BIOT Science Expedition 2014 - Dr Melita Samoilys BIOT Science Expedition 2014 Initial Report
Diversity and population abundance of reef fishes
Melita Samoilys, CORDIO and Heather Koldewey, ZSL
Objectives
The objectives of the reef fish surveys were to:
i) measure the diversity of reef fishes in Chagos using a standardised method used in the Western Indian Ocean (WIO) based on an inventory of all species from a pre-defined list of 19 families;
ii) measure abundance and biomass of a broad cross section of the fish community to determine the densities and biomass of key trophic groups considered important in coral reef resilience;
iii) estimate the abundance and biomass of groupers as taxa that are generally highly vulnerable to overfishing and consequently depleted and threatened in most coral reef regions. Chagos may represent some of the most pristine populations of coral reef groupers in the world
Having recently quantified coral reef fish diversity and population abundance in Madagascar, Comoros, Mozambique and Tanzania, a core region representing the highest biodiversity in the West Indian Ocean (WIO), the Chagos survey provided a unique opportunity to collect comparable data to this WIO dataset. Chagos potentially may provide baseline or reference values as Chagos reefs arguably represent reefs least damaged by humans in the Indian Ocean.
Methods
Species diversity
Reef fish diversity measures were recorded using a method standardised for the WIO which is designed to provide sufficient breadth of species sampling, while remaining practically feasible for relatively rapid dive surveys across a number of locations. It is based on compiling a complete species inventory of 19 families at each location. Family selection was based on a number of diversity criteria, encompassing a) most speciose; b) known indicators of aspects of fish communities; c) fishery importance; and d) taxonomic diversity. These families include potentially around 460 species in total from the WIO (Allen in McKenna and Allen 2005, Davidson et al. 2006, Obura 2004, Samoilys 1988). This was considered to be broad enough and diverse enough to capture patterns in diversity of fishes within the WIO region. For example, the potential species list represents 51% of the total number of coral reef species from 92 families reported from Madagascar (Allen 2005), one of the most diverse coral reef areas in the WIO. At each location two 30-40 minute dives were completed recording presence/absence of all species from the 19 families. In total 31 dive sites were surveyed giving species richness measures for 16 locations.
Fish abundance and biomass
Fish abundance and size class data were recorded across pre-selected taxa that can be assigned to different trophic groups with a strong focus on herbivory (after Green and Bellwood 2009), but also including other functional groups of fish, to provide a broad and representative cross section of the reef fish community. The data can be used to ask general questions about the status of reef fish populations as well as specific questions about the densities and biomass of key trophic groups considered important in coral reef resilience. Thus, fish species were categorised across seven broad functional trophic groups (Samoilys and Carlos 2000, Obura and Grimsditch 2009), with the herbivores further broken down into the six functional groups (Green and Bellwood 2009). Most fish were recorded to species level so that they could be easily assigned to the correct trophic group. Fish densities and size classes were estimated using 50 x 5 m transects on SCUBA, a standard method set since the 1980s. Sizes of all species that were >10cm TL were estimated to obtain biomass values based on published length–weight relationships. In total 31 dive sites were surveyed giving 13 fish abundance and biomass measures for 13 locations. A further restricted list of species were counted using long timed swims (10 mins) to estimate the abundance and biomass of species that are a) roving or pelagic taxa which are generally though not exclusively large that associate with coral reefs: the Scombridae, Carangidae and Carcharinidae; b) contribute significant biomass on a reef such as Cheilinus undulatus, Bolbometapon muricatum and other large excavating Scarinae, are in the top trophic categories of piscivores or omnivores such as large (> 56cm TL) serranids, lutjanidae and lethrinids.
Preliminary results and observations
Species diversity
Species diversity is lower in Chagos compared with the northern Mozambique Channel (NMC) of high diversity in the WIO, with total species counts in the order of 102 per site, compared with 135-150 in Cabo Delgado, Mozambique and Mafia island, Tanzania. Around 214 species known from the WIO were not observed in Chagos, with a total species richness of around 226 recorded across all sites, compared with 335 in the NMC. Data are still being analysed so these figures are provisional and not final. 2-3 species ranges have been extended and the Chagos Fish List compiled by Sheppard from various surveys since the 1970s will be compared.
Fish abundance
The fish abundance and biomass data require analysis, but provisional observations suggest that fish abundance was generally higher than observed elsewhere in the WIO, particularly for scarids and acanthurids, but with certain key taxa absent or rarely observed, overall biomass was not as high as expected. However, what was noticeable was the large size of the Plectropomid groupers, with 100cm TL frequently observed in Plectropomus laevis, which is the maximum size this grouper attains. This suggests little to no fishing pressure on this species. These grouper were also abundant, being present at almost every site. Reef sharks were relatively common – seen on every dive - when compared to other locations in the WIO, where they are very rarely observed. This shows the generally dire situation for reef sharks in the WIO, as these fish are known to be very depleted in Chagos (Graham et al., 2010) and poaching continues to be a problem. Grouper densities were patchy and require analysis to try and determine the reasons for this. Notable was the camouflage grouper, Epinephelus polyphekadion, which was very common and may reflect natural population abundance of this species in the absence of fishing. This fish is extremely unwary of divers and therefore very vulnerable to spearfishing. It is rarely or never seen in mainland east Africa.
Figure 19: Chlororus enneacanthus and other scarids in Middle Brother lagoon.