable home ranges (median=0.85 m2) that increased with body size. Observational and experimental data documented aggressive defense of home ranges against conspecific and to a lesser extent congeneric, A. bahianus, juveniles (about 7.5 approaches and attacks per hour directed at intruders). Home range locations were structurally more complex and closer to a vertical face than expected by chance. Although juvenile blue tang territories overlapped considerably with those of larger and more aggressive Stegastes damselfish, which are believed to exclude solitary adult Acanthurus spp. from reef crest and spurs, the tangs avoided Stegastes and were rarely chased (

Movement of coral reef fishes across marine reserve boundaries subsequent to their initial settlement from the plankton will affect the ability of no-take reserves to conserve stocks and to benefit adjacent fisheries. However, the mobility of most exploited reef species is poorly known. We tagged 1443 individuals of 35 reef fish species captured in Antillean fish traps in the Barbados Marine Reserve and adjacent non-reserve over a two-month period. Trapping and visual surveys were used to monitor the movements of these fish during the trapping period and the subsequent two months. Estimates of distances moved were corrected for the spatial distribution of sampling effort and for the number of recaptures of individual fish. Recapture rates for individual species ranged from 0–100% (median=38%). Species mobility estimated by recapture and resighting were highly correlated. Most species were strongly site attached, with the majority of recaptures and resightings occurring at the site of tagging. However, only one of 59 tagged jacks (Caranx latus, C. ruber) was ever resighted, suggesting emigration from the study area. All species were occasionally recorded away from the sites where they had been tagged (20–500 m), and several species, including surgeonfish, Acanthurus bahianus, A. coeruleus, filefish, Cantherhines pullus, butterflyfish, Chaetodon striatus, angelfish Holocanthus tricolor and parrotfish, Sparisoma viride, ranged widely within reefs. In contrast, few movements were observed between reefs separated by more than 20 m of sand and rubble, and no emigration from the Reserve was recorded. Most reef fishes vulnerable to Antillean traps appear sufficiently site-attached to benefit from reserves. However, many species move over a wide enough area to take them out of small reserves on continuous reef. Use of natural home range boundaries could minimize exposure of fishes in reserves to mortality from adjacent fisheries.

Reserves are being used increasingly to conserve fish communities and populations under threat from overfishing, but little consideration has been given to how fish behavior might affect reserve function. This review examines the implications of how fish use space, in particular the occurrence and size of home ranges and the frequency and direction of home range relocations. Examples are drawn primarily from the literature on coral reef fishes, but the principles apply to other habitats. Reserves can protect fish species only if individuals restrict their movements to a localized home range during at least part of the life cycle. Home range sizes increase with body size. In small reserves, a significant proportion of fish whose home ranges are centered within the reserve can be exposed to fishing mortality because their home ranges include non-reserve areas. Relocation of home ranges following initial settlement increases exposure to the fishery, especially if habitat selection is frequency-dependent. Distance, barriers, and costs of movement counter such redistribution. These considerations lead to predictions that population density and mean fish size (1) will form gradients across reserve boundaries with maxima in the center of the reserve and minima outside the reserve away from the boundary; (2) will increase rapidly in newly established reserves, only later providing ‘spillover’ to adjacent fisheries as density-dependent emigration begins to take effect; and (3) will be higher in reserves that are larger and have higher area:edge ratios, more habitat types, natural barriers between reserve and non-reserve areas, and higher habitat quality inside than outside the reserve. (4) Species with low mobility and weak density-dependence of space use will show the greatest increase in reserves and the strongest benefit for population reproductive capacity, but those with intermediate levels of these traits will provide the greatest spillover benefit to nearby fisheries.

Reductions in fishing mortality within no-take coral reef marine reserves can produce gradients in the density and size of fishes across reserve boundaries. Such gradients may be affected by other factors, however, including differences in habitat quality between reserve and non-reserve areas and the movement of fish across reserve boundaries. To examine the effects of protection from fishing mortality and of habitat quality on an assemblage of exploited reef fishes, we measured the spatial patterns of fish density and size on fringing reefs near the boundary of the Barbados Marine Reserve (Barbados, West Indies) and statistically controlled for habitat correlates of fish density and size. Reserve sites supported a higher total density and size of fishes than non-reserve sites. Most species had a non-significantly higher mean density and size at reserve sites. The density and/or size of many species were correlated with the depth, rugosity, and/or substrate composition of sites. After statistically controlling for the effects of habitat correlates, the difference in total density between reserve and non-reserve sites remained significant, and the mean density and size of most species remained non-significantly higher at reserve sites. Neither the mobility of species nor their vulnerability to capture by Antillean fish traps was correlated with their relative difference in density or size between reserve and non-reserve sites. Spearfishing target species had a significantly higher relative difference in size between reserve and non-reserve sites than non-target species. Protection from fishing mortality and higher habitat quality appear to contribute to the increased density and size of fishes on study reefs in the Barbados Marine Reserve, and this difference is not compromised by emigration from the reserve.

Surgeonfish (Acanthuridae) are prominent, herbivorous members of coral reef communities that occur as dispersed individuals and small, loose groups ('non-schooling fish') or as members of large, highly aggregated, mixed-species schools ('schooling fish'). We examined the relationships among fish size, habitat use and schooling in two species of surgeonfish on a fringing reef in Barbados, West Indies. Both ocean surgeonfish, Acanthurus bahianus, and blue tangs, A. coeruleus, appeared to show ontogenetic habitat shifts. The density of juvenile ocean surgeonfish was highest in the back reef (inshore), lower on the reef crest (intermediate) and lowest in the spurs and grooves (offshore) zone, but schooling adults were most abundant in the spurs and grooves zone. In a multiple regression considering the effects of depth, algal cover, rugosity and distance from shore, the density of non-schooling ocean surgeonfish was positively associated with percent algal cover on the substratum and negatively with distance from shore. Newly settled blue tangs occurred only in the reef crest and spurs and grooves zones, but larger juveniles were more common in the back reef, while adults were more evenly distributed across zones. The density of non-schooling blue tang was positively associated with rugosity, distance from shore, and percent algal cover. In both species, schooling occurred primarily in adults; small juveniles never participated in the large, dense schools. The proportion of adults that were schooling increased from the back reef to the reef crest to the spurs and grooves zone. These results are consistent with the hypothesis that schooling permits adult surgeonfish access to higher quality food in the territories of damselfish (Pomacentridae) that predominate on the reef crest and spurs.

We examined how spatial distribution and the use of aggressive behavior by blue gouramis, Trichogaster trichopterus (Belontiidae), in the presence of a concentrated food source were affected by group size and by the short-term presence and absence of food. Gouramis aggregated in the area of the food source, and the frequency of aggressive acts per fish was higher for fish near the food source than for fish away from it. The frequency of aggressive acts per fish near the food source decreased with group size and was about 50 times higher in groups of 2 and 4 than it was in groups of 16 and 32. In group sizes 2 and 4, the frequency of aggression was unaffected by the presence and absence of food, but in larger group sizes aggression increased during short intervals without food. The mean proportion of time spent near the food source increased from group size 2 to group size 8 and decreased from group size 8 to group size 32 and was greater during intervals of food availability than during intervals in which food was not available. We suggest that gouramis adjust their relative use of contest and scramble competition according to the costs and benefits of aggression as determined by the number of competitors and by the potential for missed feeding opportunities. The average proportion of time spent in the vicinity of the food source appears to be influenced by both the rate of aggression and by the net rate of gain available.

We examined the relationship between the intestine length and the amount of plant material in the diet of 21 species of fish from forest streams in Panama. Alimentary tract analyses supplemented by literature reports showed that four loricariid catfish species and one poeciliid were specialized herbivores consuming almost exclusively periphyton and detritus. Four species, including one erythrinid, one characid, one trichomyctycterid and one eleotrid, were carnivores consuming almost entirely food of animal origin. Twelve species, including five characids, one lebiasinid, two pimelodelids, three cichlids and one poeciliid, were omnivores consuming food of both plant and animal origin, but the average proportion of food of plant origin (detritus and algae plus higher plant parts) varied from 4–60%. Most omnivores increased plant food consumption with increasing size. Because intestine length increases allometrically with body size and the pattern of increase differs considerably among species and is influenced by length:mass relationships, we compared species at the same size and took both length and mass into account. At a given size, intestine lengths of herbivores were longer than those of omnivores, and these were longer than those of carnivores. Differences in intestine length among the dietary categories were greater at larger body sizes and when the common size was defined by body mass than when it was defined by body length. There was no trend for the average proportion of plant material consumed to be related to intestine length among the omnivores, when confounding effects of body mass were taken into account. The slopes of the allometric equations relating log10 intestine length to log10 body size for herbivores tended to be higher than for omnivores and higher for omnivores than for carnivores, but both herbivores and omnivores showed extensive variation and overlap with the other dietary categories. Among the omnivores, there was no trend for slopes to be steeper for species consuming more plant material on average or for species showing larger ontogenetic increases in plant consumption. These results permit increased precision in describing diet-intestine length relationships, but indicate that the widely held belief that intestine length reflects diet in fishes should only be applied to broad dietary categories and not to finer divisions among omnivores.

We examined the ontogenetic allometry of intestine length in relation to body length and body mass in 21 species of fish from forest streams in Panama. The relationships between log10 intestine length and log10 body length and mass were well described by linear regressions, although some species showed slight curvilinearity. Slopes and intercepts of the linear regressions varied considerably among species. Intestine length was positively allometric in most species, with slopes of the intestine length:body length relationship ranging from 1.09 to 2.11. Relative intestine lengths (intestine length/body length) varied by two orders of magnitude (0.39–38.44) in the data set as a whole, but the variation was about one order of magnitude when species were compared at a common body size. Species in which body mass increased more rapidly with body length had more rapid increases in intestine length with body length. Among omnivorous and carnivorous species compared at the same body length, heavier species had longer intestines. Interspecific comparisons of relative intestine length may produce misleading conclusions unless comparisons are made at a common size and account for differences in relative mass.

Recently emerged brook trout (Salvelinus fontinalis) foraging is still-water pools varied considerably in the proportion of time they spent moving, speed while moving, and distance travelled during pursuit of prey. Trout that exhibited a high proportion of time spent moving, fast speeds while moving, and long pursuit distances directed more foraging attempts toward the upper portion of the water column, ate more insect prey and fewer crustaceans, and made more foraging attempts per minute than trout that exhibited a low proportion of time spent moving, slow speeds while moving, and short pursuit distances. The variation in movement was related less strongly to spatial heterogeneity in the locations (pool sizes, water depths) where individuals were observed. It was not significantly related to measures of temporal variation in the environment (date, time-of-day, water temperature), body size and shape, or hunger (the amount of food in the stomach). In these clear, shallow pools, differences in the spatial distribution and behaviour of prey types apparently facilitate divergent foraging behaviour and diet among the trout. Our findings are valuable for understanding the development and maintenance of the individual specializations in water-column use and diet reported previously for several salmonids.

This study examines aspects of the seasonal limnology of an intermittent stream in the dry forest region of northwest Costa Rica. It focuses on annual water level fluctuations and both seasonal and among pool variation in dissolved oxygen concentration and water temperature. Dry season pools differed in morphometry and the rate of decline in water levels subsequent to the seasonal floods. Rate of water level decline was related both to pool depth and to the exposure of the pool to the sun. Oxygen concentration was generally low in the dry season, but increased during the rainy season in association with rain events and seasonal flooding. A repeated-measures analysis of variance indicated that seasonal effects accounted for 40% of the variance in oxygen concentration for residual pools. Differences among pools were also significant. However, there was strong evidence for a significant interaction between seasonal and spatial influences on oxygen values in the system. Variation in water temperature was small, but differences among pools, sampling dates, and their interaction were all significant. Periphyton production increased significantly between the late wet season sample in November and the dry season sample in February. Incident light intensity explained 76% of the variation among pools in net periphyton production in the dry season.

It is commonly accepted that more active fishes have a greater proportion of red muscle in their trunk musculature than do less active fishes. Further, the proportion of red muscle has been used to classify fish species into functional groups reflecting different activity patterns. Nevertheless, existing measures of both red muscle and mobility have several limitations, and the relationship between these parameters has never been evaluated quantitatively. Using data from the literature, we demonstrate a positive, statistical association between the proportion of red muscle in the caudal peduncle of marine fishes and a qualitative measure of mobility (categorization as sedentary vs. mobile based on natural-history accounts). Analyses of the frequency distribution of the proportion of red muscle also provide evidence for two subdistributions. However, this bimodality does not correspond with sedentary vs. mobile or sit-and-wait vs. active search dichotomies.

We examined the old, but untested hypothesis that territory size limits the maximum population density of salmonids in streams. We used published data to derive an interspecific regression of territory size (m2) on fork length (cm) (log10 territory size = 2.61 log10 length—2.83, r2 = 0.87, n = 23). Growth and mortality trajectories of salmonid cohorts from eight experimental studies were compared to the maximum-density regression, the inverse of the territory–size regression. In shallow habitats, such as riffles and raceways, the cohort trajectories followed the maximum density regression quite closely and were consistent with the territory–size hypothesis. In addition, natural densities in eight other studies did not exceed the predicted maximum density and tended to fail within the 95% C.L. of the maximum-density regression. Data from shallow habitats, therefore, provide strong support for the territory–size hypothesis. A linear logistic response model showed that the probability of observing density-dependent growth, mortality, or emigration increased significantly with increasing values of an index of habitat saturation, developed from the territory–size regression. Our results suggest that the territory–size regression has practical value for predicting the maximum densities of stream-dwelling salmonids in shallow habitats and the occurrence of density-dependent population responses.

Use of the surface water for aquatic respiration (aquatic surface respiration, ASR) is one of the few alternatives to aerial respiration which allow fish to survive extreme hypoxia, yet it has received very little attention. This report examines three generalizations concerning ASR on a phylogenetically and geographically diverse range of tropical freshwater fishes. It demonstrates that ASR greatly enhances survival in hypoxic water, even in fish not morphologically specialized to use the surface film, that ASR is initiated at a distinct threshold oxygen concentration, with time spent at the surface increasing rapidly as O2 declines, and that with extreme deoxygenation fish perform ASR over 90% of the time. Ninety-four percent of the 31 species of non-air breathing fish tested showed ASR., with the threshold oxygen concentration ranging from 6 to 40 torr.

When dissolved oxygen concentration was near saturation in a laboratory experiment, guppies that spent (a) more time at the surface and (b) more time moving had a higher probability of being captured by a predatory cichlid fish. With decreasing oxygen concentration surface time and percent time moving increased, but prey risk decreased. In addition, the qualitative correlates of risk changed; under hypoxic conditions, predation risk was lower for prey which spent more time at the surface and in motion. Thus, dissolved oxygen concentration influences both quantitative and qualitative aspects of risk from water-breathing predators.

The bimodally respiring catfish Clarias macrocephalus Gunther responded to a toxic extract of Croton tiglium (Euphorbiaceae) seeds by increased air breathing under both normoxic (8.1 ± 0.4 mgO2 l−1) and hypoxic (0.7 ± 0.1 mgO2l−1) conditions. Fish in hypoxia survived longer than those in normoxia when surface access was provided. When air breathing was prevented, survival time in toxin was greatly reduced at both levels of dissolved oxygen, and fish in normoxia survived longer than those in hypoxia. Non-toxin controls without surface access survived in normoxia but in hypoxia died at the same time as the fish in toxin. These results suggest that air breathing increases the resistance offish to toxins by permitting a decrease in the rate of gill ventilation and hence the rate at which toxins are absorbed.

We describe the short-term patterns of egg production and release in the lemon tetra, Hyphessobrycon pulchripinnis (Characidae) as observed over a six month aquarium study and then use our results and those of others to both describe general patterns and derive comparative predictions. Female lemon tetras ovulated about once every four days; differences among individuals were small and inconsistent. As in other species, the probability of ovulation depended strongly on time since last ovulation, indicative of an ovarian cycle; furthermore we found no obvious long-term patterns of ovulation within females and batch fecundity was independent of the length of the two previous interovulation intervals and of the one following. Each batch of ovulated eggs was released over an average of 23.1 spawning acts, beginning as soon as the lights went on in the morning. The number of spawning acts increased with ovulation fecundity but decreased with the number of other females spawning in the tank on that day. The mass of eggs produced every four days was greater than that of the remaining ovary, and the mass of eggs that could be produced in a season was greater than that of the female. This latter observation highlights the most important consequence of repeated reproduction within a season - increased reproductive output - and leads to predictions associating it with less seasonal environments (e.g. low latitudes), as well as with small ovaries and small body size. The significance of releasing a batch of eggs over many spawning acts remains unknown.

Eighty six species of non-air breathing fish from Panama were tested for aquatic surface respiration (ASR), a behavioral response to extreme hypoxia that involves aquatic respiration at the air-water interface. ASR was well developed in 93% and 72% of species from potentially hypoxic freshwater and marine habitats, respectively, but only 42% of species from consistently well oxygenated marine habitats. This provides evidence that ASR is a specific adaptation to hypoxia and that in fishes of shallow, tropical waters, respiratory responses which maintain oxygen supply are much more widespread than those which eliminate oxygen demand (anaerobic respiration).

Aquatic respiration at the air-water interface, herein termed aquatic surface respiration (ASR), is used by the guppy,Poecilia reticulata (Poeciliidae) to meet oxygen demand in hypoxid water. A specific position in which the head contacts the surface and the jaws open just beneath the surface is adopted. ASR is initiated at a P02 of about 50 torr and the percent time spent rises in a steep, linear fashion as PO2 decreases. Below 4 torr more than 90% of the animal's time is spent in ASR. Males spend less time in ASR than do females. The percent time in ASR increases with increasing size of female guppies, but decreases with increasing size of male guppies. At low oxygen (18 torr) laboratory-born guppies derived from stocks likely to experience deoxygenation spend less time in ASR than do guppies derived from stocks less likely to experience deoxygenation. The percent time in ASR increases with temperature when PO2 is held constant. Acclimation to low oxygen decreases the percent time in ASR. Guppies not permitted ASR die in 6–41 h at 14–17 torr and 10–15 min at 1–4 torr. Guppies performing ASR survive the duration of experiments at 13–35 torr (13 days), 14–17 torr (96 h), and 1–4 torr (9.5 h). Activity, courtship, and dive duration in response to a shadow stimulus are all reduced by low oxygen. ASR is an effective alternative to aerial respiration as an adaptation to hypoxic waters, but is probably energetically and temporally more costly.

Zebrafish (Brachydanio rerio) (Cyprinidae) were exposed to a toxic extract of Croton tiglium (Euphorbiaceae) seeds (4 mg∙L−1) at 0.8, 1.1 and 8.1 mg O2∙L−1, with and without surface access. Equilibrium loss occurred more quickly at lower dissolved oxygen concentrations. At 0.8 and 1.1 mg O2∙L−1, equilibrium loss also occurred more quickly when access to the water surface was denied. Surface access is therefore an important factor in studies of the interaction between toxicity and dissolved oxygen concentration. We suggest that, by performing aquatic surface respiration, zebrafish with surface access in hypoxic water reduce their ventilation rate and hence their uptake rate of toxins.

A simple semi-mechanical analog-to-digital convertor allows a dissolved oxygen meter to control the flow of nitrogen gas (N 2 ) into a deoxygenation tank. The ensuing N 2 /air mixture is then recirculated through this tank, reducing the quantity of pure N 2 required. The system can easily be modified to allow additional aeration and/or an alarm when dissolved oxygen concentration falls below a preset value.

Juvenile guppies (Poecilia reticulata) were fed ad libitum and maintained for 11 d at oxygen concentrations ranging from 0.5 to 8 mg/L (25–26 °C). With access to the surface, guppies showed high growth rates and low mortality at all concentrations. Without surface access, they exhibited a progressive reduction in growth rate below 2–3 mg/L and did not survive below 1 mg/L. Activity was influenced by oxygen concentration and surface access. Feeding rates declined sharply with oxygen concentration for fish without surface access and to a lesser extent for fish with access. Percent time in contact with the surface increased as oxygen decreased. Aquatic surface respiration is used even when this behavior is not necessary for survival. We suggest that aquatic surface respiration increases the energy available for growth under hypoxic conditions by augmenting oxygen supply, which permits greater ingestion rate, and possibly by decreasing the cost of ventilation.

In juvenile Clarias macrocephalus, distance travelled to breathe air increased strongly with depth, but growth rate was not significantly affected. Forty-seven fish (4.60 ± 0.12 g, 8.4 ± 0.07 cm, mean ± SE) were initially raised for 28 d in individual cages (30, 60, 90, and 118 cm deep) at 27 °C and 0.72 mg O2∙L−1, with a daily ration of 6% body weight. Distance travelled increased from 15.5 m∙h−1 at 30 cm to 51.2 m∙h−1 at 118 cm. When fish were subsequently maintained without food for a further 14 d, at 0.15 mg O2∙L−1, distance travelled averaged 6.6 and 22.7 m∙h−1 at 30 and 118 cm, respectively. Despite the increase in activity, depth had no significant effect on specific growth rate of either fed (3.62 ± 0.08%∙d−1) or fasting (−0.70 ± 0.03%∙d−1) fish. Strong effects of depth on the growth of air-breathing fishes may not be as general as previously suggested.

This essay reviews the behavioral responses of fish to reduced levels of dissolved oxygen from the perspective of optimization theory as used in contemporary behavioral ecology. A consideration of oxygen as a resource suggests that net oxygen gain per unit of energy expenditure will be the most useful currency for ecological models of breathing. In the process of oxygen uptake, fish always expend energy on perfusion, usually on ventilation and often on locomotion. These costs, and the risk of predation, will vary with oxygen availability and the type of behavioral response shown. The principal categories of behavioral response to reduced external availability of dissolved oxygen are (1) changes in activity, (2) increased use of air breathing, (3) increased use of aquatic surface respiration, and (4) vertical or horizontal habitat changes. Fish should choose whichever combination of responses minimizes the costs of meeting their oxygen demands. A small number of studies provides qualitative support for this prediction.

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