Evaluation of a class of possible simple interim management procedures for the Namibian hake fishery

During 1997, considerable scientific differences arose about the status of the Namibian hake (Merluccius capensis and M. paradoxus) resource, and as to whether the hake Total Allowable Catch (TAC) should be substantially decreased or increased. These differences revolved primarily around whether or not abundance estimates from the swept-area trawl surveys by the Norwegian research vessel Dr Fridtjof Nansen should be considered as reliable measures of biomass in absolute (as distinct from relative) terms. The paper relates the computations underlying the Interim Management Procedure (IMP) approach that was put forward at that time as a basis to resolve this impasse. The anticipated performance, in terms of catch and risk of resource depletion, of a number of simple candidate IMPs for the Namibian hake resource is evaluated. The IMPs depend on two parameters, whose values are to be chosen by decision-makers, and adjust the TAC up or down from one year to the next according to whether trends in recent commercial catch rate and survey indices of abundance are positive or negative. Performances are evaluated across the then current wide range argued for resource abundance and status. Trade-offs in performance across the candidates considered are discussed. One of the candidates was subsequently selected by a joint meeting of scientists, industry and Ministry officials in February 1998 and served as the basis for scientific recommendations for the TAC for the hake resource for the following three years.     

The structure of demersal assemblages off Namibia in relation to abiotic factors

Groundfish surveys carried out between October 1992 and October 1996 were used to describe the demersal assemblages of the northern Benguela along the Namibian coast. Multivariate analyses indicated a clear separation between shelf and slope habitats, which were divided into three and two assemblages respectively. These divisions were distinct, with clear distributional boundaries separating them. Species determining the structure of these divisions and subdivisions and their relative abundances were identified. Depth, bottom temperature, bottom salinity and dissolved oxygen were all significantly correlated with the spatial distribution of the assemblages. Minor changes or shifts in the distribution of assemblages were observed over the study period, although patterns of species associations remained relatively constant.     

Long-term sea-level variability in the eastern South Atlantic and a comparison with that in the eastern Pacific

The data set of pressure-corrected monthly mean sea level from sites on the coast of the eastern South Atlantic Ocean has now been extended to cover the years from 1959 to 1985. The length of this data set is now comparable to those used in studies of long-term variability in sea level in the eastern Pacific Ocean. Comparison of the data sets reveals a qualitative agreement in the character of the variability in sea level between the two oceans. In particular, the possibility of high sea-level events propagating polewards from the equatorial Atlantic in the manner of the Pacific El Niño is explored and confirmed. The sea-level record, supported by evidence from published studies of variability in sea surface temperature, shows that the years 1963, 1974 and 1984 should be considered to be years of anomalously high sea level along the entire eastern South Atlantic Ocean.     

Making sense of censuses and dealing with missing data: trends in pup counts of Cape fur seal Arctocephalus pusillus pusillus for the period 1972–2004

Trends in the population of Cape fur seals Arctocephalus pusillus pusillus were estimated from counts of pups on aerial photographs of colonies taken between 1972 and 2004 to determine trends in the overall population and subpopulations. Incomplete coverage resulted in missing data in some years. Various methods of determining proxy values for missing data were assessed, and it was concluded that different methods were applicable to Namibian and South African colonies. This reflected variation in trends of pup counts between the countries, which was associated with differences in productivity between the southern and northern Benguela ecosystems. In Namibia, temporal changes in pup numbers were non-linear in some years and there was correspondence in fluctuations at most colonies. This appeared to be on account of an effect of periodic, wide-scale prey shortages that reduced birth rates. There was a northward shift in the distribution of seals in the northern Benguela system. In South Africa, pup counts were much less variable between years, probably on account of a relative stability of food supply. A linear approach was therefore suitable for determining proxy values for missing data at South African colonies. Pup counts suggest that there has been little change in the overall population of the Cape fur seals since 1993, when it was estimated at about two million animals.     

The orange roughy fishery of Namibia: lessons to be learned about managing a developing fishery

Exploration for orange roughy Hoplostethus atlanticus in Namibia started in 1994 and within 12 months several aggregations had been discovered, suggesting the existence of a biomass sufficient to support a viable fishery. At that early stage it was realized that few, if any, recognized management procedures existed for newly developing fisheries, especially with the paucity of data such as existed on Namibian orange roughy. The development of the Namibian orange roughy fishery is reviewed to document the management strategies implemented and how the management of the fishery evolved. The first six years of the fishery are described, including the three-year exploration phase, several years of profitable exploitation, and the severe decline in catch rates. Whether the decline is attributable to fishing mortality or to change in the aggregating behaviour of orange roughy, or both, is not clear. Although many aspects of the precautionary approach were followed, a risk analysis applied and a number of innovative management methods implemented (e.g. incentives to promote exploratory fishing, use of Bayesian statistical methods, implementation of a management plan for long-term total allowable catches), the aggregating biomass declined to between 10 and 50% of virgin levels within the six years. The management methods applied are evaluated in the light of the severe decline in catch rate experienced in 1998 and 1999, so that others may learn from the experience.     

Recent population trends of African penguins Spheniscus demersus in Namibia

The African penguin Spheniscus demersus is endemic to southern Africa and is listed overall as "vulnerable". Over the past century, however, the Namibian population has been severely reduced and is currently listed as "critically endangered". Recent trends at Possession, Halifax, Ichaboe and Mercury islands, which account for 97% of the Namibian population, were examined using counts of moulting adults and active nests at peak breeding. Since 1996, the adult population has decreased at a rate of 2.6% per year. Since 1990, the breeding population has decreased by 3.7% per year. Mercury Island is the only Namibian breeding site where penguin numbers are increasing. Improving the conservation status of the species is critical. Better management strategies need to be identified and implemented.     

In Situ Measurements of the Acoustic Target Strength of Cape Horse Mackerel Trachurus Trachurus Capensis Off Namibia

As it is not always possible to analyse nutrient samples on board ship during research cruises, it is necessary to preserve them for analysis ashore, and freezing is by far the simplest and seemingly the most accurate means of preservation. To verify this statement, a series of experiments on samples analysed both at sea and later ashore (after freezing) was conducted. Freezing is apparently a viable method for preserving samples destined for analysis of nitrate and low silicate (<30–40 mmol·m^?3), but phosphate samples are seemingly not well preserved when frozen. In the latter case, the deficits between analysis of fresh samples aboard ship and frozen duplicates ashore vary randomly. To determine normal trends in nutrient distribution it seems feasible to store samples in the frozen state, but for highly accurate work, as would be necessary for WOCE studies, it is advisable to analyse samples at sea immediately after collection.     

An economic comparison of the commercial and recreational linefisheries in Namibia

The most important Namibian linefish species, the silver kob Argyrosomus inodorus, is currently heavily exploited, and in order to ensure its survival catch restrictions are being introduced. However, kob are exploit ed oth by recreational anglers and by commercial vessels, and it is important to examine the economics of these fisheries in order to determine where catch restrictions will do the least harm to the economy. Data from a survey of commercial fishing vessels are compared with results from earlier surveys of recreational anglers to determine economic values and impacts from both fisheries. The economic benefits are shown to be greatest in recreational angling, less in commercial fishing by large vessels and least in commercial skiboat fishing. This sudy also shows that catch restrictions would do less harm to the economy if applied to the commercial linefishing sector rather than to recreational angling.     

Biochemical genetic variation in the Cape anchovy Engraulis capensis Gilchrist

The extent of biochemical genetic variation in the Cape anchovy Engraulis capensis was estimated from starch-gel protein electrophoresis. The gene products of 31 protein-coding loci were identified and Mendelian polymorphisms were observed at 21 (68 per cent). Ten of these loci (32 per cent) had common-allele frequencies less than 0,95 and hence were useful for testing hypotheses of geographic stock structure. The average population heterozygosity, a measure of genetic variation, ranged from 0,110 to 0,128 and averaged 0,115 over samples from 31 locations. Cape anchovies and other clupeiform fishes appear, on average, to have higher levels of genetic variation than many other marine fishes. This may result from the large population sizes and weak population subdivision that characterize them.