Assessment of methods to estimate abundance and population structure of the spider crab Maja brachydactyla in soft substrates

A good sampling method for collecting data on the abundance, spatial and demographic structure of a marine population would enable a researcher to: (1) obtain unbiased abundance estimates (absolute or relative), (2) accurately represent the spatial and demographic structure, and (3) obtain enough samples at a reasonable cost to analyze size-frequency distributions and other demographic parameters. Different methods for estimating the abundance and population structure of the spider crab Maja brachydactyla in the Ría de A Coruña (NW Spain) were assessed. Sampling was carried out with different procedures—tangle-nets, beam trawls, diving and traps. The use of tangle-nets caused serious problems as both the specimens sampled and the gear itself were damaged. These nets also provided less spatial resolution of the small-scale distribution of organisms. Sampling by means of diving in this area was unviable due to limited visibility, high turbulence and strong currents. The catches obtained during monthly samplings using traps and beam trawls in the same area during one yearly cycle (June 1998–June 1999) were compared. Trap catches were much higher, in terms of actual quantities and catch per unit of effort (CPUE), although the seasonal pattern was similar with both methods. A significant linear relationship was found between the trap and beam trawl catches: CPUEtraps = 3.34 + 3.987CPUEtrawl (R2 = 0.54, p = 0.02). The beam trawl catches contained a higher proportion of very small crabs (specimens < 60 mm carapace length, CL, comprised 23% of the total catch taken by beam trawling, compared to only 0.7% when traps were used), and very few large specimens (CL > 130 mm), especially adults. The catchability of adult crabs using the beam trawl accounted for only 12% as compared to that of traps. The fishing area of a trap was estimated to be 1440 m2 for juveniles (CL = 70 mm). A comparison of these two sampling methods in terms of cost-effective sampling (in days) showed that the beam trawl is almost four times as expensive as the traps, for the same number of organisms collected.