Browsing by Author "Silvert, William"
Now showing 1 - 2 of 2
Results Per Page
Sort Options
- Review of recent carrying capacity models for bivalve culture and recommendations for research and managementPublication . McKindsey, Christopher W.; Thetmeyer, Helmut; Landry, Thomas; Silvert, WilliamModels and tools for assessing the carrying capacity of an area of interest for bivalve culture can be classified according to their level of complexity and scope. In this report, we discuss and outline four hierarchical categories of carrying capacity studies: physical, production, ecological, and social carrying capacity. The assessment of carrying capacity for progressively higher categories of models is based on a sound understanding of preceding categories. We discuss each in brief and the third in more detail as this is the level at which knowledge is the most lacking and for which science may make the most advances. (1) Physical carrying capacity may be assessed by a combination of hydrodynamic models and physical information, ideally presented and analysed within a Geographic Information System (GIS). (2) Most scientific effort to date has been directed towards modelling production carrying capacity and some of the resulting models have been used successfully to this end. Further development of these models should pay attention to (i) better modelling of feedback mechanisms between bivalve culture and the environment, (ii) a consideration of all steps in the culture process (seed collection, ongrowing, harvesting, and processing), and (iii) culture technique. (3) The modelling of ecological carrying capacity is still in its infancy. The shortcomings mentioned for models for production carrying capacity estimates are even greater for ecological carrying capacity models. GIS may be employed to consider interactions between culture activities and sensitive habitats. (4) It is recommended that social carrying capacity be evaluated only after the preceding levels have been completed so that an unbiased assessment is obtained. This however does not exclude direction from managers for scientists as to which factors (such as water clarity, specific habitats, etc.) should be evaluated. The use of expert systems to aid in management decisions is briefly discussed with a suggested application of a fuzzy expert system to this end. (c) 2006 Elsevier B.V. All rights reserved.
- The efficacy of releasing caught Nephrops as a management measurePublication . Castro, Margarida; Araújo, Artur; Monteiro, Pedro; Madeira, Ana Margarida; Silvert, WilliamThe objective of this work was to examine if a policy of releasing caught lobsters, Nephrops, could be used to protect the Nephrops population off the south coast of Portugal. Such a measure, as an alternative to closing the fishery completely, would allow the continued operation of the fleet in targeting other species, resulting in a reduced economic and social impact compared with a complete closure. For such a proposal to be considered, information on survival rates of released lobsters is required. Nephrops were sampled randomly from the catch of commercial trawlers and put in cages, in an area with similar conditions to the area where they were caught, in order to evaluate survival rates. The cages were immersed for periods of 5–9 days. Several factors were considered to potentially affect survival, including duration of the tow and season, and biological characteristics of the individuals (e.g. size, sex and ovigerous condition). Survival was only affected by season (increased mortality inwarm months).Aglobal estimate of survival of released lobsters, taking into consideration survival and proportion of the catches for each season, was 35%. Simulation models were used to investigate the potential effect of a release measure on the population. Assuming the number of recaptured individuals is negligible, Z (instantaneous total mortality rate)would be considerably reduced if a release policy was implemented. This is particularly important if the levels of exploitation are high, which seems to be the case in this stock. It is our belief that the implementation of amanagement measure of returning all caught lobsters to the sea is worth considering as a means of aiding recovery of the resource. This measure would have the side effect of discouraging the choice of Nephrops as a target species, potentially reducing the fishing mortality even more significantly.