The spectrum decision concept in Cognitive Radio Ad-Hoc Networks (CRAHNs) introduces important challenges. These include the time-dependent SNR observations of individual CRAHN users due to the fading and shadowing effects in the licensed channels, the necessity of fusion mechanisms for accurate decisions, and the difficulties depending on multi-hop deployment. Considering these challenges, in this paper, we propose a dynamic, cooperative and distributed spectrum decision mechanism in order to decide the channel usage in CRAHNs accurately. The proposed mechanism introduces the SNR Tracking System which considers the time-varying local SNR observations and decisions of the CRAHN users. The proposed mechanism employs a distributed Weighted Fusion Scheme (WFS), to combine the individual decisions and hence, to obtain the cooperative decision. The proposed spectrum decision mechanism adapts itself dynamically to the multi-hop architecture of the network. The performance of the proposed mechanism is compared to some conventional fusion mechanisms based on the AND, OR and MAJORITY rules, and it is shown that the proposed weighted mechanism gives lower false alarm and higher detection probabilities compared to the conventional fusion mechanisms. (C) 2011 Elsevier B.V. All rights reserved.