Individual movements and population density estimates for moray eels on a Caribbean coral reef

Observations of moray eel (Muraenidae) distribution made on a Caribbean coral reef are discussed in the context of long term population trends. Observations of eel distribution made using SCUBA during 1978, 1979–1980, and 1984 are compared and related to the occurrence of a hurricane in 1979. An estimate of the mean standing stock of moray eels is presented. The degree of site attachment is discussed for spotted morays (Gymnothorax moringa) and goldentail morays (Muraena miliaris). The repeated non-aggressive association of moray eels with large aggregations of potential prey fishes is detailed.     

Distribution and habitat associations of the California moray (<Emphasis Type="Italic">Gymnothorax mordax</Emphasis>) within Two Harbors,Santa Catalina Island,California

While kelp forests are some of the best-surveyed ecosystems in California, information on cryptic inhabitants and their role within the community are lacking. Kelp itself provides overall structure to the habitat; however the rocky reef to which the kelp attaches is known to provide additional structure for cryptic species. Gymnothorax mordax, the California moray, is an elusive predatory species that is considered abundant in the waters around Catalina Island. However, no life history data exists for this species. We examined habitat composition, relative abundance, size pattern distributions, and biomass of G. mordax within Two Harbors, Catalina Island. Habitats were sampled using a combination of baited trap collection and transect surveys using SCUBA. A total of 462 G. mordax were captured, primarily in shallow (< 10 m) waters. Individuals of G. mordax were associated with mostly boulder and cobble substrates. Measurements of relative abundance and density indicate that G. mordax is more prevalent than reported in previous studies. We also discovered that the 6 trapping sites from which all morays were collected, differed in size structuring and density while the relatively high biomass did not change across sites. In general, southern facing sites exhibited higher densities of morays, while northern facing sites showed more size structuring. We show how the structural complexity of the rocky reef habitat in an already diverse kelp forest ecosystem, can support a high biomass of a cryptic elongate predatory fish.     

The structure of a nearshore fish community of Western Australia: diel patterns and the habitat role of limestone reefs

Synopsis Extensive limestone reefs are a characteristic feature of much of the coastline of Western Australia, and potentially represent a major habitat feature influencing the structure of the coastal fish community. The structure and temporal dynamics of the fish fauna and its relationships to nearshore patch reefs and surrounding habitat near Dongara, Western Australia, were examined using (1) diel gill-netting and (2) quantitative rotenone sampling of enclosed areas of substratum. Long-term and day-to-day variability of the fauna was low. Dominant species of gill-net collections were either associated with reefs or occurred in similar abundances at both reefs and surrounding sand/seagrass flats. The overall abundance, number of species and biomass of netted fishes was higher around reefs. Rotenone collections of the more sedentary species showed a similar pattern, but suggested, however, that a simple reef versus surrounding sand and seagrass habitat comparison is complicated by the canopy-forming seagrass Amphibolis that occurs on reef tops. Time of day had an important effect on overall fish abundance and number of species, with peaks occurring at crepuscular periods. This reflected dusk and dawn activity peaks of a dominant species rather than overlapping activities of many diurnal and nocturnal species. Diel switches between reef-edge habitat and surrounding sand/seagrass flats were uncommon despite expectations (based on literature examples) that patch reefs would function primarily as sheltering habitats and surrounding non-reef areas act as foraging habitat. High catches at reef-edge sites suggest that the majority of fishes forage on or near limestone patch reefs. Fish densities of around 0.8 individuals per m^-2 of bottom on these Western Australian reefs are relatively high in comparison to visual census estimates obtained for temperate reef systems in South Australia and New Zealand, but similar to those obtained using comparable netting methods in temperate Australian seagrass systems.     

Comparative spatial ecology of fished spiny lobsters <Emphasis Type="Italic">Panulirus argus</Emphasis> and an unfished congener <Emphasis Type="Italic">P. guttatus</Emphasis> in an isolated marine reserve at Glover's Reef atoll,Belize

Palinurid lobsters are being exploited with increasing intensity in coral reef ecosystems, but marine protected areas may play a key role in preventing overfishing and local extinctions. In order to define the spatial requirements for protection, we compared the spatial and temporal patterns in distribution, density, biomass, size structure, and reproductive seasonality of Caribbean spiny lobsters Panulirus argus and the congeneric spotted lobsters P. guttatus on coral patch reef, forereef, and deep reef habitat at Glover's Reef, Belize. The relative impact of fishing on P. argus was also examined in an isolated marine reserve and adjacent fished habitats, in comparison with the relatively unfished distribution of P. guttatus. Over a 5-year period, both species co-occurred in all major reef habitats, but aspects of their population dynamics differed markedly due to both habitat and fishing effects. All size classes of spiny lobsters P. argus occupied shallow patch reefs, but large adults were predominant on the deep wall reef. Panulirus guttatus also occupied patch reefs in the lagoon, but spur-and-groove forereef appeared to be the primary habitat of this species. Density and exploitable (adult) biomass of P. argus increased significantly over time in the protected patch reef habitat of the lagoon but remained stable on deep reef habitat. The biomass of spotted lobsters P. guttatus in all habitats was at least an order of magnitude less than that of exploitable P. argus. Reproductive activity by both species was evident most of the year in all habitats, but breeding P. argus females were concentrated on the deep reef. Commercial fisheries for spotted lobsters P. guttatus are currently being considered for development, but data from this and other studies suggest that such a fishery may be relatively unproductive and may lead to rapid localized extinctions. Spiny lobsters P. argus used a variety of coral reef habitats, but spotted lobsters P. guttatus were habitat specialists restricted to shallow reef habitat. The protection needs of both species are similar in one aspect: large protected areas. However, P. argus required large areas with heterogeneous habitats including coral reefs and seagrass beds, whereas P. guttatus required large areas of coral reef habitat.     

Functional morphology of the pharyngeal jaw apparatus in moray eels

Moray eels (Muraenidae) are a relatively large group of anguilliform fishes that are notable for their crevice-dwelling lifestyle and renowned for their ability to consume large prey. Morays apprehend their prey by biting and then transport prey by extreme protraction and retraction of their pharyngeal jaw apparatus. Here, we present a detailed interpretation of the mechanisms of pharyngeal jaw transport based on work with Muraena retifera. We also review what is known of the moray pharyngeal jaw apparatus from the literature and provide comparative data on the pharyngeal jaw elements and kinematics for other moray species to determine whether interspecific differences in morphology and behavior are present. Rather than comprising broad upper and lower processing tooth plates, the pharyngeal jaws of muraenine and uropterygiine morays, are long and thin and possess large, recurved teeth. Compared with the muraenines, the pharyngobranchials of the uropterygiines do not possess a horn-shaped process and their connection to the fourth epibranchial is dorsal rather than medial. In addition, the lower tooth plates do not exhibit a lateral groove that serves as a site of muscle attachment for the pharyngocleitheralis and the ventral rather than the lateral side of the lower tooth plate attaches to the fourth ceratobranchial. In all morays, the muscles positioned for protraction and retraction of the pharyngeal apparatus have undergone elongation, while maintaining the generalized attachment sites on the bones of the skull and axial skeleton. Uropterygiines lack a dorsal retractor muscle and we presume that retraction of the pharyngeal jaws is achieved by the pharyngocleitheralis and the esophagus. The fifth branchial adductor is greatly hypertrophied in all species examined, suggesting that morays can strongly adduct the pharyngeal jaws during prey transport. The kinematics of biting behavior during prey capture and transport resulted in similar magnitudes of cranial movements although the timing of kinematic events was significantly different and the duration of transport was twice as long as prey capture. We speculate that morays have evolved this alternative prey transport strategy as a means of overcoming gape constraints, while hunting in the confines of coral reefs.     

Native Predators Do Not Influence Invasion Success of Pacific Lionfish on Caribbean Reefs

Biotic resistance, the process by which new colonists are excluded from a community by predation from and/or competition with resident species, can prevent or limit species invasions. We examined whether biotic resistance by native predators on Caribbean coral reefs has influenced the invasion success of red lionfishes (Pterois volitans and Pterois miles), piscivores from the Indo-Pacific. Specifically, we surveyed the abundance (density and biomass) of lionfish and native predatory fishes that could interact with lionfish (either through predation or competition) on 71 reefs in three biogeographic regions of the Caribbean. We recorded protection status of the reefs, and abiotic variables including depth, habitat type, and wind/wave exposure at each site. We found no relationship between the density or biomass of lionfish and that of native predators. However, lionfish densities were significantly lower on windward sites, potentially because of habitat preferences, and in marine protected areas, most likely because of ongoing removal efforts by reserve managers. Our results suggest that interactions with native predators do not influence the colonization or post-establishment population density of invasive lionfish on Caribbean reefs.     

Indo-Pacific lionfish are larger and more abundant on invaded reefs: a comparison of Kenyan and Bahamian lionfish populations

The invasion by Indo-Pacific lionfish (Pterois volitans and P. miles) of the western Atlantic, Caribbean and Gulf of Mexico is emerging as a major threat to coral reef communities across the region. Comparing native and introduced populations of invasive species can reveal shifts in ecology and behaviour that can accompany successful invasions. Using standardized field surveys replicated at multiple sites in Kenya and the Bahamas, we present the first direct comparisons of lionfish density, body size, biomass and behaviour between native and invaded coral reefs. We found that lionfish occur at higher densities with larger body sizes and total biomass on invaded Bahamian coral reefs than the ecologically equivalent species (P. miles) does on native Kenyan reefs. However, the combined average density of the five lionfish species (Pterois miles, P. antennata, P. radiata, Dendrochirus brachypterus and D. zebra) on Kenyan reefs was similar to the density of invasive lionfish in the Bahamas. Understanding the ecological processes that drive these differences can help inform the management and control of invasive lionfish.     

Ecosystem engineering by a canopy‐forming kelp facilitates the recruitment of native oysters

Ecosystem engineers are species that influence the abiotic and biotic environment around them and may assist the restoration of associated species, including other habitat‐forming species. We deployed an array of 28 artificial reefs with transplanted Ecklonia radiata, the dominant canopy‐forming kelp species across southern Australia, to investigate how the patch size and density of E. radiata influenced the establishment of the associated communities of plants and animals. Many of the reefs were rapidly colonized by Ostrea angasi, a critically depleted reef‐forming oyster. Over the 24‐month deployment of the reefs, thick oyster mats formed across the entire surface of many of the reefs with estimated biomass densities exceeding 5 kg of live oysters/m²; however, oyster density was dependent on E. radiata patch size and density. Increasing patch size and the presence of kelp resulted in significantly higher densities of oysters 5 months after the reefs were deployed and at the end of the experiment, where oysters were approximately three times more numerous on reefs with kelp compared to those without kelp. E. radiata appeared to facilitate the establishment of O. angasi largely through its capacity to reduce benthic light and thus suppress competition from turfing algae. These results may inform the development of novel approaches to tackle recruitment bottlenecks affecting the restoration of O. angasi reefs.     

Edge effects influence the composition and density of reef residents on subtidal restored oyster reefs

Within estuarine and coastal ecosystems globally, extensive habitat degradation and loss threaten critical ecosystem functions and necessitate widescale restoration efforts. There is abundant evidence that ecological processes and species interactions can vary with habitat characteristics, which has important implications for the design and implementation of restoration efforts aimed at enhancing specific ecosystem functions and services. We conducted an experiment examining how habitat characteristics (presence; edge vs. interior) influence the communities of resident fish and mobile invertebrates on restored oyster (Crassostrea virginica) reefs. Similar to previous studies, we found that restored reefs altered community composition and augmented total abundance and biomass relative to unstructured sand habitat. Community composition and biomass also differed between the edge and interior of individual reefs as a result of species-specific patterns over small spatial scales. These patterns were only weakly linked to oyster density, suggesting that other factors that vary between edge and interior (e.g. predator access or species interactions) are likely more important for community structure on oyster reefs. Fine-scale information on resident species' use of oyster reefs will help facilitate restoration by allowing decision makers to optimize the amount of edge versus interior habitat. To improve the prediction of faunal use and benefits from habitat restoration, we recommend investigations into the mechanisms shaping edge and interior preferences on oyster reefs.     

Comparison between community structures of fishes in <Emphasis Type="Italic">Enhalus acoroides</Emphasis>- and <Emphasis Type="Italic">Thalassia hemprichii</Emphasis>-dominated seagrass beds on fringing coral reefs in the Ryukyu Islands,Japan

To clarify differences in community structures and habitat utilization patterns of fishes in Enhalus acoroides- and Thalassia hemprichii-dominated seagrass beds on fringing coral reefs, visual censuses were conducted at Iriomote and Ishigaki islands, southern Japan. The numbers of fish species and individuals were significantly higher in the E. acoroides bed than in the T. hemprichii bed, although the 15 most dominant fishes in each seagrass bed were similar. Cluster and ordination analyses based on the number of individuals of each fish species also demonstrated that fish community structures were similar in the two seagrass beds. Species and individual numbers of coral reef fishes which utilized the seagrass beds numbered less than about 15% of whole coral reef fish numbers, although they comprised about half of the seagrass bed fishes. Of the 15 most dominant species, 5 occurred only in the two seagrass beds, including seagrass feeders. Ten other species were reef species, their habitat utilization patterns not differing greatly between the two seagrass beds. Some reef species, such as Lethrinus atkinsoni and L. obsoletus, showed ontogenetic habitat shifts with growth, from the seagrass beds to the coral areas. These results indicate that community structures and habitat utilization patterns of fishes were similar between E. acoroides- and T. hemprichii-dominated seagrass beds, whereas many coral reef fishes hardly utilized the seagrass beds.     

Quantitative assessment of fish assemblages on artificial reefs using acoustic and conventional netting methods,in Xiangshan Bay,Zhejiang Province,China

Artificial reefs are often deployed by many countries for the purpose of enhancing fishing after a period of several years. The objective of this study was to analyze the effect of a pilot artificial reef system deployment in enhancing the demersal fish assemblages, investigated using both acoustic methods and bottom trammel nets, during four years between 2011 and 2017, in Xiangshan Bay, Zhejiang Province, China. Comparisons of community indices, including fish biomass, species richness and species diversity, indicated relatively consistent trends between the control sites and the artificial reefs following their deployment in 2012. Fish density, represented by the value of the nautical area scattering coefficient, and fish biomass were significantly higher on the artificial reefs than at the control sites in 2015 and 2017, and species richness and diversity were significantly greater at the reefs from 2013 to 2017. Blackhead seabream (Acanthopagrus schlegelii) and false kelpfish (Sebastiscus marmoratus) were the dominant fishes on the artificial reefs, and the average body lengths of these two species were significantly greater 40 and 64 months after deployment. Thus, construction of the artificial reefs appears to have achieved their primary purpose as fish-attraction devices, thereby contributing to ecological restoration in Xiangshan Bay.     

Distribution,movements and diet of nocturnal fishes on temperate reefs

We counted nocturnal fishes both day and night, and monitored the position of tagged individuals on temperate reefs in New South Wales, Australia. Pempheris affinis and P. multiradiata were the most abundant nocturnal planktivores on Sydneys rocky reefs and showed great differences in diel migration behaviour. Both species were observed in deep shelter sites during the day (5–10m), and most emerged into the water column at night. P. multiradiata was found to undergo extensive vertical and horizontal migrations. In contrast, P. affinis remained within daytime depth strata, with tagged individuals often moving less than 20m at night. Tagged adult P. affinis returned to tagging sites for up to 7weeks, indicating high site fidelity. Dietary analysis demonstrated that small and large pempherids differed in diet and the timing of foraging, suggesting a size-based transition from diurnal to nocturnal foraging. Stratified sampling of planktonic assemblages at different depths during the day and night showed spatial variation in the availability of prey items at different times of the day. Amphipods, the main prey of large fish, were only available during the night, and concentrated in shallow water, whereas decapod larvae, consumed mainly by small fish, were abundant day and night. Large P. affinis also fed on polychaetes, which were never found in the stomachs of P. multiradiata, suggesting that these species may have different prey requirements, or that these polychaetes are only found in deep water where foraging P. affinis were abundant. We found no general model for the Pempheridae. The movements and behaviour of nocturnal fishes varied greatly by species, and this may be due to differences in body size, and/or physiological (e.g. visual ability) and ecological constraints.     

Diel Habitat Partitioning by Bull Charr and Cutthroat Trout During Fall and Winter in Rocky Mountain Streams

We used underwater observation to determine diel habitat partitioning between bull charr, Salvelinus confluentus, and cutthroat trout, Oncorhynchus clarki, during fall and winter (0.1–8.3°C) in two Rocky Mountain streams that differed in habitat availability. The majority (>70%) of both species emerged from concealment cover at night, though bull charr exhibited a greater tendency for nocturnal behavior than cutthroat trout. Differences in day and night counts were most pronounced at temperatures <3°C, when very few fish of either species were observed in the water column during the day, but both species were common at night. Both species used concealment cover of large woody debris and boulder substrate crevices in deep pools during the day. At night, fish emerged from cover and habitat use shifted to shallow water with low cover. Microhabitat partitioning among species and size classes occurred at night, cutthroat trout moving into shallower, faster water that was farther from cover compared to bull charr. Smaller fish of both species occupied focal positions in slower, shallower water closer to the substrate than larger fish. Large, mixed-species aggregations also were common in beaver ponds both day and night. High variation in diel and site-specific winter habitat use suggests the need for caution in developing habitat suitability criteria for salmonids based solely on daytime observations or on observations from a few sites. Our results support the need to incorporate nocturnal habitat use and partitioning in studies of salmonid ecology.     

Rapid changes in fish utilization of mangrove habitat in Western Madagascar

Fish use of a mangrove habitat was studied in a small mangrove forest on the West coast of Madagascar. A sand bar near the inlet retains water in parts of the channel (pools) at low tide. Fishes in four of these pools were examined daily at all phases of the tidal cycle for 3 weeks using underwater visual census. During week 1, fishes were diverse and abundant in all pools: the dominant species were cardinalfish (related to Apogon lateralis); monos, Monodactylus argenteus; black spotted snappers, Lutjanus ehrenbergi; double bar bream, Acanthopagrus bifasciatus; emperors, Lethrinus lentjan and L. sp., surgeon fish, Acanthurus nigricauda; red-lined sweetlips, Plectorhinchus plagiodesmis; and butterflyfish, Chaetodon kleini. Some species were more abundant in shaded pools; others in more open pools. During week 2 a dramatic difference was noted: the only fishes found were schools of cardinalfish and one moray eel. This week had neap tides, with high tides in the morning and low tides in the afternoon. As the week progressed and during week 3 (spring tides), fishes slowly repopulated the habitat and diversity increased. Monos, absent in week 2, now had increasing numbers of small individuals. While large emperors were scarce, small individuals appeared. The larger butterflyfish and surgeonfish seen in week 1 were replaced by small ones during week 3. Species that had been rare in week 1 were more abundant, including pipefish and small barracudas. While species richness increased during week 3, the community was strikingly different from that seen 2 weeks earlier. Only Pool 1, closest to the entrance, recovered its original species richness. Abundance was much lower than in week 1. Our snapshot study apparently captured a time when older juveniles left the mangrove forest and smaller fishes recruited into it. Utilization of this habitat will likely vary throughout the year depending on the reproductive cycle of the different species whose juveniles utilize it. Longer studies are needed to learn about cycles in fish use of the mangroves.     

Reef fish structure and distribution in a south-western Atlantic Ocean tropical island

The community structure of the reef fish fauna of Trindade Island, a volcanic oceanic island located 1160 km off the coast of Brazil, is described based on intensive visual censuses. Seventy-six species were encountered in 252 censuses, with mean ± S.E. of 99 ± 3 individuals and 15.7 ± 0.3 species 40 m(-2) transect. The average fish biomass, calculated from length-class estimation, was 22.1 kg 40 m(-2) transect. The species contributing most to biomass were, in decreasing order, Melichthys niger, Cephalopholis fulva, Kyphosus spp., Holocentrus adscensionis, Sparisoma amplum, Sparisoma axillare, Acanthurus bahianus and Epinephelus adscensionis. Carnivorous fishes were the largest trophic group in terms of biomass, followed by omnivores and roving herbivores. The two predominant types of reef habitat, fringing reefs built by coralline algae and rocky reefs made of volcanic boulders, showed significant differences in the biomass and the abundance of the trophic guilds. Within each habitat type, significant differences in species richness, density and biomass were detected among crest, slope and interface zones. Although similar in overall species composition to coastal reefs in Brazil, the fish fauna of Trindade Island shares certain characteristics, such as a high abundance of planktivores, with other Brazilian oceanic islands. Despite comparatively high fish biomass, including the macro-carnivorous species habitually targeted by fisheries, signs of overfishing were evident. These findings highlight the urgency for a conservation initiative for this isolated, unique and vulnerable reef system.     

Spatiotemporal variations in density and biomass of rocky reef fish in a biogeographic climatic transition zone: trends over 9 years,inside and outside the only nearshore no-take marine-protected area on the southern Brazilian coast

Biogeographical transition zones are important areas to investigate evolutionary ecological questions, but long-term population monitoring is needed to better understand ecological processes that govern population variations in such edge environments. The southernmost Brazilian rocky reefs are the southern limit of distribution for 96% of the tropical ichthyofauna of the western Atlantic. The Arvoredo Marine Biological Reserve is the only nearshore no-take marine-protected area (MPA) located in this transition zone. The main aim was to investigate how the populations of rocky reef fish species vary in density and biomass in space and over time, inside and outside the Arvoredo MPA. This study presents results based on a 9 year (2008–2017) underwater visual census monitoring study to evaluate the density and biomass of key fish species. Variations in density and biomass were detected for most species. Factors and mechanisms that may have influenced spatial variation are habitat structural complexity and protection from fisheries. Temporal variations, otherwise, may have been influenced by species proximity to their distributional limit, in synergy with density-dependent mechanisms and stochastic winter temperature oscillations. The MPAs harbour higher density and biomass for most species. Nonetheless, a prominent temporal decline in the recruitment of Epinephelus marginatus calls into question the continuous effectiveness of the MPA.     

Habitat correlates of the distribution and biomass of Seychelles' reef fishes

Synopsis Relationships between quantitative measures of habitat type and the biomass of Chaetodon, Scarus and Parupeneus species were investigated across 35 reef sites in the Inner Seychelles Group. Multiple regression was used to determine the proportion of variance in biomass between sites which could be explained by depth, exposure, vertical relief, topographic complexity, live coral cover, coral rubble cover, rock cover, sand cover, underlying carbonate substrate, underlying sand substrate, underlying rock substrate and an index of fishing intensity. A significant proportion of the variance in biomass was explained by habitat variables and the index of fishing intensity for 7 of 12 Chaetodon species (23–52% of variance explained), 3 of 6 Parupeneus species (33–40%), and 10 of 13 Scarus species (14–46%). Within genera, different groups of habitat variables explained the variance in biomass for different species and, of the variables studied, only the proportion of underlying sand substrate failed to explain a significant proportion of the variance in biomass for any species. Quantitative relationships between the biomass of Chaetodon and habitat were often in accordance with those suggested by previous studies of their ecology, life-history and distribution at other Indo-Pacific locations. However, the habitat associations of the Parupeneus and some Scarus species have not been studied at other locations and clearly warrant further investigation. It was concluded that habitat was an important determinant of the distribution of many Seychelles reef fishes, but that the habitat variables examined were rarely the most important determinant of biomass. However, the inclusion of a procedure to collect habitat data provided a useful means by which to reduce the unexplained variance associated with visual census biomass estimates and therefore improves the possibility of elucidating the effects of other factors on the biomass of Seychelles reef fishes.     

Habitat selection by ice-associated pinnipeds near St. Lawrence Island,Alaska in March 2001

Aerial surveys of ice-associated pinnipeds were conducted south of St. Lawrence Island in March 2001. The observed distributions of bearded seals (Erignathus barbatus), ribbon seals (Phoca fasciata), ringed seals (P. hispida), spotted seals (P. largha), and walruses (Odobenus rosmarus) were compared to the distributions of ice habitat types and benthic communities. Randomization tests were used to investigate habitat selection for each species. Both ringed seals and walruses preferred large ice floes (>48 m in diameter) that were common in the interior ice pack. Spotted seals favored smaller ice floes (<20 m in diameter) common near the ice edge, and bearded seals avoided large floes and preferred transitional habitat between small and large floes. Ringed seals also seemed to prefer areas with greater than 90% sea ice coverage, and bearded seals preferred 70–90% sea ice coverage while avoiding areas with greater than 90% coverage. All species, except spotted seals, were seen most frequently in a region of high benthic biomass, and randomization tests suggested that bearded seals actively selected that region.     

Alien algae <Emphasis Type="Italic">Undaria pinnatifida</Emphasis> causes habitat loss for rocky reef fishes in north Patagonia

Since the introduction of Undaria into Nuevo Gulf, Argentina, around 1992, this alien seaweed has now colonized different sites over 700 km of coast, forming dense seasonal forests in waters from 0 to 15 meters in depth. In the spring it is common for plants of Undaria to break away from the substrate and be transported by sea currents. As Undaria gets stuck onto reefs it has the potential to reduce habitat quality for reef fish by physically obstructing refuges. This study aims to assess the impact of Undaria on the abundance of four species of rocky-reef fishes by an observational experiment. Fish abundance on reefs with and without Undaria was estimated by underwater visual census methods. Sites were classified according to their topographical relief, as this was expected to influence the effect of Undaria on the abundance of reef fishes. Fish abundance decreased markedly in low-relief reefs that had been covered by Undaria. In contrast, the drifting Undaria had no effect on the abundance of any of the fish species considered in high-relief reefs, where it tends to cover only the lowest-lying areas, leaving much of the refuges for fish unaffected. In conclusion, the presence of Undaria off the coast of Argentina results in transitory habitat loss for reef fishes inhabiting low-relief reefs during late spring and early summer. Although we do not know how much of a threat this habitat loss represents for the conservation of reef fish populations of northern Patagonia, the documented local impact of Undaria within the gulfs is strong and may affect a number of recreational and commercial activities which are centered on the reefs and their fish assemblages.