Epibenthic Habitats Research Articles

Reversible shifts between interstitial and epibenthic habitats in evolutionary history: Molecular phylogeny of the marine flatworm family Boniniidae (Platyhelminthes: Polycladida: Cotylea) with descriptions of two new species.

Tiny animals in various metazoan phyla inhabit the interstices between sand and/or gravel grains, and adaptive traits in their body plan, such as simplification and size reduction, have attracted research attention. Several possible explanations of how such animals colonized interstitial habitats have been proposed, but their adaptation to this environment has generally been regarded as irreversible. However, the actual evolutionary transitions are not well understood in almost all taxa. In the present study, we show reversible evolutionary shifts from interstitial to epibenthic habitats in the lineage of the polyclad flatworm genus Boninia. In addition, we establish two new species of this genus found from different microhabitats on a single beach in Okinawa Island, Japan: (i) the interstitial species Boninia uru sp. nov. from gravelly sediments and (ii) the epibenthic species Boninia yambarensis sp. nov. from rock undersurfaces. Our observations suggest that rigid microhabitat segregation exists between these two species. Molecular phylogenetic analyses based on the partial 18S and 28S rDNA sequences of the new Boninia species and four other congeners, for which molecular sequences were available in public databases [Boninia antillara (epibenthic), Boninia divae (epibenthic), Boninia neotethydis (interstitial), and an unidentified Boninia sp. (habitat indeterminate)], revealed that the two interstitial species (B. neotethydis and B. uru sp. nov.) were not monophyletic among the three epibenthic species. According to ancestral state reconstruction analysis, the last common ancestor of the analyzed Boninia species inhabited interstitial realms, and a shift to the epibenthic environment occurred at least once. Such an "interstitial to noninterstitial" evolutionary route seems to be rare among Animalia; to date, it has been reported only in acochlidian slugs in the clade Hedylopsacea. Our phylogenetic tree also showed that the sympatric B. uru sp. nov. and B. yambarensis sp. nov. were not in a sister relationship, indicating that they colonized the same beach independently rather than descended in situ from a common ancestor that migrated and settled at the beach.

Fish associated with a subsea pipeline and adjacent seafloor of the North West Shelf of Western Australia

Information on the potential ecological value of offshore oil and gas infrastructure is required as it reaches the end of its operational life and decisions must be made regarding the best practice option for decommissioning. This study uses baited remote underwater stereo-video systems to assess fish assemblages along an offshore subsea pipeline and in adjacent natural seabed habitats at ∼140 m depth on the North West Shelf of Western Australia. A total of 955 fish from 40 species and 25 families were recorded. Species richness was, on average 25% higher on the pipeline (6.48 ± 0.37 SE) than off (4.81 ± 0.28 SE) while relative abundance of fish was nearly double on the pipeline (20.38 ± 2.81 SE) than in adjacent natural habitats (10.97 ± 1.02 SE). The pipeline was characterised by large, commercially important species known to associate with complex epibenthic habitat and, as such, possessed a biomass of commercial fish ca 7.5 × higher and catch value ca. 8.6 × ($65.11 ± $11.14 SE) than in adjacent natural habitats ($7.57 ± $2.41 SE). This study has added to the knowledge of fish assemblage associations with subsea infrastructure and provides a greater understanding of the ecological and fisheries implications of decommissioning, helping to better inform decision-making on the fate of infrastructure.

Diel shifts and habitat associations of fish assemblages on a subsea pipeline

Nocturnal studies of fish assemblages are relatively rare, particularly at depths greater than 100 m, despite the relevance of diel shifts in habitat usage to fisheries management. This study assesses fish diversity and abundance from remotely operated vehicle (ROV) video recordings that were collected by industry during the day and at night in the course of a subsea pipeline survey, at 130 m depth. A total of 34,862 fish from 41 species and 25 families were recorded along the 23 km of pipeline. The pipeline was characterised by a high abundance of commercially important snapper (Lutjanidae) and grouper (Epinephelidae) species. The fish assemblage sampled along the pipeline during the day differed markedly to that sampled at night time. Several ubiquitous predatory species, e.g. Epinephelus areolatus, Lutjanus quinquelineatus, Lutjanus russellii, where present during the day but not at night, likely moving off the pipeline to feed in nearby habitats. Structurally complex mesophotic epibenthic habitat forming invertebrates were observed on the pipeline including; mesophotic corals, crinoids (featherstars), gorgonocephalids (basket stars), hydroids, true anemones and sponges, but elsewhere in the region, historical trawling effort is thought to have removed such organisms and extensively modified the original habitat. These complex epibenthic habitats were considered to be important to commercial target species and the modification or loss of these habitats is thought to have negatively impacted the valuable commercial fisheries in the region. This study suggests pipelines can offer a significant epibenthic habitat and refuge for fish, potentially comparable to the historical habitats lost to trawling. Fish diversity and abundance was observed to be consistently greater where a gap/crevice existed beneath the pipeline and many species were frequently observed in conjunction with the complex invertebrate matrix above the pipeline, under spanning sections beneath the pipeline and at the pipeline-sediment interface, regardless of time of day. Further dietary analysis, spatially explicit fisheries modelling and off-pipeline surveys on the natural seafloor are required to further investigate the ecological value of pipelines and its influence in fish behaviour. The study builds knowledge of mesophotic coastal fish ecology and will help to inform discussions regarding the ecological and fisheries implications of decommissioning and the removal of subsea infrastructure.

Identifying epibenthic habitats on the Seco de los Olivos Seamount: Species assemblages and environmental characteristics

High habitat diversity was observed on the Seco de los Olivos Seamount (SW Mediterranean Sea), a Site of Community Importance belonging to the Spanish marine Natura 2000 Network. Thirteen epibenthic habitats were identified by analysing 55 Remotely Operated Vehicle (ROV) transects from 76 m to 700 m depth and derived data from multibeam bathymetry and high resolution seismic profiles. Habitat identification was based on a combination of assemblages of habitat-forming species and the environmental characteristics supporting their distribution. Depth and slope were identified as the main significant factors structuring epibenthic assemblages. The high diversity and patchiness of habitats found on the Seco de los Olivos Seamount can be explained by the high environmental variability resulting from its wide geomorphologic diversity, where flat summits, steep flanks, rocky outcrops and sedimentary moats are combined. The distribution of benthic habitats at this seamount is likely a combination of suitable ecological conditions, local recruitment, feeding strategies and attachment mechanisms. Knowledge on the occurrence of habitats in areas of natural importance is crucial to species and habitats conservation and to develop proper monitoring and management programs aimed at fulfilling European regulation requirements.

Links between epibenthic community patterns and habitat characteristics in the Parila lagoon (Croatia)

The aim of the study was to describe epibenthic macroinvertebrates and demersal fish and to identify links between spatial patterns and habitat characteristics (sediment, salinity, oxygen and temperature). The research was performed in the Parila lagoon in Croatia ; an important nursery ground where conditions in epibenthic community can have significant direct and indirect impact on fisheries. Diversity (30 species) and abundance (from 0 to 7 ind m−2) of epibenthic assemblages was low on all stations and showed remarkable seasonal changes. Such patchiness could support the estuarine quality paradox idea, i.e. the most abundant species are well adapted to changing conditions in a variable environment. The ecological analysis showed that total phosphorus concentration (TP), temperature (T) and salinity (S) were the main environmental parameters affecting community structure of the Parila lagoon. Six species listed as frequent (Nerophis ophidion, Zosterisessor ophiocephalus, Syngnathus typhle, Cerastoderma glaucum, Palaemon adspersus and Carcinus aestuarii) have a potential to be used as indicators of habitat quality. In terms of trophic diversity, community was dominated by carnivores and omnivores. Fish trophic index (above 2.7) indicated upward shift of the local food chain and towards mostly carnivorous species, while the absence of herbivorous species pointed to low environmental quality. The present study gives an insight into the current ecological conditions in the Parila lagoon. Further research about the sensitivity of these potential indicators to different types of pressure is necessary in order to select robust early-warning indicators framework.

Coastal Fish Assemblages Reflect Geological and Oceanographic Gradients Within An Australian Zootone

Distributions of mobile animals have been shown to be heavily influenced by habitat and climate. We address the historical and contemporary context of fish habitats within a major zootone: the Recherche Archipelago, southern western Australia. Baited remote underwater video systems were set in nine habitat types within three regions to determine the species diversity and relative abundance of bony fishes, sharks and rays. Constrained ordinations and multivariate prediction and regression trees were used to examine the effects of gradients in longitude, depth, distance from islands and coast, and epibenthic habitat on fish assemblage composition. A total of 90 species from 43 families were recorded from a wide range of functional groups. Ordination accounted for 19% of the variation in the assemblage composition when constrained by spatial and epibenthic covariates, and identified redundancy in the use of distance from the nearest emergent island as a predictor. A spatial hierarchy of fourteen fish assemblages was identified using multivariate prediction and regression trees, with the primary split between assemblages on macroalgal reefs, and those on bare or sandy habitats supporting seagrass beds. The characterisation of indicator species for assemblages within the hierarchy revealed important faunal break in fish assemblages at 122.30 East at Cape Le Grand and subtle niche partitioning amongst species within the labrids and monacanthids. For example, some species of monacanthids were habitat specialists and predominantly found on seagrass (Acanthaluteres vittiger, Scobinichthys granulatus), reef (Meuschenia galii, Meuschenia hippocrepis) or sand habitats (Nelusetta ayraudi). Predatory fish that consume molluscs, crustaceans and cephalopods were dominant with evidence of habitat generalisation in reef species to cope with local disturbances by wave action. Niche separation within major genera, and a sub-regional faunal break, indicate future zootone mapping should recognise both cross-shelf and longshore environmental gradients.

The foraging ecology of Arctic cod (Boreogadus saida) during open water (July–August) in Allen Bay, Arctic Canada

Arctic cod (Boreogadus saida) is a schooling fish providing a critical link between lower and upper trophic levels in the Arctic. This study examined foraging of Arctic cod collected from Allen Bay, Cornwallis Island, Canada (~75 N 95 W), during summer 2010 using temporal indicators of diet including stomach content, and carbon (δ13C) and nitrogen (δ15N) stable isotopes of liver and muscle. Foraging at the time of capture reflected sympagic and epi-benthic habitats indicated by the prevalence of cyclopoid and harpacticoid copepods in stomachs, whereas stable isotope data, which provide an estimate of feeding over a longer period, indicated pelagic prey as important. Prey selection of juveniles differed from adults based on stable isotopes, while large adults showed the most separation based on stomach contents, suggesting size-related diet shifts. Compared to studies near Resolute in the 1970s, 1980s, and 1990s, growth and pre-spawning gonadal conditions of Arctic cod have not changed.

Mosasaur predation on a nautiloid from the Maastrichtian Pierre Shale, Central Colorado, Western Interior Basin, United States

Mosasaurs were common predators on the ammonites that inhabited the upper water column of the Late Cretaceous Western Interior epicontinental seaway of North America. Mosasaurs developed predictable behaviour patterns for feeding on ammonite prey. There are no previous reports of mosasaur predation on the much less common Cretaceous nautiloids, possibly because of the prey's predominantly deep, epibenthic habitat, as deduced from modern Nautilus life habits. A single specimen of the highly inflated nautiloid, Eutrephoceras dekayi (Conrad), prey to a small adult mosasaur, likely Platycarpus, Prognathodon or Mosasaurus, is reported herein from the Pierre Shale of Colorado in the Early Maastrichtian biozone of Baculitesgrandis transitional to the biozone of B. clinolobatus. The nautiloid was killed in the same manner as described previously for discoid ammonites (Placenticeras, Sphenodiscus) from coeval strata in the USA and Canada.

Phylogenetic study of benthic, spine‐bearing prorocentroids, including Prorocentrum fukuyoi sp. nov.

SUMMARYSpecies of prorocentroid dinoflagellates are common in marine benthic sediment and epibenthic habitats, as well as in planktonic habitats. Marine planktonic prorocentroids typically possess a small spine in the apical region. In this study, we describe a new, potentially widely distributed benthic species of Prorocentrum, P. fukuyoi sp. nov., from tidal sand habitats in several sites in Australia and from central Japan. This species was found to possess an apical spine or flange and was sister species to P. emarginatum. We analyzed the phylogeny of the group including this new species, based on large subunit (LSU) rDNA sequences. The genus contained a high level of divergence in LSU rDNA, in some cases among sister taxa. P. fukuyoi and P. emarginatum were found to be most closely related to a clade of generally planktonic taxa. Several morphological features may constitute more informative synapomorphies than habitat in distinguishing clades of prorocentroid species.

Diel and seasonal habitat and food segregation of three sympatric Coregonus lavaretus forms in a subarctic lake

Habitat use of three sympatric whitefish Coregonus lavaretus forms was determined using hydroacoustics, pelagic trawling and epibenthic gillnetting in the subarctic Lake Muddusjärvi during the day and night in June, August and September. Whitefish constituted 97% of the numerical catches and whitefish with high number of gill rakers (DR) were the most abundant whitefish form. Forms with low numbers of gill rakers used only epibenthic habitats during both the day and night in all study periods: large whitefish with low numbers of gill rakers (LSR) dwelled mainly at depths 0–10 m, whereas small whitefish with low numbers of gill rakers (SSR) used deeper (>10 m) habitats. LSR and SSR whitefish consumed mainly benthic macroinvertebrates during all study occasions. The planktivorous DR whitefish used both epibenthic and pelagic habitats, but vertical habitat selection varied both over time of day and season. In June, when light intensity was continuously high, DR whitefish did not perform diel vertical migrations. In August and September, when dark nights were distinguishable, DR whitefish ascended from the bottom to the pelagic at dusk to feed on zooplankton, and descended at dawn. DR whitefish used pelagic habitats only at the lowest light intensities during the night, which was probably related to the high predation risk from brown trout Salmo trutta.

Piscivory and prey selection of four predator species in a whitefish dominated subarctic lake

According to logistic regressions derived for pikeEsox luciusand burbotLota lota, the probability of ingesting fishes in Lake Muddusjärvi, northern Finland, was 50% at 19·3 and 22·1 cmLT, whereas Arctic charrSalvelinus alpinusand brown troutSalmo truttashifted to piscivory at the lengths of 25·7 and 26·4 cmLT. The specialist piscivores, pike and burbot, consumed more prey species and took a wider range of prey sizes than Arctic charr and brown trout. The prey length for all predators increased in relationship to predator length. WhitefishCoregonus lavaretuswas the dominant prey species in the lake and in the diet of all the piscivorous species. The whitefish population was divided into three forms, of which the slow‐growing, and the most numerous densely rakered whitefish form (DR), was selected by all predator species. This form also had the smallest average size and widest habitat range, utilizing both pelagic and epibenthic habitats. Two sparsely rakered whitefish forms (LSR and SSR) occupied only epibenthic habitats and had lower relative densities than DR. These forms, LSR and SSR, had a minor importance in the diet of predator species.

Brown trout (Salmo truttaL.) and Arctic charr (Salvelinus alpinus(L.)) as predators on three sympatric whitefish (Coregonus lavaretus(L.)) forms in the subarctic Lake Muddusjärvi

Abstract – Brown trout (Salmo truttaL.) and Arctic charr (Salvelinus alpinus(L.)) use whitefish (Coregonus lavaretus(L.)) as their main prey in the subarctic Lake Muddusjärvi. Brown trout dwelled in littoral and pelagic habitat, whereas Arctic charr lived only in epibenthic habitat. Both species shifted to whitefish predation at a length of 20–30 cm. At this size, brown trout fed on larger whitefish than Arctic charr. Whitefish occur in three sympatric forms, differing in their habitat, ecology and morphology. Both the predators preyed primarily upon the small‐sized, densely rakered whitefish form (DR), which was the most numerous whitefish form in the lake. DR used both epibenthic and pelagic habitat, whereas two sparsely rakered whitefish forms dwelled (LSR and SSR) only in epibenthic habitat: LSR in littoral and SSR in profundal areas. Sparsely rakered whitefish forms had minor importance in predator diet.

Recent benthic foraminiferal assemblages in deep high-energy environments from the Gulf of Cadiz (Spain)

The benthic environment in the Gulf of Cadiz, north-eastern Atlantic, is strongly affected by the Mediterranean outflow water undercurrent (MOW) which flows northwards along the western Iberian Margin at 500–1500 m water depth. Foraminiferal census counts of living and dead assemblages from 27 surface samples ranging from 103 to 1917 m water depth, and the examination of hard substrates reveal a close correlation of the fauna with the local hydrography and sediment facies. Four different faunal groups are separated by factor analysis of the living fauna. Assemblage 1 contains typical lower slope species and dominates samples from the lower MOW core layer and in the North Atlantic deep water below. Shelf edge foraminifera are common in assemblage 2a which shows the highest proportions in samples from 103 to 272 m. Assemblage 2b is dominated by upper slope species and suspension-feeders that are frequent in the upper MOW core layer and in distal settings between 396 and 901 m. Species from assemblage 3 prefer epibenthic habitats and are recorded with high proportions exclusively in the immediate flow paths of the upper MOW between 496 and 881 m. Colonisation structures and species composition of epibenthic assemblages from the proximal facies largely differ from those in distal settings. In general, epibenthic foraminifers only use elevated substrates under the influence of near-bottom flow. Under high current velocities, epibenthic foraminifers prefer large and heavy objects. They colonise high attachment levels where a maximum yield of advected food particles can be achieved. In distal settings at lower flow velocities, the elevation height does not exceed 20 mm above the surrounding sediment surface. This level is related to a hydrologic transition layer with high concentrations of suspended particles. The comparison of microhabitat preferences and faunal structure under high and low current velocities reveal that substrate stability may be a confining environmental variable for endobenthic and shallow epibenthic foraminifers. The observations also indicate that the preferential settling height of epibenthic foraminifera is related to the highest lateral flux rates of food particles within reach from the sea floor. A dynamic selection of elevated microhabitats is only used by 7.8% of all species recognised in the Gulf of Cadiz area.

Diversity of epibenthic habitats of gammaridean amphipods in the eastern Weddell Sea

Diversity of epibenthic habitats of gammaridean amphipods in the eastern Weddell Sea

SYRINGAMMINA CORBICULA SP. NOV. (XENOPHYOPHOREA) FROM THE CAPE VERDE PLATEAU, E. ATLANTIC

A new xenophyophore species, Syringammina corbicula, is described and illustrated from live specimens collected at 3106 m on the Cape Verde Plateau, off West Africa. This species is characterized by its epibenthic habitat and the unique basket-like depressions that rim the periphery of its hemispherically-shaped test of anastomosing, agglutinated tubes. Morphological features of the test and cytoplasm of S. corbicula are described in detail and contrasted with other known xenophyophore species grouped in the genus Syringammina. Laboratory observations of the pseudopodia extruded by live individuals are presented and the potential feeding behavior of S. corbicula is discussed briefly.

Compensatory Restoration of Mangrove Habitat Following the Tampa Bay Oil Spill

ABSTRACT In 1993, an incident involving three vessels near the entrance to Tampa Bay, near St. Petersburg, Florida, resulted in a discharge approximately 300,000 gallons of No. 6. fuel oil into the waters off Egmont Key. The oil contacted the shores from Egmont Key to locations approximately 14 miles to the north. Oil also entered Boca Ciega Bay through John's Pass and impacted four small islands that supported mature overwash mangrove forest. A cooperative damage assessment process was developed between the Responsible Parties (RPs) and the trustees for the natural resources: the state of Florida, the National Oceanic and Atmospheric Administration (NOAA), and the U.S. Fish and Wildlife Service (USFWS). Through the cooperative process, several alternatives for primary and compensatory restoration were developed. In considering alternatives to compensate for impacts to epibenthic communities, fish and bird habitats, wetlands, and mangrove communities, the trustees and the RPs considered purchase of shoreline habitat and restoration of mangrove forest at several sites. The RP developed a proposal to purchase and deed into public ownership in perpetuity, a 10.67-acre parcel of land on the west bank of Cross Bayou in Boca Ciega Bay that had been used as a disposal site for dredge spoil in the past. This site consisted of approximately 5.0 acres of uplands, 4.4 acres of mangrove forest, and 1.4 acres of nonmangrove intertidal and subtidal habitat. The objective of the project was to establish a typical Tampa Bay mangrove forest and a roadside buffer free of exotic plant species. The secondary goals were to establish a typical Tampa Bay salt marsh dominated by smooth cordgrass (Spartina spp.) as a successional precursor to mangrove recruitment by seeds and seedlings. Tidal exchange through the site was reestablished to improve water quality and increase export of mangrove detritus and import of high-quality tidal waters. The project was designed and constructed by the RPs with the approval and supervision of the trustees. The project was completed and title transferred to Pinellas County, Florida in summer 1999. A monitoring program was developed and performance criteria established by trustee representatives and the RPs. The monitoring program currently is being conducted and has met performance criteria to date. This project demonstrates the positive result of trustees and the RPs working together to provide compensation to the environment.

Taxon-specific responses to high-flow disturbance in streams:implications for population persistence

AbstractThis study examined how the leptophlebiid mayfly Deleatidium spp., the mudsnail Potamopyrgus antipodarum, and stony-cased caddisflies Pycnocentrodes aeris and Hudsonema amabilis, each with different flow habitat preferences and motilities, respond to elevated flows. Our aim was to learn more about how refuge-seeking behavior and life-history traits affect species persistence along the flow disturbance gradient. All taxa responded to incrementally increased flows in a recirculating laboratory flume with layered substrata by moving to deeper, more stable layers. However, snails made the most pronounced shift to protected sublayers. Caddisflies that remained on epibenthic surfaces during high flows laid draglines on surface rocks to effectively resist dislodgment. Very few individuals of any taxon were washed from epibenthic habitats by high flows alone. Simulated flash floods in artificial stream channels revealed that dislodgment was linked to substrate stability and that likelihood to be dislodged...

Benthic shear stress and sediment condition

This article presents a literature review and analysis which finds that the conditions in a shallow waterway are dependent on the underlying shear stress. Circulation and mixing have been recognised as somehow important to the health of the lower water column and sediment of a pond. Benthic shear stress is identified as one of the principal governors of sediment condition. Benthic shear stress regulates the convection of volatile substances such as oxygen across the sediment–water interface. Benthic shear stress is the primary determinant of where particulate matter scours, rolls, saltates, settles, and consolidates. The critical shear stress of very small particles tends to be determined by density rather than size. Excessive shear stress may cause organic material to become buried under a fallout of silt in locations with lower shear stress. Such organic matter trapped in a sludge blanket consumes oxygen faster than it can diffuse through the pores between grains of soil, resulting in anaerobic soils. The objective of pond aeration and circulation systems should be to optimise conditions for the growth of cultured stock. Better management of the epi-benthic habitat of shrimp and prawns would concurrently reduce the use of energy and soil. The finding is that pond sediment condition may be assessed in terms of benthic shear stress. Control of benthic shear stress would enable resuspension and oxygenation of organic detritus without making the situation worse by intermixing organic matter with mineral soil. Desirable benthic shear stresses are expected to exist in the range of 0.003–0.03 N/m 2 where organic matter is gently maintained in an aerobic state. Locations having less than 0.001 N/m 2 benthic shear stress are subject to burial under accumulating mineral silts. Excessive scour occurs in locations having more than 0.1 N/m 2. High stress locations are the source of suspended solids which fallout on locations experiencing lower rates of benthic shear stress. The declining productivity of shrimp mariculture ponds is associated with excessive stocking and feeding because these practices necessitate intensive aeration, which results in high rates of pond circulation. The circulation causes soil to be scoured in some places while fallout covers the bottom in other places. Consequently intensive cropping of a pond causes feed pellets and other organic material to be buried in the soil, creating anaerobic conditions whereby bacterial metabolites are produced and released into the water column.

Habitat use and life history of brown trout (Salmo trutta) and Arctic charr (Salvelinus alpinus) in some low acidity lakes in central Norway

Habitat utilization and the life history of browntrout Salmo trutta and Arctic charr Salvelinus alpinus were investigated in fivesympatric populations and five allopatric brown troutpopulations in Hoylandet catchment, a atmosphaericlow deposition area in Mid Norway. There was asignificant inverse correlation in abundance ofepibenthic Arctic charr and brown trout in theselakes, indicating that the latter species is dominant.The largest numbers of sympatric brown trout andArctic charr were caught in epibenthic habitat. In twolakes, brown trout to some extent also occurredpelagically, while pelagic individuals of Arctic charrwere found in all five lakes. The main food items forboth epibenthic and pelagic brown trout wereterrestrial surface insects and chironomid pupae.Zooplankton was the primary food item for Arctic charrin both habitats. Although the age distribution wasvery different in the populations, neither speciesseem to suffer from recruitment failure. There was nosignificant difference in survival rates betweensympatric populations of brown trout and Arctic charr.We found a significant inverse correlation betweenepibenthic catches of brown trout and the mean weightof 4+ fish, the most abundant age group. However, ifusing weight data for three-year-old fish, no suchrelationship was found for Arctic charr. Brown troutand Arctic charr reached asymptotic lengths of197–364 mm and 259–321 mm, respectively. Both speciestypically reached sexual maturity at age 2–3, and nomaturation-induced mortality was evident. We concludethat fish populations in Hoylandet lakes areregulated throughout their lifes by inter- andintraspecific competition.

Differences in habitat utilization among native, native stocked, and non-native stocked brown trout (Salmo trutta) in a hydroelectric reservoir

Native and native-stocked brown trout (Salmo trutta) in Lake Tesse, a regulated hydroelectric reservoir (southern Norway), were spatially segregated according to size: small individuals occurred mainly in the epibenthic habitat and larger individuals mainly in the pelagic habitat. In contrast, all size groups of non-native stocked brown trout were mostly restricted to the epibenthic habitat. Age-specific lengths were generally larger for non-native than for native stocked trout, which were larger than native fish. However, growth rate between age 3 and 4 was significantly lower for non-native stocked fish than for native and native stocked fish. Differences in body length were mainly due to strain but also to some extent to habitat. Native fish had significantly fuller stomachs in the pelagic than in the epibenthic habitat in summer. Epibenthic non-native fish had significantly fuller stomachs than native and native stocked fish in August but not in July. Native and native stocked fish fed mainly on surface insects and planktonic crustaceans in both habitats. We hypothesize that the non-native brown trout stocked in Lake Tesse do not use the pelagic habitat in the home lake and are therefore less adapted to utilize such habitat than populations originating from lakes where pelagic habitat is available.