Astrid Jarre

Trophic flows in the southern Benguela during the 1980s and 1990s

Mass-balanced models of trophic flows in the southern Benguela ecosystem suggest a 10% increase in zooplankton biomass between the 1980s and the 1990s, in agreement with observed trends of increased zooplankton abundance off South Africa over the last few decades. Minimum hake biomass in balanced trophic models is substantially larger than survey and other model estimates,suggestingundersamplingofhakesinsurveysandunderestimationofjuvenilehakemortality.Modelbiomassandmean annual production of five important small pelagic fish groups were larger in the 1990s, and total catches were smaller than in the 1980s. Estimates of biomass per trophic level, transfer efficiencies, mixed trophic impacts and many other ecosystem attributes suggest that trophic functioning of the southern Benguela ecosystem was similar in the 1980s and 1990s. Because catches were lowerandmodelzooplanktonandsmallpelagicfishbiomasseswerelargerinthe1990s,theecosystemwaslesstightlyconstrained by predators (including fishers) and food availability than in the 1980s. Fishing took place at low trophic levels compared to other systems. Despite smaller total catches in the 1990s, fishing was ecologically more expensive (from higher trophic levels) during the 1990s than in the 1980s because snoek and hake catches were large. There was greater shared niche overlap of small pelagic fish predators in the 1990s than in the 1980s. Mean transfer efficiency was 12%. Transfer of biomass at trophic levels III–V appears to be more efficient in the southern Benguela than in other upwelling ecosystems. Primary production required to sustain catches in the southern Benguela ecosystem is 4% of total primary production, i.e. more similar to estimates for open ocean and coastal regions than for other upwelling or shelf systems averaging more than double this value. D 2002 Elsevier Science B.V. All rights reserved.

Application of the sequential t-test algorithm for analysing regime shifts to the southern Benguela ecosystem

Long-term ecosystem changes, such as regime shifts, have occurred in several marine ecosystems world-wide. Multivariate statistical methods have been used to detect such changes. A new method known as the sequential t-test algorithm for analysing regime shifts (STARS) is applied to a set of biological state variables as well as environmental and anthropogenic forcing variables in the southern Benguela. The method is able to correct for auto-correlation within time-series by a process known as prewhitening. All variables were tested with and without prewhitening. Shifts that were detected with both methods were termed robust. The STARS method detected shifts in relatively short time-series and identified when these shifts occurred without a priori hypotheses. Shifts were generally well detected at the end of time-series, but further development of the method is needed to enhance its performance for auto-correlated time-series. Since 1950, two major long-term ecosystem changes were identified for the southern Benguela. The first change occurred during the 1960s, caused predominantly by heavy fishing pressure but with some environmental forcing. The second change occurred in the early 2000s, caused mainly by environmental forcing. To strengthen these findings, further analyses should be carried out using different methods.

St Helena Bay (southern Benguela) then and now: muted climate signals, large human impact

The development of suitable reference states for ecosystem-based management requires documentation of changes in structure and functioning of marine ecosystems, including assessment of the relative importance of bottom-up and top-down processes as drivers of change. We used monitoring data available from St Helena Bay, the most productive bay and an important nursery area situated on the west coast of South Africa, during 1950–2010 to reveal changes in the abiotic and biotic components. St Helena Bay in the 1950s showed similarities to 2000–2010 in terms of wind patterns, hydrology and phytoplankton. Upwelling, oxygen and nutrient concentrations in subthermocline water displayed pronounced decadal-scale variability. Primary production in St Helena Bay is variable, but consistently higher than that on the adjacent Namaqua shelf. Zooplankton size composition and biomass in August have changed markedly since the 1950s. During 2001–2010, mesozooplankton biomass in autumn was considerably lower than in summer, probably due to predation by small pelagic fish. Pelagic fish catch patterns and distributions have altered dramatically. Conservation measures, implemented to reverse past negative human impact, have benefitted marine mammals, the abundance of which has increased in the area, but additional conservation measures are necessary to reverse the decline in African penguins Spheniscus demersus. St Helena Bay shows a muted response to long-term change in the southern Benguela, with marked decadal variability but no clear long-term trend in oceanography and biogeochemistry. Changes in ecosystem boundary conditions and fishing pressure cannot be ignored as important drivers of change in the southern Benguela since the 1950s.

Changes in demersal fish assemblages on the west coast of South Africa, 1986–2009

Research survey data collected over 24 years (1986–2009) were used to explore long-term changes in demersal fish assemblages on the west coast of South Africa. Differences in spatial (latitude and depth) and temporal (seasonal and annual) factors were examined using multivariate analyses. Fish assemblages are clearly influenced by depth, with a distinct change in the region of the shelf break between 300 m and 400 m. There are also geographic differences in fish assemblage from north to south. Multivariate analyses show two clear temporal changes in assemblages, first in the early 1990s and second in the mid-2000s, although the latter change may be confounded by a concurrent change in survey trawl gear. The abundance of three fast-growing, early-maturing species increased over the study period whereas that of two slow-growing, long-lived species decreased, supporting the hypothesis of an increase in fast-growing, early-maturing species and a decline in slow-growing, long-lived species in fished systems. Shifts in demersal fish assemblages coincide temporally with spatial shifts observed in West Coast rock lobster Jasus lalandii and with regime shifts in the pelagic ecosystem. The changes in demersal fish assemblages detected are probably a reflection of long-term indirect effects of fishing in combination with environmental changes.

A fuzzy-logic tool for multi-criteria decision making in fisheries: the case of the South African pelagic fishery

The present study presents an electronic decision-support tool that uses a fuzzy-logic model of expert knowledge to assist in multi-criteria decision-making in the context of an Ecosystem Approach to Fisheries (EAF). The prototype model integrates the multiple goals and objectives related to the evaluation of the ecosystem performance of the South African sardine Sardinops sagax fishery into a NetWeaver knowledge base and provides intuitive visual outputs to communicate results to managers and stakeholders. The software tool was developed in a consultative process with key experts and follows the hierarchical tree approach recommended in the FAO guidelines for responsible fisheries. Input variables are based both on quantitative data and expert opinion. We evaluated the model in terms of robustness to input changes, influence of system structure, and appropriateness of input scales for parameters based on expert opinion. Results show that the model is robust and conservative. The strength of the approach lies in the ability to include variables that are difficult to measure. It provides a means of rendering value judgements explicit and transparent. The tool synthesises a large amount of information and aims at improving understanding rather than achieving precision. The system has the potential to have wide application in the context of EAF.

11 Developing a basis for detecting and predicting long-term ecosystem changes

Abstract Long-term ecosystem changes in the Benguela region include species alternations and regime shifts, which are sometimes obscured by large intra- and inter- annual variability in the ecosystem. This chapter proposes that no single model or approach can resolve this variability and effectively detect and predict long-term ecosystem changes; a coherent, robust, transparent and reproducible synthesis framework is required. Indicators and models are described that can be used to identify some aspects of the current state of ecosystem structure and to detect and monitor long-term change. A short-term challenge is to synthesize these varied sources of multidisciplinary (and sometimes contradictory) information in a logical and consistent fashion. An expert system approach is proposed to do this, consolidating results of different indicators and models within a dynamic process that uses feedbacks to validate predictions of the expert system, and to improve it. It is suggested that such an approach should be initiated in the short term, even as models and indicators are being developed further. In parallel, multivariate statistical tools should be refined and applied to existing time series, to identify past periods of ecosystem change. Current data gaps should be filled, including time series of primary production and the abundance of gelatinous zooplankton. In the medium term, the expert system model should evolve to a point where its results can be used to inform various management groups about the state of the ecosystem. Part of this evolution requires that ecosystem indicators be presented with error estimates or formal assessments of quality.

Cooperation between scientists, NGOs and industry in support of sustainable fisheries: the South African hake Merluccius spp. trawl fishery experiencea

This paper examines the increasingly close interaction between natural and social scientists, non-governmental organizations (NGO) and industry, in pursuit of responsible ecosystem-based management of fisheries. South Africa has committed to implementing an ecosystem approach to fisheries management. Management advice stems from multi-stakeholder representation on government-led scientific and management working groups. In the hake Merluccius capensis and Merluccius paradoxus fishery, the primary management measure is an annual total allowable catch (TAC), the level of which is calculated using a management procedure (MP) that is revised approximately every 4 years. Revision of the MP is a consultative process involving most stakeholders, and is based on simulation modelling of projected probable scenarios of resource and fishery dynamics under various management options. NGOs, such as the Worldwide Fund for Nature in South Africa (WWF-SA), have played an important role in influencing consumers, the fishing industry and government to develop responsible fishing practices that minimize damage to marine ecosystems. Cooperation between industry, government and scientists has helped to improve sustainability and facilitated the meeting of market-based incentives for more responsible fisheries. Research includes ecosystem modelling, spatial analysis and ecosystem risk assessment with increasing research focus on social and economic aspects of the fishery. A four-year cooperative experiment to quantify the effect of trawling on benthic community structure is being planned. The food requirements of top predators still need to be included in the TAC-setting formulae and more social and economic research is needed. This paper also demonstrates how NGO initiatives such as Marine Stewardship Council certification and the Southern African Sustainable Seafood Initiative, a traffic light system of classifying seafood for consumers, have contributed to responsible fishing practices, increased ecosystem research and public awareness. This fishery appears to have a good future, provided that the monitoring, control and surveillance systems continue to function, TACs remain within ecologically sustainable limits and the effective collaboration between government, industry, scientists and NGOs continues to drive positive change.

Modelling regime shifts in the southern Benguela : a frame-based approach

This study explores the usefulness of a frame-based modelling approach in the southern Benguela upwelling ecosystem, with four frames describing observed small pelagic fish dominance patterns. We modelled the dynamics of sardine Sardinops sagax and anchovy Engraulis encrasicolus populations under the influence of the small pelagic fishery and climate variability, performed extensive sensitivity analyses and exercised the model prototype in scenarios based on realistic management objectives. Model outputs were seen to roughly parallel observed dynamics. Frame dynamics were reasonably sensitive to frame switching rules. Reduced sardine recruitment in the ‘low’ frames had a strong negative impact on stock recovery, and bycatch of juvenile sardine from the modelled anchovy fishery could also greatly impair sardine recovery. Specific tests included exploring the robustness of fisheries management strategies under highly stochastic recruitment and uncertainty around long-term environmental forcing. The respective frame of the model is suggested as a composite indicator of ecosystem state. The model approach shows promise for evaluating small pelagic fisheries management strategies in an ecosystem context incorporating long-term environmental variability.

Biannual otolith‐zone formation of young shallow‐water hake Merluccius capensis in the northern Benguela: age verification using otoliths sampled by a top predator

Otoliths collected at least monthly from scat samples of Cape fur seals Arctocephalus pusillus are used to show that shallow-water hake Merluccius capensis from the northern Benguela develop three translucent zones in their first 1·5 years of life. The novel sampling approach provided otoliths that belonged to four M. capensis cohorts of approximate known age (hatched in 1996, 1998, 2002 and 2005), allowing age verification. Following spawning in austral winter, translucent zones consistently formed first in summer and autumn (T1), then in winter and spring (T2) and again in summer and autumn (T3), with no difference in appearance of the zones (biannuli) for the four cohorts considered. The second translucent zone is usually the first true annulus (year mark). It forms during July to September in fish of 15-20 cm total length (LT ). Formation of the translucent zones appears to be determined by fish length or age, rather than by exogenous cues. It is suggested that length measurements should be used to help determine the first age group; fish with a translucent zone marked at otolith lengths >7·5 mm should be termed 1 year-old fish. Ages of M. capensis used in previous stock assessment models have been overestimated. Biannuli are an unusual occurrence in fish otoliths in general, but have been observed in other Merluccius species.

Applying a decision tree framework in support of an ecosystem approach to fisheries: IndiSeas indicators in the North Sea

Applying a decision tree framework in support of an ecosystem approach to fisheries: IndiSeas indicators in the North Sea Emma M. Lockerbie*, Christopher P. Lynam, Lynne J. Shannon, and Astrid Jarre Marine Research (MA-RE) Institute and Department of Biological Sciences, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa Statistics in Ecology, Environment and Conservation (SEEC), University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa Lowestoft Laboratory, Centre for Environment, Fisheries and Aquaculture Science, Pakefield Road, Lowestoft, Suffolk NR33 0HT, UK