Education
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Danish master student, Nha Phu Bay 2008The educational aspect is very important in the project and 2 young Vietnamese students will come to Denmark to complete a Master’s degree in Marine Biology at Aarhus and Copenhagen University. Arranging workshop s in Vietnam where more Vietnamese students and researchers can attend, has a very high priority in the project. Danish students are inspired to do their ;)
Growth experiment on P. viridis - part of master thesisMasters Project in Vietnam as well. There are all kinds of facilities at the laboratory in Nha Trang as well as easy access to the major type of Vietnamese ecosystems, as estuaries, coral reefs, and the open ocean to mention some. At the moment we have one Danish Master student, Rasmus, who will carry out research about fish populations and diversities in the Nha Phu Estuary.
On-going master project
Work schedule for priliminary sampling
Christian Westergaard, Aarhus University, Denmark
Fish composition survey at a Vietnamese estuary
Supervisor: Kurt Thomas Jensen
The present project is aimed at determining the fish composition in the estuary of Nha Phu Bay north of Nha Trang, Vietnam by using stable isotope analysis as well as gut content analysis. This particular area has seen a severe decrease in mangrove cover, as well as an increase in shrimp farming activites within the last 25 years which, along with a high fishing pressure, has put a great pressure on the fish community. Therefore, knowledge on the fish composition is needed to assess the future health of the ecosystem.
Fish and prey items have been collected at two sites, the head and the mouth of the estuary, respectively. Immediately after landing each fish, the stomach of each fish is removed, and preserved in alcohol for further analysis of prey items. Each fish is identified, measured, and a small tissue-sample is removed, dried and later used for stable isotope analysis. Stable isotopes are an efficient way of determining the structure of a food web, as the N- and C-isotopes behave in a predictable manner. The N15 isotope is enriched approximately 3-4 % between trophic levels, and can be used to determine the trophic position of each species. C14 on the other hand, does not chance much between trophic levels (<1%) and therefore can be used to determine the food sources. N15 and C14 analysis combined with stomach content analysis, should provide insight in both the trophic position of important fish species as well as knowledge on the primary food items in the estuary.
The present survey was officially initiated in Denmark on the 1st of November 2010, with field-work being conducted from 15th of November 2010 to the 13th of December 2010 in Nha Phu, Viet Nam.
On-going master project
Work schedule for priliminary sampling
Nguyen Thi Kim Hanh, Aarhus University
Viet Nam: August 2010-January 2011; Denmark: Februray-August 2010
The influence of organic matter loading from floating fish farms on nitrogen cycling in Nha Phu Bay sediments
Supervisors:Lars Chresten Lund-Hansen (Arhuss University), Erik Kristensen (Danmark Southern University)
Introduction
Nha Phu,;)
Cage culture in Nha Phu, Photo credit -Hanh2010 one of the biggest bays in Khanh Hoa Province, Viet Nam, is a socially as well as economically important place since fisheries production has brought benefits to society. Floating fish farms have been used as a favourite model of aquaculture in this area. Besides the profits of these farming activities, there are also local environmental impacts. Fish food and excreta from fish cages sinking to the bottom of estuary are likely to affect mineral cycles, especially N cycling, in sediment due to the increase of organic matter loading. Three important process in N cycling, namely nitrification, denitrification and dissimilatory nitrate reduction to ammonium (DNRA) are affected by fish floating farming activities, leading to a problem of nitrogen retention and loss dynamics. DNRA is expected to be favoured by eutrophication from fish farming and therefore competes with denitrification for NO3- (Nizzoli D. et al, 2006). As a result, the amount of dinitrogen produced from denitrification decreases in the nutrient-rich environment. Denitrification removes N from the system and potentially reduces eutrophication problems. If DNRA takes over, such N removal will not occur and the produced NH4 can contribute to eutrophication.
In a study of Christensen et al. (2000), it was shown that DNRA was under fish cages can be almost 7-fold greater than denitrification. It is likely that while denitrification was limited by carbon-rich and high organic matter loading environment, DNRA happened strongly instead. Consequently, ammonium efflux increased in the water column whereas nitrogen removal by denitrification decreased. Denitrification only removed 0.1% nitrogen of nitrogen loading from fish farms which was 12% of total nitrogen from the land during summer. Nizzoli et al. (2006) also mentioned about the dominance of DNRA under a mussel farm due to a high organic matter inputs.
In this project, isotope pairing technique and a 15N technique will be used to measure denitrification and DNRA, respectively. Nitrogen isotope pairing technique is carried out by first adding 15NO3- in water column of sediment core. The single-labelled (14N15N) and double-labelled (15N15N) formed through denitrification are meas;)
Field studies, sediment cores were collected with divers, photo: Huong '10ured by mass spectrometry. The total denitrification then is calculated according to labelled and unlabelled N2. By using this method, denitrification using nitrate from water column (Dn) or from nitrification (Dw) are also calculated (Nielsen 1992).
The effect of floating fish farms in Nha Phu Bay on sediment-water fluxes, denitrification and DNRA is becoming a crucial and necessary topic which needs studied since no studies have addressed it so far.
Thesis objective
To determine the impact of marine aquaculture on sediment metabolism, with emphasis on nitrogen cycling
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On-going master project
Work schedule for priliminary sampling
Vietnam Sep-Oct 2009
An ecological study of fish in the inner part of Nha Phu Bay (KTJ & RB, Marine Ecology, AU, Denmark)
Objective
- To test standard methods used in lakes and fjords for catching fish in the inner part of Nha Phu Bay (for quantitative and qualitative description of the fish fauna)
- To determine the trophic position of dominant fish species from the lagoon (sampling of tissue from dominant fish and collecting samples of possible C-sources in the lagoon)
Materials & methods
Re 1. By using gillnet[1] (Fig 1a) and ‘fish traps’(Fig 1b) at selected positions in Nha Phu Bay it should be possible to catch a variety of the fish present in the examined part of the bay. The net and traps should be positioned in a standardized way with respect to water current and tidal cycle (perhaps necessary to examine the effect of orientation with respect to the dominant bottom current). The fishing time should be at least 4 hours but we consider 6-8 hours as more optimal (however practical conditions may be decisive). If possible it would also be interesting to examine the effect of day/night on catch (species and numbers).
We have 8 gillnets and 2 ‘fish traps’. The numbers placed at certain time and position will depend on the practical possibilities
Fig. 1a and 1b
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At the end, the catch will be placed in plastic bags according to the mesh size, where they were caught, and brought to the laboratory for further handling. To keep the fish fresh ice should be added to the bags. For large fish (if they are few) it may be more feasible to handle them onboard.
Handling of the individual fish will include:
- Identification (species, genus, family)
- Length measurement
- Weight measurement
- Photo
- Sampling of stomach content & identification of food remains (preservation in ethanol)
- Sampling of muscle tissue for analysis of stable isotopes (to be dried & in small plastic bags)
- Sampling of tissue for DNA-identification (Eppendorf tube & 100% ethanol)
Re 2. Samples of fish and possible C-sources from the lagoon will be dried at 55˚C or 60˚C for 70 to 100 hours (it varies slightly between the different items). Dried samples will analysed in Denmark for stable isotopes[2] to determine the trophic position (food chain length) of the examined fish.
Preserved stomach content will be examined by using a stereomicroscope for identifiable prey items.
Possible C-sources (for fish) in the bay will be collected. This includes items like:
- Leaves and litter from macrophytes
- Mangrove leaves & mangrove litter
- Epiflora associated with macrophytes and rock, stones etc
- Phytoplankton
- Detritus
- Zooplankton
- Epizoobenthic and benthic species
Equipment and organisation of the study at Institute of Oceanography, Nha Trang
1) Booking of a ship/boat (‘x’ (?) full day trips) (depends on catch size, weather, practical conditions)
2) Buoy, flags/markers, lead and rope for setting and stabilizing gill nets and fish traps
3) Square box or small table to be used for handling the catch onboard
4) Ice to keep caught fish fresh until treatment
5) Large plastic bags (for fish caught at each mesh size)
6) Laboratory/locality where we can handle the catch
7) 100% ethanol for preserving stomach and DNA-samples
8) Balance and ruler for fish measurements
9) Binocular microscopes for examining stomach content
10) Heater to dry fish and ‘C’-sources
[1] Gill nets are basically a series of panels of meshes with a weighted "foot rope" along the bottom, and a "headline", to which floats are attached. They can therefore be set to fish at any height in the water column. The meshes of a gill net are uniform in size and shape, hence highly selective for a particular size of fish. Fish which are smaller than the mesh of the net are able to pass through unhindered, while those which are too large to push their heads through the meshes as far as their gills are not retained. This gives a selectivity ogive which is skewed towards medium sized fishes, unlike active gears such as trawling, in which the proportion of fish entering the net which are retained increases with length.
[2] The stable isotopes of N, C, and O are commonly used to assess diet, trophic position, and habitat in modern and ancient organisms, both terrestrial and aquatic. Numerous factors affect the isotopic signature (e.g., d13Corg) of an organism including the isotopic signature of the primary producers at the base of the food chain (d13Cbase), the number of food chains within the food web (base1 and base2), the relative contributions of each food chain to the diet of the organism (α), the trophic position of the organism (torg), and the trophic fractionation (Δc high for N, low for C): d13Corg = d13Cbase2 – [ α (d13Cbase2 - d13Cbase1)] - Δctorg. Our ability to resolve changes in diet (α) depends on our understanding of (1) the isotopic disparity of the two dietary sources of carbon or end members (e.g., C3 vs C4 plants, littoral vs pelagic producers), (2) the amount of variation in isotopic signature around those end members, (3) the magnitude of the observed isotopic difference, and (4) the experimental error around those measurements. The d13C values of modern aquatic primary producers exhibit great heterogeneity and shift with lateral changes along shore, onshore-offshore gradients, water depth, lake size, primary productivity, latitude, seasonality, etc. In the fossil record we must consider these factors in addition to time averaging and long-term trends such as changes in the pCO2 of the atmosphere. Where isotopic disparity of the end members is large, and the variation around those end members is small or quantified using an independent proxy, modest differences in the observed isotopic signature between samples (~2-3‰) may be considered robust. However, where end member disparity is low, or variation around those end members is not addressed, and the observed differences are small (~1‰) specific interpretations related to diet, habitat, or organism behavior may be suspect. (CASEY, Michelle M., et al, 2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies)