Study of inter-habitats connectivity in Ouano Lagoon

Context of the study

In New Caledonia, the lagoon environments includes mangroves, seagrass beds, algal colonies, soft silty bottoms as well as different types of coral reefs. The size and distribution of these habitats can influence the movement of organisms, affect their abundance, and have an impact on the functioning of the overall system.

Studies have shown that while groupings of fish species may differ from habitat to habitat, some of these species could be observed in several habitats, sometimes at different stages of development. For example, mangroves function as important nurseries for some species of reef-dwelling fish, the mangrove-raised juveniles migrating to deeper water when they reach adult stage. However, there is little hard evidence to support this theory of adult migration from one habitat to another. Similarly, there is a lack of information concerning the importance of the role of mangroves, silt bottoms or seagrass beds in terms of feeding grounds for many coral-reef dwellers.

Sustainable management of lagoon resources requires a better understanding of how fish make use of space over periods of time, and of the functional links between the different lagoon habitats. This is what we mean here by "connectivity".

The study area

The study area is located in the Southern Province, in the Ouano district. The site was chosen because it regroups the various types of habitat of interest to the study ( seagrass flats, silt bottoms, mangroves, fringing reefs, mid-lagoon reefs and inner face of the barrier reef), on account of the large quantity of biological data already collected since the creation of the marine reserve, and on account of a perceived need for further information relating to the specific needs of the region (being registered with the UN World Heritage list, while undergoing increasing pressure from human activity).

How to study the movement patterns of fish in their natural environment?

In order to be able to study the movement patterns of fish in their environment one must be able to identify them individually and durably. Much technological progress has been made in this domain, giving the scientist a wide variety of possible markers, particularly electronic: acoustic transmitters, radio transmitters, satellite transmitters, etc.
Once fitted with such transmitters, fish can be followed using specialised receivers (stationary hydrophones, boat-mounted mobile hydrophones, satellite tracking, etc.)

For this particular study, acoustic transmitters were selected, with a network of 28 VR2 multidirectional hydrophones permanently set up within the study area.

The signal broadcast by each individual marker is unique and can be identified. The markers remain active for about 6 months. Each hydrophone can detect these signals within a radius of approximately 250 m. The computer records the identification number of the marker, and the date and time of the recording.

As the number of VR2 units (stationary hydrophones) is insufficient to cover the entire study area (> 40 km2), the monitoring of fish location will also rely on a boat-mounted mobile hydrophone (VR100 Vemco). By careful planning of the boat outings, it will be possible to (a) narrow down the observations of fish activity and habitats visited, (b) to locate individual fish having strayed from the range of the fixed hydrophone network, and (c) to test certain hypotheses concerning loss of detection.

Target species

The study focuses on 3 species of commercial interest (60 individuals) regularly observed in several of the habitats present in the study area, and therefore likely to cross over habitats within the Ouano lagoon. They are one common species (Scarus ghobban or Bluebarred parrotfish), one species likely to travel in search of food (Lethrinus atkinsoni, or Yellowtail emperor), and one species which changes habitat during stages of its development (Lutjanus fulviflamma, or black spot snapper).

Identifying markers

Several techniques for implantation of acoustic markers in fish have been described: external implantation, gastric insertion, internal insertion via the oviduct (or the urogenital tract in the male), and intra-peritoneal implantation. The present study uses the latter technique, as it presents a higher rate of survival, a lower rate of rejection, and therefore the possibility of longer observations. Implantation is carried out under anaesthesia. The fish are kept in a tank to which a few drops of clove oil have been added (anaesthetic) to obtain the required concentration of 180 mg/litre. When the fish has reached the stage of clinical anaesthesia (typically < 2 minutes), it is taken out of the water for the procedure. Surgical implantation is quickly done (< 5 minutes), and no further anaesthetic is added.

Once anaesthetised, the fish is placed in a dissection tray for the operation. An incision is made with the scalpel along the ventral median line, 1 to 3 cm forward of the anus. The marker is disinfected, then delicately inserted with care taken not to injure the adjacent organs. The incision is then sutured. A healing cream (Lotagen®) is applied to the cut, and no further medication given. Once implanted, the fish are measured and weighed.

Following the operation, the fish are kept for a few hours in a recovery enclosure for observation of their state of health. They are then released by divers at the place where they had been collected, typically an area rich in sheltered holes and nooks. The fish, unavoidably stressed by having been handled, can then take shelter while they recover, thus minimising the risk of immediate predation.

The main points

  • Sustainable management of the resource requires a knowledge of habitat connectivity within the lagoon.
  • One way to study connectivity is to observe the movements of fish through acoustic monitoring.
  • Acoustic monitoring uses markers (transmitters) implanted in the fish, and hydrophone receivers.
  • These markers are surgically implanted within the abdominal cavity of the fish
  • Whenever a fish is within its range, each hydrophone detects the signal and records the event.

Summary in figures

  • Extent of the study area: > 40 km²
  • Number of species studied: 3
  • Number of acoustic markers implanted: 60
  • Number of stationary hydrophones installed: 28
  • Number of mobile hydrophones (boat-mounted): 1

Expected outcomes

The study is expected to yield valuable information on the ecology and behaviour of 3 species of fish of commercial interest in New Caledonia. It aims at identifying the interactions which exist between the various lagoon habitats, and should form the basis for further investigations (using more species, introducing other methods) with a view to optimising resource management and establishing the size of the required marine reserves, particularly the one in Ouano Lagoon.

Joint research with the University of Technology, Sydney

Through the implantation of this experimental hydrophone network in Ouano Lagoon, the Aquarium des Lagons and the Université de Nouvelle Calédonie were able to develop a joint research project with the University of Technology, Sydney, aimed at studying the migrations and larval dispersion patterns of some fish species between different habitats. The outcome of this work will be matched with similar studies being carried out as part of a thesis on species of temperate waters, to derive a comparison at different latitudes.