Abstract. To a certain degree, Eastern Boundary Current (EBC) ecosystems are similar: Cold bottom water from moderate depths, rich in nutrients, is transported to the euphotic zone by a combination of trade winds, Coriolis force and Ekman transport. The resultant high primary production fuels a rich secondary production in the upper pelagic and nearshore zones, but where O 2 exchange is restricted, it creates oxygen minimum zones (OMZs) at shelf and upper slope (Humboldt and Benguela Current) or slope depths (California Current). These hypoxic zones host a specifically adapted, small macro-and meiofauna together with giant sulphur bacteria that use nitrate to oxydise H 2 S. In all EBC, small polychaetes, large nematodes and other opportunistic benthic species have adapted to the hypoxic conditions and co-exist with sulphur bacteria, which seem to be particularly dominant off Peru and Chile. However, a massive reduction of macrobenthos occurs in the core of the OMZ. In the Humboldt Current area the OMZ ranges between <100 and about 600 m, with decreasing thickness in a poleward direction. The OMZ merges into better oxygenated zones towards the deep sea, where large cold-water mega-and macrofauna occupy a dominant role as in the nearshore strip. The Benguela Current OMZ has a similar upper limit but remains shallower. It also hosts giant sulphur bacteria but little is known about the benthic fauna. However, sulphur eruptions and intense hypoxia might preclude the coexistence of significant mega-und macrobenthos. Conversely, off North America the upper limit of the OMZ is considerably deeper (e.g., 500-

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The Callinectes arcuatus population of the Gulf of Nicoya and its current level of exploitation were investigated based on size frequency analysis of trap and trawl catches. Von Bertalanffy growth parameters (K = 0.89; CW ∞ = 142 mm for males) are in the range reported for other species of this family and suggest that the male size at first maturity (CW mat = 94.3 mm) is reached in about a year. Total and natural mortality (Z yr = 2.49; M yr = 1.32) were derived from catch curve analysis and age at first maturity, respectively, and indicate that the stock is below full exploitation (E = 0.47). A yield per recruit analysis suggests that yield could be maximised, if E was increased to 0.7 under the precondition that recruitment was independent of stock size. Until this has been verified, the use of a precautionary exploitation rate (E 0.1 ) of 0.57 is advised allowing for a 20% increase in fishing effort (from 300 traps currently being fished to around 360 traps). A maximum effort of 1600 traps, as recommended by the Instituto Costarricense de Pesca y Acuicultura (INCOPESCA), is unlikely to be sustained by the population, since a decrease in the proportion of large males in the catches has already been observed over the past years under the current fishing regime. This resource is as yet only sold locally, but present catches of around 145 t seem to already cause market saturation. A greatly increased catch (at E > 0.57) would thus not only be detrimental to the stock but also to the market price of the resource. Future developments of the fishery should be based on a co-management approach and should involve the exploration of new market opportunities such as the "soft crab market".

Vertical distribution and diel vertical migration of a zooplankton community were studied at two stations oV Central Peru in April 2006. Zooplankton was collected at Wve depth strata by vertical hauls with Hydo-Bios multinet (300-m mesh, 0.25-m 2 mouth size). The zooplankton community was distributed in relation to a strong, shallow oxycline (1 ml l ¡1 oxygen isopleth generally above 36 m). The highest total abundance was always in the upper, welloxygenated layer. The most important species were: Acartia tonsa (72.86%), Centropages brachiatus (7.5%), and Paracalanus parvus (3.1%); Acartia tonsa was the dominant species at all times. Larvae of the polychaete Magelona sp. (7.5%) and larvae of the brachiopod Discinisca lamellosa (3.5%) were numerically dominant in April and small copepods e.g. Oncaea venusta (3.88%) were numerically dominant during August. Five distinct patterns of vertical distribution and migration in relation to the oxygen minimum layer were distinguished in this study: (1) Ontogenetic vertical migration through the oxycline (Acartia tonsa adults, nauplii, and copepodids), (2) permanent limitation to layers above the oxycline (e.g. Oikopleura sp., most invertebrate larvae), (3) distribution mostly below the oxycline with occasional migration into the layers just above the oxycline (Eucalanus inermis), (4) Diel Vertical Migration (Centropages brachiatus), and (5) reverse Diel Vertical Migration (larvae of the polychaete Magelona sp.).