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  • 1.
    Andersson, Mathias H.
    et al.
    Stockholm University, Faculty of Science, Department of Zoology.
    Berggren, Matz
    Wilhelmsson, Dan
    Stockholm University, Faculty of Science, Department of Zoology.
    Öhman, Marcus C
    Stockholm University, Faculty of Science, Department of Zoology.
    Epibenthic colonization of concrete and steel pilingsin a cold-temperate embayment: a feld experiment2009In: Helgoland Marine Research, ISSN 1438-387X, E-ISSN 1438-3888, Vol. 63, no 3, p. 249-260Article in journal (Refereed)
    Abstract [en]

    With large-scale development of offshore wind farms, vertical structures are becoming more common in open water areas. To examine how vertical structures of different materials may be colonized by epibenthic organisms, an experiment was carried out using steel and concrete pilings constructed to resemble those commonly used in wind farm constructions as well as in bridges, jetties and oil platforms. The early recruitment and succession of the epibenthic communities were sampled once a month for the first 5 months and then again after 1 year. Further, the fish assemblages associated with the pillars were sampled and compared to natural areas. The main epibenthic species groups, in terms of coverage, diVered between the two materials at five out of six sampling occasions. Dominant organisms on steel pillars were the barnacle Balanus improvisus,the calcareous tubeworm Pomatoceros triqueter and the tunicate Ciona intestinalis. On the concrete pillars, the hydroid Laomedea sp. and the tunicates Corella parallelogramma and Ascidiella spp. dominated. However, there was no different in coverage at different heights on the pillars or in biomass and species abundance at different directions (north-east or south-west) 5 months after submergence. Fish showed overall higher abundances and species numbers on the pillars (but no difference between steel and concrete)compared to the surrounding soft bottom habitats but not compared to natural vertical rock walls. Two species were attracted to the pillars, indicating a reef effect; Gobiusculus flavescens and Ctenolabrus rupestris. The bottom-dwelling gobies, Pomatoschistus spp., did not show such preferences.

  • 2.
    Andersson, Mathias H
    et al.
    Stockholm University, Faculty of Science, Department of Zoology, Department of Animal Ecology.
    Dock-Åkerman, Emily
    Stockholm University, Faculty of Science, Department of Zoology, Department of Animal Ecology.
    Ubral-Hedenberg, Ramona
    Stockholm University, Faculty of Science, Department of Zoology, Department of Animal Ecology.
    Öhman, Marcus C
    Stockholm University, Faculty of Science, Department of Zoology, Department of Animal Ecology.
    Sigray, Peter
    Stockholm University, Faculty of Science, Department of Meteorology .
    Swimming Behavior of Roach (Rutilus rutilus) and Three-spined Stickleback Gasterosteus aculeatus) in Response to Wind Power Noise and Single tone frequencies2007In: Ambio, ISSN 0044-7447, E-ISSN 1654-7209, Vol. 36, no 8, p. 636-638Article in journal (Refereed)
    Abstract [en]

    Human-induced underwater noise is drastically increasing as the result of offshore installations and human activities in the marine environment. Many of these structures and activities produce low-frequency noise that could potentially disturb or have harmful effects on several species of teleost fishes. Within the next decade, thousands of wind turbines will be in use in coastal and offshore waters and there is increasing concern on how they may influence marine life. The aims of this study were to examine how swimming behavior of roach (Rutilus rutilus) and three-spined stickleback (Gasterosteus aculeatus) were influenced by single-frequency sounds and noise generated by an offshore wind turbine, and the function of sound pressure level.

  • 3.
    Andersson, Mathias H
    et al.
    Stockholm University, Faculty of Science, Department of Zoology, Animal Ecology.
    Gullström, Martin
    Stockholm University, Faculty of Science, Department of Zoology, Animal Ecology.
    Asplund, Maria E
    Department of Marine Ecology, Göteborg University.
    Öhman, Marcus C
    Stockholm University, Faculty of Science, Department of Zoology, Animal Ecology.
    Importance of Using Multiple Sampling Methodologies for Estimating of Fish Community Composition in Offshore Wind Power Construction Areas of the Baltic Sea2007In: Ambio, ISSN 0044-7447, E-ISSN 1654-7209, Vol. 36, no 8, p. 634-636Article in journal (Refereed)
    Abstract [en]

    It is standard procedure that an Environmental Impact Assessment (EIA) is conducted before larger constructions are built. To adequately describe the impact, methods used in an EIA should be carefully adapted considering both the character of the constructions under development and the environment that will be affected. Various sampling techniques are applied to estimate fish abundances and species composition. Methods used, including trawling, seine and gill netting, angling, echo-sound sampling, fishery data, video recordings, dredging, and visual counts using SCUBA, will all give different estimates of fish community composition.

  • 4.
    Andersson, Mathias H.
    et al.
    Stockholm University, Faculty of Science, Department of Zoology, Animal Ecology.
    Öhman, Marcus C.
    Stockholm University, Faculty of Science, Department of Zoology, Animal Ecology.
    Fish and sessile assemblages associated with wind-turbineconstructions in the Baltic Sea2010In: Marine and Freshwater Research, ISSN 1323-1650, E-ISSN 1448-6059, Vol. 61, no 6, p. 642-650Article in journal (Refereed)
    Abstract [en]

    Offshore wind farms are being built at a high rate around the world to meet the demand for renewable energy. We studied fish and sessile communities on and around offshore wind-turbine foundations in the southern Baltic Sea, 7 years after construction, using visual census techniques to determine how fish, sessile-invertebrate and algal communities are affected by the introduction of such structures. Fish assemblages were dominated by two-spotted gobies (Gobiusculus flavescens) that were found in large shoals in close association with the vertical surface. At the seabed, close to the foundation, the black goby (Gobius niger) was recorded in large numbers. The most obvious difference in fish densities was found between wind-power foundations extending through the entire water column and the surrounding open waters. Fouling assemblages on the vertical foundation surfaces and at the seabed just below differed from those at the seabed further away by having higher coverage of blue mussel (Mytilus edulis) and less algal growth. The results from the present study suggest that the introduction of offshore wind turbines in marine waters could have a positive effect on fish numbers and the presence of sessile invertebrates

  • 5.
    Eklöf, J.S.
    et al.
    Stockholm University, Faculty of Science, Department of Systems Ecology.
    Gullström, Martin
    Department of Zoology. Ekologi.
    Björk, Mats
    Department of Botany.
    Asplund, M.E.
    Hammar, L.
    Dahlgren, A.
    Öhman, Marcus
    Department of Zoology. Ekologi.
    The importance of grazing intensity and frequency for physiological responses of the tropical seagrass Thalassia hemprichii2008In: Aquatic botany, Vol. 89, p. 337-340Article in journal (Refereed)
    Abstract [en]

    Seagrass grazing is an intrinsic disturbance in primarily tropical and subtropical areas. While there is a general parabolic response in seagrass growth to grazing intensity, there is less knowledge on the role of grazing frequency, as well as potential interactions between grazing intensity and frequency. This study experimentally investigated physiological responses in Thalassia hemprichii to simulated (leaf cutting) grazing regimes with different intensities (25% vs. 75%) and frequencies (I times vs. 3 times) over 35 days in Chwaka Bay (Zanzibar, Tanzania). The results showed that the two high-intensity treatments (75% removal) had 37-41% lower growth rate than the low-intensity/low-frequency treatment, and rhizome sugar and starch content were both affected in a similar way. A 36% lower starch content in the simulated low-intensity/high-frequency regime (25% x 3) compared to the one of low-intensity/low-frequency (25% x I) also shows an interaction between grazing intensity and frequency. This suggests that high-intensity (and to some extent frequency) grazing regimes, in comparison to low-intensity regimes, could negatively affect T. hemprichii growth, energy reserves, and thereby the ability to deal with additional stress like light limitation or grazing.

  • 6.
    Garpe, Kajsa C.
    et al.
    Stockholm University, Faculty of Science, Department of Zoology.
    Yahya, Saleh A.S.
    Stockholm University, Faculty of Science, Department of Zoology.
    Lindahl, Ulf
    Stockholm University, Faculty of Science, Department of Zoology.
    Öhman, Marcus C.
    Stockholm University, Faculty of Science, Department of Zoology.
    Long-term effects of the 1998 coral bleaching event on reef fish assemblages2006In: Marine Ecology Progress Series, ISSN 0171-8630, E-ISSN 1616-1599, Vol. 315, p. 237-247Article in journal (Refereed)
    Abstract [en]

    Coral bleaching events constitute compound disturbances often resulting in coral death as well as successive degradation of the reef framework. The 1997/1998 El Niño Southern Oscillation (ENSO) was the most severe on record and affected coral reefs worldwide. The present study examined the response of fish assemblages in plots of transplanted coral before and after the 1998 bleaching. Multidimensional scaling ordinations (MDS) demonstrate significant changes in assemblage composition related to habitat alteration. Within-site variability increased with disturbance, the increase being most apparent following substrate erosion. The differences in long-term responses as opposed to short-term responses were striking. Six mo after coral death, total abundance as well as taxonomic richness had increased at one of the sites, but not the other, whereas 6 yr later, both measures had decreased significantly at both sites. Functional groups, with documented affiliations with coral, were significantly influenced by the habitat alteration. Herbivore abundance increased as an immediate response to bleaching, but was subsequently decimated in eroded habitat. The loss of structural complexity had major detrimental effects on the entire fish community. In conclusion, we present evidence of severe and long-lasting secondary impacts of a catastrophic bleaching event, with no apparent recovery. The discrepancies between short-term and long-term responses underline the importance of long-term monitoring of fish assemblages following habitat alteration.

  • 7.
    Garpe, Kajsa C
    et al.
    Stockholm University, Faculty of Science, Department of Zoology.
    Öhman, Marcus C
    Stockholm University, Faculty of Science, Department of Zoology, Animal Ecology.
    Non-random habitat use by coral reef fish recruits in Mafia Island Marine Park, Tanzania2007In: African Journal of Marine Science, ISSN 1814-232X, Vol. 29, no 2, p. 187-199Article in journal (Refereed)
    Abstract [en]

    The habitat use by nearly 3 000 reef fish recruits, comprising 56 taxa, at seven sites in Mafia Island Marine Park, Tanzania, were examined. The study was carried out following the 1998 global coral bleaching event and all sites but one were dominated by dead coral and rubble. Mean recruit densities ranged between 0.1 m-2 and 0.7 m-2 among sites. Although live coral represented only 15% of the overall substrate composition, almost half of all observed recruits were found associated with this substrate. Pooled across all sites, 46% of the recruits used live coral cover in disproportion to availability. Principal component analyses were applied to explore microhabitat use by the 11 most common recruit taxa in comparison to availability. Among these taxa, 10 exhibited nonrandom microhabitat use and six associated with live coral in disproportion to availability. A comparison with the adult fish community revealed that adult abundances of four of the six coral selective recruit taxa were significantly correlated with live coral. The study demonstrated that reef fish recruits use microhabitats non-randomly and that a substantial proportion is selective towards live coral.

  • 8. Graham, Nicholas A. J.
    et al.
    Chabanet, Pascale
    Evans, Richard D.
    Jennings, Simon
    Letourneur, Yves
    MacNeil, M. Aaron
    McClanahan, Tim R.
    Öhman, Marcus C.
    Stockholm University, Faculty of Science, Department of Zoology.
    Polunin, Nicholas V. C.
    Wilson, Shaun K.
    Extinction vulnerability of coral reef fishes2011In: Ecology Letters, ISSN 1461-023X, E-ISSN 1461-0248, Vol. 14, no 4, p. 341-348Article in journal (Refereed)
    Abstract [en]

    P>With rapidly increasing rates of contemporary extinction, predicting extinction vulnerability and identifying how multiple stressors drive non-random species loss have become key challenges in ecology. These assessments are crucial for avoiding the loss of key functional groups that sustain ecosystem processes and services. We developed a novel predictive framework of species extinction vulnerability and applied it to coral reef fishes. Although relatively few coral reef fishes are at risk of global extinction from climate disturbances, a negative convex relationship between fish species locally vulnerable to climate change vs. fisheries exploitation indicates that the entire community is vulnerable on the many reefs where both stressors co-occur. Fishes involved in maintaining key ecosystem functions are more at risk from fishing than climate disturbances. This finding is encouraging as local and regional commitment to fisheries management action can maintain reef ecosystem functions pending progress towards the more complex global problem of stabilizing the climate.

  • 9. Graham, Nicholas A. J.
    et al.
    McClanahan, Tim R.
    MacNeil, M. Aaron
    Wilson, Shaun K.
    Polunin, Nicholas V. C.
    Jennings, Simon
    Chabanet, Pascale
    Clark, Susan
    Spalding, Mark D.
    Letourneur, Yves
    Bigot, Lionel
    Galzin, Rene
    Öhman, Marcus C.
    Stockholm University, Faculty of Science, Department of Zoology.
    Garpe, Kajsa C.
    Stockholm University, Faculty of Science, Department of Zoology.
    Edwards, Alasdair J.
    Sheppard, Charles R. C.
    Climate Warming, Marine Protected Areas and the Ocean-Scale Integrity of Coral Reef Ecosystems2008In: PLOS ONE, ISSN 1932-6203, Vol. 3, no 8, p. e3039-Article in journal (Refereed)
    Abstract [en]

    Coral reefs have emerged as one of the ecosystems most vulnerable to climate variation and change. While the contribution of a warming climate to the loss of live coral cover has been well documented across large spatial and temporal scales, the associated effects on fish have not. Here, we respond to recent and repeated calls to assess the importance of local management in conserving coral reefs in the context of global climate change. Such information is important, as coral reef fish assemblages are the most species dense vertebrate communities on earth, contributing critical ecosystem functions and providing crucial ecosystem services to human societies in tropical countries. Our assessment of the impacts of the 1998 mass bleaching event on coral cover, reef structural complexity, and reef associated fishes spans 7 countries, 66 sites and 26 degrees of latitude in the Indian Ocean. Using Bayesian meta-analysis we show that changes in the size structure, diversity and trophic composition of the reef fish community have followed coral declines. Although the ocean scale integrity of these coral reef ecosystems has been lost, it is positive to see the effects are spatially variable at multiple scales, with impacts and vulnerability affected by geography but not management regime. Existing no-take marine protected areas still support high biomass of fish, however they had no positive affect on the ecosystem response to large-scale disturbance. This suggests a need for future conservation and management efforts to identify and protect regional refugia, which should be integrated into existing management frameworks and combined with policies to improve system-wide resilience to climate variation and change.

  • 10.
    Gullström, Martin
    et al.
    Stockholm University, Faculty of Science, Department of Systems Ecology. Stockholm University, Faculty of Science, Department of Zoology.
    Berkström, Charlotte
    Stockholm University, Faculty of Science, Department of Systems Ecology.
    Öhman, Marcus C.
    Stockholm University, Faculty of Science, Department of Zoology.
    Bodin, Maria
    Dahlberg, Mattis
    Stockholm University, Faculty of Science, Department of Zoology.
    Scale-dependent patterns of variability of a grazing parrotfish (Leptoscarus vaigiensis) in a tropical seagrass-dominated seascape2011In: Marine Biology, ISSN 0025-3162, E-ISSN 1432-1793, Vol. 158, no 7, p. 1483-1495Article in journal (Refereed)
    Abstract [en]

    Although herbivorous fish form critical linkages between primary producers and higher trophic levels, the major factors regulating their spatial structure in seagrass systems remain poorly understood. The present study examined the parrotfish Leptoscarus vaigiensis in seagrass meadows of a tropical embayment in the western Indian Ocean. Stomach content analysis and direct field observations showed that L. vaigiensis is an efficient grazer, feeding almost exclusively on seagrass leaves. Seagrass shoot density was highly correlated to all density variables (total, juvenile and subadult) and juvenile biomass of L. vaigiensis, while subadult biomass was predicted by distance to neighbouring coral habitat. Moreover, density and biomass of predatory fish (piscivores) were predicted by seagrass canopy height and the distribution patterns of predators followed those of L. vaigiensis. Hence, factors at local (seagrass structural complexity and feeding mode) and landscape scale levels (seascape context and distribution of piscivores) likely mutually structure herbivorous fish communities. The findings underscore the importance of incorporating multiple scale-dependent factors when managing coastal seagrass ecosystems and their associated key species.

  • 11.
    Gullström, Martin
    et al.
    Stockholm University, Faculty of Science, Department of Zoology. Ekologi.
    Bodin, Maria
    Nilsson, Per G.
    Öhman, Marcus
    Stockholm University, Faculty of Science, Department of Zoology. Ekologi.
    Seagrass structural complexity and landscape configuration as determinants of tropical fish assemblage composition2008In: Marine ecology - progress series, Vol. 363, p. 241-255Article in journal (Refereed)
    Abstract [en]

    Seagrass meadows are regularly used by fish as resident, transient, or nursery habitat. However, there is a long-standing debate on how spatial variability of seagrass fish assemblages is determined. We examined the influence of seagrass structural complexity, physical water conditions, and proximity of neighboring shallow-water habitats on tropical fish assemblage composition in a shallow seagrass-dominated embayment at Zanzibar Island in the western Indian Ocean. Sampling of fish assemblages was carried out in seagrass meadows dominated by Enhalus acoroides or Thalassia hemprichii (3 localities each), 1 mixed meadow, and 1 unvegetated area. Overall, the density and biomass of fish were dominated by juvenile and subadult herbivores, either stationary seagrass residents or fish associated with coral reef and seagrass habitats. In terms of number of fish species, the majority were either carnivorous or omnivorous, and mainly coral-seagrass-associated. Multiple regression analysis indicated that canopy height was the foremost predictor for density, biomass, and species richness of juvenile fish, whereas adult and subadult fish densities were predicted by water depth. Moreover, distance-based correlation analyses revealed that fish assemblage structure was significantly correlated with the distance to neighboring mangrove and coral-reef habitats, shoot density, and (although weaker) canopy height. Based on these findings, attributes of seagrass structure and the location of a seagrass habitat within the seascape context appear to be important determinants of spatial patterns and variability of seagrass fish assemblages. This kind of information is important for spatial coastal management and for the selection of marine protected areas.

  • 12.
    Gullström, Martin
    et al.
    Stockholm University, Faculty of Science, Department of Zoology.
    de la Torre-Castro, Maricela
    Stockholm University, Faculty of Science, Department of Systems Ecology.
    Bandeira, Salomao O.
    Björk, Mats
    Kautsky, Nils
    Rönnbäck, Patrik
    Öhman, Marcus C.
    Stockholm University, Faculty of Science, Department of Zoology.
    Seagrass ecosystems in the Western Indian Ocean2002In: Ambio, ISSN 0044-7447, E-ISSN 1654-7209, Vol. 31, no 7, p. 588-596Article in journal (Refereed)
    Abstract [en]

    Seagrasses are marine angiosperms widely distributed in both tropical and temperate coastal waters creating one of the most productive aquatic ecosystems on earth. In the Western Indian Ocean (WIO) region, with its 13 reported seagrass species, these ecosystems cover wide areas of near-shore soft bottoms through the 12 000 km coastline. Seagrass beds are found intertidally as well as subtidally, sometimes down to about 40 m, and do often occur in close connection to coral reefs and mangroves. Due to the high primary production and a complex habitat structure, seagrass beds support a variety of benthic, demersal and pelagic organisms. Many fish and shellfish species, including those of commercial interest, are attracted to seagrass habitats for foraging and shelter, especially during their juvenile life stages. Examples of abundant and widespread fish species associated to seagrass beds in the WIO belong to the families Apogonidae, Blenniidae, Centriscidae, Gerreidae, Gobiidae, Labridae, Lethrinidae Lutjanidae, Monacanthidae, Scaridae, Scorpaenidae, Siganidae, Syngnathidae and Teraponidae. Consequently, seagrass ecosystems in the WIO are valuable resources for fisheries at both local and regional scales. Still, seagrass research in the WIO is scarce compared to other regions and it is mainly focusing on botanic diversity and ecology. This article reviews the research status of seagrass beds in the WIO with particular emphasis on fish and fisheries. Most research on this topic has been conducted along the East African coast, i.e. in Kenya, Tanzania, Mozambique and eastern South Africa, while less research was carried out in Somalia and the Island States of the WIO (Seychelles, Comoros, Reunion (France), Mauritius and Madagascar). Published papers on seagrass fish ecology in the region are few and mainly descriptive. Hence, there is a need of more scientific knowledge in the form of describing patterns and processes through both field and experimental work. Quantitative seagrass fish community studies in the WIO such as the case study presented in this paper are negligible, but necessitated for the perspective of fisheries management. It is also highlighted that the pressure on seagrass beds in the region is increasing due to growing coastal populations and human disturbance from e.g. pollution, eutrophication, sedimentation, fishing activities and collection of invertebrates, and its effect are little understood. Thus, there is a demand for more research that will generate information useful for sustainable management of seagrass ecosystems in the WIO.

  • 13. Gullström, Martin
    et al.
    Lundén, Bengt
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Bodin, Maria
    Kangwe, Juma W.
    Stockholm University, Faculty of Science, Department of Botany.
    Öhman, Marcus C.
    Stockholm University, Faculty of Science, Department of Zoology.
    Mtolera, Matern S. P.
    Björk, Mats
    Stockholm University, Faculty of Science, Department of Botany.
    Assessment of vegetation changes in seagrass communities of tropical Chwaka Bay (Zanzibar) using satellite remote sensing2006In: Estuarine, Coastal and Shelf Science, ISSN 0272-7714, E-ISSN 1096-0015, Vol. 67, no 3, p. 399-408Article in journal (Refereed)
    Abstract [en]

    Spatial and temporal dynamics of submerged aquatic vegetation (SAV) cover were studied in the relatively pristine and seagrass-dominated area of Chwaka Bay, Zanzibar (Tanzania) by using satellite remote sensing. Through complementary field work the potential of the technique for change detection was verified. The general changes in SAV cover were examined using Landsat images from 1986, 1987, 1998, 2001 and 2003. Two of these images, from 1987 (Landsat TM) and 2003 (Landsat ETM+), were specifically analysed to create a map of the change in SAV cover. Overall, the general distribution of SAV stayed fairly stable over the period investigated, but the result also showed regions where significant alterations, both losses and gains, had occurred between the two years. Based on our findings and anecdotal information from local fishermen and seaweed farmers potential causative factors are discussed. It was concluded that a repeated mapping with satellite remote sensing is a suitable tool to monitor changes of seagrass and seaweed distribution in shallow tropical environments.

  • 14.
    Lokrantz, Jerker
    et al.
    Stockholm University, Faculty of Science, Department of Systems Ecology.
    Norström, Albert
    Stockholm University, Faculty of Science, Department of Systems Ecology.
    Nyström, Magnus
    Stockholm University, Faculty of Science, Department of Systems Ecology.
    Öhman, Marcus
    Stockholm University, Faculty of Science, Department of Zoology.
    Muhando, Christopher
    Institute of Marine Sciences, University of Dar es Salaam.
    Jiddawi, Narriman
    Institute of Marine Sciences, University of Dar es Salaam.
    A comparison of functional groups in coral reefs around Zanzibar Island (Tanzania)Manuscript (Other academic)
    Abstract [en]

    Species’ traits determine how biodiversity interacts with ecosystem processes and influence the capacity of ecosystems to respond to and recover from disturbances. Classifying species with regards to their traits, i.e. functional groups, and analyzing their distribution provides a mechanistic approach to investigate the resilience in ecosystems. This study investigates the status of functional groups of coral, fish and sea urchins on reefs on the west coast of Zanzibar Island, Tanzania. The classification of functional groups is based on traits reflecting important community processes or properties which underpin ecosystem resilience. The results show that reefs with high accessibility, i.e. close to shore and open to fisheries, have lower abundance and diversity of functional groups of both coral and fish compared to more remote or protected reefs. More specifically, highly accessible reefs display lower abundances of herbivorous fish (macroalgae browsers in particular), large-bodied fish, structurally complex corals and corals with certain reproduction strategies. Based on these findings we speculate what this means in terms of ecosystem functioning and vulnerability. This study also provides a first “baseline” of functional group distribution and although it represents an already degraded state it may serve as an important comparison when evaluating further degradation and effects of impending management interventions.

  • 15. Mwandya, A. W.
    et al.
    Mgaya, Y. D.
    Öhman, M. C.
    Stockholm University, Faculty of Science, Department of Zoology.
    Bryceson, I.
    Gullström, M.
    Stockholm University, Faculty of Science, Department of Zoology.
    Distribution patterns of striped mullet Mugil cephalus in mangrove creeks, Zanzibar, Tanzania2010In: AFR J MAR SCI, ISSN 1814-232X, Vol. 32, no 1, p. 85-93Article in journal (Refereed)
    Abstract [en]

    Spatial and seasonal variations in density of striped mullet Mugil cephalus were investigated in four mangrove creeks in Zanzibar, Tanzania, during a one-year cycle. Fish were collected monthly in the lower, intermediate and upper reaches of each creek using a beach-seine net. All fish collected were juveniles between 2 and 16 cm standard length. The density of juvenile mullet differed significantly among the creeks, but the spatial patterns within them were consistent with higher densities upstream in three of the creeks. Generally, small mullet (2-10 cm) were more abundant in the upper reaches compared to the lower and intermediate reaches. Seasonal patterns were weak, although mullet densities were high during the period of heavy rains (March-May). Principal component analysis showed that a muddy substrate with microphytobenthos was positively correlated with high mullet densities, although site-specific variables such as shallow water depth and water clarity were also significantly correlated. Our findings suggest that the densities of juvenile striped mullet vary among sites and creeks in response to refuge availability from turbid, shallow water and the accessibility of food from benthic microalgae.

  • 16.
    Mwandya, Augustine
    et al.
    Stockholm University, Faculty of Science, Department of Zoology.
    Gullström, Martin
    Stockholm University, Faculty of Science, Department of Zoology. Stockholm University, Faculty of Science, Department of Systems Ecology.
    Andersson, Mathias H.
    Stockholm University, Faculty of Science, Department of Zoology.
    Öhman, Marcus C.
    Stockholm University, Faculty of Science, Department of Zoology.
    Mgaya, Yunus D.
    Ian, Bryceson
    Spatial and seasonal variation of fish assemblages in mangrove creek systems in Zanzibar (Tanzania)2010In: Estuarine, Coastal and Shelf Science, ISSN 0272-7714, E-ISSN 1096-0015, Vol. 89, no 4, p. 277-286Article in journal (Refereed)
    Abstract [en]

    Spatial and seasonal variations of fish assemblage composition were studied in three non-estuarine mangrove creeks of Zanzibar (Tanzania). Fish were collected monthly for one year at three sites (lower, intermediate and upper reaches) in each creek using a seine net (each haul covering 170 m(2)). Density, species number and diversity of fish were all higher at sites with dense cover of macrophytes (seagrass and macroalgae) than over unvegetated sandy sites. In general, fish assemblages mainly comprised juveniles of a few abundant taxa, e.g. Mugil cephalus, Mugilidae spp. and Leiognathus equulus at sites with mud substratum and Germs oyena, Lethrinus harak and Sillago sihama at sites dominated by macrophytes. Multivariate analyses revealed significant separations in fish assemblage composition within the two creeks where the bottom substratum differed among sites. Overall, season seemed to have little effect on density, species number, diversity index (H') and assemblage structure of fish. Water condition variables were also relatively stable across the season, although a short-term fluctuation primarily induced by decreased salinity, occurred during the heavy rains in April and May. Fish assemblage structure was not significantly affected by any of the abiotic factors tested. However, significant regressions were found between the other fish variables and environmental variables, but since these associations were mostly species-specific and generally inconsistent, we suggest that the overall distribution patterns of fish were mainly an effect of particular substrate preferences of fish species rather than contemporary water conditions.

  • 17.
    Mwandya, Augustine
    et al.
    Stockholm University, Faculty of Science, Department of Zoology.
    Gullström, Martin
    Stockholm University, Faculty of Science, Department of Zoology.
    Öhman, Marcus C
    Stockholm University, Faculty of Science, Department of Zoology.
    Andersson, Mathias H
    Stockholm University, Faculty of Science, Department of Zoology.
    Mgaya, Yunus
    Fisheries and Aquaculture.
    Fish assemblages in Tanzanian mangrove creek systems influenced by solar salt farm constructions2009In: Estuarine, Coastal and Shelf Science, ISSN 0272-7714, E-ISSN 1096-0015, Vol. 82, no 2, p. 193-200Article in journal (Refereed)
  • 18.
    Mwandya, Augustine
    et al.
    Stockholm University, Faculty of Science, Department of Zoology.
    Mgaya, Yunus
    Fisheries and Aquculture.
    Öhman, Marcus
    Stockholm University, Faculty of Science, Department of Zoology.
    Ian, Bryceson
    International Environment and Development Studies.
    Gullström, Martin
    Stockholm University, Faculty of Science, Department of Zoology.
    Distribution patterns of the striped mullet (Mugil cephalus) in mangrove creeks of Zanzibar, TanzaniaManuscript (Other academic)
    Abstract [en]

    Spatial and seasonal variations in density of the striped mullet Mugil cephalus were investigated in four mangrove creeks in Zanzibar (Tanzania) during a one-year cycle. Fish were collected monthly in the lower, intermediate and upper reaches of each creek using a beach seine net (each haul covering 170 m2). All individuals collected were juveniles with a mean size of 2 to 16 cm (standard length). The density of juvenile mullets inhabiting mangrove creeks differed significantly among the different creeks, but the patterns within creeks were consistent, with higher densities upstream in three of the creeks. In general, small-sized juvenile mullets (2-10 cm) were more abundant in the upper reaches compared to the lower and intermediate sites in most creeks. Seasonal patterns were fairly weak, although high mullet densities were observed during the period of heavy rains (from March to May). Principal component analysis (PCA) showed that a muddy bottom with microphytobenthos was likely important to explain high mullet densities, although site-specific variables like low water depth and water clarity may also be important. Our findings suggest that the densities of juvenile striped mullet vary among sites and creeks in response to refuge availability from turbid shallow waters and the accessibility of food resources from benthic microalgae.

  • 19.
    Ohman, Marcus C.
    et al.
    Stockholm University, Faculty of Science, Department of Zoology.
    Sigray, Peter
    Stockholm University, Faculty of Science, Department of Meteorology .
    Westerberg, Hakan
    Offshore windmills and the effects electromagnetic fields an fish2007In: Ambio, ISSN 0044-7447, E-ISSN 1654-7209, Vol. 36, no 8, p. 630-633Article in journal (Refereed)
    Abstract [en]

    With the large scale developments of offshore windpower the number of underwater electric cables is increasing with various technologies applied. A wind farm is associated with different types of cables used for intraturbine, array-to-transformer, and transformer-to-shore transmissions. As the electric currents in submarine cables induce electromagnetic fields there is a concern of how they may influence fishes. Studies have shown that there are fish species that are magneto-sensitive using geomagnetic field information for the purpose of orientation. Thin implies that if the geomagnetic field is locally altered it could influence spatial patterns in fish. There are also physiological aspects to consider, especially for species that are less inclined to move as the exposure could be persistent in a particular area. Even though studies have shown that magnetic fields could affect fish, there is at present limited evidence that fish are influenced by the electromagnetic fields that underwater cables from wind-mills generate. Studies on European eel in the Baltic Sea have indicated some minor effects. In this article we give an overview on the type of submarine cables that are used for electric transmissions in the sea. We also describe the character of the magnetic fields they induce. The effects of magnetic fields on fish are reviewed and how this may relate to the cables used for offshore wind power is discussed.

  • 20.
    Wilhelmsson, Dan
    et al.
    Stockholm University, Faculty of Science, Department of Zoology.
    Malm, Torleif
    Öhman, Marcus
    Stockholm University, Faculty of Science, Department of Zoology.
    The influence of offshore wind power on demersal fish2006In: ICES Journal of Marine Science, ISSN 1054-3139, E-ISSN 1095-9289, Vol. 63, no 5, p. 775-784Article in journal (Refereed)
    Abstract [en]

    A significant expansion of offshore windpower is expected in northwestern Europe in the near future. Little is known about the impacts it may have on the marine environment. Here, we investigate the potential for wind turbines to function as artificial reefs and fish aggregation devices (FADs), i.e. whether they would locally increase fish densities or alter fish assemblages. Fish communities and habitat composition were investigated using visual transects at two windpower farms off the southeastern coast of Sweden, central Baltic Sea. Fish abundance was greater in the vicinity of the turbines than in surrounding areas, while species richness and Shannon–Wiener diversity (H') were similar. On the monopiles of the turbines, fish community structure was different, and total fish abundance was greater, while species richness and diversity (H') were lower than on the surrounding seabed. Blue mussels and barnacles covered most of the submerged parts of the turbines. On the seabed, more blue mussels and a lesser cover of red algae were recorded around the power plants than elsewhere. Results from this study suggest that offshore windfarms may function as combined artificial reefs and fish aggregation devices for small demersal fish.

  • 21.
    Wilhelmsson, Dan
    et al.
    Stockholm University, Faculty of Science, Department of Zoology.
    Yahya, Saleh
    Stockholm University, Faculty of Science, Department of Zoology.
    Öhman, Marcus
    Stockholm University, Faculty of Science, Department of Zoology.
    Effects of high-relief structures on cold-temperate fish assemblages: a field experiment2006In: Marine Biology Research, ISSN 1745-1000, E-ISSN 1745-1019, Vol. 2, no 2, p. 136-147Article in journal (Refereed)
    Abstract [en]

    High-relief structures may influence the abundance and diversity of reef-associated fish. We conducted a field experiment to investigate whether the presence of vertical structures (PVC pipes) affects fish communities on artificial reefs. The effect of the height of the structures (1 and 3 m) was also tested. Furthermore, the effects on fish of placing artificial reefs on otherwise featureless bottoms were quantified. Algal and macro-invertebrate colonization of the reefs was also recorded. The experiment was carried out on the west coast of Sweden over a period of 1 year. The vertical structures had a positive effect on fish abundance but not on diversity. The height of the structures did not, however, influence the fish communities. Natural as well as urban vertical structures on the seafloor could have a positive effect on local fish abundance. The positive effects of artificial reefs on total fish abundance and diversity were immediate. Of the 10 species recorded, two, the black goby Gobius niger and the goldsinny wrasse Ctenolabrus rupestris , dominated over the whole survey period. There were significant temporal differences in fish abundance, and diversity increased with time.

  • 22. Wilson, S. K.
    et al.
    Adjeroud, M.
    Bellwood, D. R.
    Berumen, M. L.
    Booth, D.
    Bozec, Y. -Marie
    Chabanet, P.
    Cheal, A.
    Cinner, J.
    Depczynski, M.
    Feary, D. A.
    Gagliano, M.
    Graham, N. A. J.
    Halford, A. R.
    Halpern, B. S.
    Harborne, A. R.
    Hoey, A. S.
    Holbrook, S. J.
    Jones, G. P.
    Kulbiki, M.
    Letourneur, Y.
    De Loma, T. L.
    McClanahan, T.
    McCormick, M. I.
    Meekan, M. G.
    Mumby, P. J.
    Munday, P. L.
    Öhman, Marcus C.
    Stockholm University, Faculty of Science, Department of Zoology.
    Pratchett, M. S.
    Riegl, B.
    Sano, M.
    Schmitt, R. J.
    Syms, C.
    Crucial knowledge gaps in current understanding of climate change impacts on coral reef fishes2010In: Journal of Experimental Biology, ISSN 0022-0949, E-ISSN 1477-9145, Vol. 213, no 6, p. 894-900Article in journal (Refereed)
    Abstract [en]

    Expert opinion was canvassed to identify crucial knowledge gaps in current understanding of climate change impacts on coral reef fishes. Scientists that had published three or more papers on the effects of climate and environmental factors on reef fishes were invited to submit five questions that, if addressed, would improve our understanding of climate change effects on coral reef fishes. Thirty-three scientists provided 155 questions, and 32 scientists scored these questions in terms of: (i) identifying a knowledge gap, (ii) achievability, (iii) applicability to a broad spectrum of species and reef habitats, and (iv) priority. Forty-two per cent of the questions related to habitat associations and community dynamics of fish, reflecting the established effects and immediate concern relating to climate-induced coral loss and habitat degradation. However, there were also questions on fish demographics, physiology, behaviour and management, all of which could be potentially affected by climate change. Irrespective of their individual expertise and background, scientists scored questions from different topics similarly, suggesting limited bias and recognition of a need for greater interdisciplinary and collaborative research. Presented here are the 53 highest-scoring unique questions. These questions should act as a guide for future research, providing a basis for better assessment and management of climate change impacts on coral reefs and associated fish communities.

  • 23.
    Yahya, Saleh A. S.
    et al.
    Stockholm University, Faculty of Science, Department of Zoology.
    Gullström, Martin
    Stockholm University, Faculty of Science, Department of Zoology. Stockholm University, Faculty of Science, Department of Systems Ecology.
    Öhman, Marcus C.
    Stockholm University, Faculty of Science, Department of Zoology.
    Jiddawi, Narriman
    Andersson, Mathias H.
    Stockholm University, Faculty of Science, Department of Zoology.
    Mgaya, Yunus D.
    Lindahl, Ulf
    Coral bleaching and habitat effects on colonisation of reef fish assemblages: an experimental study2011In: Estuarine, Coastal and Shelf Science, ISSN 0272-7714, E-ISSN 1096-0015, Vol. 94, no 1, p. 16-23Article in journal (Refereed)
    Abstract [en]

    Degradation and mortality of corals is increasing worldwide and is expected to have significant effects on coral reef fish; hence studies on these effects are essential. In the present study, a field experiment was set up within Mafia Island Marine Park in Tanzania (East Africa) to examine the effects of bleaching and habitat structure on colonisation of coral reef fish assemblages. Live and bleached staghorn coral Acropora formosa was transplanted onto plots in a site dominated by sand and rubble, and the experimental design comprised of three treatments: live coral, bleached coral and eroded coral rubble. There was an immediate increase (within 24 h) in fish abundance and diversity in the two treatments with standing corals. Overall, live and bleached coral plots showed similar effects, but differed from the eroded coral plots which had a much lower abundance and diversity of fish. In general, fish species diversity changed with time over the study period while fish abundance did not. Multivariate analyses showed that while there were differences in fish assemblage structure between standing corals and the eroded coral treatment, there was neither a difference between live and bleached coral treatments nor any temporal effects on fish assemblage structure. Our findings suggest that physical structure and complexity of habitat have stronger effects on colonisation of reef fish assemblages than changes in coral health (such as bleaching) which do not affect coral structure. This may have important implications for appropriate coral reef management.

  • 24.
    Yahya, Saleh A.S.
    et al.
    Stockholm University, Faculty of Science, Department of Zoology.
    Gullström, Martin
    Stockholm University, Faculty of Science, Department of Zoology.
    Öhman, Marcus
    Stockholm University, Faculty of Science, Department of Zoology.
    Jiddawi, Narriman
    Mgaya, Yunus
    Fish assemblages in relation to quality, structure and configuration of staghorn coral reefs at Mafia and Zanzibar islands, TanzaniaManuscript (Other academic)
  • 25.
    Öhman, Marcus C.
    Stockholm University, Faculty of Science, Department of Zoology.
    Coral reefs in northwestern Sri Lanka; biology and human disturbances1993In: Proceedings of the Colloquium on Global Aspects of Coral Reefs: Health, Hazards and History / [ed] Ginsburg, R.N., Miami: Rosentiel School of Marine and Atmospheric Science , 1993, , p. 420p. 474-480Conference paper (Other academic)
  • 26.
    Öhman, Marcus C.
    et al.
    Stockholm University, Faculty of Science, Department of Zoology.
    Rajasuriya, Arjan
    National Aquatic Resources Agency, Sri Lanka.
    Relationships between habitat structure and fish communities on coral and sandstone reefs1998In: Environmental Biology of Fishes, ISSN 0378-1909, E-ISSN 1573-5133, Vol. 53, p. 19-31Article in journal (Refereed)
    Abstract [en]

    The influence of habitat structure on reef-fish communities at Bar Reef Marine Sanctuary, Sri Lanka, was investigated. The relationship between habitat characteristics and the distribution and abundance och 135 species of fishes was examined on two reef types: coral and sandstone reefs. Results suggested that the reef fish communities were strongly influenced by various aspects of reef structure. However, relationships between habitat variables and fish communities structure, varied between the two reef types. Fish species diversity was correlated with a number of habitat variables on the sandstone reefs, although structural complexity seemed to play the dominant role. There were no correlations between habitat structure and fish diversity on the coral reefs. Total abundance was not related to any one habitat parameter on either reef type. However, abundances of some species, families and trophic groups were correlated with habitat features. These specific correlations were commonly related to food and shelter availability. For example, coral feeders were correlated with live coral cover, and pomacentrid species, which used branching corals for protection, showed a significant relationship with the density of Acropora colonies. This shows that a summary statistic such as total abundance may hide important information. Effects of habitat structure on the distribution patterns of the fish communities was further investigated using multidimensional scaling ordination and the RELATE-procedure. With the MDS-ordinations for both habitat and fish community composition it was possible to show that the multivariate pattern between the two ecological components was clearly correlated.

  • 27.
    Öhman, Marcus C.
    et al.
    Stockholm University, Faculty of Science, Department of Zoology.
    Rajasuriya, Arjan
    National Aquatic Resources Agency.
    Lindén, Olof
    Stockholm University, Faculty of Science, Department of Zoology.
    Human disturbances on coral reefs in Sri Lanka: a case study1993In: Ambio, ISSN 0044-7447, E-ISSN 1654-7209, Vol. 22, no 7, p. 474-480Article in journal (Refereed)
    Abstract [en]

    The degradation of coral reefs in Sri Lanka has increased substantially over the last decades. Human activities causing this degradation include: mining for lime production, sewage discharges, discharges of oil and other pollutants in connection with shipping and port activities, destructive fishing practices, land and mangrove destruction, tourism and the collecting of fauna such as fish, shells and corals. In this study, three adjacent coral reefs: Bar Reef, Talawila Reef, and Kandakuliya Reef, which are widely scattered patch reefs off Kalpitiiya Peninsula, northwestern Sri Lanka, were surveyed and compared in terms of their fish and coral diversity and abundance as well as human and natural disturbances. Information was gathered by snorkelling in visual overview surveys and by scuba diving in detailed transect surveys. When each reef was ranked according to the extend of live coral cover, and chaetodontid diversity, the results indicated that Bar Reef was in excellent condition, Talawila Reef was intermediate, and Kandakuliya Reef was in poor condition. The diversitity of coral genera, the topographic relief and the proportion of coral rubble, did not follow the same pattern. The number of coral genera found was 49, while 283 fish species belonging to 51 families were recorded. Human disturbance factors on the reefs were found to be net fishing, boat anchoring and ornamental fish collection for the aquarium trade. Bottom-set nylon nets in particular were found to have a very destructive impact on the bottom fauna

1 - 27 of 27
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