The Northeast Fisheries Science Center (NEFSC) has been conducting standardized bottom trawl surveys in the fall since 1963 and spring since 1968. The surveys follow a stratified random design. Fish species and several invertebrate species are enumerated on a tow by tow basis (Azarovitz 1981). The data are housed in the NEFSC’s survey database (SVDBS) maintained by the Ecosystem Survey Branch.
Direct pulls from the database are not advisable as there have been
several gear modifications and vessel changes over the course of the
time series (Miller et
al. 2010). T R package survdat was developed as
a database query that applies the appropriate calibration factors for a
seamless time series since the 1960s.
The R package survdat is used to pull and process the
data. survdat identifies those research cruises associated
with the seasonal bottom trawl surveys and pulls the station and
biological data. Station data includes tow identification (cruise,
station, and stratum), tow location and date, as well as several
environmental variables (depth, surface/bottom salinity, and
surface/bottom temperature). Stations are filtered using a station,
haul, gear (SHG) code for tows prior to 2009 and a tow, operations,
gear, and aquisition (TOGA) code from 2009 onward. The codes that
correspond to a representative tow (SHG <= 136 or TOGA <= 1324)
are the same used by assessment biologists at the NEFSC. Biological data
includes the total biomass and abundance by species, as well as lengths
and number at length.
survdat applies the calibration factors. There are four
calibrartion factors applied (Table 64.1). Calibration factors are
pulled directly from SVDBS. Vessel conversions were made from either the
NOAA Ship Delaware II or NOAA Ship Henry
Bigelow to the NOAA Ship Albatross IV which
was the primary vessel for most of the time series. The Albatross was
decommissioned in 2009 and the Bigelow is now the primary vessel for the
bottom trawl survey.
| Name | Code | Applied |
|---|---|---|
| Door Conversion | DCF | <1985 |
| Net Conversion | GCF | 1973 - 1981 (Spring) |
| Vessel Conversion I | VCF | Delaware II records |
| Vessel Conversion II | BCF | Henry Bigelow records |
Describe a little about the survey process that pertains to the data.
eg.
Since 19xx all species landed on the survey vessel are measured for length. A subset of these are then individually weighed, sexed, assessed for stage of maturity, and undergo a stomach content analysis
At the tow level … expanded number, expanded weight etc..