The peninsula of Baja California has a very interesting and unique marine environment. More than 600 miles long, the peninsula stretches from cool California waters all the way into the tropics, indeed, the Tropic of Cancer crosses the road from San Jose del Cabo to La Paz at the far southern point of land.

As a result, the marine life varies from the cold rocky kelp forests in the north on the Pacific coast near San Diego to a true coral reef at Cabo Pulmo on the Sea of Cortez side between La Paz and Cabo San Lucas.

The Sea of Cortez is millions of years old, formed by the slow north-west movement of the Pacific Plate carrying Baja California away from the Mexican mainland and splitting open the land to form the long finger-like Gulf of California. The Sea of Cortez used to have a great estuarine delta system at the northern end where the Colorado River used to empty into the sea, however this flood has dried to a trickle as the US Southwest has diverted the flow of water for agriculture and cities.

Most of Baja is in the temperate zone, and water temperatures can vary a lot seasonally. At the northern Gulf (Puerto Penasco, Puertocitos, and San Felipe), water temperatures can dip to 54 degrees F in the winter. This is just as cold as Southern California waters on a very cold day in January, so thick wetsuits are necessary.

In contrast, in the summer, the waters in the northern Gulf are stagnant, highly saline (very salty), and baked in the sun. In mid-summer, the same region can get water temperatures up to 90 degrees F, essentially a warm bathtub.

In the mid-Gulf, during the winter the water is cool (down to 60 degrees F) and a wetsuit is quite necessary.

In the far southern Gulf, the winter water is also cool (down to 68 degrees F) and a thin wetsuit is recommended.

In all regions in the summer the water is quite warm enough to swim without a wetsuit; although if one is diving below about 50 feet, the water can turn quite cold and a thin wetsuit is necessary.

The water starts warming substantially in April and stays warm through the fall. Visibility in the region is highly variable, but generally good, and there is excellent diving most of the year round in the lower Gulf.

The diversity of fishes and invertebrates increases substantially as you head down the Baja peninsula. The Pacific side is relatively uninteresting once you leave the kelp beds at the northern edge, because there are mostly sandy shores essentially all the way down to just above Cabo San Lucas. In addition, the shore is open to the oceanic swells and there is often large surf.

While the beaches are beautiful, the surfing is great, and there are whales to watch and fish to catch, the habitat variety is low and snorkeling or diving is limited. At almost all latitudes, the Gulf side is better.

In the far northern Gulf there are extensive mud and sand bottoms (left over from the days of the great Colorado River outflow). A few small areas have interesting rocky tidepools to explore, primarily around Puerto Penasco and just south of San Felipe. There are huge tidal changes in the upper Gulf - as much as 30 feet at times, and this too limits the diversity of organisms.

In the central Gulf there are abundant rocky shores, craggy islands, and a more tropical appearing flora and fauna. Excellent snorkeling and diving is available from Isla de la Guardia down through Santa Rosalia, Mulege, and Loreto to La Paz.

The lower Gulf from La Paz to Cabo has the most spectacular underwater habitats, where the marine environment takes on a tropical feel, reminiscent of Hawaii and the Galapagos, although not nearly as lush or diverse as the Caribbean and the Indo-Pacific. Coral bottoms are common in this region, and there is a "true" coral reef at Cabo Pulmo, where the coral is more than just a layer on top of a rocky reef. Fish and invertebrate diversity increases substantially in this region and visibility is usually good to excellent.

It is important to note that there is also a prominent inshore-offshore gradient of habitat quality in this area. For example, within the Bay of La Paz there is very low diversity of habitats and organisms, and as you go out to the points (Tecolote and Coyote) the snorkeling improves, while at the offshore islands (Cerralvo, Espiritu Santo, and Los Islotes) the diving can be superb. This is true even for the rocky points along most of the Baja shoreline: the habitat near the parking spots is relatively unimpressive, but the snorkeling after a five minute walk to the tip of the point is excellent.

There are about 600 species of fishes along the coasts of Baja: about 10% of them are northern cool-water species that occur both on the Pacific coast up to Southern California and in the northern Sea of Cortez.

On the Gulf side, the cold-water fauna extends down about half-way to Golfo San Francisquito, while on the Pacific side it extends about two-thirds of the way down to the San Ignacio lagoon. The line moves up and down depending on how cold the winter is, and there can be vast fish-kills when water temperatures change abruptly.

The populations of fishes on the two sides of the northern peninsula are similar and there is some very interesting research going on at present looking at the genetic changes between the two sides. The results should elucidate how long it has been since the two populations diverged, i.e. since cold water extended to the tip of Baja allowing mixing of these populations, probably during the recent ice ages.

About 20% of the fishes in the Gulf are endemic, meaning found only there. This is a relatively high proportion, indicating that some fishes either do not disperse out of the Gulf or are adapted to living only in the temperature ranges and habitats of the Gulf. There are a number of research projects underway at present to understand why certain fishes are endemic to the Gulf, that is whether they have limited dispersal of larvae or restrictive temperature or habitat requirements. The remainder of the fish species extend south to Panama, and many extend even further, to Galapagos and Peru.

The eastern Pacific region is the epicenter of a periodic weather change that affects the whole world and particularly Baja. Every so often, and perhaps at increasing frequency because of global warming, a very warm dome of water accumulates in the central eastern Pacific, preventing cool water upwelling and changing currents, winds and weather world-wide.

Heavy rains flood the coast of the Americas and drought occurs in Africa and Australia. In Baja, there is lots of rain and flooding and the water stays warm throughout the winter. Unfortunately, it is the cool water that carries nutrients to the surface and the presence of warm clear water without plankton reduces the baitfish populations and ruins fishing in Baja.

The usual runs of dorado and tuna up the Gulf from May to July disappear or are delayed, and the large reef gamefish, such as pargo (snappers) and groupers (cabrilla and mero) spread out and become hard to catch. During the last El Niño, in 1997-1998, fishing was so bad on the East Cape that it was quite possible to spend all day fishing and catch nothing (trust me, I did it); quite astonishing for Baja where fishing is still excellent compared to the rest of the world.

The 1997-1998 El Niño was considered one of the most severe of the century, and perhaps the millenium, and coral populations throughout the eastern Pacific were wiped out by the excessively warm waters. It is unknown whether these corals will recover or if the reefs have been destroyed. Surveys of corals in Baja are being done at present to document the extent of damage and recovery.

Also during El Niño, some warm water coral reef fishes from further south show up in the southern Gulf, and those that tend to be found only around Cabo and Cabo Pulmo extend further north. In 1997-1998, several coral reef fish species never before recorded in Baja arrived in the east Cape, extending all the way up to La Paz and beyond. There is usually a particularly dry cold year right after the El Niño that has been named the La Niña. In Baja, La Niña is associated with marked cooling of the waters in the winter and drought.

Most books are written about fishes, since they are what most people are attracted to underwater. There are a few books that focus on invertebrates and plants. The books dealing with the fishes of Baja are:

• Fishes of the tropical eastern Pacific.
    G. R. Allen and D. R. Robertson
    University of Hawaii Press, Honolulu. 332 pages (1994)

  This is the best book on the fishes of the eastern Pacific including Baja. It is complete for the lower Gulf region with virtually all known fishes in the region illustrated in color photographs and with a scientific-oriented listing. It does not include the cool water species of the northern Gulf and Southern California region.

• Reef fishes of the Sea of Cortez.
    D. A. Thomson, L. T. Findley and A. N. Kerstich
    University of Texas Press, Austin. 353 pages (2000)

  A book on reef fishes, and by "reef" they mean excluding the cool water fishes of the upper Gulf and sand or mud bottom fishes. The listing is not comprehensive and there are only a few color photographs. It does include a good background section on the details of the marine environment of Baja and does come in a paperback, so a better handbook for fish identification than the more complete Allen and Robertson book.

• The Baja Catch
    N. Kelly and J. Kira
    Apples and Oranges, Inc. Valley Center CA. 246 pages (1993)

  A paperback "good-old boy" handbook guide to fishing in Baja with a surprising amount of detail, many maps, and remarkably useful for beach access, camping details, and more.

• Sea of Cortez Marine Animals
    D. W. Gotshall
    Sea Challengers, Monterey, CA. 110 pages (1998)

  A thin paperback guide to marine animals around Baja- not at all comprehensive but with excellent color photographs. A good starter fish guide.

Books on invertebrates include:

• Common Intertidal Invertebrates of the Gulf of California
    R. Brusca
    University of Arizona Press, Tucson. 513 pages (1980)

  Excellent invertebrate guide, but probably hard to find.

• Guide to Marine Invertebrates: Alaska to Baja California
    D. W. Gotshall
    Sea Challengers, Monterey, CA. 105 pages (1994)

  A thin paperback guide to invertebrates including many around Baja California- also not comprehensive, but with good photographs.

• Sea of Cortez Marine Invertebrates- A Guide for the Pacific Coast, Mexico to Ecuador
    A. N. Kerstich
    Sea Challengers, Monterey, CA. 114 pages (1989)

  Another thin guide with good pictures, more appropriate to the lower Gulf.

I am a coral reef fish biologist as well as a medical doctor, specializing in pathology. I received my PhD from UC Santa Barbara in 1986 where I studied the population biology and larval ecology of the bluehead wrasse from the Caribbean. Subsequently I went to UC Irvine Medical School and trained at Stanford and UCI Hospitals. In addition I recently finished the MBA program at UC Irvine. I am a consultant and a medical director and continue a variety of research projects on reef fish biology. My research sites have included Baja, Panama, Galapagos, Hawaii, Palau, Bali, and Brazil. More information on my research can be found at coralreeffish.com.

In Baja, I have worked for about fifteen years on recruitment (settlement of larval fishes from the plankton to the bottom) of reef fishes, primarily the wrasses. In addition, I do research on the razorfishes of the eastern Pacific Ocean, and I have discovered a new species of razorfish in the Galapagos (Xyrichtys victori) and photographed the Baja Cape razorfish (Xyrichtys mundiceps) for the first time. More recently, I have studied the effect of El Niño on fish distributions and recorded two species of tropical wrasses new to Baja California. I am also collaborating with other scientists on the genetic heterogeneity of a set of Baja blenny species with very different dispersal abilities with a view to relating dispersal and genetic variability. Another collaborative project I am working on with a group from Scripps Institute of Oceanography is a study on the spawning and larval settlement of the Gulf cabrilla, or leopard grouper.