« Food Web Dynamics Program


The FWDP has two major sources of data. The first, and most extensive, is the standard NEFSC Bottom Trawl Survey Program. During these surveys, food habits data are collected for a variety of species. These multi-species surveys were designed to monitor trends in abundance and distribution, and study the ecology of fishes and invertebrates inhabiting the northeast U.S. continental shelf.

Additionally, "process-oriented" cruises are conducted periodically to address specific questions related to feeding ecology of fishes in this ecosystem. Both sources provide primarily stomach content information; composition, total and individual prey weights or volumes, and length of prey. Additional information associated with each fish predator is also collected. Other databases have encompassed the prey fields of these fish, and include zooplankton, ichthyoplankton, and benthic surveys.

The broad scale trawl surveys cover continental shelf waters from Cape Hatteras, NC to Nova Scotia, and extend from 1963 to present; however systematic fish diet sampling began in 1973 and has continued through today. The sampling program was initiated in the autumn of 1963; a spring survey was initiated in 1968; seasonal surveys have also been conducted in summer and winter on an intermittent basis. Process oriented cruises have been undertaken sporadically throughout this time series, and are geographically more focused (e.g. Georges Bank).

The Surveys

The surveys have generally utilized a #36 Yankee (or similar) bottom trawl towed at approximately 3.5 knots for 30 minutes (1963-2008) and a 4-seam bottom trawl towed at 3.0 knots for 20 minutes (2009-present) at each station. The survey is based on a stratified random design. Strata were defined (in 1963) based on water depth, latitude, and historical fishing patterns. Within each stratum, stations are assigned randomly; the number of stations allotted to a stratum is in proportion to its area (approximately one station per 200 square nautical miles). The surveys are conducted in depths of approximately 27 m to 366 m; however, greater depths are occasionally sampled in the deep canyons along the continental shelf break. Once onboard, predators are sorted to species (see pictures), weighed (to the nearest 0.1 kg) and measured (to the nearest cm), sex and maturity are determined, and subsamples of key species are eviscerated for feeding ecology studies. For a more detailed description of the survey program see Grosslein (1969), NEFC (1988) or Reid et al. (1999).

The Sampling

Sampling Photo

During the earliest years of the program (1963-68), the sampling protocol for fish diet studies required that technicians aboard the vessel examine a random selection of fish species from each station and identify at sea any prey found in the stomachs. This protocol was modified significantly in 1969 when the requirements were to collect samples from up to 20 (but not less than 5) stomachs from each species caught at each station. Each stomach sample was individually preserved at sea in 10% buffered formalin for later prey identification. Individual stomach preservation was continued until 1981, but several changes in protocol were implemented during the intervening years: the list of species selected for sampling was refined, including the addition of important squid species, the total number of stomachs preserved from each station limited, and for the period 1973-76 the number of total samples collected by broad geographical region (e.g. Gulf of Maine, Georges Bank, etc.) was limited. Prey weight (0.01 g), number, percent composition, total stomach weight (0.01g), and lengths (mm) of fish prey were determined upon examination in the laboratory. Prey identification was to the lowest taxon feasible.

In the 1981, another significant change to the at sea sampling protocol was made. While the stomachs of major species such as Atlantic cod, haddock, silver hake, yellowtail flounder, winter flounder, Atlantic herring, and Atlantic mackerel continued to be individually preserved, stomach contents of all other species were examined and identified at sea. In addition, the volumetric measurement of prey (0.1 cc) was initiated. The protocol also required that a specific number of fish per length class (specified by species) for major fish species be sampled at every station. Shipboard processing included percent composition, prey number, and prey lengths. These changes were implemented because laboratory processing of large numbers of samples proved too costly and fish prey identification is assumed to be more accurate when stomach contents are fresh.

Since 1985, all stomach samples have been processed and prey identified at sea. We continue to individually preserve stomachs for data quality assurance during our bottom trawl survey for all routinely sampled species approximately every 25th station, and from selected species during process oriented cruises. To further ensure that the highest possible quality of data be collected at sea, the Food Web Dynamics Program has conducted prey identification workshops, provides many aids and guides to assist sea-going personnel in prey identification, and attempts to have staff trained in prey identification participate on as many survey cruises as is practical.

Identification aid 1 Identification aid 2
At sea prey identification aids.

The Food Habits Data

Once collected, whether at sea or in the laboratory, the data are part of a computerized database (currently Oracle), audited, documented, and archived for analysis. This extensive database is maintained on the NEFSC network computer system. We continue to research and address various methodological issues related to these data, including: basic identification of prey, contrasting stomach volumes versus weights, interpretation of empty stomachs (are they blown, regurgitated, or truly empty?), unbiased estimators of stomach content data, at-sea data entry, effect of time of day of sampling on stomach contents, etc.

Food Habits Metadata

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(File Modified Jan. 08 2018)