Hydroacoustic data indicate that the benthic red crab Pleuroncodes planipes (Stimpson, 1860) rises from the bottom towards the surface at dusk from very compacted patches settled on the continental shelf. The behaviour is fast and abrupt, lasting 32 mm in standard carapace length, they become strictly benthic (Boyd, 1967). The seasonal inshore-offshore movements of benthic P.planipes are well studied (AuriolesGamboa, 1992), as well as the inshore-offshore larval drift (G6mez-Gutierrez and Sanchez-Ortiz, 1997). However, the diel movements of this red crab are not well understood. Although migration from the bottom to mid-water during the night is a common behaviour (Boyd, 1967; Kato, 1974), P.planipes may also be found on the surface during the day (Alvarifio, 1976; Blackburn, 1977). We have observed surface swarms arranged in bands at Magdalena Bay (24°N) during summer, like those observed with Munida gregaria (New Zealand) by Jillet and Zeldis (1985) aggregated by internal waves and surface Langmuir circulation. Boyd (1967) reported that the red crabs do migrate towards the sea surface during the night; however, Aurioles-Gamboa (1992) observed that only a fraction of the benthic red crabs migrate towards the surface. Pelagic and benthic phases can be found simultaneously only inshore and during the seasonal reproductive months (spring, February-March), the benthic phase may migrate into the pelagic zone for reproduction (Gomez-Gutierrez and Sanchez-Ortiz, 1997). Our personal observations using hydroacoustic records indicate horizontal movements and dispersion during the night in the water column. During the day, however, the pattern is less regular; the red crabs may be found compacted and very close to © Oxford University Press 2009 C-f.Robinson and J.G6mez-Gutierrez the bottom, compacted and in continuous layers in deep water, in dense groups in mid-water or near to the surface. One of the problems when assessing pelagic stocks and behaviour of anchovies and sardines in the west coast of Baja California is the sudden, huge appearance of red crabs in middle water during dusk in areas where they were absent during the day. Although hydroacoustically it is possible to discriminate between the anchovy's target strength and the red crab's (MacLennan and Simmonds, 1992; Robinson et al., 1995), the problem remains when it is necessary to obtain mid-water net samples to validate the observations. A few minutes of trawl are more than enough for a net to be lost or damaged due to the presence of these crabs. Their sudden appearance may be an expansion of the red crabs observed in the middle water layer, they are very active swimmers during the night, or perhaps they rise from the bottom. The aim of this study is to find out from where, and how, the red-crab swarms appear suddenly in the middle water. In March 1995, considered as the peak of reproduction of the red crab, three transects each 18 km long, perpendicular to the coast and separated 36 km from each other, were defined from the RV 'El Puma' off the west coast of Baja California near Punta Eugenia, between 27°30'N, 115°00'W and 26°00'N, 114°00'W (Figure 2). Continuous hydroacoustic survey of the water column was obtained on each transect, starting from the neritic zone, heading to the oceanic zone and returning to the neritic zone -4 h later. Observations started on 18 March in transect one at 10:35 h and finished the next day at 09:00 h. Transect two started on 19 March at 12:00 h and finished the next day at 08:00 h. Transect three started on 20 March at 11:30 h and finished the next day at 07:00 h (standard Pacific time). A Simrad EY-200, single-beam echosounder with a working frequency of Pleuroncodes planipes Fig. 1. Adult of the red crab Pleuroncodes planipes (Stimpson, 1860).