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Identification and implications of fish nurseries in tropical and subtropical seascapes
Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences. (Berkström Lab)ORCID iD: 0000-0003-1222-2033
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Many species of reef fish reside in specific nursery habitats as juveniles. Seagrass meadows, and mangroves are examples of well-recognized nursery habitats, but only recently canopy-forming seaweeds have been found to provide important habitats for some fish species in the tropics. Availability of nurseries can have effects on the abundance and spatial distribution of adult fish, which is why it is important to recognize key nursery habitats for proper management. Information on reef fish nurseries is largely lacking in the South Western Atlantic (SWA), while information in the Western Indian Ocean (WIO) and elsewhere is more extensive. However, more information on the  consequences of nursery availability on adult fish populations is needed. This thesis studies nursery habitat use of reef fish on tropical and subtropical reefs in the SWA and in seagrass and reef systems in the WIO. The hypothesis that seagrass and canopy-forming macroalgae meadows function as a nursery habitat for reef fish is tested in the SWA. The aim of this thesis is also to understand distribution patterns of fish arising from the arrangement of the seascape, using a seascape ecology approach, linking patterns to non-reef nursery habitat use (mangroves and seagrass systems). Results showed that spatial and temporal patterns of juvenile reef fish abundance were weak on rocky, subtropical reefs in the SWA (Paper I), while there was a stronger preference for certain habitats on SWA tropical biogenic reefs, especially seaweed beds dominated by Sargassum (Paper II). The widely accepted paradigm that seagrass meadows function as nursery habitats for reef fish was not supported by the results from the study site in the tropical SWA (Paper II). This may be related to habitat availability in the seascape. In the SWA, seagrass meadows are spatially small, fragmented and less complex, compared to in the WIO, where they display high structural complexity and cover large areas. At the WIO study site (Bazaruto Archipelago), the juvenile fish assemblage in the seagrass meadows encompassed a number of reef fish species from a range of trophic groups and families, as well as resident seagrass species (Paper III). Key variables and extent of spatial scales that structure ontogenetic migrations were identified in both seagrass and reef habitats. Fish distribution patterns in the seagrass seascape was strongly influenced by seascape configuration and distance to adjacent habitats, highlighting that not all seagrass meadows are equally productive as nursery habitats. Variables important for distribution patterns of fish were identified, which in most cases were species-specific, and related to life history and functional traits of species. Effects of two small protected areas on the fish assemblage was also linked to geographical placement of reserves in the seascape. Likewise, the adult fish community composition on the reefs was found to be structured by the spatial arrangement of nursery habitats in the seascape, and presence of stretches of sand acting as isolating barriers (Paper IV). Nursery fish species were less abundant on reefs far from nurseries, resulting in differences in community and functional group composition along distance gradients in the seascape. Depending on functional traits of the nursery fish assemblage, seagrass and mangroves can enhance certain ecological functions on reefs. Both community structure and ecosystem functioning may therefore change depending on nursery habitat availability, highlighting the need to adopt a holistic seascape approach in management.

Place, publisher, year, edition, pages
Stockholm: Department of Ecology, Environment and Plant Sciences, Stockholm University , 2019. , p. 48
National Category
Ecology
Research subject
Marine Biology
Identifiers
URN: urn:nbn:se:su:diva-165468ISBN: 978-91-7797-606-6 (print)ISBN: 978-91-7797-607-3 (electronic)OAI: oai:DiVA.org:su-165468DiVA, id: diva2:1283806
Public defence
2019-03-15, Vivi Täckholmsalen (Q-salen), NPQ-huset, Svante Arrhenius väg 20, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 1: Manuscript. Paper 3: Submitted. Paper 4: Manuscript.

Available from: 2019-02-20 Created: 2019-01-29 Last updated: 2019-02-15Bibliographically approved
List of papers
1. Identifying reef fish nursery habitats on subtropical rocky reefs in the Southwestern Atlantic
Open this publication in new window or tab >>Identifying reef fish nursery habitats on subtropical rocky reefs in the Southwestern Atlantic
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Availability of nursery habitats can potentially limit adult populations of reef fish. Suitable nursery habitats are often shallow and located close to shore, and in many cases overlap with anthropogenic stressors and human activities. For proper management of reef fish communities it is therefore important to identify these habitats and minimize conflicts of habitat use in time and space. To identify potential spatial and temporal patterns of nursery habitat use, the reef fish communities on subtropical rocky reefs in the South western Atlantic were surveyed with under water visual census. Surveys were performed in summer and winter months during two years, along a depth gradient on marginal reefs and in seasonal Sargassum macroalgal beds. No clear patterns in total juvenile abundance and distribution were distinguished between seasons, although some families occurred in significantly higher abundances in summer months (Haemulidae and Sparidae). There were large variances in juvenile abundance between the two surveyed summers, with low total abundance in the summer of 2017, which could be linked to weak upwelling in the area and hampered growth of Sargassum. Similar to in other locations globally, climatic events that affect biomass of Sargassum may hence have large effects on fish recruitment also in the South western Atlantic. In general, shallow areas in sheltered bays had higher abundances of juvenile fish, especially when Sargassum was present in these locations. Spatial patterns of acanthurids and labrid scarinae, were more homogenous compared to those of tropical reefs in the South western Atlantic, where Sargassum beds harbor significantly more juveniles than other habitats. We suggest that fish in sub-tropical rocky reef environments in the South western Atlantic have access to less habitat diversity and complex substrate to use as nursery areas, and thus shallow, sheltered and vegetated habitats are critical for many species. Additionally, those habitats are also the most threatened by human uses demanding proper management.

National Category
Biological Sciences
Research subject
Marine Ecology
Identifiers
urn:nbn:se:su:diva-165463 (URN)
Available from: 2019-01-29 Created: 2019-01-29 Last updated: 2019-02-11Bibliographically approved
2. Seaweed beds support more juvenile reef fish than seagrass beds in a south-western Atlantic tropical seascape
Open this publication in new window or tab >>Seaweed beds support more juvenile reef fish than seagrass beds in a south-western Atlantic tropical seascape
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2017 (English)In: Estuarine, Coastal and Shelf Science, ISSN 0272-7714, E-ISSN 1096-0015, Vol. 196, no 5, p. 97-108Article in journal (Refereed) Published
Abstract [en]

Seascape connectivity is regarded essential for healthy reef fish communities in tropical shallow systems. A number of reef fish species use separate adult and nursery habitats, and hence contribute to nutrient and energy transfer between habitats. Seagrass beds and mangroves often constitute important nursery habitats, with high structural complexity and protection from predation. Here, we investigated if reef fish assemblages in the tropical south-western Atlantic demonstrate ontogenetic habitat connectivity and identify possible nurseries on three reef systems along the eastern Brazilian coast. Fish were surveyed in fore reef, back reef, Halodule wrightii seagrass beds and seaweed beds. Seagrass beds contained lower abundances and species richness of fish than expected, while Sargassum-dominated seaweed beds contained significantly more juveniles than all other habitats (average juvenile fish densities: 32.6 per 40 m2 in Sargassum beds, 11.2 per 40 m2 in back reef, 10.1 per 40 m2 in fore reef, and 5.04 per 40 m2 in seagrass beds), including several species that are found in the reef habitats as adults. Species that in other regions worldwide (e.g. the Caribbean) utilise seagrass beds as nursery habitats were here instead observed in Sargassum beds or back reef habitats. Coral cover was not correlated to adult fish distribution patterns; instead, type of turf was an important variable. Connectivity, and thus pathways of nutrient transfer, seems to function differently in east Brazil compared to many tropical regions. Sargassum-dominated beds might be more important as nurseries for a larger number of fish species than seagrass beds. Due to the low abundance of structurally complex seagrass beds we suggest that seaweed beds might influence adult reef fish abundances, being essential for several keystone species of reef fish in the tropical south-western Atlantic.

Keywords
Nursery grounds, Reef fish, Habitat choice, Seaweed, Ontogeny Connectivity
National Category
Ecology
Research subject
Marine Ecology
Identifiers
urn:nbn:se:su:diva-151230 (URN)10.1016/j.ecss.2017.06.041 (DOI)
Available from: 2018-01-09 Created: 2018-01-09 Last updated: 2019-02-21Bibliographically approved
3. Where is the grass greenest? Influence of seascape structure and marine protected areas on fish distribution patterns in a seagrass-dominated landscape
Open this publication in new window or tab >>Where is the grass greenest? Influence of seascape structure and marine protected areas on fish distribution patterns in a seagrass-dominated landscape
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(English)In: Ecography, ISSN 0906-7590, E-ISSN 1600-0587Article in journal (Refereed) Submitted
Abstract [en]

Tropical seagrass beds are critical habitats for many resident- and nursery fish species. While numerous studies have explored factors that structure reef fish assemblages, few have investigated the relative influence of multiple factors at fine- and large spatial scales as well as MPAs on seagrass fish. To understand which are the most important factors structuring fish assemblages in tropical seagrass beds, and how this is related to life history of species, we investigated fish distribution patterns at 20 sites in 13 different seagrass beds across the Bazaruto Archipelago, Mozambique. Using boosted regression tree modelling, we assessed the influence of fine-scale variables (seagrass meadow characteristics) and seascape variables (distance to adjacent habitats) on abundance of four nursery taxa (Lutjanus fulviflamma, Lethrinus spp., Scarus ghobban and Gerres spp.) and two resident species (Pelates quadrilineatus and Leptoscarus vaigiensis). We found that seascape variables were generally more important than seagrass characteristics, and that the influence of different variables was highly taxon-specific. Fish distribution patterns in seagrass-dominated seascapes were related to life history traits of the species; nursery fish taxa were negatively correlated with distance to adult habitats, while resident species occurred in higher abundances far from reefs. Proximity to mangroves was important for taxa that utilised mangroves in addition to seagrass as nurseries. Most seascape variables influenced fish abundances on a large spatial scale (km). The influence of protected areas was taxon-specific, with stronger effects on resident species than on nursery species, with geographical placement shadowing potential effects of protection on fish abundance. Our results indicate that protection efforts in seagrass-dominated seascapes can have varying impacts on fish distribution, depending on the geographical location of the reserve. This highlights the importance of considering seascape arrangement and the ecology of targeted species for conservation and marine spatial planning in seagrass-dominated systems.

Keywords
fish, seagrass, nursery, seascape ecology, marine protected areas
National Category
Biological Sciences
Research subject
Marine Ecology
Identifiers
urn:nbn:se:su:diva-165467 (URN)
Available from: 2019-01-29 Created: 2019-01-29 Last updated: 2019-02-11Bibliographically approved
4. The arrangement of nurseries within a tropical seascape structure fish communities on nearby reefs
Open this publication in new window or tab >>The arrangement of nurseries within a tropical seascape structure fish communities on nearby reefs
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Ecosystems are linked by the movement of organisms across habitat boundaries and the arrangement of habitat patches can affect species abundance and composition. In tropical seascapes many coral reef fishes settle in adjacent habitats and undergo ontogenetic migrations to coral reefs as they grow. Few studies have attempted to measure at what distances  from nursery habitats these fish migrations (connectivity) cease to exist and how the abundance, biomass and proportion of nursery species change on coral reefs along distance gradients away from nursery areas. The present study examines seascape arrangement, including distances between habitats, and its consequences on connectivity within a tropical seascape in Mozambique using a seascape ecology approach. Fish and habitat surveys were undertaken in 2016/2017 and a thematic habitat map was created in ArcGIS, where cover and distances between habitat patches were calculated. Distance to mangroves, seagrasses and channels were significant for most nursery species and both abundance, biomass and proportion of nursery species were highest in the south of the archipelago, where mangroves were present and decreased with distance to nurseries (mangroves and seagrasses). Some nursery species were absent on reef sites furthest from nursery habitats (80km) and at 8km from seagrass habitats the proportion of nursery/non-nursery species as well as abundance and biomass of seagrass nursery species drastically changed, indicating a threshold distance at which migrations may cease. A similar pattern was found between 3 and 6 km from channels. Threshold distances were found where ontogenetic movement from nurseries to reefs appeared to cease and these distances differed between fish families. Isolation and arrangement of nursery habitats were also found to structure adult fish communities on reefs, highlighting the importance of considering the matrix (sand and deep water) as barriers for fish migration.

National Category
Biological Sciences
Research subject
Marine Ecology
Identifiers
urn:nbn:se:su:diva-165464 (URN)
Available from: 2019-01-29 Created: 2019-01-29 Last updated: 2019-02-11Bibliographically approved

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