| 1. |
- Corell, Hanna, 1977, et al.
(författare)
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Depth distribution of larvae critically affects their dispersal and the efficiency of marine protected areas
- 2012
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Ingår i: Marine Ecology Progress Series. - : Inter-Research Science Center. - 0171-8630 .- 1616-1599. ; 467, s. 29-46
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Tidskriftsartikel (refereegranskat)abstract
- This study aims to improve estimates of dispersal by including information on larval traits, and in particular to explore how larval depth distribution affects connectivity and MPA (marine protected area) functionality in the Baltic Sea. A field survey showed that both invertebrates and fish differed in their larval depth distribution, ranging from surface waters to >100 m. A biophysical model of larval dispersal in the Baltic Sea showed that decreased depth distribution increased average dispersal distance 2.5-fold, decreased coastal retention and local recruitment, and substantially increased connectivity. Together with pelagic larval duration (PLD), depth distribution explained 80% of total variation in dispersal distance, whereas spawning season, and geographic and annual variations in circulation had only marginal effects. Median dispersal distances varied between 8 and 46 km, with 10% of simulated trajectories dispersing 30 to 160 km depending on drift depth and PLD. In the Baltic Sea, the majority of shallow Natura 2000 MPAs are <8 km in diameter. In the present study, only 1 of the 11 assessed larval taxa would have a recruitment >10% within MPAs of this size. Connectivity between MPAs was expected to be low for most larval trait combinations. Our simulations and the empirical data suggest that the MPA size within the Natura 2000 system is considerably below what is required for local recruitment of most sessile invertebrates and sedentary fish. Future designs of MPA networks would benefit from spatially explicit biophysical models that consider dispersal and connectivity for complex circulation patterns and informed larval traits.
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| 2. |
- Moksnes, Per-Olav, 1965, et al.
(författare)
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Larval behavior and dispersal mechanisms in shore crab larvae: Local adaptations to different tidal environments?
- 2014
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Ingår i: Limnology and Oceanography. - : Wiley. - 0024-3590. ; 59:2, s. 588-602
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Tidskriftsartikel (refereegranskat)abstract
- Using a combination of empirical and model studies we tested whether European shore crab larvae (Carcinus maenas) from environments with different tidal regimes in the North Sea area have different swimming behaviors, and whether this affects connectivity and settlement success of larvae. Laboratory studies demonstrated the presence of an inherited tidal migration rhythm in newly hatched crab larvae from the mesotidal Danish Wadden Sea, and field studies showed that postlarvae swam in surface water almost exclusively during flood tides, suggesting that larvae use selective tidal stream transport to control the dispersal process. In contrast, shore crab larvae from microtidal Skagerrak displayed a nocturnal vertical migration behavior that appeared to switch to a diurnal behavior at the end of the postlarval phase, indicating an adaptation to avoid visual predators and to use wind-driven transport to reach shallow settlement areas. A biophysical model showed that tidal-migrating larvae in the Wadden Sea had two times higher settlement success than larvae with a diel behavior. However, no differences in settlement success were found between the two larval behaviors in microtidal Skagerrak, where lower fitness is suggested for tidal-migrating larvae due to higher predation mortality from visual predators. We suggest that the differences in inherited larval behavior in larvae from meso- and microtidal regions reflect local adaptations maintained through natural selection of successful recruits. Consistent with recent population genetic studies, modeled connectivity of shore crabs indicated an oceanographic dispersal barrier to gene flow in Eastern Wadden Sea that may facilitate such adaptations.
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| 3. |
- Moksnes, Per-Olav, 1965, et al.
(författare)
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Larval connectivity and ecological coherence of marine protected areas (MPAs) in the Kattegat-Skagerrak region
- 2014
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Marine protected areas (MPAs) are considered effective instrument to mitigate the loss of biodiversity in the sea. However, the management of MPA-networks is challenged by a lack of information of habitat distribution, and of how populations are connected between habitats and MPAs through dispersal of pelagic larval stages. In this study, the effect of larval connectivity on the ecological coherence of the MPA-networks in the Kattegat-Skagerrak area of the North Sea is investigated with special focus on the OSPAR-MPAs. By using biophysical models, the larval dispersal and connectivity of benthic organisms in the Kattegat-Skagerrak area is assessed. The report also aims to assess if a series of new model tools can be applied to identify optimal MPA-networks for benthic communities, and evaluate the existing MPA-networks with regards to larval connectivity. This report was prepared on request by the Swedish Agency for Water and Marine Management.
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| 4. |
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| 5. |
- Svensson, Carl Johan, 1976, et al.
(författare)
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Temporal mismatches in predator-herbivore abundance control algae blooms in nutrient-enriched seagrass ecosystems.
- 2012
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Ingår i: Marine Ecology Progress Series. - : Inter-Research Science Center. - 0171-8630 .- 1616-1599. ; 471, s. 61-71
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Tidskriftsartikel (refereegranskat)abstract
- Blooming filamentous algae recurrently overgrow macroalgae and seagrass in many coastal ecosystems, leading to potential losses in the functions and services these habitats provide. The seasonal reoccurrence of algal blooms is partly caused by excessive input of nutrients into coastal ecosystems. At the same time, overfishing of top predators has led to increased densities of smaller predators that reduce herbivore abundance. Managers of coastal ecosystems are in urgent need of information on how overfishing may act synergistically with eutrophication to promote algal blooms. In the present study, we model the interaction between the opportunistic filamentous algae Ulva spp. and the effective mesograzer (small invertebrate herbivore) Gammarus locusta L. under different nutrient regimes and predation rates by intermediate-size fish (4 to 12 cm). The aim is to assess how productivity, herbivory and predation interact to regulate seasonal algal growth under a range of scenarios, including temporal matching or mismatching between trophic groups. The model is parameterised with nutrient and demographic data from a seagrass ecosystem on the west coast of Sweden. Model simulations show that both nutrient level and herbivore abundance regulate seasonal algal biomass in the seagrass meadow. Their relative importance is dynamic and sensitive to the arrival times of herbivores in relation to the start of algal development in spring. Thus, herbivore presence and timing are crucial factors for the control of algal biomass, particularly during times when resource levels are ambient to slightly elevated. Simulations also show that it is mainly the predation rate and timing of predators, rather than food limitation, that limit herbivore abundance. As predation rates by invertebrates and smaller fish can be linked to the presence of larger and commercially targeted fish, overfishing may have similar effects to eutrophication in the seagrass ecosystems.
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| 6. |
- Baden, Susanne P., 1952, et al.
(författare)
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Relative importance of trophic interactions and nutrient enrichment in seagrass ecosystems: A broad-scale field experiment in the Baltic-Skagerrak area.
- 2010
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Ingår i: Limnology and Oceanography. - 1541-5856. ; 55:3, s. 1435-1448
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Tidskriftsartikel (refereegranskat)abstract
- The interaction of eutrophication and predation in structuring seagrass Zostera marina L. ecosystems was assessed in a field experiment in three regions along an estuarine salinity gradient, from southern Finland to the Skagerrak area of the Swedish west coast. All regions are considered to be affected by eutrophication and overfishing but differ in the abundance of intermediate predators (e.g., small fish, shrimp, and crabs), mesograzers, and the biomass of epiphytic algae. Using transplanted Zostera (eelgrass), nutrient levels and intermediate predator abundance were manipulated in a full-factorial cage experiment. On the Swedish west coast, where ambient densities of mesograzers are very low, epiphytic algae responded strongly to nutrient enrichment, resulting in significantly reduced growth of eelgrass. At the Baltic sites however, where ambient densities of mesograzers are high, no significant growth of epiphytic algae was detected, and only grazer biomass responded to nutrient enrichment. Predation from small fish, shrimp, and crabs decreased the biomass of mesograzers by . 98% on the Swedish west coast, but natural predators had no significant effect on mesograzers biomass at the Baltic sites. Predation and nutrient enrichment interacted to affect the growth of eelgrass by controlling the biomass of mesograzers and nuisance algae. The differing effect of nutrient enrichment and grazing in the three regions may therefore be a result of the prevailing low and high predation pressure on mesograzers in Zostera. This absence or presence of predation may derive from interregional changes in trophic interactions, possibly caused by a combination of eutrophication and overfishing.
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| 7. |
- Duffy, J. E., et al.
(författare)
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Ecology of seagrass communities
- 2014
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Ingår i: Marine Community Ecology and Conservation, M. D. Bertness et al. (eds). - Sunderland, USA : Sinauer Associates Inc.. - 9781605352282 ; , s. 271-297
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Bokkapitel (refereegranskat)
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| 8. |
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| 9. |
- Mirera, David Oersted, 1975-
(författare)
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Capture-based mud crab (Scylla serrata) aquaculture and artisanal fishery in East Africa- Practical and ecological perspectives : Mud crab ecology and aquaculture
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Mud crab Scylla serrata is a crustacean that spends most of its life cycle in the mangrove environment throughout its range. Fishery and aquaculture of this crab are significant economic activities in coastal areas in the tropics and sub-tropics because of the meat quality and nutritional value. However there is a significant shortage of information on the ecology, fishery and aquaculture of these crabs in sub-Saharan Africa. This impacts the development of a sustainable aquaculture and fishery for the benefit of coastal communities. The present study analyses various aspects of mud crab ecology, fishery, aquaculture and social economics in East Africa using multidisciplinary approaches. The results are given in seven papers based on field and laboratory studies. The study established for the first time that high intertidal mangrove back-flats constitute a key habitat for the earliest instars of S. serrata (4 -30 mm CW). It also showed that diurnal tidal migration behaviour occurs in small juveniles that migrate to sub-tidal habitats during the day, possibly due to variable predation risks. Monthly sampling of juveniles in Kenya and Tanzania indicated continuous recruitment throughout the year. The large numbers of juvenile crabs along mangrove fringes indicate that these habitats could serve as sites suitable for collection of juvenile crabs for aquaculture. However, these areas must also be managed and protected to support the recruitment to the wild crab populations. An assessment of the crab fishery indicated that artisanal crab fishers possess significant traditional knowledge mainly inherited from their parents that enabled them to exploit the resource. Such knowledge could be useful for the development of the aquaculture and in management of the fishery. Mud crab fishing was found to be a male dominated activity, and fishers on foot practiced fishing in burrows at spring low tides. Interviews indicated that the average size of marketable crabs has declined over the years and a weak management system was observed with most fishers operating without a license. Due to the knowledge required regarding the local conditions, fishers are unable to shift to new areas. Furthermore fishers and could not fish at neap tides. Such limitations provide a “natural closure” of the fishery. Also foot fishers cover fairly limited distances in their daily operations, an aspect that can be utilized to effect site-specific management for the fishery if necessary. Laboratory and field experiments indicated that cannibalistic interactions are heavily influenced both by size differences of crabs and the availability of shelter but no significant effect was found for different stocking densities. Such information is of direct importance for crab farmers in East Africa, where seed from the wild are of multiple sizes and there is a need to grade juvenile crabs and provide shelter at stocking to ensure maximum survival. Experimental studies in earthen pond and mangrove pen cultures indicated high mortality rates. Comparing growth in earthen pond and mangrove pen systems indicated that growth rates were generally high in both systems, but significantly lower in pen systems without shelter, suggesting that shelter may have a stronger effect on growth than has been previously thought. Similar to artisanal mud crab fishery, an assessment of small-scale mud crab farming by organized community groups in Kenya indicated low level of women participation. A good knowledge of the market existed among the mud crab farming groups where hotels and exporters offered the highest prices. However there is a need for national policies to be directed to support small-scale aquaculture development by ensuring training and capacity building for women, operation and management of groups, data management and provision of user rights for communities working in the mangrove environment. Market analyses showed that the common market size of crabs in East Africa ranged between 500-1000 g and are thus larger than in Southeast Asia where the average size is reported at 300 g. Prices for mud crabs were over 50 % lower in Tanzania than in other East African countries and most of the profit was earned by middlemen and exporters. Cost revenue analysis showed that it would be more profitable to farm smaller commercial crabs, and develop a market for 300 g crabs to increase the profitability of crab farming in East Africa. Also, the same analyses found that farming large crabs in individual cages, which is the dominant culture form in East Africa today, had very low profitability due to high labor costs and low growth rates. Using a step-wise function of natural growth it was shown that growth rates of S. serrata cultured in cages was 40 % of the growth rates obtained in experimental pond and pen cultures, which were similar to natural growth. Therefore the good performance of grow-out cultures of juvenile mud crabs in earthen ponds and mangrove pens showed a potential to develop into a profitable and sustainable intervention. However, more work is needed to improve survival in culture systems and address the identified limitations of crab seeds and feed to enable development of sustainable mud crab aquaculture in East Africa.
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| 10. |
- Mirera, O. D., et al.
(författare)
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Cannibalistic interactions of juvenile mud crabs Scylla serrata: the effect of shelter and crab size
- 2013
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Ingår i: African Journal of Marine Science. - : National Inquiry Services Center (NISC). - 1814-232X .- 1814-2338. ; 35:4, s. 545-553
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Tidskriftsartikel (refereegranskat)abstract
- In the culture of mud crab Scylla serrata, cannibalism is often the greatest cause of mortality. A laboratory study was conducted to compare the influence of size class differences and shelter on cannibalism and limb loss in juvenile mud crabs (20-70 mm internal carapace width; ICW). Four size classes of juvenile crab (A: 21-30 mm, B: 31-40 mm, C: 41-50 mm and D: 51-70 mm ICW) were tested in all possible combinations using four different substrata with varying degree of shelter (seaweed, plastic strings, bamboo tubes and open sand substratum) in 48 h trials. Results suggest that cannibalistic interactions are heavily influenced both by size differences of crabs and the availability of shelter. Cannibalism on the smallest size class (20-30 mm ICW) increased about 10 times in the presence of the largest crab (51-70 mm ICW) compared with treatment with only same-size crabs (control treatment). Shelter provided little refuge for the smallest crabs, whereas cannibalism in larger size classes decreased by >50% in all the shelters compared with the sand substratum. The findings suggest that both size-grading and provision of shelter could minimise cannibalism in the culture of mud crabs.
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