The Gulf of California or Sea of ​​Cortez is among the five marine ecosystems with the highest productivity and biodiversity in the world. It borders the states of Sonora, Sinaloa and Nayarit in continental northwestern Mexico, as well as the Baja California peninsula. The presence of over 800 islands and the geographical characteristics of the peninsula make the region “spotted” with isolated habitats ideal for a high incidence of endemic species. The Pacific coast, along the state of Baja California Sur (Mexico), is also highly productive and diverse due to the upwelling phenomena and other oceanographic processes taking place in the area.

In this natural inlet, in its waters and rocks, 23 sites are priorities for marine biodiversity, 42 sites are priorities for terrestrial biodiversity and 62 sites are priorities for the conservation of birds.

To understand the ecological value of this area, we can also say that five of the seven species of sea turtles existing in the world are present here. The Gulf of California is also known worldwide for cetacean diversity, as nearly 40% of all known cetaceans are found in the region. Vaquita (Phocoena sinus), the only endemic cetacean in Mexico, inhabits the northern end of the Gulf of California and has the most limited distribution range of any marine cetacean in the world. 40 sea lion colonies are spread across the region, with an estimated population of over 30,000 individuals.

Obviously not to forget is that in the area as well as having an extraordinary animal biodiversity, there are towns and local populations. The human footprint is certainly visible, and in some areas it is more, in others less. The impact given by man can be sought in fishing, in sewage and obviously in tourism (just to mention some of the impacts).

But in the last 20-30 years the landscape value of the area and the importance of biodiversity has been recognized, so much so, that various sectors of society have committed themselves to the conservation of these so-called amenities. Obviously the basis of conservation is scientific research which must also resort to raising awareness of those who live in the area and gulf all year round: the population.

An exemplary case is the population of a locality in the south of the Baja, which populates the waters of the Pacific. We are talking about Cabo Pulmo whose citizens have been integrated for a quarter of a century with the concept of the Marine Protected Area and have recognized both the ecological and economic benefits (favored tourism and local fishing).

Cabo Pulmo was declared a Natural Area Protected by the Mexican government in 1995. The bay is affected by the cold, low salinity water (34.6) of the Californian current in winter, and by the hot and high salinity water (> 34, 9) coming from the tropical Pacific, in summer. This coral reef is naturally divided into three sections:

• a northern section, characterized by sandy bottoms, low coral cover and strong seasonal influence of fresh water in summer and autumn;
• a central section, with large areas of a hard substrate (granite, sandstone and conglomerate) which support the main structure of the coral reef;
• a southern section, where hydrodynamism is not excessive.

When we talk about coral reefs we don’t just refer to the profound beauty of these places, but we do it mainly because of the fact that these ecosystems are highly productive and heterogeneous enough to host a vast biodiversity. The assemblies of associated fish are dozens of species, of different sizes, shapes and colors and therefore contribute to the complexity of these ecosystems.

Although the Pacific Ocean is the region where coral reefs are the most developed in the world, these structures are however rare in the eastern Pacific where they are found mainly from Ecuador and Perù, south of the Gulf of California, Mexico and are mostly distributed in “spots”.

The latitudinal section with the highest abundance and richness of corals in the Gulf of California is 23 ° N, where the Cabo Pulmo coral reef is located, considered the only coral reef in this region. Cabo Pulmo is the northernmost coral reef in the eastern Pacific and is located near the entrance to the Gulf of California in a transitional zone, between the tropical and temperate Pacific.

From the ichthyological point of view, there are 236 species of fish, belonging to 155 genera and 60 families. Most of these fish are typical of the Panama province, but some taxa are from the Indo Pacific and others are endemic of the Gulf of California.

Thalassoma lucasanum and Chromis atrilobata are the dominant species with an onset frequency of 97.2% and 72.2%, respectively, and together they accounted for 62.5% of the total overabundance. On the contrary, 50 species can be considered “rare” because their abundance is <1% of the total.

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The two species of dominant groups are a labride (Thalassoma lucasanum) and a pomacentride (Chromis atrilobata). They are small in size (about 15 cm) but differ in habitat and life histories. C. atrilobata lives in water column and is a planctivora species, while T. lucasanum is demersal and omnivorous, in particular it feeds on benthic organisms. Probably both species can coexist in great abundance as they do not compete for the same resources.

The Climate Diagnostic Center shows that the surface temperature range with varying seasonality is at least 3°C lower in Cabo Pulmo, probably due to the continuous influence of the warm currents of the South. So the most stable oceanographic conditions, see these 2 species of fish and more generally, the assemblies of fish, more stable in the composition during the year.

In contrast to this stability of the fish communities, there is the seasonality that characterizes the plant part of the ecosystem instead. In fact, there is an increase in chlorophyll a and plankton that characterizes the southern Gulf of California in the winter months. Furthermore, there are blooms of benthic macroalgae which show peaks in spring and winter.

The presence of seasonality for plant biomass, and the absence of seasonality for the ichthyofauna in Cabo Pulmo indicate that the latter is relatively independent from primary productivity and could be a sign of a Top Down type control of the trophic cascade in the reef.

It has also been observed that deeper waters are those with a greater number of “typical species”. It is evident that depth has a decisive effect on the composition and structure of sets of fish, but this factor is unlikely to affect individuals and species directly. The depth could act as a proximal indicator of another variable that changes in parallel. Among the many potential abiotic factors (temperature, pressure, light intensity, available space) that are related to the depth and can influence the biology of the fish, it can be thought that turbulence is the one that contributes most to the differences in the structure of the fish community between the depth layers of Cabo Pulmo. The movement of the water is in fact very important for the fauna associated with the coral reef so much that it has been highlighted in the classic vision for the zoning of this ecosystem.

Fish tend to avoid the highest points of the coral reef due to the fact that the waves, here, are more energetic. Furthermore, local upwelling phenomena are observed along these reefs in winter; during this season individuals of many species tend to avoid shallow waters.

Another important point to consider is the exposure of coral reefs and therefore their intrinsic morphology. In fact, it has been observed that if these are not exposed there are fewer species than those exposed. But how can such a thing be explained ?? In this case, “opening” is the one that allows the refueling of eggs and larvae transported more easily by the currents.

Today I don’t want to add much, I leave you to reflect on how everything can be explained from a biological point of view. And in fact all the parameters taken into consideration, observed from the DNA point of view, are easily explained. The key of everything, that resists to everything, is precisely this nucleic acid that has been successful in the various geological eras, always.

P.S. Cabo Pulmo seems to be really cool. I love when nature is protected and conserved. And this is one of those places where preservation is the basis of the life of these populations.

Maria Bruno