Project Costa Rica Osa Peninsula Crocodylus acutus Survey

Preliminary Survey-September 2004


Preliminary Assessment of the Status of the American Crocodile in Southwestern Costa Rica: A basis for Further Study.


The American crocodile’s range extends from the Southern tip of Florida through Northwestern South America. As such, it is the second most widely spread crocodilian in the new world (second only to the spectacled caiman). Over the course of the past 8 years guiding eco-tourists throughout the Osa Peninsula, wildlife biologist and guide Mike Boston has observed substantial numbers of American crocodiles (Crocodylus acutus) throughout the rivers and tributaries of the Area Conservation de Osa (ACOSA).

Due to a lack of available information on these large archosaurs, it became apparent that little study has been made on the populations of this remote and unique area of Costa Rica. The majority of studies have focused on the much drier, easier-to-access northern regions of Costa Rica. After discovering recurring nests of Crocodylus acutus during two previous trips to Corcovado National Park, as well as numerous other individuals, it became apparent that a thorough study should be undertaken. Private funding has allowed us to complete a preliminary assessment of 3 rivers in the ACOSA. In addition, we reconnoitered a fresh-water lagoon along the Pacific Coast of the Osa Peninsula, home to a large population of crocodiles. It is estimated that only 10,000 to 20,000 American crocodiles exist throughout their range. As their skins are covered with very small osteoderms (relative to other species) they are often hunted for the skin trade.

This article summarizes the preliminary survey, and the results to support initiating a long-term census in 2005. Also, accompanying us on this survey was Zack Kerstetter, a naturalist from Philadelphia, whom we met on our April 2004 Corcovado trip. His father is a landowner of some property at the tip of the Peninsula near Matapalo. Several conservation-oriented landowners are working to restore the tropical forests near Matapalo, and they happened to be down there for 10 days as well. In all, over 80 crocodiles ranging in size from 35cm to over 3-meters were observed. Two rivers were surveyed along 4 to 6 kilometers of their length, from a landmark up-river, down to their mangrove deltas where they empty into the Golfo Dulce. One survey yielded 29 crocodiles, while another yielded 19 crocodiles each on two successive nights. A third river, smaller was only surveyed near its mouth, yet held an astonishing 13 crocodiles in just under a half-of-a-kilometer! One crocodile was observed killing and eating a greater fishing bat!


Preparation. The weeks leading up to the trip since early June when we received word that we would get funding for the trip had been chaotic. Trying to plan this project was hectic, especially when you're trying to rely on local services in Costa Rica that tend to be unreliable. In one sense, ticos (native Costa Ricans) can be the most reliable people there in terms of navigation, work, etc....but they just don’t go about it in a timely fashion. On the other hand, most U.S. transplants are a different story...unreliable, lazy, just there because of the laid back lifestyle. Planning the trip around what rivers we wanted to survey was easy enough, but so many factors came into play that one little snafu could mean disaster for the entire project…the weather. This is the beginning of the main rainy season. If it rained cats and dogs, we very well may never get out at night at all to do any surveys. The main protocol for surveying rivers for crocodiles entails going up river a known distance and locating crocodiles by eye-shine as you navigate back down river. The crocodile possesses a highly reflective retina, which glows red when a light is shined on it, making them much easier to spot than during the daytime. In a complete downpour, the spotlight beam (usually a 1-million candlepower) will penetrate only a few feet reflecting off the raindrops.

As the departure day approached, I still had no idea as to the itinerary for the project. Oh well, more often than not, plans on the Osa usually work themselves out, and as is customary with travel in less developed countries, even the best plan gets messed up at some point. Because of this, you need to be flexible and be able to deal with adversity-that’s the number one rule in traveling to foreign countries, not to mention chasing large crocodiles in remote rainforests!

I booked a ticket in advance, but for the purposes of this trip, had hoped to arrive in San Jose and just catch a charter flight directly to Puerto Jimenez. Unfortunately, most of the cheaper flights all arrived at noon, and the only charter flight going to PJ that day, was at 1PM. Whew! That would be cutting it close to get through customs, get my luggage and get to the charter terminal in less than an hour. Well, as usually happens, 2 weeks before the trip, airfares dropped and I was able to switch my ticket for an American Airlines flight that arrived at 10:30AM! Much better, now I'd have plenty of time. No problem…or at least I thought. As the day approached, I kept a close watch on Hurricane Frances, which was churning over the Bahamas. I was watching it closely, and fortunately the original ticket I had bought was going through Houston, so I actually didn't cancel it until the day before I would leave, just in case the hurricane hit early.

Day 1: Thursday, September 2nd. Today was a travel day. With Hurricane Frances well off the coast, my flight thru Miami should have been uneventful...and it was. I got into San Jose right on time. My only concern was that the luggage arrived safely as well. In my duffle bag, I had most of the supplies we'd need for data recording, but more importantly a water test kit. My biggest fear was that it would be flagged by the airlines as it contained some chemicals such as sulfuric acid. Also, there were some delicate glass test tubes, beakers and a refractometer in there for measuring the salinity of seawater. My luggage arrived and I passed through customs. I was so early, that I sat for over an hour and a half at the SANSA terminal before we boarded for the 1PM flight to Jimenez. The flight itself is only 45 minutes, but this time of year it can be bumpy, and because there are fewer travelers, they piggy-back it with a stopover in Golfito as well. The flight was uneventful, other than a few downdrafts to shake the plane.

Day 2: Friday, September 3rd. Today constituted an organization day. While we were more than ready to get going, there were several items to attend to. Initially, Mike had arranged for a local gringo to provide boat services on the rivers, and we'd work directly out of Puerto Jimenez. This plan, as usual, did not work out. Most gringos function on their own schedule, so we couldn't rely on anyone to be there each night when we were ready to go survey. After breakfast, Zack and I walked down to a lagoon behind Parrot Bay Village to collect some water samples. We wanted to do a few trial runs with all of the various water sample tests so we’d be ready to go once we started taking actual samples. We would be testing for nitrates, phosphates, salinity and conductivity to determine if any fertilizers or pesticides are affecting the crocodile populations, as well as the effects of salinity on the preferred habitats of hatchlings, juveniles and adults. Behind the Parrot Bay lodge is a large brackish mangrove lagoon. We walked down the road that leads past the docks and the beach in Puerto Jimenez, before winding around towards the airstrip. Right before the airstrip, you turn into the Parrot Bay driveway.

The lagoon behind the lodge is actually situated between the lodge and the airstrip, and is home to several VERY large spectacled caiman—some of which are over 6 feet long! Occasionally, a 2-meter-plus crocodile lives here as well. We took the path to the lagoon, and as soon as we reached the bank, we both noticed a 4-to-5 foot caiman lying along a log. Immediately, the caiman noticed us, as they are use to people feeding them here. The caiman was about 10 feet away from where I had to jump down to the waters edge to fill the test tubes. I asked Zack to just hand me the test tubes and I’d fill them and hand them back, while keeping my eye on the caiman. It had already started coming closer. Sure enough, as I stooped down to fill a test tube, the caiman swam closer but stopped about 6 feet away. I never took my eye off of it. I filled a couple more smaller test tubes and the caiman came a little closer, now about 3-4 feet away. At least I could see it…if there were any others, or worse yet, a hidden crocodile in the murky water, I couldn’t tell! I filled the last test tube and hopped up on the bank.

We got back to Mike’s apartment and ran several of the samples through each of the tests. It took us a good 2 hours to go through the entire suite of tests and by then it was around noon, and we were all pretty hungry. We walked into the main street of town to Carolina’s for lunch. I had my usual arroz con pollo (stir fried rice with chicken). We decided the night before that we’d initiate the study on the Rio Coto, which empties into the northeastern corner of the Golfo Dulce. We wouldn’t leave for Zancudo, the village at the mouth of the Rio Coto, until the following morning, when one of the lodge owners would come pick us up by boat for the 45-minute boat ride across the gulf. We had some time to kill, so we decided to hike up the beach to the mouth of the Rio Platanares. This river is a smaller river that empties into the Golfo Dulce, right in Puerto Jimenez. Mike had thought about surveying it as he has seen crocodiles in it before, and it would lend an idea as to how viable and critical the smaller rivers are in supporting substantial crocodile populations. We thought, if we could get an idea of how far up the river we could go, and/or if we could access it by canoe, then maybe we could survey this river some night in between rivers when we were in Puerto Jimenez?

We hiked up the beach a ways, but could only go as far as the docks of Crocodile Bay, as the tide was in and we couldn’t get past a narrow stretch of beach that was now inundated with water. It was now around 4:30PM and Mike remembered he had to meet two guides that were leading one of his tours through Corcovado. They were due back in town around 5PM and he was anxious to hear how it went. We started to hike back into town but stopped at Parrot Bay for a coke. We downed the drinks pretty fast because it was hot that day, and Mike and Zack headed back into town. I told them I would catch up because I wanted to go check the lagoon again, in case any crocodiles might be there. By then I was really getting to know my way around Puerto Jimenez, so I wasn’t concerned about getting lost. It’s not that big of a town anyway!

I walked down the path to the lagoon and saw nothing in terms of crocodilians. The water was perfectly calm. I scanned the mangroves and found a small 3-foot long caiman in the same position it was in 8 hours earlier. I happened to scan the far bank and noticed a big hulk underneath some overhanging branches, laying about three-quarters of the way out of the water. I could tell right away by the light tan color of its tail that it was a crocodile and a very fat one at that! This animal was around 2 to 2.25 meters in length, and judging by its bulk, was very well fed. I tried to figure out how to get over to the opposite side of the lagoon to get a closer look but since I wasn’t sure if it was private property, I didn’t risk trespassing in a foreign country. I stared at it for a few minutes to get a good mental image, and then turned and headed back to town.

That evening we met up with Jorgé and Rebecca, (the two guides who led Mike’s tour) and since their trip went so well, we went out to a Mexican cantina and bar in PJ. It was quite the happening place, and boasted being the only place in Jimenez that carried the 2004 summer Olympics in their entirety via satellite. We turned in early, as we were going to be heading to Zancudo in the morning to get this project underway!!

Day 3: Saturday, September 4th. Mike and I awoke early and started planning for the trip to Zancudo. The first stage would take us to the Rio Coto for 3 days. If the weather cooperated, we’d get two surveys done… but I was realistic, and figured if we got just one done, that we’d be lucky. This river marked an area where about a year ago, Mike and a lodge owner from Zancudo had counted 27 crocodiles during an hour-long boat ride up the river. The mouth of the Rio Coto contains an extensive mangrove system at its delta, where a tributary, the Rio Saballos, cuts through it, to create a system of natural canals, which a small boat can navigate. In recent years, silt from agricultural activities up-river (grazing and banana plantations) has washed down stream and created massive sand bars at the mouth of the river extending out into the Golfo Dulce. These sand bars eventually allow mangroves to take root and form small islands, providing a prime basking site for large crocodiles at low tide. We tried to contact our original boat operator, but after several futile attempts, we contacted Susan and Andrew, owners of Los Cocos Cabinas in Zancudo, and made arrangements for Andrew to come pick us up at 10AM. They are wonderful hosts, who regularly run boat and kayak trips up the Rio Coto, so they know the river very well.

We had time to pack and go get breakfast. Andrew arrived at the dock right on time, and we headed across the gulf. The gulf was pretty calm, and was an easy ride in their 21-foot boat. As we neared the mouth of the Rio Coto, Mike spotted a spinner dolphin surface for air and go under. Andrew stopped the boat and turned around heading back out toward the dolphins. Suddenly another appeared, and yet another. It didn’t seem to be a large pod, but may have had a dozen individuals. Mike said some commercial billfish captains report seeing pods of dolphins with as many as a thousand animals in the rich waters further out in the colder Peruvian current, which runs along the Pacific coast. We saw several more fins break the surface, but the school (or pod) didn’t seem to be in a hurry. Andrew maneuvered the boat around a massive sandbar, which extends far out into the gulf. The sandbar is not visible from out in the gulf. However, some clue as to its existence can be guessed from the waves that appear to break in the middle of the open water, when seen farther out in the gulf. As we rounded the tip and entered the river channel, Andrew mentioned that this sandbar sometimes has basking crocodiles on it at low tide. We confirmed this as we entered the river, spotting two large crocodiles approximately 2-meters in length lying on the bank. One immediately got up and did the typical “high walk” down to the water's edge and slid into the river. The other was already in the river and was swimming up-stream as we tried to get near them for a closer look. We got no more than 20 yards from them when they submerged. We proceeded upstream, passing several small mangrove islets, and turned into the Rio Saballos where the Los Cocos dock is located. As we passed an islet that had actually grown into a large sandy mound, Zack spotted a HUGE crocodile lying on top of it. This animal was easily 3-meters long, probably a little bigger. As we got closer, it slid right into the Rio Saballos. The problem with the American crocodile is that the adults are so shy, that getting close enough for an accurate size estimate can be difficult. Seeing the 3 large adults at the river mouth now had us all excited about the project, and what lay ahead. It was good to finally get started and be exploring new territory!

The rest of the day was spent settling into our cabinas. These were nice large spacious cabins right on the beach in Zancudo. Each had a nice large kitchenette, which Mike and I found to be perfect for setting up a little laboratory to test the water samples. I took a walk down the beach, and then we went and ate lunch at a little cantina up the beach called Café Sol y Mar. It’s a nice little open-air bar with a great menu. We ate lunch and then took a siesta. We decided that Andrew would take us up the river around 8PM for tonight’s survey. It didn’t rain that afternoon so we were optimistic. After dinner Andrew arrived and we headed to the dock. We started down the Rio Seballos and immediately spotted 2 sets of eyes on the far bank of the Rio Saballos. We hadn’t even gotten into the Rio Coto and we found crocodiles. As this was our maiden voyage, we knew it would take at least one night to get organized and get the survey protocol down pat. We tried to get close to these crocodiles to estimate their size, but they went under rather quickly. This, and the fact that their eyes were quite separated from one another, told us that they were definitely adults over 2-meters in length. Andrew slowly steered the boat along the mangroves along the right bank (as you look up-river). Mike and Zack were in the front as spotters and I was in the middle of the boat recording data. Mike spotted another adult, and then found another a little ways up-river. This animal didn’t seem to move and we approached it very closely. We got right up to it to reveal a 2-meter crocodile that had managed to wedge itself fairly well into the mangrove roots, and was stuck there. We got so close that Andrew nearly hit it with the boat! When we were nearly on top of it and suddenly it made a splash and disappeared into the muddy depths, but not before we saw it from only 3-4 feet away. There’s something magical about staring directly into the bright glowing eye of a crocodile; especially when it has its mouth partially open, giving you that toothy smile! This was such a neat animal, but I didn’t have time to get the camcorder to capture some video.

Suddenly, no sooner did we get out into the river, than the skies opened up and it came down in buckets! Andrew threw the top up, and we headed back to the dock. Luckily we weren’t far and in 5 minutes were back at the dock and heading back to the cabinas. So much for that night, but at least we saw crocodiles!

Day 4: Sunday, September 5th. Today we had planned to have Susan take us for a boat ride upstream to get familiar with the layout of the river during daylight. This would help us identify landmarks and features so we could estimate our distance covered at night, as well as estimate how fast we were surveying the river. Also, any croc sightings would tell us where to concentrate our efforts and whether certain habitats were preferable over others. The sun was shining, and we were up at 6AM. I made coffee (Nothing beats Costa Rican coffee, and their pure unrefined cane sugar…I prefer Café Rey, but Café Britte is not bad either!). I made a pot of “Tar” and drank most of it before Mike woke, so I made another pot. We walked down for breakfast at 7AM, but the Café Sol y Mar wasn’t open yet. Since Susan was meeting us at 8AM, we didn’t have time for breakfast. We got a pack of cookies at the pulperria, which would hold us until we got back for lunch. We met Susan and headed to the docks. We started out by going up the Rio Saballos, and Susan took us through a natural canal that led back to the Rio Coto about a kilometer up river. We motored upstream, and then turned into a little tributary that Susan wanted to show us. Much of the land along the Coto has been cleared for grazing or teak plantations, but along these little tributaries, patches of second growth forest with large oil palms and giant hardwoods still stand. These forest patches attract a variety of bird life. This area had a tract of pasture with trees, which die at the tops, but grow at the bottom. The dead snags provide important nesting sites for Red-lored Amazon parrots. It contained several large sweeta palms, from which leaves for thatch roofs are grown. We went up the tributary to the point where a boat could go no further, and turned around. As we entered the Rio Coto, we spotted a couple of small crocodiles, maybe a meter or more in length.

We headed upriver and passed a long stretch of river where Susan said she used to regularly see a large crocodile, but she hadn’t seen it in so long that she no longer bothered to look for it. No sooner did she say this when Mike yelled “THERE’S ONE THERE!” Hiding among the logs along the bank was a fairly large croc giving us a toothy grin. It was probably 2.5m long, and may have feasted on small cattle that grazed along this stretch of river. Susan pointed out a teak plantation growing on the slopes of the hills beyond the pastureland. We continued upriver nearly 12 km where the drainage canals dug in by the United Fruit Company in the middle of the century, drain the Rio Coto valley. These canals provided drainage and access to large banana plantations. Today, only remnants of these plantations still stand, but many smaller private plantations line the banks of the upper Rio Coto where it splits into the Rio Coto Colorado and the Rio Colorado. When navigating upriver, one must keep a lookout for the ferry crossing. This is an auto ferry that transports cars across the Rio Coto that are traveling from Golfito to Zancudo, Pavones and Panama. The ferry cables that run across the river are only 4-5 feet above the river at high tide. An unknowing boater could get “close-lined” if they were traveling up there at high speed, and weren’t aware of the cables. At low speed we were able to easily duck underneath them.

We saw a few small crocs along the river up to the crossing, but nothing above it save for a few medium sized caiman. We stopped at the ferry crossing on the way down, as there is a restaurant in a large red-domed circular building. We ate breakfast there consisting of beans, rice, and eggs. We headed back down river and now the sun was burning very intensely. We neared the mangroves about 4km up river and started to notice more crocodiles basking along the banks since the tide was on its way out. We counted nearly a dozen large adults on the banks, and watched several slide down their 10-foot high mud tracks into the river. As we entered the Rio Saballos, the entire mouth of the river was nearly above water. We had to navigate way out around the shallow areas, where the night before we were able to go over…though we did bottom out in one spot.

As we entered the Rio Saballos, Zack mentioned that the large adult we saw yesterday was off to our right, on its island again. At the same moment, I had been tapping Mike on the shoulder to point out the three 2 meter adults basking on the exposed mud flats 100 yards to the left of us. We docked the boat, and went to our cabins. After lunch, I ran the water samples I had collected up river through the various tests, and then fell asleep in the hammock for an afternoon siesta. I woke up a little later and Susan had invited us to their house for dinner that evening. We would eat around 6:30 and then Andrew would take us out after to do a survey, weather permitting.

After dinner, we headed out on the Rio Coto, but this time we went upriver all the way to the first tributary that Susan showed that afternoon. We picked this because it was an easily identifiable landmark on a map and was about 6km upriver. We took about 2 hours to cover the distance to the mouth of the river. In all we counted 29 crocodiles, but oddly only spotted 2 large adults with the majority of crocodiles being hatchlings and juveniles less than a meter long. This was strange since we had seen nearly a dozen or so large ones during the day! Where were they? Well, being that this was our first survey, we couldn’t really draw any conclusions, but it did tell us maybe our counts would be underestimates? At least we were able to get a decent survey done for relative comparisons, and we were getting practice at our technique. It’s not terribly difficult, but takes practice and a keen eye to spot the glowing eyes. It was very important to get at least one good night on the Rio Coto for comparison purposes, and we accomplished that!

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