Notes about the mapping
Mapping procedure: The mapping used to create these movies uses an inverse-distance weighting interpolation scheme that includes a distance penalty for depth differences between the interpolated point and the sample station. This interpolation procedure is intended to produce interpolated maps that better represent the distributions of species that are associated with bathymetric features. An example of this is sand lance in the Gulf of Maine which are associated with Stellwagen Bank, a shallow feature surrounded by deeper waters. Interpolation techniques not informed by bathymetry would suggest that sand lance are collected in moderate abundance in Stellwagen Basin, a deep water area to the west of Stellwagen Bank. In fact the adundance of sand lance in trawl samples in this area is low. The depth-informed interpolation is intended to reduce the influence of adjacent areas that are separated by a sharp depth gradient.
Mapping range: We interpolate only in NEFSC strata that contain at least 1 sample station to ensure that we do not interpolate outside of the sampling domain (e.g. Long Island Sound).
Annual maps: The annual maps are in fact informed by multiple years of data to prevent the movies from being to jumpy. The movies are thus best for looking at long-term trends rather than year to year variability. A five year window of data is used, with data from the mapped year having the highest weight in the interpolation and data from 2 years prior to and after the mapped year having much lower weights. For example a map for the year 2000 is informed by data from 1998-2002, with the weighting of the 1998 and 2002 data low, the 1999 and 2001 data intermediate and the 2000 data the highest. Any distribution change due to a single anomoulous year in these maps will thus be dampened.
Calibrations: In 2009 the R/V Bigelow took over the trawl survey from the R/V Albatross. This transition included a change in sample gear. For most species this resulted in an increase in catchability and thus the raw survey indices increased. For others such as sand lance there was a dramatic decrease in catchability. A calibration study did accompany the transition between vessels. For most species there are calibration coefficients for the total weight and number of fish caught on a tow. For some species there are also length-specific calibration coefficients which in many case show the notable changes in the selectivity of the gear. So far we have not included the new Bigelow data in these movies. We will in the future be applying the calibration coefficients as they are applied in the stock assessments for each species, length-based or otherwise.
Length-Based movies: Some species include movies of distribution by 1 cm length bin to show size based changes in the locations occupied. One of the dominant effects of high fishing mortality rates is a truncation in the size structure of a population. If the species exhibits length-specific distributions this truncation in size structure may also result in a change in distribution (e.g. the mean center of biomass) that is not associated with environmental factors.