African Wildlife and Environment Issue 65

FAUNA, FLORA & WILDLIFE

FAUNA, FLORA & WILDLIFE

crocodiles encountered, but generally we will also catch specific size classes of crocodiles to collect biological samples for other studies we are running concurrently to our population work. The catching of crocodiles is hugely labour intensive and requires experienced hands and excellent team work. With a team of three, each person has a very specific role in the process. We have a driver, a ‘spot-lighter’ and a catcher. The spot-lighter identifies the crocodile and indicates by hand signal to the driver, who steadily directs the boat toward the crocodile. The catcher is now responsible for either catching the animal (if it is within the size class we are targeting for samples) or visually estimating the size of the animal. These jobs are rotated throughout the night in order to limit the possibilities of fatigue and mistakes. Because this is a mobile survey (i.e. no base camp to return to), it is essential that our gear is kept to a minimum as all of our camp gear needs to transported with us every night. There also needs to be space on deck to load a 3 meter crocodile should this be required! At the end of our survey the team had successfully captured a number of adult crocodiles, covered 260 km of river and counted a total of 433 crocodiles. The population density comparative study allows us to understand the success (or failures) of previous conservation and management regulations. This will allow officials to remedy any shortfalls in regulation and policy around the management of this keystone species. As a recently enlisted UNESCO World Heritage Site, the Government is required to closely monitor wildlife species numbers and this study will provide some of the most comprehensive population monitoring data for any wildlife species in the Okavango. As our efforts are focused in the unprotected areas of the Okavango, this study serves as a critical assessment of the wildlife and human co existence challenges. The initial spotlight survey carried out in 2008 provided a population estimate of 1 793 crocodiles. The 2016 spotlight survey showed a slight increase in numbers with a total estimate of 1 931 crocodiles. These surveys represent a 7.7% increase in the population, which is encouraging. While the increase is not substantial, it is certainly a strong enough indication that the population is at the very least stabilizing and possibly showing signs of increasing. The proportion of crocodiles in each size class has remained somewhat constant over time. One notable exception is the decline in the proportion of sub-adults in 2016 (suggesting a decline of 74% relative to 2008; 5% versus 21%). The reasons for this are unclear. Conditions between the two surveys were fairly constant with regard to: timing; observers; survey methodology; survey transects and water levels. Our previous research on crocodile diet indicates

that this size class is heavily reliant on fish in its diet. Casual spotlight observations made during the nocturnal surveys seemed to indicate a decline in both fish abundance and diversity in the lagoon systems. This anecdotal evidence may support a theory that the decline in sub-adults encountered on the 2016 survey could be related to dwindling fish numbers in these lagoon habitats. Commercial fishing activities along the Okavango Panhandle are heavily focused on lagoon systems where nylon monofilament gillnets are placed adjacent to the lagoon edges to capture and trap the fish species within this habitat type. Sub-adult crocodiles target the same species of fish in these lagoon systems and this fact is likely to bring fisherman and sub-adult crocodiles into conflict. Further anecdotal reports from recreational fishermen, lodges and residents in the region support the notion that fish catch rates have decreased substantially. Reports of large-scale commercial fishing for export markets has been suggested as the reason for this. While it is understood that the Okavango fish stocks have always been healthy enough to sustain a local commercial fishing industry, an aggressive commercial export industry has developed over the last three years which sees large numbers of fish dried and packed for export to Zambia and the Democratic Republic of Congo (DRC). Our findings on the matter of over-fishing are anecdotal at best, but what cannot be ignored is the drastic decline in sub-adult crocodiles observed and the fact that these crocodiles prefer lagoon habitats and the fish found in them. The lagoon habitats are the very same areas currently being targeted for this commercial fishing industry. The possible decline in the sub-adult portion of the population is of some concern, and definitely merits further investigation by our research group and the wildlife department. Our research team will continue to carry out this critically important population density work in the Okavango Panhandle. Along with this work, our research project continues to work on a number of other aspects of crocodile conservation including human crocodile conflict, nesting surveys, environmental education at Primary Schools and scientific studies on crocodile behavior. A long lived reptile requires long term monitoring and if we are to ensure the health of this critical aquatic keystone predator, we will need to continue with this important research work.

formulated factor which we use to provide a general estimate of population density, using a single spotlight survey of the study area. So how are we able to detect a cryptic species, such as the crocodile, in the river for these surveys? The crocodile has a reflective layer in the eye called the tapetum lucidum and this reflective layer allows us to see crocodiles at night from a boat using a powerful spotlight. Our first spotlight survey of this population took place in 2008 and this survey allowed us to calculate a baseline population figure for the surveyed area. Using this initial population figure we are now able to repeat the survey to understand whether this figure is increasing or decreasing. Our surveys include total counts of crocodiles encountered but each crocodile encountered is also grouped into the relevant size

myself, Dr Sven Bourquin and Professor Lee Fitzgerald, began our spotlight survey from a small riparian island called Etsatsa in the southern part of the Okavango Panhandle. For six consecutive nights, between 8pm and 4am, our team travelled upstream at 20km per hour, scanning the river for crocodiles. Our route was broken up into five transects covering over 250km of river channel. At the end of each evening, our research vessel, the ‘red eye’ was carefully tied up to a tree on an open island, where we would set up camp for the day to rest and clean our gear as we prepare for the next evening of surveying. Although these surveys are short (only six nights) they are hugely demanding as work is carried out through the early and darkest hours of the morning. Nights on the Okavango River are incredible, with excellent sightings of lesser seen

class into which it falls. This allows us to understand and assess the size class distribution of the population after each survey. In order for the results to be comparable, the methods of the survey need to be as standardized as possible. With baseline population figures established, we were now ready to carry out our second spotlight survey which would ultimately give us a good idea of the pattern of recovery of this population. On 21 August 2016, our research teammade up of

species such as Pels Fishing Owls, White-backed Night Herons, Papyrus scrambling Genets and a number of fish species which rise to the surface to feed at night. Our team make routine stops on our favourite islands along the way, to boil a kettle and refresh ourselves with a strong coffee. These are great moments for reflection and discussion on how the night has gone and which sections of river we can look forward to before reaching camp. Our surveys aim to simply count and ‘size’ the

Vincent Shacks General Manager Ecotourism: WESSA www.wessa.org.za

48 |

49 | African Wildlife & Environment | 65 (2017)