Multi-Species Bottom Trawl Surveys
Staff at the St. Andrews Biological Station have led and coordinated DFO's annual Multi-Species Bottom Trawl Surveys since they began on the Scotian Shelf/Bay of Fundy in 1970, and on Georges Bank in 1987. Primarily, these surveys are used as fisheries independent tools to estimate stock abundance (the magnitude of the marine populations) and recruitment (the abundance of juveniles) over time for a number of fish and invertebrate species. This information is then used along with fisheries catch data to assess the status of commercial species such as cod, haddock, pollock, halibut, offshore lobster, shrimp etc. An annual survey review document is produced to summarize the results of the survey and is provided to Fisheries Management to assist them in deciding which stocks require a more complete assessment. Stocks with no detailed assessment, like sand lance and squid, are also reviewed with the information.
Although surveys have been conducted on the Scotian Shelf and Bay of Fundy since the 1950s, it wasn't until 1970 that an annual survey was created using a stratified random design where stations are randomly selected within unvarying specifically defined areas. Standard sampling protocols were also initiated in 1970 to ensure that the data collected on each species was unbiased and comparable from year to year. This sampling design is now used on both the current Scotian Shelf/Bay of Fundy and Georges Bank surveys.
The multi-species surveys also provide valuable information on the ecosystem of the areas sampled using three techniques: bottom trawls, hydrographic samples, and vertical plankton tows. Data is collected on non-commercial fish species, the physical and chemical properties of seawater, and the type and abundance of phytoplankton and zooplankton. For the users of this information, the surveys provide an opportunity to collect the data that they work with firsthand.
Aboard a DFO research vessel, staff work around the clock throughout the surveys, collecting approximately 200 species with the bottom trawl. All fish are identified, as are most invertebrates. For most bottom trawl catches, every fish is sampled in biological detail, however if time is short and there is an excess of a species, sub-sampling is required. Depending on the type of sample required, information on length, weight, sex, and maturity is collected. Otoliths (earbones) or scales are taken from many commercial fish species for ageing and stomachs are removed and the contents are analysed and identified. For some species such as shrimp, no detailed data are collected on individuals, and for others, like sponges and tunicates, identification is done only to a higher taxonomic group. Specimens that cannot be identified are photographed, frozen and retained for later identification.
As technology has advanced, the extent of the hydrographical samples that are collected has changed immensely. Originally, only surface and bottom water samples and temperature profiles were taken. Since 1990, temperature, salinity, and oxygen data have been collected with a SEABIRDTMCTD (conductivity, temperature, depth). In the last decade, nutrients and chlorophyll have been measured. Vertical zooplankton and phytoplankton tows have also been added. During the survey, CTD and water sampling are done at every station, whilst plankton sets are only done at select stations. This combination of hydrographical and trawl data is useful for a wide variety of ecosystem studies.
Providing Reliable, Consistent Data
The survey methodology is designed to be consistent with the previous years' surveys. This reliability is important in ensuring the collection of consistent and comparable data for the entire time series. The Surveys Manual is a bottom trawl surveys reference for participants from Science Branch, Maritimes Region and also for those who use the data. The manual includes information for procedures and protocols, sampling techniques, data and error checking, species identification, and codes. It also contains general information for common situations, although it is not exhaustive, and is not meant to be a training manual. To master the skills needed for at-sea data collection and editing, considerable on-the-job training is needed
With consistency in mind, technological advances have been incorporated into the surveys through the years. These include switching from a Yankee 36 side trawl to a Western IIA stern trawl and the use of electronic balances that can accurately weigh to 1/10 of a gram instead of within 25 grams. In 1995, the Groundfish Surveys Entry System (GSE) was developed and integrated as an at-sea data collection system. The GSE provides a direct connection to the database and has built-in error checks allowing the production of more reliable, accurate and timely data reports. The data can now be edited and ready for use just one week after the completion of the survey.
In the future, the annual Multi-Species Bottom Trawl Surveys will continue to move away from solely monitoring commercial groundfish species to include more information relating to entire ecosystems. More Species at Risk, non-commercial species, and invertebrates will be sampled in detail and the survey's scientific staff will be preparing for a new survey research vessel. Protocols and techniques will continue to be reviewed to improve the quantity and quality of data collected whilst still ensuring consistency and comparability. Other changes, such as an increase in the number of deep sea stations, will also be considered.
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