A comparative study of seven grabs used for sampling benthic macroinvertebrates in rivers
Authors: ELLIOTT J. M. , DRAKE C. M. 
Published: May 2006

SUMMARY.
After considering the large number of grabs described in the literature, seven grabs of weight < 25 kg were chosen for manual operation from a small boat: Van‐Veen grab, weighted and unweighted Ponar grabs, Friedinger version of the Petersen grab, Dietz‐La Fond mud‐snapper, pole‐operated Birge‐Ekman grab and pole‐operated Allan grab.Random samples (number of sampling units n= 10) were taken in a large tank with a known number of 2‐mm cylindrical plastic pellets amongst stones of uniform size. Separate experiments were performed with four sizes of stones (model ranges: 2–4 mm, 8–16 mm, 16–32 mm, 32–64 mm). Stratified random samples (n= 10) were taken in rivers and the modal particle sizes at four sites were 0.004–0.06 mm, 0.5–2 mm, 16–64 mm and 64–128 mm. All grabs usually took a representative sample of the substratum at each site with no strong bias towards a particular particle size. The general performance of the Friedinger, Dietz‐La Fond and Allan grabs was poor, except on a muddy bottom, with frequent failure to operate, small samples of substratum and a mean depth of penetration < 3 cm in all substrata except mud for the Dietz‐La Fond and Allan grabs. The Van‐Veen and Birge‐Ekman grabs sampled to a mean depth < 3 cm in mud and fine gravel (2–4 mm), but the Birge‐Ekman jammed frequently in fine gravel. Both Ponar grabs operated well and sampled to a mean depth ≥ 5 cm in mud and fine gravel, > 3 cm when small stones (8–16 mm) were present and 2 cm (weighted Ponar only) when larger stones (> 16 mm) were present in a gravel bottom. The mean depth was <0.8 cm for all grabs when larger stones (>16 mm) were predominant on the bottom.In the tank experiments with pellets, the efficiencies for the total catches of the Friedinger, Dietz‐La Fond and Allan grabs were low with values <45% for fine gravel (2–4 mm), < 22% for small stones (8–16 mm) and <5% for a substratum of larger stones (>16 mm). If 50% is the minimum acceptable efficiency, then the Ponar, Van‐Veen and Birge‐Ekman grabs were adequate for fine gravel, only the two Ponar grabs were adequate for small stones and no grabs were adequate for sampling a substratum of larger stones (>16 mm).In field trials, the relative abundances of major taxa were similar for most grabs at each site; Friedinger and Dietz‐La Fond grabs were the major exceptions. In terms of both mean number of taxa and mean number of invertebrates m−2 the Ponar, Birge‐Ekman and Allan grabs performed well on the predominantly muddy substratum at site 1, but only the weighted Ponar grab performed adequately on the predominantly gravel bottom with some large stones (>16 mm) at site 2. All grabs performed badly when larger stones (>16 mm) were predominant on the bottom (sites 3, 4).The relationship between the variances and means of the samples taken with each grab followed a power law for the catches of pellets in tank experiments, and for major taxa and total numbers at each site in field trials. Values of exponents in the power law lay within the range 1.14–2.34. The coefficient of variation was also frequently related to the sample mean and was an unreliable statistic for comparing the precision of grabs.