Fish assemblages in the Lower Congo differ in stretches of rapids, in runs, pools, and backwaters, and in near-shore habitats with intermixed rocky and sandy substrates. Because of the marked habitat heterogeneity in our study areas effective sampling is challenging and a range of collection techniques are necessary to effectively capture the diversity of the region. The greatest challenge is collecting in the rapids, and effective sampling is possible only at low water and even then it is often only possible to collect at the edges of the rapids.
For an overview of standard ichthyological collecting techniques we recommend the chapter on collecting fishes by Baldwin et al., in Lang and Baldwin (Eds.) 1996. Methods and Techniques of Underwater Research. Proceedings of the American Academy of Underwater Sciences Scientific Diving Symposium. Smithsonian Institution, Washington DC. 236 pp.
Along the margins of the rapids themselves, large dip nets, cast nets, and hook and line have all proven effective. Cast nets are used to sample within the current and along the surface of boulders. Dive lights and dip nets are used to capture nocturnally active fishes as well as species that are diurnal and more-or-less immobile at night. This method has proven to be extremely effective in collecting in still water regions. Along sandy beaches in slower water, seines can be used to sample a variety of taxa, particularly schooling characins, clupeids, and mormyrids. In circumscribed backwaters open to the current at a single point, rotenone, a biodegradable, plant-based fish toxicant, is highly effective. Following careful application, over the course of approximately 2-4 hours, fishes are disabled and are easily captured. Many rapids adapted species that hide in rock rubble are brought to the surface during rotenone application, allowing them to be captured with a dip net. Rotenone allows for capture of species that are rarely collected otherwise (see Bayley and Peterson, 2001. An approach to estimate probability of presence and richness of fish species. Transactions of the American Fisheries Society 130(4): 620-633). With prior permission, we use rotenone in amenable locations away from human settlement, and this has proven been by far the most effective sampling method for certain species.
Near villages, local fishermen are involved in our sampling efforts. Specimens are purchased from fishermen who use cast nets, hook-and-line, gill nets, barriers and fish traps, allowing us to benefit from the aptitude of people who fish the river on a daily basis. In addition to multiplying the total fishing effort expended, involving local fishermen provides us with essential insight into the life-histories and distributions of many species, knowledge of which has been acquired over years of continual interaction with fishes as a food source.
GPS data: Recent technological advances allow exceptionally detailed geospatial data to complement our specimen data. Using a Ricoh Caplio Pro G3 digital camera with GPS (Global Positioning System) capability at all localities, samples are directly linked to high-resolution images of their environs. Each image, imprinted with precise coordinates, directionality, and elevation readings, can in turn be linked to satellite maps of the region. These GIS augmented collection data provides precise information for studies of biogeography, aquatic ecology, and conservation assessment (see MAPPING section of this site).
Euthanizing and Preservation: Upon capture, fishes are anesthetized with MS-222 and representative specimens are photographed. Tissue samples, such as fin clips or muscle plugs, are taken from as many specimens as possible and preserved in 95% ethanol in labeled cryotubes. Whole specimens are then fixed in 10% formalin and later transferred to 70% ethanol for long-term storage.
Tagging Voucher Specimens: Once a tissue sample has been taken it is important that that tissue can readily be linked with its voucher specimen. To do this we employ a rather cheap but effective method using a Dymo letter press and a garment Tag Gun. For each tissue voucher an individual Dymo tag number is made (corresponding to the numbers on and in each cryotube) and the needle of the tag gun is passed through that label (Fig. 1 A, above). The needle is then passed through the lateral muscles of the voucher specimen just above the anal fin (Fig. 1B, above). The specimen is then tagged with a permanent label (Fig. 1C, above) that ties it directly to the corresponding tissue sample. This is a cheap and efficient way to individually tag tissue specimen vouchers.