Saturday, 31 October 2009

Juvenile stages of coastal fish (some results)

With all the surface trawls having been completed, we have had a preliminary look at some of our data on juvenile fish stages. During a total of 19 trawls, some 151 taxa were distinguished, photographed and prepared for bar-coding and identification. The taxa ranged from species with potential commercial importance such as scombrids (2 or 3 tuna species), anchovies and carangids (10 kingfish species) to coral reef inhabitants such as parrot fish, rock cods and surgeon fishes. To get an idea of the variety of forms and colours, click on the image below. Any guessed on the species in question? Some are easier to associate to their families than others. If you get it right, we will let you know once identification has been completed.

In terms of juvenile species richness (number of species), distribution and abundance the current cruise data must be interpreted with great care. The number of trawls was relatively low and no attempts have yet been made to link our sample compositions to the physical environment. In order to strengthen the data, samples from previous ASCLME cruises as well as upcoming regional cruises need to be added to the analysis. Nonetheless some patterns do seem to emerge (but for now, need to be treated with a pinch or salt).
The number of species caught in each trawl did not vary dramatically between locations (see below). On average 21 species were distinguished per trawl. The fact that this is only a tiny fraction of the total number of species identified, suggests that we have either seriously under sampled (likely) or that different species are found in different localities (apparently true in some cases). Only a more complete analysis of all data sets will allow for a less subjective interpretation.
While the number of taxa collected show little variation between locations, the number of individuals per unit water volume vary more dramatically (see below). The highest densities of juveniles were generally caught along the Madagasi shelf, with fewer being observed in the Comoros region and the lowest overall abundances along the Mozambique coast. Again, additional samples need to be added to this analysis to see whether this pattern stands up to scrutiny.
The juveniles caught during this cruise will add to a regional juvenile identification guide, will allow us to better understand larval and juvenile origins and dispersal (with the help of genetics) and should eventually allow for more informed regional management strategies (pertaining to commercial fisheries as well as coastal ecosystem health).

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