The interpretation process located 7,214 hectares of land surface classified as mangrove in the study area. The distribution of the mangroves is shown in the map below.
Despite the difficulty and expense involved in arriving at a figure of 7,214 hectares, the mere measurement of the area of mangroves is of limited interest. The true value of this work is found in comparing this result with other manifestations of the landscape. That is the purpose of the remainder of this section.
The first of these that comes to mind is a comparison of present distributions to past distributions. Such a comparison would provide valuable insight into rates and causes of change. Analysis of the 1970 aerial photographs would provide the requisite data. It is our intention to pursue this investigation, but, unfortunately, we have not yet been able to obtain the necessary resources.
A second data set that provides an interesting contrast is found in the Ordenamiento Ecológico de la Costa Norte de Nayarit [Bojórquez Tapia, L.A. et al, 1997], which is hereafter referred to as the OECNN. This is a planning document produced for the Secretaría del Medio Ambiente y Recursos Naturales (SEMARNAT). It contains the most current information on the state of the environment in the northern half of Nayarit State and is considered by planners as the definitive source for current environmental data.
One of the data layers in the GIS that underlies the OECNN is a land use/land cover classification derived from a Landsat Thematic Mapper image, without field verification. The areas identified as mangrove in the OECNN are shown in grey on the map below. Roads are included as red lines to provide spacial reference.
The OECNN interpretation in the map above identifies 9,160 hectares of mangrove in the study area, or 27 percent more than this study was able to distinguish. The location of these areas is shown in the map below. Furthermore, only 5,437 hectares (75% of the current study's results) were identified as mangrove in both studies. Forty one percent of the area identified as mangrove by the OECNN study (3,723 hectares) is not so classified in this study.
A large proportion of the disagreement between the OECNN interpretation and the current study seems to be a failure in the former to discriminate dry brushland (monte seco) from mangrove. The difficulty in discriminating the two on the orthophotos was discussed above. This problem can only be more pronounced in the lower resolution satellite imagery. The arrows labeled "1" in the map above point to areas where this is a pronounced misclassification. Field verification reveals that the entire coastline has a strip of dry brushland (monte seco) immediately behind the beach. The OECNN interpretation identifies all of this as mangrove. Note that the mangrove classification extends beyond the coastline near the southern end of the study area on the map. This is because the study area limits were defined from the 1970 topographic map data. Since that time there has been considerable accretion of the coast behind a jetty constructed in 1976 to protect the San Blas port from siltation. The OECNN imagery reflects this change in the coastline.
A second area of disagreement between the two interpretations is labeled "2" in the map above. This is an area where the AquaNova shrimp production ponds have recently been constructed. The imagery used by OECNN predates this construction.
A third area of interpretation disagreement, labeled "3" in the map above, undoubtedly comes from a difference in registration between the images. These are water courses (estuaries, lagoons, etc.) Which are narrow features, small enough that a relatively minor change in position is significant. Confirmation of this fact is that these inconsistencies also appear labeled with the number "1" in the map below, which shows areas classified as mangrove in the current study, but not in the OECNN. Such areas amount to 1,777 hectares, or approximately 25% of the area identified as mangrove in the current study, and are indicated as the red areas in the map below. Areas identified as mangrove in both studies are marked as yellow, and the grey dot pattern signifies areas classified by the OECNN as mangrove but not so classified by this study.
In addition to the misregistration discussed in the previous paragraph, there are possible explanation for the discrepancies in two other areas, labeled "2" and "3" in the map above. Area "2" was found to consist of mostly white mangrove (mangle blanco) that has been heavily modified by cattle grazing. It is quite possible that the impact of the cattle on the vegetation was sufficient to alter the spectral signature enough to fall outside the bounds of the image classifier. Area "3" consists of relatively narrow bands of mangrove, separated by bare ground or other vegetation. It is probable that the resolution of the satellite imagery was insufficient to pick these long, narrow features out of the background.
No immediate explanation springs to mind for the other discrepancies between the two studies. The only solution is to return to the field and verify the facts on the ground. That will be one of the highest priorities in the coming field season.
In summary, comparing the results of the two studies leads to the conclusion that there is a very high degree of confidence that there are at least 5,437 hectares of mangrove in the study area. This is the area identified as mangrove in both studies. Furthermore, there is a fairly high degree of confidencethat there is as much as 7,214 hectares, the number arrived at in this study. The possibility exists that there is more. Investigating this possibility will require further field work. It is, however, very unlikely that the ultimate figure will approach the 9,160 hectares identified in the OECNN.
Another interesting data set, also obtained from the OECNN study, is land tenure. The mangroves are under the control of three different kinds of entities: 1) private proprietors, 2) The Federal Government (Zona Federal), and 3) communal owners known as ejidos. Private property is subject to the open market and is held by a variety of owners, from large commercial conglomerates to small holders. The cadastre in Mexico is very obscure, making it difficult to locate and contact private property owners to either sample their attitudes or influence their activities. Unfortunately, nearly seventy percent (4,947 hectares) of the area identified as mangrove in this study is under private ownership and thus the most difficult to influence.
The Federal Government, like all governments everywhere, is bureaucratic and subject to political influences. This fact opens up the possibility of influencing the management of mangroves under Federal control through the political process. In this light one is especially encouraged to observe the current opening of the political process to democratic influence that is now underway in Mexico. The down side is that only about four percent (271 hectares) of the mangrove is in the Zona Federal.
The last type of land tenure, ejidos, control approximately twenty seven percent (1,911 hectares) of the study area's mangroves. Ejidos are communally owned properties in which ejido members are typically assigned inheritable rights to individual parcels. Since ejido lands cannot be alienated, they are not available as collateral to secure capital investment. This fact is the source of much current debate in Mexican political circles over the future of ejidos. For the moment, however, ejidos are by far the most accessible of the three tenure types by virtue of their centralized administrative structures and communitarian cultures. Undoubtedly, this is a major reason that the aforementioned Diagnůstico Socioambiental de la Zona Estuarina y de Manglar del Municipio de San Blas, Nayarit (Socio-environmenta