Fisheries Research 83 (2007) 297–305 Assessment of methods to estimate abundance and population structure of the spider crab Maja brachydactyla in soft substrates Antonio Corgos, Juan Freire ∗ Grupo de Recursos Marinos y Pesquer´ ıas, Universidade da Coru˜ na, Campus da Zapateira s/n, E-15071 A Coru˜ na, Spain Received 21 December 2005; received in revised form 2 October 2006; accepted 11 October 2006 Abstract 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 R´ ıa de A Coru˜ na (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: CPUE traps = 3.34 + 3.987CPUE trawl (R 2 = 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 m 2 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. © 2006 Elsevier B.V. All rights reserved. Keywords: Abundance estimation; Maja brachydactyla; Sampling; Size-frequency distribution; Spider crab; Trawl; Trap 1. Introduction The spider crab, Maja brachydactyla (see Neumann, 1998 for taxonomic status, corresponding to the North Atlantic species previously known as Maja squinado) is a species of a high com- mercial interest in the North East Atlantic (Kergariou, 1984; Le Foll, 1993; Freire and Garc´ ıa-Allut, 2000; Freire et al., 2002). Juveniles inhabit shallow bottoms (<20 m) and differ- ent types of substrates: soft (Kergariou, 1971; Kergariou and Veron, 1981; Le Foll, 1993; Meyer, 1992; Sampedro et al., 2003), rocky (Kergariou, 1971; Kergariou and Veron, 1981; Gonz´ alez-Gurriar´ an and Freire, 1994; Stevcic, 1967, 1968) or mixed (Rodhouse, 1984; Sampedro et al., 1999; Sampedro and Gonz´ alez-Gurriar´ an, 2004). During the summer of their second ∗ Corresponding author. Tel.: +34 981 167000; fax: +34 981 167065. E-mail address: jfreire@udc.es (J. Freire). or third year of life, depending on the recruitment season, they carry out their terminal moult (Corgos, 2004; Le Foll, 1993). The animals then attain sexual maturity and cease growth (Corgos and Freire, 2006; Gonz´ alez-Gurriar´ an et al., 1995; Le Foll, 1993; Sampedro et al., 1999), after which they proceed to migrate towards deep waters (Gonz´ alez-Gurriar´ an and Freire, 1994; Gonz´ alez-Gurriar´ an et al., 2002; Hines et al., 1995; Latrouite and Le Foll, 1989; Le Foll, 1993; Stevcic, 1973). Various sampling methods have been used in previous stud- ies on the biology of M. brachydactyla—namely, traps (Meyer, 1992, Rodhouse, 1984), tangle-nets (Rodhouse, 1984), the “glass box” (a wooden box in the shape of a truncated pyra- mid open at the top and bottom). In the larger, square base, a piece of glass is installed (Gonz´ alez-Gurriar´ an and Freire, 1994; Sampedro et al., 1999; Sampedro and Gonz´ alez-Gurriar´ an, 2004), or dredges (Latrouite, 1990; Le Foll, 1993). This is the first assessment to be carried out on the efficiency of the differ- ent methods, despite the aggregative behavior and high mobility 0165-7836/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.fishres.2006.10.011