Cetacean habitats in the northern Gulf of Mexico - Statistical Data Included

Fishery Bulletin, April, 2001 by Mark F. Baumgartner, Keith D. Mullin, L. Nelson May, Thomas D. Leming

Water depth was extracted from a digital bathymetric data set compiled from NAVOCEANO's DBDB5 5-minute x 5 minute gridded bathymetry, National Ocean Service's high resolution coastal bathymetric data set and Texas A&M University's digitized bathymetric charts (Herring(9)). This depth data set was provided on a 0.01 [degrees] x 0.01 [degrees] linear latitude-longitude grid with a nominal resolution of 1.1 km for the entire Gulf of Mexico. Depth gradient or sea floor slope was derived from the depth grid by using a 3 x 3 pixel Sobel gradient operator. The resulting product had the same base resolution and spatial coverage as the bathymetry data set. For descriptive purposes, the following physiographic terms will be used to denote specific depth ranges or features: continental shelf (0-200 m), shelf break (~200 m), continental slope (200-2000 m), upper continental slope (200-1000 m), lower continental slope (1000-2000 m), and deep Gulf ([is less than] 2000 m).

A single descriptor of the vertical temperature structure in the upper ocean was selected to quantify the influence of mesoscale features such as eddies on cetacean distribution. Reilly (1990) chose the depth of the 20 [degrees] C isotherm as an approximate indicator of thermocline depth in his study of cetacean habitat in the eastern tropical Pacific. We used a similar approach by extracting the depth of the 15 [degrees] isotherm from each CTD and XBT profile. This variable is not intended to represent the depth of the thermocline, however. The low-frequency, large-scale temperature variability along this isotherm is associated with the mesoscale features of interest and it occurs deep enough that it never reaches the sea surface during the spring in the northern Gulf of Mexico.

The discrete samples of the depth of the 15 [degrees] isotherm, surface chlorophyll concentration and zooplankton biomass from each cruise leg (9-17 days in duration) were interpolated on a regular 0.1 [degrees] x 0.1 [degrees] linear latitude-longitude grid by using the kriging method (Golden Software, 1994). Surface chlorophyll was log-transformed before interpolation because the observed chlorophyll concentrations had a log-normal distribution and spanned several orders of magnitude (0.02-13.02 mg/[m.sup.3]). The interpolation method provided consistent results when compared with other data sets (e.g. Fig. 2). Because no interpolation method will capture the true spatial structure of these variables, the accuracy of the interpolated values in the effort and sighting datasets is undoubtedly low. Despite these errors, however, the horizontal variability associated with mesoscale oceanographic features is much larger than these errors and therefore the interpolated fields represent these features reasonably well (e.g. Fig. 2).

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The base unit of effort for this study was defined as 1 km of actively surveyed transect during adequate sighting conditions. To conform to this definition, each contiguous transect in the effort data set was broken into 1-km linear sections and all the environmental variables measured along each 1-km section were averaged. This provided a single set of observed environmental variables for each unit of effort. Only those 1-km sections that were actively surveyed (i.e. those where the observers were on-effort) during adequate sighting conditions (defined as Beaufort sea states of 3 or less) were used for analysis. Similarly, only those cetacean sightings that occurred while observers were on-effort and in Beaufort sea states of 3 or less were used for analysis. All of the following analyses were conducted on cetacean group sightings and therefore do not account for group size.