Vietnam Sep-Oct 2009

An ecological study of fish in the inner part of Nha Phu Bay (KTJ & RB, Marine Ecology, AU, Denmark)

Objective

1. 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)
2. 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

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

Work  plan (Nha Trang)

[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., d¹³C_org) of an organism including the isotopic signature of the primary producers at the base of the food chain (d¹³C_base), 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 (t_org), and the trophic fractionation (Δ_c high for N, low for C): d¹³C_org  = d¹³C_base2 – [ α (d¹³C_base2 - d¹³C_base1)] - Δ_ct_org. 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)