In a recent paper [1], a new type of watershed (WS) transform was introduced: the tie-zone watershed (TZWS). This region-based watershed transform does not depend on arbitrary implementation and provides a unique (and thereby unbiased) optimal solution. Indeed, many optimal solutions are sometimes possible when segmenting an image by WS. The TZWS assigns each pixel to a catchment basin (CB) if in all solutions it belongs to this CB. Otherwise, the pixel is said to belong to a tie-zone (TZ). An efficient algorithm computing the TZWS and based on the Image Foresting Transform (IFT) was also proposed. In this article, we define the new concept of "bottlenecks" in the watermerging paradigm. Intuitively, the bottlenecks are the first contact points between at least two different wave fronts. They are pixels in the image where different colored waters meet and tie and from which may begin, therefore, the tie-zones. They represent the origin points or the access of the tie-zones (regions that cannot be labeled without making arbitrary choices). If they are preferentially assigned to one or another colored water according to an arbitrary processing order, as occurs in most of watershed algorithm, an entire region (its influence zone -the "bottle"!) is conquered together. The bottlenecks play therefore an important role in the bias that could be introduced by a WS implementation. It is why we show in this paper that both tie-zones and bottlenecks analysis can be associated with the robustness of a segmentation. © 2005 IEEE.2005 5562Audigier, R., Lotufo, R., Couprie, M., The tie-zone watershed: Definition, algorithm and applications (2005) IEEE Proceedings of ICIP'05, , Genova, Italy, Sept, In pressBeucher, S., Lantuéjoul, C., Use of watersheds in contour detection (1979) International Workshop on Image Processing, Real-Time Edge and Motion Detection/Estimation, , Rennes, FranceCouprie, M., Bertrand, G., Topological grayscale watershed transformation (1997) SPIE Vision Geometry VI Proceedings, 3168, pp. 136-146Dijkstra, E., A note on two problems in connexion with graphs (1959) Numerische MathematikFalcão, A., da Cunha, B., Lotufo, R., Design of connected operators using the image foresting transform (2001) SPIE on Medical Imaging, 4322, pp. 468-479. , Feb. 17-23Falcão, A., Stolfi, J., Lotufo, R., The image foresting transform: Theory, algorithms, and applications (2004) IEEE Trans. on Pattern Analysis and Machine Intelligence, 26 (1), pp. 19-29. , JanLotufo, R., Falcão, A., The ordered queue and the optimality of the watershed approaches (2000) 5th International Symposium on Mathematical Morphology, pp. 341-350. , Palo Alto CA, USA, June, Kluwer AcademicLotufo, R., Falcão, A., Zampirolli, F., IFT-watershed from gray-scale marker (2002) Proceedings of the 15th Brazilian Symposium on Computer Graphics and Image Processing, pp. 146-152. , Fortaleza CE, Brazil, October, IEEE Computer SocietyMeyer, F., Topographic distance and watershed lines (1994) Signal Processing, 38 (1), pp. 113-125Meyer, F., Beucher, S., Morphological segmentation (1990) Journal of Visual Communication and Image Processing, 1 (1), pp. 21-46Najman, L., Couprie, M., Watershed algorithms and contrast preservation (2003) Lecture Notes in Computer Science, 2886, pp. 62-71. , Discrete geometry for computer imagery, of, SpringerRoerdink, J., Meijster, A., The watershed transform: Definitions, algorithms and parallelization strategies (2000) Fundamenta Informaticae, 41 (1-2), pp. 187-228. , JanuaryVincent, L., Soille, P., Watersheds in digital spaces: An efficient algorithm based on immersion simulations (1991) IEEE Trans, on Pattern Analysis and Machine Intelligence, 13 (6), pp. 583-598