Remaining questions in the case for balanced harvesting

Balanced harvesting – harvesting all species and sizes in an ecosystem in proportion to their productivity – is a fisheries management strategy that has been suggested recently to increase yields, while reducing overall ecosystem impact. However, some aspects of balanced harvesting are controversial, including its call for extensive harvesting of juveniles and forage fish. Balanced harvesting also calls for targeting species and size‐classes that are not currently marketable, possibly at a significant economic cost. Some have argued that this cost is outweighed by the ecological benefits of maintaining the ecosystem size and trophic structures and by the benefits of extra yield for food security. There is broad consensus that balanced harvesting would require major changes to fishery management institutions and consumer behaviour, and it is unclear to what extent it is physically possible with current technologies. For this reason, we argue that steps to implement balanced harvesting are difficult to justify until the case for it is more clearly resolved. We outline some of the pivotal questions that must be answered to make a convincing case for or against balanced harvesting, many of which can be answered empirically. In identifying these questions, we hope to offer a constructive path forward in resolving some of the key issues in the balanced harvesting debate.     

Development and use of a framework for characterising computer models of silvoarable economics

A review of existing computer models of silvoarable¹ economics was undertaken for a project, entitled Silvoarable Agroforestry for Europe (SAFE), which aims to reduce uncertainty regarding the introduction and management of silvoarable systems in Europe. Because the published literature describing and comparing such models is sparse, a framework was developed and then used to characterise five computer models: POPMOD, ARBUSTRA, the Agroforestry Estate Model, WaNuLCAS, and the Agroforestry Calculator. Key characteristics described for each model were the background, systems modelled, objective of the economic analysis, economic viewpoint, spatial and temporal scales, generation and use of biophysical data, model platform and interface, and input requirements and outputs. Each of the models could produce a partial budget of the profitability of a silvoarable, arable, or forestry system at a one-hectare level using discounted cost–benefit analysis. Whilst the research models undertook the analysis from a viewpoint of a generic farmer, models developed for decision-support also included appraisals from the perspectives of tenants, share-croppers, and participants in a joint-venture. The two farm-scale models, ARBUSTRA and the Agroforestry Estate Model, could also be used to examine the feasibility of silvoarable systems on an existing business, and to determine the effects of heterogeneous land types and phased planting. The framework allows users to identify the pertinent issues for selecting or developing a particular model. The word ‘silvoarable’ is synonymous with the word ‘agrisilviculture’ described by Nair (An Introduction to Agroforestry, Kluwer Academic, 1983) and describes the same type of production system. An erratum to this article is available at .     

Multiplication of the V4 strain of Newcastle disease virus in Madin Derby bovine kidney cells

This study describes a reproducible cell culture system that permits the growth and secondary multiplication of the V4 strain of Newcastle disease virus. Allantoic fluid, magnesium chloride and diethylaminoethyl dextran were incorporated in Dulbecco's modified Eagle's medium to encourage secondary viral multiplication without adversely affecting healthy Madin Derby bovine kidney cell growth.     

Detection of immunological tolerance to bovine spumavirus (BSV) with evidence for salivary excretion and spread of BSV from the tolerant animal

A group of 17 Friesian-Holstein steers held in individual pens was examined for evidence of infection with bovine spumavirus (BSV). Serum was examined for specific antibody by 2 serological procedures, and circulatory leucocytes and throat swabs were examined for the presence of circulatory leucocyte-associated BSV (CLAB) and saliva-associated BSV (SAB). Initial tests showed that 7 of the 17 steers had specific antibody to BSV by both serological procedures, and a further steer developed such antibody during the first 3 months of holding the animals in single contiguous pens. All 8 of these specific antibody-positive steers were CLAB positive and SAB negative. Nine steers showed no specific antibody to BSV by either of the 2 serological procedures; 8 of these 9 steers showed no evidence of CLAB or SAB. The exception was one steer which was CLAB- and SAB-positive at each of 30 samplings taken over a period of 9 months observation, whilst remaining specific-antibody free. This steer was classed as immunologically tolerant of BSV, and epidemiological data suggested that lateral spread of infection had originated from this animal through the agency of saliva.