Survey description

Fish and invertebrate abundance and distribution are collected by standardized NEFSC bottom trawl surveys. The surveys are conducted in spring and fall and cover the area from Cape Hatteras, North Carolina, to Nova Scotia, Canada, at depths >27m. For more information on the survey, visit the NEFSC surveys website. The spring survey data are from 1968 to 2025 and the fall survey data are from 1963 to 2025 (Website). Additional surveys were conducted during summer (1991-1995) and winter (1992-2007) but are not analyzed here.

Survey statistics

We focused on samples from the western Gulf of Maine (within 150 km of Appledore Island). The first survey year was 1963 and the last survey year currently available is 2025. During this time interval, a total of 4274 bottom trawl tows were conducted. The minimum number of tows conducted in a year was 22 and the maximum number was 128 (Figure 1). Most spring survey tows were completed in April and May and most fall survey tows were completed in October and November (Figure 2). Spatial coverage was excellent except that nearshore areas of New Hampshire and Maine were not sampled (Figure 3).

Figure 1: The number of samples completed each year in the western Gulf of Maine.

Figure 2: The number of samples completed each month in the western Gulf of Maine.

Figure 3: Sample locations completed in the western Gulf of Maine.

Fish and invertebrate species

A measure of animal (bottom fish or invertebrate) density (catch (kg) per tow) was computed for each bottom trawl tow by species. This computed value is a measure of animal abundance. We log₁₀-transformed densities because their distribution has a long-right tail; this transformation tends to normalize the distribution. We also added ‘1’ to the densities before log-transformation because densities of ‘0’ occurred and log₁₀ of ‘0’ is not finite. The log-transformed values can be interpreted as: log-transformed values around 1 indicate the original density was around 10 kg per tow, 2 indicates around 100, 3 indicates around 1,000 and 4 around 10,000. This simplification of “tens”, “hundreds” and “thousands” nicely approximates bottom fish densities.

The number of bottom fish and invertebrate species caught in the western Gulf of Maine is 186. We selected the 16 most abundant species of each taxa to compute average densities by species and year. We chose a threshold of 16 so as to create a symmetric (4 X 4) matrix of plots of the most abundant species. We plotted density by species and year (Figure 4) as well as percent frequency of occurrence by species and year (Figure 5). In general, the time trends are similar for both statistics by year, implying that when a species density is higher, the species also is more widespread. Fish and invertebrate sizes range from about 10 to 100 cm and the ranges vary among species (Figure 6).

Figure 4: Average catch (kg) per tow in the western Gulf of Maine.

Figure 5: Percent Frequency of occurrence (FO) in the western Gulf of Maine.

Figure 6: Length frequency in the western Gulf of Maine.

Forage species

We classified forage species as forage fish (silver sided fish like herring), squid, and a few species found in neuston nets near Appledore Island (e.g., fourbeard rockling). We selected the 16 most abundant forage species to compute average densities by species and year. We plotted density by species and year (Figure 7) as well as percent frequency of occurrence by species and year (Figure 8). In general, the time trends are similar for both statistics by year, implying that when a forage species density is higher, the species also is more widespread. Forage species sizes range from about 10 to 40 cm and the ranges vary among species (Figure 9).

Figure 7: Average catch (kg) per tow in the western Gulf of Maine.

Figure 8: Percent Frequency of occurrence (FO) in the western Gulf of Maine.

Figure 9: Length frequency in the western Gulf of Maine.

Acknowledgements

We thank NOAA Northeast Fisheries Science Center for making this data publicly available and for the many field scientists who carefully collected this data.