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Habitat Dependent Catch Composition and Food Web Dynamics in the Western Gulf of Maine Closure, Stellwagen Bank

Project Summary

Funded by the National Marine Fisheries Service, this project investigated the effects of trawling versus trawling exclusion on groundfish assemblages, using the Western Gulf of Maine Closed Area (WGOMCA) as an opportunistic experimental system. The focus was on the ways that targeted groundfish species function in the food web and how this might be influenced by trawling exclusion. Eighty-seven trawls were conducted during three sampling periods, both inside and outside the WGOMCA, and among three benthic habitats: sand, gravel, and mixed sand and gravel. The contents of each trawl were counted, resulting in data matrices for catch composition by numbers and weight, weights of livers and ripening ovaries for a subset of the sample, and length-frequency data. Up to five individuals spanning the full size range for each species caught in each tow were sampled for stable isotope analysis, and archival DNA samples were taken from a subset of cod and haddock. The stomachs of these five individuals were also removed for stomach contents analysis on a subset of the samples. Reference isotope samples were taken from invertebrates and algae caught opportunistically in the trawls. In addition, a small sample of cod was jigged from boulder habitats, and two exploratory trawls were made on deep mud, to initiate study of fishes on these bottom types.

BU grad student Kevin Blinkoff and Captain Paul Vitale sorting the day's catch.

Importantly, the data demonstrated that there is an overwhelming pattern driven by the occurrence of large catches of dogfish, particularly inside the WGOMCA. There was a trend for there to be more dogfish inside than outside the closure in all three habitats, but the pattern was forced mostly by dogfish distribution on sand and mixed bottoms. The catch data support fishermen’s observations using normal commercial gear, that dogfish are concentrated on Stellwagen Bank and aggregate there during the late summer and fall, particularly within the Western Gulf of Maine Closed Area.

Bu grad student, Elizabeth Soule, recording data

Preliminary results indicate marked, species-specific patterns associated with each of the cardinal variables: habitat, sampling period, and area management. Stable isotopic signatures revealed spatial and temporal patterns of variation in the food web, and fishes' position in it. The presence of significant differences in isotopic signature for fishes of the same size and species over such small geographical distances means that abundant species exhibit appreciable site fidelity, remaining within a small area at least long enough for management area effects to appear. Stable isotope analysis has the potential to provide replicable, relevant and useful information in future ecosystem-based management decisions in the Gulf of Maine. In sum, there is a strong correlation between the Western Gulf of Maine Closed Area and fish catches, both for dogfish specifically, and for the community as a whole. The original hypothesis of higher biomass and fish numbers within the Closure across the board was rejected: some fish species were at higher biomass inside the Closure, others were at higher biomass outside the Closure, and many either showed no effect or could not be assessed with the limited numbers caught during this study.

Captain Leo Vitale and BU grad student measuring catch.

Project partners:
Dr. Les Kaufman, Boston University
Elizabeth Soule, Graduate Student, Boston University
Jason Link, Senior Scientist, Northeast Fisheries Science Center
 William Ojwang, Boston University and Kenya Marine and Fisheries Research Institute (KMFRI)
Briana Brown, Boston University
Jean-Francois Bertrand, Boston University
Captain Paul Vitale, F/V Angela and Rose
David Bergeron, Massachusetts Fishermen's Partnership
Olivia Free, Massachusetts Fishermen's Partnership

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