- This layer has been derived from the Massachusetts Division of Marine Fisheries (DMF) Resource Assessment Trawl Survey collected from 1978-2007. The survey allocates its sampling effort according to a series of 23 strata, based on five bio-geographic regions and six depth zones. Trawl sites are allocated in proportion to stratum area and randomly chosen in advance within each sampling stratum. Tows of standard speed and duration are made with a bottom trawl throughout Massachusetts Waters in the spring (May) and fall (September). Total weight and length-frequency are collected for all finfish and a subset of invertebrates. Additional information such as age, sex and maturity are collected from a list of priority species. To create this layer, 22 species were selected for consideration, based on 2 criteria: 1) they were adequately sampled by the Resource Assessment Trawl Survey and 2) they were considered to be an important component of commercial or recreational fisheries within the planning area. For two species, Atlantic cod and black sea bass, it was also considered important to include the abundance of young-of-year fish as a separate piece of information, since the biomass index may not adequately represent the areas important to YOY fish for these species. For each year/species/season/stratum combination, the mean biomass per tow was calculated, with the exception of jonah crabs, where the mean number per tow was used. The tri-mean ([1st quartile + 2*median + 3rd quartile] / 4) of the annual values for each species/season/stratum was then calculated. The tri-mean of the annual values was chosen because it is less influenced by outlier years than the mean and less sensitive to zero years than the median. For each species/season considered, one or more 'sets' of survey strata were identified, over which the data were normalized. For a stratum to be included in a set for a particular species/season, it had to have at least 6 years of non-zero catches. Furthermore, more than one strata-set was defined for some species, as the planning area encompasses multiple stock units. The normalization algorithm involved dividing each tri-mean value by the total of the values in its strata set. This has the effect of giving each species approximately equal influence in the model. The median of the normalized tri-mean values for all species/seasons was then determined for each stratum and used to create a 250 x 250-meter raster grid. The final normalized tri-mean values were re-classified into high (value=4), medium (value=3) and low (value=2) categories based on top 25%, middle 50% and bottom 25% of the data. As with the Fisheries Activity data, the re-classification routine occurred separately within 2 regions: north of Cape Cod, and south of Cape Cod.
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