Estimation of krill (Euphausia pacifica Hansen) biomass within semi-enclosed bodies of water
Date
1999
Authors
Romaine, Stephen John
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Abstract
A methodology combining acoustics and net tows was tested and evaluated on euphausiid biomass and distributions in two B.C. inlets adjoining the Strait of Georgia from 1991 to 1996. Better information concerning the reliability of seasonal and interannual variability of euphausiid stock size estimates as well as the within-time-period statistical reliability of these estimates was the goal of this thesis. Repeated surveys were conducted primarily in the spring and fall in both Jervis and Saanich Inlets, and monthly surveys were conducted in Jervis Inlet between June 1994-June 1995. Acoustic samples were the primary data source for biomass estimates, with net tows providing species composition and size-frequency distributions.
Transects provided closely-spaced acoustic sample points along lines. Estimation of total stock size from samples required point observations to be converted into maps, and maps to be converted to integrated biomass estimates for the survey area. Block averages estimated stock sizes from a mosaic of equally weighted sample averages integrated over the survey area. The geostatistical interpolation method of kriging estimated unknown points within an area based on a function of spatial autocovariance and unique weightings applied to neighbouring measurements. Both acoustic estimation methods provided similar results, but kriging was the preferred method of choice as data points were considered spatially dependent for estimates and high resolution spatial distributions could be further displayed and analyzed.
One source of uncertainty in stock size estimates based on linear acoustic transects was their repeatability. This uncertainty was evaluated by comparing results from independent survey grids between both transects and days. On a daily basis, estimates agreed to within 31-59% (2 standard deviations) and between survey grid agreement was within 46-63%.
Seasonal variation of stock size due to predation, growth, and migration was on average about 13-2.6x greater than within time period variations due to statistical uncertainty. Monthly surveys of Jervis Inlet noted a bimodal biomass peaking in spring, followed by a large drop in early summer. A second peak occurred in October followed by a large winter decline. Less frequent sampling of Saanich Inlet showed higher average euphausiids stocks in the fall compared to the spring, and a similar winter decline. Broader coverage of Jervis Inlet noted a similar distribution and over-winter biomass decline to Saanich Inlet.
Conversion from acoustic return to biomass was an additional source of uncertainty. A hybrid acoustic target strength model was developed to minimize this uncertainty by using seasonal sizeĀ frequency information combined with Macaulay's (1994b) model for euphausiid acoustic returns. The strong seasonal size-frequency target strength conversion factors varied up to a magnitude of four.