P.A.M. van Zwieten

Scenarios for Resilient Shrimp Aquaculture in Tropical Coastal Areas

We contend there are currently two competing scenarios for the sustainable development of shrimp aquaculture in coastal areas of Southeast Asia. First, a landscape approach, where farming techniques for small-scale producers are integrated into intertidal areas in a way that the ecological functions of mangroves are maintained and shrimp farming diseases are controlled. Second, a closed system approach, where problems of disease and effluent are eliminated in closed recirculation ponds behind the intertidal zone controlled by industrial-scale producers. We use these scenarios as two ends of a spectrum of possible interactions at a range of scales between the ecological, social, and political dynamics that underlie the threat to the resilience of mangrove forested coastal ecosystems. We discuss how the analytical concepts of resilience, uncertainty, risk, and the organizing heuristic of scale can assist us to understand decision making over shrimp production, and in doing so, explore their use in the empirical research areas of coastal ecology, shrimp health management and epidemiology, livelihoods, and governance in response to the two scenarios. Our conclusion focuses on a series of questions that map out a new interdisciplinary research agenda for sustainable shrimp aquaculture in coastal areas.

Are the Lake Victoria fisheries threatened by exploitation or eutrophication? Towards an ecosystem-based approach to management.

Lake Victoria’s ecosystem has shown fundamental changes over its past recorded history in terms of nutrient loadings, productivity, faunal composition and fisheries. As yet, however, no attempt has been made to link the driving processes of eutrophication and fisheries to understand the feedback observed in fish stocks, food webs, exploitation patterns and trade. Single- and multi-species stock assessments, based on steady-state models with effort (and/or predation) as the only driver – still used in the region to advise on management – uniformly indicate overfished stocks of Nile perch that are in danger of collapse. These current views of overfishing are not validated by empirical observations. This chapter presents a holistic integrated ecosystem approach which combines a phenomenological analysis of key processes with a comprehensive set of simple indicators, covering physical, biological and human development, where directionality in time is made explicit to understand ongoing processes in the changing ecosystem. This new approach results in: (i) no signs of overfishing in any of the verifiable indicators; and (ii) biological production increasing over time together with effort and yield as a function of increased eutrophication. The results indicate that continued eutrophication presents a much graver risk to the resource base and thus livelihoods of Lake Victoria’s coastal populations than fishing pressure. Lake Victoria can serve as an interesting case study for the inherent risk of using traditional fish stock assessment in changing ecosystems, and for the development of holistic monitoring systems for ecosystem-based management.

Aquatic food production in the coastal zone: data-based perceptions on the trade-off between Mariculture and fisheries production of the Mahakam delta and estuary, east Kalimantan, Indonesia

In less than two decades, from 45,000 to 70,000 ha, or up to 70%, of the mangroves in the Mahakam Delta, East Kalimantan, were converted to shrimp ponds. This is expected to affect the productive and buffering function of intact mangroves, observable as shifts in composition and a possible reduction in productivity of the coastal fisheries. The trade-off between mariculture and fisheries is explored with data from fisheries statistics, surveys and reported information. Analysis of trends and developments in total catch, catch by species category, catch rate, fishing effort, pond production and productivity gave no direct quantitative evidence of reduced coastal production and productivity. Shrimp-pond productivity (125 kg/ha/year) is low, stable and highly variable (CV = 69%) at the aggregated level of the delta. Since 1989, fishing efforts have increased and patterns diversified, but aggregated catch rates did not decrease. Catches of rays and sharks decreased and the pelagic/demersal catch ratio increased. A shift towards more resilient species categories with a high turnover rate took place from 1993 to 1999, 4 to 10 years after the boom in pond construction. However, these clear shifts are not self-evidently related to mangrove conversion. Reasons for this are discussed. The potential for detection of changes in resource outcome and assessment of the trade-off between mariculture and fisheries, at both the local level and through aggregated fisheries statistics, is limited because of the high variability in outcome. This implies a limited capacity for resolution of resource-use conflicts when evaluating competing claims informed by existing data and information on resource change.

Nile perch distribution in south-east Lake Victoria is more strongly driven by abiotic factors, than by prey densities

We studied the effects of environmental driving factors (maximum depth, visibility, oxygen, temperature, and prey densities) on the distribution and diet composition of Nile perch (Lates niloticus) in south-east Lake Victoria from 2009 to 2011. We tested the hypotheses that (i) Nile perch distribution is regulated by the same environmental factors on a local scale (Mwanza Gulf) and on a regional scale (Mwanza Gulf, Speke Gulf and the open lake in Sengerema district), and (ii) driving factors act differently on different Nile perch size classes. Fish were sampled with gillnets. Nile perch densities were highest in the shallow part of the Mwanza Gulf and during the wet seasons, mainly caused by high densities of juveniles. The environmental driving factors explained Nile perch distributions on both regional and local scales in a similar way, often showing non-linear relationships. Maximum depth and temperature were the best predictors of Nile perch densities. Prey densities of shrimp and haplochromines did not strongly affect Nile perch distributions, but did explain Nile perch diet on a local and regional scale. We conclude that abiotic variables drive Nile perch distributions more strongly than prey densities and that feeding takes place opportunistically.