Worldwide Cephalopod Distribution

Andy Bliss June 11th, 1999

Abstract: The purpose of this investigation is to find out what correlations might exist between the distributions of modern cephalopods and environmental conditions, such as the primary productivity of the ocean. The first phase of the study, completed this year, was to convert a series of paper range maps into digital form to allow manipulation of the data. A few preliminary conclusions have been drawn, but more substantial results should be achieved from phase two, to be completed next year.

Procedure:

The procedure followed was relatively simple. The starting point was the Food and Agriculture Organizationís (FAO) Species Catalogue. This book contains range maps for 173 different species of cephlapods along with typical depth ranges and a number of other facts. For now, I just concentrated on the maps, not worrying about other information. Using ESRIís ArcView the maps were entered into a computer, eyeballing off the paper map and using a mouse to trace out a digital map. ArcView stores this information in a shapefile (see example of Alloteuthis africana). I then had to convert this shapefile into a grid database (grid), a map with ones in all areas with the species (black) and zeros everywhere else (white). Next, I used the Map Calculator tool to combine all of the individual species maps into one composite map (composite). Essentially, this was just adding up the number of species per cell. The only problem that I ran into was with the species around Antarctica. While ArcView did have a polar map projection, it did not allow me to draw a polygon around the pole. Due to their small number, I have eliminated the Antarctic species that were plotted in polar coordinates in the book. For a different look, I created a contour map of the data (without including the numbers). This was more just an exercise than something I expected to use. A map of slope could also be made, for comparison with bathymetry, for example. The final step was to enter the areas of low primary productivity as identified from Chlorophyll a measurements taken by the SeaWiFS satellite. I started with their rectangular projection of the global biosphere averaged over September 1997 through August 1998 (it's pretty). I identified the low productivity areas using Adobe Photoshop, selecting for the colors closest to the blue end of the spectrum (see map). The areas I selected have Chlorophyll a concentrations of approximately .15 mg/m3 or less. Once I had a reasonable boundary determined, I entered it into ArcView just as I had done for the ranges and then pasted it into the Distribution map.

Data Analysis:

Although I do not have a good way to check my inputs, I believe that they are within a reasonable error margin when looking at individual maps, and probably very reasonable over the whole globe. One problem inherent in the use of this data set is that it is based on maps made for fisheries. This could bias the record against certain species. Species too small to eat might not even be included; Arctic and Antarctic areas, which are harder to fish, probably are under represented; and areas heavily populated with people that rely on fishing as a main food source would be over represented i.e. around Japan, where we observe the greatest diversity of species.

The chlorophyll line that I drew probably is about as accurate to the data as the species maps. However, as Berger pointed out in his work, there are numerous problems with making maps of global ocean productivity. The main problem that this data set has is that it was collected over such a short period of time (1 year) and, in addition, it was a strong El Niño year which could greatly change the observed productivity.

Observations / Conclusions:

Setting aside all the problems, I do not see many positive correlations. There does not seem to be a very strong relationship between areas of low productivity and areas of few cephalopod species. Note the major exceptions Ė areas of low primary productivity but high cephalopod diversity: southwest of Japan, in the Mediterranean Sea, and the southern Caribbean. Also note that these areas tend to be big fisheries. For reasons unknown to me, there is a notable absence of cephalopods all along the western coast of North and South America. More study needs to be done in order to effectively model the distribution of cephalopods. The first place to start would be to look at other variables that affect the population dynamics of cephalopods. Water depth, salinity, water temperature, sediment load, and other biologic variables should be considered.