In May 1992 an experiment was carried out on North Cape Bank in the Barents Sea in order to answer the following questions:
- Does seismic shooting with air guns affect catch and catch-availability of cod and haddock?
- How far away from the seismic shooting area can possible effects be demonstrated?
- How long after the conclusion of seismic shooting can possible effects be demonstrated?
This was done by means of fishing trials with trawl and longline and acoustic rnapping of the fish distribution before, during and after the seismic shooting.
The fishing trials were conducted with a trawler and with an autoline vessel within an area of 40 x 40 nautical miles before (7 days), during (5 days) and after (5 days) seismic shooting. Both vessels used commercial fishing gear. In total, 62, 67 and 60 trawl hauls were made respectively before, during and after the seismic shooting. The trawl hauls were distributed over four distances from the seismic shooting area: 1) within the shooting area, 2) 1-3 nautical miles from the shooting area, 3) 7-9 nautical miles from the shooting area, and 4) 16-18 nautical miles from the shooting area. Longline fleets were placed at four comsponding positions in relation to the shooting area. In total 56, 40 and 35 longline fleets were hauled respectively before, during and after the seismic shooting.
Within the same area and time period, the fish distribution was mapped and abundance estimated by another trawler. The acoustic mapping was executed by crisscrossing. the shooting area along transects out to 20 nautical miles. In addition, detailed mapping within the shooting area was carried out before and during the seismic shooting. Sarnples of the acoustically registered fish were taken with a standard sampling trawl.
The seismic air-gun shooting (5 days) was performed within an area of 3 x 10 nautical des in the center of the area where the fishing trials were performed. The rigging of the air-gun array and the practical execution of the shooting was performed in accordance with the same guidelines that are followed in an ordinary three-dimensional survey for the oil industry. The acoustic mapping and catchhg trials with trawl and longhe on North Cape Bank show that the seismic shooting with air guns affects the fish distribution and catch rates for cod and haddock, not only locally within the area where the shooting is carried out, but also in significant surrounding areas.
The catches by trawl and longline consisted principally of cod and haddock, with cod as the dominant species. The trawl catch rates both for cod and haddock declined over the entire investigation area, even to the border, 18 nautical miles from the shooting area. On average for the whole area, the catch rate was halved when the shooting began. The reduction was greatest in the center, that is, in the seismic shooting area. Here the average catch for both species was reduced by about 70% during the shooting. The reduction in the trawl catches generally agreed with the acoustic observations, which showed a reduction of about 45% in the total quantity of cod and haddock within the investigation area. The reduction in acoustic quantity was also greatest in the central area.
The reduction in catch rates of cod by longline were lower than by trawl. The decrease was 44% in the seismic shooting area, with a gradually declining infiuence on the catches toward the border of the investigation area. For the longline fleets set furthest away from the shooting area (16-18 nautical miles), no decline in catch rates for cod-was observed. For haddock the weight reduction per longline haul was about 50% over the entire investigation area.
In both the trawl and longline catches and in the acoustic abundance estimates a relatively greater reduction was found in large (>60 cm) than in small (<60 cm) fish. However, the number of small fish was reduced with a single exception: the quantity of small cod increased in the longline catches during the shooting.
Neither the acoustic mapping nor the trawl trials showed that the quantity of cod and haddock increased during the five days after the end of the seisrnic shooting. A change in the length distribution of fish by trawling toward the condition before shooting was observed. Longlining showed an increase in cod catches at the end of the trial period, but not in haddock catches.