Page:Biodiversity Assessment of the Fishes of Saba Bank Atoll, Netherlands Antilles.pdf/33

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occur in the central Caribbean, but only three of these species were found on Saba Bank. In addition, the absence of mangrove vegetation and apparent lack of sea-grass beds also limits the fish fauna of Saba Bank. In their quantitative study of a number of Saba Bank habitats, Toller et al. [1] attribute the apparent lack or rarity of a number of fish species found on Saba Bank to the absence of those habitats as nurseries for the juvenile stages of these fish species. Nevertheless, diverse habitat types exist on Saba Bank ranging from coral reefs and algal flats to soft-bottom lagoon areas and scoured hard, flat, pavement-like zones. These diverse habitats support a highly diverse, fish fauna and include a number of undescribed, new species along with species rarely encountered elsewhere in the Caribbean.

Figure 198. Sphoeroides spengleri, 73.1 mm SL, photo by JT Williams.

The highly diverse, fish fauna so far reported appears to substantially under represent the species richness of fishes for Saba Bank. The actual species-accumulation curve of rotenone collections and visual surveys combined do not reach an asymptote (Figure 201). The expected species-accumulation curves (Chao2 and Jack1 estimators in EstimateS) predict total fish-species richness somewhere between 320 and 411 species.

Of the 270 species we report (Table 1) from Saba Bank, 132 (49%) were observed during visual surveys. This result is comparable to the findings for Navassa Island where 41% of the fish species were detected by visual surveys [2], for the Mona Passage islands with 43% of the fish species detected visually [3], and for BIRNM with 44% of the fish species detected visually [4]. A higher percentage of fish species was detected visually at Saba Bank than at Navassa, Mona, or BIRNM. This could possibly be a result of the higher number of visual surveys carried out at Saba Bank than at Navassa or Mona. Although the BIRNM results are based on a higher number of visual surveys (70), a comparable percentage of the fauna was detected visually at BIRNM. BIRNM ichthyocide surveys (58) appear to demonstrate the ability of ichthyocide collections to more thoroughly sample the fauna (see below). Visual censuses have numerous biases as discussed in the methods for the Pelican Cays study [5]. Despite the slight differences in methods and sampling designs between the published fish species-richness studies in different parts of the Caribbean, the methods utilized produce consistent and comparable results.

Figure 199. Chilomycterus antillarum, 180.0 mm SL, photo by JT Williams.
Figure 200. Diodon holocanthus, 104.9 mm SL, photo by JT Williams.

Ichthyocide collections at Saba Bank yielded specimens representing 155 fish species (57% of the total fish fauna). At Navassa Island where there were fewer visual surveys than at Saba Bank, over 70% of the fish species were collected with ichthyocide [2], at the Mona Passage islands, 61% of the fish species were collected with ichthyocide [3], and at BIRNM, 87% of the fish species were collected with ichthyocide despite their being more visual surveys in the BIRNM study [4]. By occupying 58 (27 at Saba Bank) ichthyocide stations covering a broader diversity of ecological habitats, the BIRNM study appears to have obtained a more thorough sampling of the resident species. The lower number of species taken at Saba Bank with ichthyocide is at least partially due to the absence of shallow habitat. Shallow habitat is required by many of the small cryptic fish species that are normally detected only with the use of ichthyocide and these species have not been found at Saba Bank. The prevalence of strong currents across Saba Bank further limits the effectiveness of ichthyocide, which is only effective when relatively high concentrations of ichthyocide remain in one place (preferably in a confined area) for more than 15 to 20 minutes. The ichthyocide-collecting methods employed at Saba Bank were the same as those used at Navassa Island and Belize. The BIRNM ichthyocide collecting methods differed from ours only in that the BIRNM study used a block net in addition to collecting outside the net. Nevertheless, the BIRNM methods were comparable to ours because the BIRNM study included those species collected outside the block nets in their results. There are inherent biases in ichthyocide collecting due to unpredictability of environmental parameters, such as currents, temperature, degree of confinement of the sampling area, ability of larger species to swim away, and variable assays of active rotenone in the powdered Derris root (batches often vary from 5% to over 11% active rotenone due to natural variation in rotenone concentration among the roots of different plants). Despite these biases, ichthyocide collections yield comparable results as described above.

Neither visual surveying nor ichthyocide collecting alone are capable of providing a comprehensive survey of coastal (or submerged atoll in the case of Saba Bank) fish species. A

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