Dynamics of the pelagic component of Namibian hake stocks

A study was undertaken to investigate the spatial, diurnal and interannual dynamics of the pelagic component of Namibian hake stocks. Data were derived from trawl surveys conducted on board the R.V. Dr Fridtjof Nansen during the period 1991–1999. In addition to trawl surveys, acoustic methods were applied concurrently to quantify fish biomass off the bottom (above the headline). The pelagic component of hake was variable in both time and space – it was denser in the north and at depths of 200–500 m. Pelagic densities also correlated well with the density of adult fish (&#8805 30 cm) at the bottom, but not with that of young hake (>18 an

Precision in systematic trawl surveys as assessed from replicate sampling by parallel trawling off Namibia

Following independence of Namibia in 1990, abundance of that country's hake stocks was monitored by trawl surveys conducted by the Norwegian F.R.V. Dr Fridtjof Nansen using a systematic survey design. Precision from such designs is considered to be better than with a random design, but it cannot be quantified through standard analysis of variance. In 1998 and 1999, the trawl surveys were duplicated in full using a commercial trawler that operated in parallel with the Dr Fridtjof Nansen. Both vessels had the same fishing gear and rigging. In both years, biomass and distribution patterns estimated by the vessels were similar. The paired datasets collected from the surveys were used to analyse variability in the point samples. Absolute differences between pairs of catches were, in general, proportional to the catch, but they varied randomly following a normal distribution around the catch level. Local variability was analysed as random noise, modelled and later reapplied on single vessel data series to evaluate the effect of sample size and replicates on the survey mean. In a survey with 200 stations, local sample noise accounted for about 4% of the variability in the survey mean. Alternatively, running series of simulated surveys by bootstrapping on pairs of catch data gave similar results, when a small systematic vessel effect was adjusted for. The main statistical techniques applied were less susceptible to outlier catches than straightforward correlations or regressions and could therefore, perhaps with some advantage, also be used to estimate the vessel factor when intercalibrating trawl survey vessels.Keywords: intercalibration, point precision, replicate sampling, trawl surveysAfrican Journal of Marine Science 2001, 23: 385–39

Towards a catchability constant for trawl surveys of Namibian hake

A trawl catches only a portion of the fish in its path. The term catchability therefore refers to the fraction of the available fish caught. A method was developed and tested to establish catchability constants for trawl surveys of Namibian hake Merluccius spp. A catchability constant can be expressed as a simple relationship between hake area densities calculated from trawl catches and acoustic biomass estimates. Initial values were on an order of magnitude of 0.8, meaning that the catch takes 80% of the hake available to the trawl. The methodology depends on careful area selection (flat bottom, homogenous fish distribution), following the same trawl lane during subsequent hauls in an area, and thorough acoustic post-processing. A pronounced and repetitive pattern in catchability within surface daylight hours was found. Early morning and early afternoon catches were low, and the best catches of the day were made around noon, a result that may influence stock assessments based on trawl data, because morning and afternoon data would under-represent actual abundance.Keywords: acoustics, catchability, hake, trawl surveysAfrican Journal of Marine Science 2001, 23: 375–38

Escapement of Cape hakes under the fishing line of the Namibian demersal sampling trawl

Swept area surveys in Namibian waters provide input data to the stock assessment model. To evaluate the model's predictions, it is important to understand the catch efficiency of sampling trawls. The objective of this study was to establish whether Cape hakes Merluccius capensis and M. paradoxus escape under the fishing line and to identify any species or length dependence of escape rates. Experiments were carried out in Namibian waters during two cruises in October 2002 and October 2003. A collection bag was mounted under the trawl to catch fish escaping below the fishing line. Environmental data and photographs of fish in front of the trawl were also taken. Escapement of hake varied by species, length, depth and year. It was generally below 5% for M. capensis but escapement in 2002 averaged 10–20% for M. paradoxus, and in 2003 it was over 50% in the shallow area (300m), decreasing to 10% in the deep area (570m). Oxygen level variation seemed to explain the marked between-year difference in escapement. Escapement of M. capensis decreased marginally with increasing fish length, whereas escapement of M. paradoxus showed a marked increase with increasing fish length in shallow waters and no length dependence in deep waters. Species differences in behaviour and vision may partly explain the observations.African Journal of Marine Science 2007, 29(2): 209–22