Study of the physical, chemical and biological processes in semi-intensive fishponds: development of a mathematical model as a tool for managing white seabream (Diplodus sargus) production

摘要

Semi-intensive aquaculture has been recognised as an “environmentally friendly” option.However, the low profitability and competitiveness of these systems compromise their economicviability. The optimization of production is thereby crucial for the sustainability of semiintensivepond aquaculture, and implies that fish yields are maximized with minimum impactson the environment. Understanding the physical, chemical and biological processes occurring infishponds is of outmost importance for defining farming strategies that optimize fish production.This knowledge is even more relevant when dealing with newly cultivated species, as the whiteseabream (Diplodus sargus). Due to the lack of information on the performance of this species inearth ponds, one of the main objectives of the present work was to study the physical, chemicaland biological processes in white seabream ponds, over a production cycle. The most relevantresults of this experimental work were that: i) the impacts of fish activity on bottom sedimentsare only noticeable above a fish biomass of 0.5 kg m-3 and a feeding rate of 5 kg d-1; ii) pondsediment and water quality was comparable to that of natural systems, suggesting that theassayed farming conditions ensure a good pond environment; and iii) pond water quality wasstrongly dependent on inflowing water and on benthic nutrient fluxes, emphasizing the relevanceof optimum water exchange rates and sediment treatment to an efficient pond management. Theother main objective of this work was to develop an ecological model to be used as a tool formanaging semi-intensive systems, to improve their economic and environmental performance.The added value of a modeling approach is that, due to their ability to integrate the complexity offishpond processes, models can be used to simulate the effect of different management scenarioson the pond environment and on the adjacent coastal systems. The model was implemented andtested with the white seabream as a case study, using data collected over the experimental work,together with literature data. Model construction was done in 3 steps: i) implementation of abiogeochemical model; ii) implementation of a fish Dynamic Energy Budget (DEB) model andiii) coupling of the two models. The biogeochemical model developed in this study is a mechanistic model that reproduces the dynamics of organic and inorganic nutrient (nitrogen andphosphorus) forms as well as of oxygen, in the pelagic and benthic compartments of an earthpond. This model not only helped understanding the interactions between pond variables andprocesses but also how pond structural features and operational parameters affect the water andsediment quality of pond systems. The fish DEB model was able to reproduce the growth ofwhite seabream as well as of gilthead seabream (Sparus aurata), a traditionally cultivatedspecies in semi-intensive ponds. This model was used to investigate which biological processesare more likely to influence fish performance and to explain inter-species growth variability. Acomparison between the DEB model parameters of the two Sparidae revealed that whiteseabream lower growth rates are presumably linked to a higher energy demand for bodymaintenance and a lower feed absorption efficiency. The coupled model was able to reproducefish pond dynamics, and was further used to simulate different management scenarios, related tostocking densities, water exchange rates and feeding strategies. Scenarios and standard farmingconditions were compared in terms of their effects on pond water and sediment quality, as wellas on final fish yields and nutrient discharges into the environment. Using the AnalyticalHierarchical Process (AHP) methodology, scenarios were ranked in order to evaluate the bestmanagement options for optimizing white seabream production. Results revealed that doublingthe standard stocking density and improving feed absorption efficiency, may enhance theperformance of semi-intensive white seabream production systems. Aside from providing a toolfor managing aquaculture systems, this work contains valuable information for definingguidelines on environmental standards (e.g. Maximum Recommended Values) for marine fishfarming.