Rikus Grobler | Oct 18, 2017 | 0
Surveying pelagic and demersal fish in Namibian sea
The National Marine Information and Research Centre in the Ministry of Fisheries and Marine Resources is responsible for conducting annual research activities on the different types of fish found in the sea.
The Subdivision Demersal Resources is tasked to conduct research and provide scientific advice to government on the sustainable utilisation of commercially important living marine resources such as Hake, Monk, Orange Roughy, Rock Lobster and Crab. In total, about 140 days are needed to conduct demersal research activities onboard the Research Vessel.
The main objective of conducting such research is to estimate the biomass of demersal resources and consequently provide scientific advice for the management of these resources, including recommendations for annual TACs (Total Allowable Catches).
Demersal fish such as hake lives in greater depth of oceans also called bottom feeders whereas horse mackerel, a type of pelagic fish, is found mostly at shallow depths and their availability decreases with depth as food material becomes scarce.
The two species of hake occur on the shelf and upper slope in the Namibian waters. Merluccius capensis occurs at depths of about 100 m to 350 m and overlaps with the shallow end of the distribution range of M. paradoxus, which occurs mainly at depths of 300 m to 600 m and even deeper, having been found at depths exceeding 900 m, but in lower catches. Three types of vessels (freezer trawlers, wet fish trawlers and long-liners) are operating in the hake fishery. Swept-area biomass surveys for hakes are conducted annually to obtain an index of abundance, determine the geographical distribution and collect biological information from the stock.
The biomass estimates from these surveys are used as relative abundance indices in the age-structured production model (ASPM) which integrates the available information on the resource regarding historic catches, indices of abundance, age and length composition data, which are collected from the commercial fishery and the scientific research surveys.
The Namibian horse mackerel fishery is split between the mid-water and purse seine-directed fisheries. The mid-water fishery is the largest by volume of all fisheries in Namibia. The main target species of the purse seine-directed fishery is sardine, which used to be the mainstay of the Walvis Bay economy.
Both horse mackerel and sardine are surveyed annually to determine the relative abundance, spatial distribution, biology and size composition of these two species, as well as the other small pelagic species such as, red-eye round herring and anchovy. The method uses hydro-acoustics in conjunction with targeted trawling.
The horse mackerel and small pelagic survey covers the area from 17°15 to 25° 00’ S, while the sardine survey follows the stock’s distribution up to 16° 00’ S in southern Angola. The surveys cover water depths from approximately 20m close to the coast up to the offshore limit of the stocks (a few thousand meters water depth).
The Ministry is also responsible for assessing the state of the marine environment through research and regular monitoring activities. Such information enables the ministry to study the relationship between Namibia’s living marine resources and the ocean as well as to identify anomalous events that might impact these resources negatively.
Although the northern Benguela Current region has been researched for many decades it has lost none of its appeal to both local and foreign scientists.