SPATIAL AND SEASONAL PATTERNS IN SPECIES COMPOSITION AND OCCURRENCE OF FISH LARVAE IN THE AREA OF THE CANARY ISLANDS, EASTERN CENTRAL ATLANTIC
4.1. SAMPLING METHOD Traditional vertical plankton sampling techniques allow to catch also considerable quantitites of fish larvae during hauling-in of the net.This method causes considerable bias regarding vertical distribution patterns (SASSA et al. 2002). LEIS (1986) used the same method with a collapsing net, however in more recent years other more elaborated methods have been adopted. For instance, JOHN et al. (2001, 2004a, 2004b) used a multiple opening-closing net (multinet) for their investigations. When trawling nets are used (JOHN et al. 2001; LOBEL and ROBINSON 1988; LOGERWELL and SMITH 2001; PALOMERA et al. 1988; RODRÍGUEZ et al. 1999; SABATÉS and MASÓ 1992) mainly horizontal and virtually no vertical resolution of the distribution is obtained. JOHN et al. (2001) found differences in the catch between neuston nets and multinets, especially with diurnal catches. While the multinet net does not reveal diurnal changes, the neuston net does, due to its higher visibility to the larvae. Another bias arises from the relative coarse mesh size. A mesh size of 2 mm in the cod end selects only larger larvae and is inappropriate for obtaining smaller ones. Also, larger slim larvae might escape through the wide meshes. Paralepidids and Cyclothone sp. can easily escape and are therefore underrepresented, although they are common in adult stages around the Islands (WIENERROITHER 2003 and 2005). Meticulous larval investigations use nets with a mesh size between 200 and 500 µm (ACEVES-MEDINA et al. 2004; BÉCOGNÉE 2006; JOHN et al. 2001, 2004a and 2004b; LEIS 1986; LOBEL and ROBINSON 1988; LOGERWELL and SMITH 2001; PALOMERA et al. 1988; RODRÍGUEZ et al. 1999; SABATÉS and MASÓ 1992; SASSA et al. 2002, 2004). A further problem lies in the non-identical realisation of the trawls. Having no identical replicates leads to statistical problems. In addition, overlapping sampling depths cause a bias in the statistical analyses. The variable ranges of depth and durations make it difficult to give exact details about the occurrence of different species (WIENERROITHER 2003). Moreover, in respect to the horizontal distribution of sampling stations a bias lies in the accumulation of stations in some areas, e.g., southeast of Fuerteventura. The non-randomly and non-evenly distribution of the stations within an area might also lead to a problem in the application and interpretation of statistical analyses. Further, the analyses of horizontal distribution might reflect the sampling pattern and not the real in-situ small scale distribution of fish larvae within the entire area of investigation.
Inf. Téc. Inst. Canario Cienc. Mar. n°13
Published on Mar 26, 2013
Published on Mar 26, 2013
Technical report consisting on a comprehensive annotated larvae taxa list with the most important taxonomic characters of this region