Bottom Trawl Surveys

The Northeast Fisheries Science Center (NEFSC) of the National Marine Fisheries Service (NMFS) began fall and spring Bottom Trawl Surveys in 1963 and 1968, respectively to provide a basis for determining relative changes in the abundance of a wide variety of marine fish species.[26] The Surveys are conducted of the continental shelf from the Gulf of Maine to Cape Hatteras. Tows were made with a #36 Yankee otter trawl rigged with rollers, 5 fathom legs and 1000 pound polyvent doors; the codend and upper belly were lined with ½-inch mesh to retain the young-of-the-year fish.[27] Tows are thirty minutes in duration and made over randomly-selected areas of bottom. The results of these surveys play an important role in the stock assessments for most of the marine fisheries that are managed in the Mid-Atlantic and New England regions. However the NEFSC surveys differ from commercial fishery trawls because the short tow times may result in lighter NEFSC survey landings relative to commercial landings, making the two sources of data difficult to compare. Also, 30-minute survey tows were made constantly over a 24-hour period with day and night catchability differences not taken into account.[28] Despite these points the survey tow data provides useful information about the distribution and relative abundance of species within the survey area.

To read about the recent problems with the NEFSC trawl survey gear please click here.

During the spring of 1999 NEFSC surveys were conducted from Cape Hatteras to the Gulf of Maine from March 1 to April 22. The largest catches were reported at stations 314 and 317 where 336 and 354 pounds of cod were caught, respectively. 23 stations recorded catches of between 1 and 10 pounds of cod while 18 stations recorded catches of between 11 and 50 pounds of cod. 9 stations recorded catches in excess of 50 pounds of cod. A total of 2,211 pounds of cod were caught at 51 stations during the length of the survey.[29]

Image adapted from http://www.nefsc.noaa.gov/esb/survey_reports/Spring%201999/spring99.pdf

During the fall of 1999 NEFSC surveys were conducted from Cape Hatteras to the Gulf of Maine from September 20 to November 10. The largest catches were reported at stations 298 and 312 where 150 and 136 pounds of cod were caught, respectively. 28 stations recorded catches of between 1 and 10 pounds of cod while 23 stations recorded catches of between 11 and 50 pounds of cod. Only 11 stations recorded catches in excess of 50 pounds of cod. A total of 1,818 pounds of cod were caught at 62 stations during the length of the survey.

Image adapted from http://www.nefsc.noaa.gov/esb/survey_reports/fall1999/9908%20entire%20file.pdf

During the spring of 2000 NEFSC surveys were conducted from Cape Hatteras to the Gulf of Maine from March 15 to May 4. The largest catches were reported at stations 195 and 244 where 474 and 1,477 pounds of cod were caught, respectively. 38 stations recorded catches of between 1 and 10 pounds of cod while 32 stations recorded catches of between 11 and 50 pounds of cod. Only 11 stations recorded catches in excess of 50 pounds of cod. A total of 3,626 pounds of cod were caught at 81 stations during the length of the survey.

Image adapted from http://www.nefsc.noaa.gov/esb/survey_reports/Spring%202000/spr2000_all.pdf

During the fall of 2000 NEFSC surveys were conducted from Cape Hatteras to the Gulf of Maine from March 15 to May 4. The largest catches were reported at stations 324 and 335 where 133 pounds of cod were caught at both stations. 23 stations recorded catches of between 1 and 10 pounds of cod while 19 stations recorded catches of between 11 and 50 pounds of cod. Only 6 stations recorded catches in excess of 50 pounds of cod. A total of 1,368 pounds of cod were caught at 48 stations during the length of the survey.

Image adapted from http://www.nefsc.noaa.gov/esb/survey_reports/Fall%202000/complete%202005.PDF

During the spring of 2001 NEFSC surveys were conducted from Cape Hatteras to the Gulf of Maine from February 26 to April 30. The largest catches were reported at stations 272 and 348 where 368 pounds of cod were caught, respectively. 27 stations recorded catches of between 1 and 10 pounds of cod while 25 stations recorded catches of between 11 and 50 pounds of cod. 15 stations recorded catches in excess of 50 pounds of cod. A total of 2,706 pounds of cod were caught at 67 stations during the length of the survey.

Image adapted from http://www.nefsc.noaa.gov/esb/survey_reports/Spring%202001/whole%20spring%20FR.pdf

During the fall of 2001 NEFSC surveys were conducted from Cape Hatteras to the Gulf of Maine from September 4 to October 23. The largest catches were reported at stations 301 and 311 where 307 and 299 pounds of cod were caught, respectively. 27 stations recorded catches of between 1 and 10 pounds of cod while 17 stations recorded catches of between 11 and 50 pounds of cod. 14 stations recorded catches in excess of 50 pounds of cod. A total of 2,817 pounds of cod were caught at 58 stations during the length of the survey.

Image adapted from http://www.nefsc.noaa.gov/esb/survey_reports/Fall%202001/full%20fall%20fishermans%20report.pdf

During the spring of 2002 NEFSC surveys were conducted from Cape Hatteras to the Gulf of Maine from March 5 to April 25. Again, tows were made identical gear from the previous years. The largest catches were reported at stations 250 and 321 where 215 and 509 pounds of cod were caught, respectively. 29 stations recorded catches of between 1 and 10 pounds of cod while 42 stations recorded catches of between 11 and 50 pounds of cod. 18 stations recorded catches in excess of 50 pounds of cod. A total of 3,140 pounds of cod were caught at 89 stations during the length of the survey.

Image adapted from http://www.nefsc.noaa.gov/esb/survey_reports/Spring%202002/complete%20document.pdf

During the fall of 2002 NEFSC surveys were conducted from Cape Hatteras to the Gulf of Maine from March 5 to April 25. The largest catches were reported at stations 225 and 326 where 1493 and 4253 pounds of cod were caught, respectively. 14 stations recorded catches of between 1 and 10 pounds of cod while 22 stations recorded catches of between 11 and 50 pounds of cod. 15 stations recorded catches in excess of 50 pounds of cod. A total of 7,876 pounds of cod were caught at 51 stations during the length of the survey.

Image adapted from http://www.nefsc.noaa.gov/esb/survey_reports/Fall%202002/FBTS%20all.pdf

When we average the weight and numbers of cod caught per tow we come up with the mean catch per tow. For the spring surveys the years with the lowest average numbers of cod per tow were 1971 and 1995 when an average of 1.43 and 1.27 cod were caught per tow. The spring surveys that produced the largest average number of cod caught per tow were 1968 and 1973 when an average of 5.44 and 7.54 cod were caught per tow. The fall surveys produced lows of an average of 0.87 and 0.84 cod per tow in 1997 and 1998 respectively. Highs of an average of 9.31 and 7.77 cod per tow were achieved for fall 1972 and fall 1982, respectively.

However, the seasons with the largest and smallest average number of cod per tow did not necessarily produce the tows with the largest and smallest average weight of cod per tow. The spring surveys that produced the lowest average weight of cod per tow came back-to-back in 1994 and 1995 when only 2.4 kg of cod were averaged for every tow. The spring surveys that produced the largest average weight of cod per tow were 1968 and 1973 when an average of 17.9 kg and 18.8 kg of cod were caught per tow. The fall surveys produced lows of an average of 1.0 kg and 1.5 kg of cod per tow in 1993 and 1998, respectively. Highs of an average of 22.8 and 19.4 kg of cod per tow were achieved in 1964 and 1968, respectively.

This animation loop shows relative cod stock weight and location 1979 - 2002. Each frame represents 3 years of survey data for cod and frames advance every 1.5 seconds. (yellow circles indicate cod are present, larger circles indicate more cod, plus sign (+) indicates sampled area where no cod were found[30]) However, this is not a measure of actual abundance nor is it a stock assessment.