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Abundance of the Gulf of Maine/Bay of Fundy harbor porpoise based on shipboard and aerial surveys during 1999
  • Published Date:
    2000
Filetype[PDF - 575.81 KB]


Details:
  • Personal Authors:
  • Corporate Authors:
    Northeast Fisheries Science Center (U.S.)
  • Series:
    Northeast Fisheries Science Center reference document ; 00-07
  • Document Type:
  • Description:
    The Northeast Fisheries Science Center (NEFSC) conducted a line transect sighting survey that covered waters from Cape Cod to the Gulf of St. Lawrence during 28 July to 31 August 1999. An airplane and ship surveyed 8,212 km of track lines within a region of 225,400 km². Over 91% of the survey time was conducted in sighting conditions less than or equal to Beaufort sea state two. In total, 12 cetacean species, 2 seal species, and 2 turtle species were detected. The abundance estimate (and CV) of harbor porpoise was 89,700 (CV=22%). This estimate is greater than previous estimates made in 1991, 1992, and 1995. The 1999 estimate is significantly different from only the smallest estimate, which was made in1991. The main difference between the 1999 and previous surveys was in 1999 an additional region of 12,500 km² had harbor porpoise present. Within the sub-region surveyed during all years, the 1999 encounter rate was less than that from 1995 and greater than that from 1991 and 1992, the estimates of g(0) and average group size were similar for all years, and the 1999 effective strip half-width, and density estimates were slightly larger than that from previous years. The inter-annual changes in abundance could be due to experimental variability, population growth, small-scale changes in distribution of harbor porpoise and/or their prey, heterogeneities due to, for example, inter-annual differences in the observer skills or sighting conditions, or, most likely, some combination of these. Because the region where harbor porpoise were detected in 1999 was slightly larger than that from previous surveys, the time interval between surveys is fairly long (4 to 9 years), and the possibility of a positive growth rate, the best current abundance estimate for the Gulf of Maine/Bay of Fundy harbor porpoise stock is 89,700 (CV=22%), the 1999 estimate not averaged with other years. The 1999 and previous abundance estimates are minimum estimates because the shipboard surveys were not corrected for effects from responsive movement or dive time.