A knowledge base for management of the capital-intensive fishery for small pelagic fish off South Africa

As a contribution to South Africa's move towards an ecosystem approach to fisheries management, this study explores the existence of common perceptions about South Africa's pelagic fishery between resource users and scientists. It represents a collaborative research effort of social and natural scientists. A brief overview is given of the southern Benguela upwelling ecosystem and small pelagic fish resources, the fishery, and management of the fishery. Stakeholder knowledge and views were determined by conducting open-ended qualitative local knowledge interviews. Candidate indicators to address five major issues raised in the interviews were selected: length-at-50% maturity, total mortality, exploitation rate, proportion of bycatch, mean length of catch, and centre of gravity of catches. The indicator approach is shown to be a useful tool to manage the South African small pelagic fishery, and can be made compatible with existing management approaches. The foundation of a good adaptive fisheries management system is a data collection system that enables multi-disciplinary analysis and provides a basis on which decisions can be made.     

Revised estimates of abundance of South African sardine and anchovy from acoustic surveys adjusting for echosounder saturation in earlier surveys and attenuation effects for sardine

Hydro-acoustic surveys have been used to provide annual estimates of May recruitment and November spawner biomass of the South African sardine Sardinops sagax and anchovy Engraulis encrasicolus resources since 1984. These time-series of abundance estimates form the backbone of the assessment of these resources, and consequently the management of the South African sardine and anchovy is critically dependent on them. Upgrades to survey equipment over time have resulted in recent surveys providing more accurate estimates of abundance, yet in order to maintain comparability across the full time-series, estimates of biomass mimicking the old equipment were used for a number of years. In this paper we develop a method to revise the earlier part of the time-series to correct for receiver saturation in the older generation SIMRAD EK400 and EKS-38 echo sounders and to account for attenuation in dense sardine schools. This is applied to provide a revised time-series of biomass estimates for the South African sardine and anchovy resources with associated variance–covariance matrices. Furthermore, the time-series presented here are based on updated acoustic target strength estimates, making this the most reliable time-series currently available for both resources.     

Abrupt environmental shift associated with changes in the distribution of Cape anchovy Engraulis encrasicolus spawners in the southern Benguela

Cape anchovy Engraulis encrasicolus spawners in the southern Benguela showed an eastward shift in their distribution on the Agulhas Bank that occurred abruptly in 1996 and has since persisted. We assessed whether this shift was environmentally mediated by examining sea surface temperature data from different regions of the Agulhas Bank, which showed that in 1996 the inner shelf of the Agulhas Bank to the east of Cape Agulhas abruptly became 0.5°C colder than in previous years and has since remained that way. In addition, signals, coherent with the 1996 shift recorded in sea surface temperatures, were also found in atmospheric surface pressure and zonal wind data for that region; interannual coastal SST variability is also shown to be correlated with zonal wind-stress forcing. As a result, increased wind-induced coastal upwelling east of Cape Agulhas is proposed as the main driver of the observed cooling in the coastal region. The synchrony between the environmental and biological signals suggests that the eastward shift in anchovy spawner distribution was environmentally mediated and arose from a change in environmental forcing that altered the relative favourability for spawning between regions to the west and east of Cape Agulhas. The results highlight how a relatively minor change in environmental conditions can lead to a drastic spatial reorganisation of the life history of one species in an ecosystem.     

Feeding ecology of South African Argyrosomus japonicus (Pisces: Sciaenidae), with emphasis on the Eastern Cape surf zone

A Monte-Carlo simulation approach is used to investigate the relative robustness of the results of the Butterworth–Andrew (B ₁ = K and B ₁ estimated) dynamic production-model observation-error estimators to different "true" values of the ratio of initial biomass B ₁ to unexploited equilibrium biomass K. The simulations use an underlying operating model typical of that appropriate for stocks of Cape hake off southern Africa. The B ₁ = K variant of the estimator results in lower estimated expected discrepancies for a number of management quantities over a wide range of the value of the "true" ratio. In addition, it is less likely to provide severe overestimates of the f _0.1 total allowable catch (TAC). Not all management quantities are equally sensitive to the value of the "true" ratio. In particular, f _0.1) TACs can be severely overestimated, although the maximum sustainable yield (MSY) is relatively well determined over a wide range of values of the ratio. It is therefore recommended that harvesting strategies which do not permit TACs to be set in excess of the estimated MSY be preferred.     

Characterising and comparing the spawning habitats of anchovy Engraulis encrasicolus and sardine Sardinops sagax in the southern Benguela upwelling ecosystem

The spawning habitats of anchovy Engraulis encrasicolus and sardine Sardinops sagax in the southern Benguela upwelling ecosystem were characterised by comparing their egg abundances with environmental variables measured concomitantly during two different survey programmes: the South African Sardine and Anchovy Recruitment Programme (SARP), which comprised monthly surveys conducted during the austral summers of 1993/94 and 1994/95; and annual pelagic spawner biomass surveys conducted in early summer (November/December) from 1984 to 1999. Eggs were collected using a CalVET net. Physical variables measured included sea surface temperature (SST), surface salinity, water depth, mixed-layer depth, and current and wind speeds; biological variables measured included phytoplankton biomass, and zooplankton biomass and production. Spawning habitat was identified by construction of quotient curves derived from egg abundance data and individual environmental variables, and relationships between these variables were determined using multivariate co-inertia analysis. SARP data showed that anchovy spawning was associated with cool water and moderate wind and current speeds, whereas sardine spawning was related to warmer water and more turbulent and unstable conditions (i.e. high wind speeds and strong currents) than for anchovy. SARP data also showed significant differences in selection of spawning habitat of the two species for all environmental variables. The relationship between anchovy egg abundance and salinity was strongly positive, but strongly negative with water depth, phytoplankton biomass and zooplankton production. Sardine egg abundance was strongly positively related to current speed. The spawner biomass survey data demonstrated that the spawning habitat of anchovy was characterised by warm water and high salinity, whereas sardine spawning was associated with cool water and low salinity. The survey data showed significant differences in spawning habitat selection by anchovy and sardine for SST, salinity and zooplankton biomass, but not for the other environmental variables. There was a positive relationship between anchovy egg abundance and SST, salinity and mixed-layer depth, and a negative relationship with water depth, phytoplankton biomass and zooplankton production. For sardine there was a strong positive relationship between egg abundance and current speed and wind speed. Differences in the results between the two survey programmes could be attributable to differences in their spatio-temporal coverage. Spawning habitats of anchovy and sardine appear to be substantially different, with anchovy being more specific than sardine in their preference of various environmental conditions.     

Comparison of assemblages and some life-history traits of seabirds in the Humboldt and Benguela systems

There are 21 and 15 species of seabirds that breed in the Humboldt and Benguela upwelling systems respectively. Only two species of gull are common to both systems, one as an endemic subspecies to the Benguela system. Eleven species and two subspecies are endemic (or nearly so) to the Humboldt system; seven species and one subspecies to the Benguela system. Each system has an endemic penguin, sulid, cormorant and tern that feed mainly on anchovy Engraulis spp., sardine Sardinops sagax or both these fish. The Peruvian pelican Pelecanus thagus also feeds primarily on these prey items. A plentiful availability of food has resulted in many of these seabirds attaining high levels of abundance. For the four pairs of species that feed on anchovy and sardine, those in the Humboldt system all have a biology that enables them to increase more rapidly than their Benguela counter-parts. This reflects the higher frequency of environmental perturbations that depress seabird populations in the Humboldt system. In addition, both systems have a small endemic cormorant that feeds near the coast and a small endemic tern that breeds in the adjacent mainland desert and feeds at the sea surface. Several seabirds endemic to a system have no obvious ecological equivalent in the other system: the pelican, a diving-petrel, four storm-petrels and a gull in the Humboldt system; a cormorant and a gull in the Benguela system. Some species with tropical or subantarctic affinities breed at the boundaries of the systems. Others breed also in freshwater systems. The grey gull Larus modestus, which feeds in the Humboldt system, breeds in montane deserts.     

Comparative trophodynamics of anchovy Engraulis encrasicolus and sardine Sardinops sagax in the southern Benguela: are species alternations between small pelagic fish trophodynamically mediated?

The results of detailed morphological, experimental, field and modelling studies on various aspects of the trophic ecology of sardine Sardinops sagax and anchovy Engraulis encrasicolus in the Benguela ecosystem are synthesised, and differences in the trophodynamics of these two species are highlighted. Anchovy possess a relatively coarse branchial apparatus; feed predominantly by particulate-feeding and maximise their net energetic gain by using this feeding mode; show higher weight-standardised clearance rates than do sardine for prey >580μm; are most efficient at assimilating nitrogen from zooplankton and excrete <50% of ingested nitrogen; feed inefficiently on phytoplankton and derive the bulk of their dietary input from larger zooplankton; and maximise their scope for growth on mesozooplankton. In contrast, sardine possess a relatively fine branchial apparatus; feed predominantly by filter-feeding and maximise their net energetic gain by using this feeding mode; show higher weightstandardised clearance rates than do anchovy for prey <580μm; are most efficient at assimilating nitrogen from zooplankton but excrete >50% of ingested nitrogen; are able to feed on phytoplankton but derive the bulk of their dietary input from smaller zooplankton; and maximise their scope for growth on microzooplankton. These differences provide compelling evidence that anchovy and sardine are trophically distinct, and indicate that the two species show resource partitioning based on zooplankton size. The implications of these trophic differences for ecosystem functioning are discussed, and support the hypothesis that species alternations between anchovy and sardine, both in the southern Benguela and in other upwelling ecosystems, are likely to be trophodynamically mediated.     

A distance meter for large swimming marine animals

An electronic distance meter for marine animals is described. The device was tested on jackass penguins Spheniscus demersus, but could easily be modified for use on a variety of animals.