PARAMETERS OF MARINE INVERTEBRATE POPULATIONS 31 



Spacing is another selective agent of the scallop dredge. The smallest sizes of 

 oysters may be either attached to large shells or 'singled', so that selection of 

 attached small oysters may be made by the dredge. Trawling for Norway 

 lobsters usually results in a higher proportion of males, possibly due to a 

 difference in the burrowing behaviour of the two sexes (Poulsen, 1946). 



Special sampling problems arise where direct mechanical sampling by 

 trawl, dredge, grab, etc. is not effective because of the rocky nature of the 

 bottom, or because of the burrowing habits or the low density of the species, 

 e.g. lobsters, whelks, crabs. Here, catching by baited traps is the commercial 

 method of capture and is also used as the method of scientific samphng. The 

 reliability of sampling the population by baited traps depends on all sections 

 of the population being equally attracted by the bait and equally readily 

 retained by the trap. Baited traps must therefore be used with an under- 

 standing of the biases which may occur. 



In obtaining data on abundance of a species from catch per unit effort, 

 especially from commercial catches, the catches may be influenced by: 

 (i) The different attractiveness of baits used. 



(2) The influence of the presence in the trap both of the desired species, and 

 of other animals attracted by the bait. Table I gives examples of (i) and (2). 

 Salted skate and dead shore crabs used together as bait for catching whelks 

 were more effective than when either was used separately (A, D, E, F). The 

 figures suggest that the attractiveness of the two baits to whelks is similar, 



Table I. Catches of whelks and shore crahs in pots containing different bait 



