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Format and Methods

Statistical Overview for Map Generation (for complete details, see Matarese et al., 2003)

The data described in the Information and Data Sources Section were combined to produce an average spatial distribution as presented in the maps on individual taxon pages. Abundance from each station was measured as catch per 10 m2 surface area. The statistical method, which we adopted, was to stratify the data into equal-sized time intervals (e.g., years), which partially controls the bias due to uneven sampling effort in time. This does not completely correct the bias because some time interval strata have no data at all, and stratification does not help in these cases. Bias for the strata that do have data is corrected by giving each stratum equal weight regardless of the number of stations. For the maps, the data within each 625 km2 grid cell were stratified by year. This removes bias due to uneven sampling between years within each cell, but only for those years for which each cell was sampled; this does not correct for uneven sampling for those years where the cell was not sampled at all.

Data Layers

The maps were produced with a geographic information system using ArcGIS software (ArcMap 10.2.1), a product of Environmental Systems Research Institute (ESRI). The ESRI Ocean Basemap is a composite of data from the following sources: ESRI, General Bathymetric Chart of the Oceans digital atlas (GEBCO), NOAA, CHS, CSUMB, National Geographic, DeLorme, and NAVTEQ.

Occurrence Map Generation

Larval Occurrence Map

The fish density data layer shows all the sample locations referenced geographically. However, samples were often taken at the same location resulting in points plotting on top of each other. Because we wanted to show the quantity and distribution of the samples, a different symbology was needed. Fish density is continuous in space even though the density may be zero at some locations. So, point locations were aggregated into a surface by overlaying the points onto a regular grid and assigning a mean value to each grid cell. The cell size of the grid is 25 km x 25 km (625 km2), or roughly 15.5 miles x 15.5 miles. The mean value at each grid cell was derived in two stages. First, the mean catch per 10 m2 was calculated per cell per year, and then the mean catch per 10 m2 was calculated per cell averaged over the years. This equalized the contribution of any single year since some years may have had a greater number of samples. The resulting data layer was a polygon showing all cells where samples were taken and their associated abundance based on catch per 10 m2.

A chloropleth map design was chosen to depict abundance. The purpose of the map is to show the general spatial extent of the data and the general trend of average larval abundance over space. Chloropleth maps shade statistical units with intensity proportional to the data values. All cells that were sampled, but contained no individuals of the taxon, are symbolized as gray, indicating absence. The remaining data were classified using quantiles: data were ranked, ordered, and divided into four categories, each containing an equal number of observations. The legend shows the range for each class and the colors are hierarchical in that lighter colors denote lower levels of abundance and darker colors denote higher levels.

Adult Occurrence Map

The occurrences of adults were derived for the most part from AFSC data residing in RACEBASE, an Oracle database. RACEBASE was developed by the Resource Assessment and Conservation Engineering Division (RACE) and comprises data from assessment, hydroacoustic, and foreign surveys conducted by federal fishery scientists from 1948 to the present. The geographic extent of RACEBASE data covers the continental shelf and slope of western North America and northeastern Asia from the Arctic Ocean (72° 14' N, 167° 52' W) south through the eastern half of the Chukchi Sea, throughout the Bering Sea (including the continental shelf of northeast Siberia), the Aleutian Basin and eastward along the Aleutian Islands, and along the U.S. Pacific coast from the Gulf of Alaska to the southern border of California (32° 28' N, 119° 18' W). Adult occurrence data points for taxa where standard RACEBASE data were insufficient were obtained from literature and unpublished sources. Maps generated with alternative data show presence only because the geographic extent of individual surveys was unknown (no gray area appears). Some maps that have a combination of both RACEBASE and alternative data will include the extent of known Race surveys. Light green triangles denote where surveys occurred, but no adults were found. Dark green triangles show the presence of the taxon.

Egg Occurrence Map

The data for the egg occurrence map was processed similarly to the adult occurrence map. A data layer was overlaid with a polygon grid having a 25 km x 25 km cell size to produce the final data layer showing presence or absence of eggs of the taxon. The result was a layout design consistent with the larval abundance map, which overcame the issue of stacked data points. Yellow circles denote where samples were taken, but no eggs were found. Orange circles show the presence of the taxon.

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last updated: August 2019